Valérie Franck

Valérie Madeline

Martine Tailleur

Amandine Reiter

Georges Billant

Christine Sartori

Joseph Benjamin

Christophe Daria

Jiaqi Lucia Christy

Aurélien Etiemble

Soufiane, Thiago Aurélien Didelot

Catalina To

In this thesis, we study the formation of vanadium dioxide (VO2) from a two-step procedure.

We demonstrate for the first time that high-quality thermochromic VO2 can be obtained from the air oxidation of sputter-deposited VN and V2N polycrystalline films. Depending on the annealing conditions (duration and temperature) VO2 and V2O5 are the two principal phases formed during the oxidation process of the nitride films. The first oxide obtained is VO2(M), which coexists with the precursor for a prolonged period at 450°C or suddenly disappeared at 525 and 550°C. For optimized durations at each temperature VO2(M) is the predominant phase.

For higher durations, the VO2 phase is oxidized into V2O5. The thermochromic performance of the vanadium oxide films is strongly influenced by the oxidation duration. TEM analysis indicates that both VN and V2N precursors follow different oxidation kinetics. Concerning the VN oxidation, there is an abrupt interface between the polycrystalline VN films and the formed VO2. On the other hand, EELS results for the V2N oxidation evidence a formation of a VN layer at the interface between the remaining V2N layer and the growing VO2 one. Besides, the influence of the V2N thickness on the optical modulation properties of the formed VO2(M) is studied. The oxidation of 100 nm-thick V2N films on Si substrate yields a negative emissivity switch (Δε). On the contrary, the annealing of 445 nm V2N samples produces a positive value for Δε. These findings indicate the possibility to control the sign of the optical modulation of the formed VO2 depending on the initial thickness of the V2N precursor. Furthermore, we investigate the oxidation of epitaxial VN thin films, which follow a different path than polycrystalline VN films, caused by the absence of grain boundaries in the epitaxial layers. This process depicts the epitaxial growth of a VO2(B) layer on the remaining VN, as shown by TEM results. Finally, we apply a simple criterion to transpose the deposition conditions between the two sputtering machines used in the thesis based on the thermochromic performance.

II

Résumé

Dans cette thèse, nous étudions la formation de dioxyde de vanadium (VO2) à partir d'un procédé en deux étapes. Nous démontrons pour la première fois que l'oxydation contrôlée de films polycristallins VN et V2N déposés par pulvérisation cathodique conduit à la formation de VO2 thermochromique de haute qualité. L'optimisation de la durée du recuit s'avère fondamentale pour former la phase désirée. VO2 et V2O5 sont, en général, les seules phases formées lors du processus d'oxydation des films polycristallins de VN et V2N. Le premier oxyde obtenu est VO2(M), qui coexiste avec le précurseur azoté pendant une période prolongée à des températures comprises entre 450 et 550 °C. Les performances thermochromiques des films oxydés sont affectées lorsque l'oxydation n'est pas complète ou lorsqu'elle est totale en raison de la formation de V2O5 pour des temps de recuit dépassant la valeur optimale. L'analyse MET indique que les deux précurseurs suivent des cinétiques d'oxydation différentes. Concernant l'oxydation de VN, il existe une interface abrupte entre les films polycristallins de VN et le VO2 formé. D'autre part, les résultats EELS pour l'oxydation de V2N mettent en évidence la formation d'une couche de VN à l'interface entre la couche de V2N restante et la couche de VO2 en croissance. De plus, l'influence de l'épaisseur de V2N sur le signe de la variation d'émissivité de VO2(M) formé a été étudiée. L'oxydation de films de V2N de 100 nm d'épaisseur déposés sur un substrat en Si donne un changement d'émissivité négatif (Δε). Au contraire, le recuit d'échantillons de V2N de 445 nm produit une valeur positive pour Δε. Ces résultats indiquent pour la première fois qu'il est possible de contrôler le signe de la variation d'émissivité de VO2 formé à partir de l'épaisseur initiale du précurseur V2N. Par ailleurs, nous avons également étudié l'oxydation des couches minces épitaxiées de VN, qui suivent un chemin différent des films polycristallins de VN, causé par l'absence de joints de grains dans les couches épitaxiées. Ce processus conduit à la croissance épitaxiée d'une couche de VO2(B) sur le VN restant, comme le révèlent les résultats TEM. Enfin, nous avons appliqué un critère simple pour transposer les conditions de dépôt entre les deux machines de pulvérisation utilisées durant cette thèse à partir des performances thermochromiques des films.

I believe that it is an impossible task to acknowledge everyone related to the conclusion of this thesis. For this reason, I will like to begin with my sincere gratitude to all of you I will not name in the following, but have made this journey more easy and enjoyable for me.

I would like to thank my co-supervisor Fabien Capon, for your conviction in my application when I didn't have a background in any specific area of the thesis proposal. Thanks for the trust, for letting me develop my ideas and the encouragement to work independently.

A special thanks to David Pilloud, without whom, this work would never have been done.

You have been a support and guidance during these years. I appreciate every one of our "only 5 minutes" discussions. You are the kindest person I have ever met, and it has been a privilege to work with you.

I would like to express my genuine admiration and gratitude to my supervisor Jean-François Pierson. For your full implication in my work, your patience and trust over the past few years. You are an amazing supervisor, a great researcher from who I have learned a lot. I appreciate the confidence and the liberty that you gave me during this thesis. One of the things that impressed me the most is how you care about your students. I always remember a talk that we had in Garmisch when you asked me if I was happy in France; if I thought that I made the right choice leaving Brasil, because I completely changed my scientific research topic. Your sincere concern during the pandemic was crucial for continuing the work in the confinement. For all your support, thanks Jeff! Furthermore, I honestly thank my former supervisors for introducing me to research and helping me enjoy it so much. If I ever have the opportunity to be a supervisor, I will have all of you like my guides.

Stéphanie Bruyère thank you so much for our priceless TEM sessions, for being always available to help, and also for the "pas mal" of phrases that you taught me during our sessions. I wish to thank the members of the CC-Microscopique: Jaafar Ghanbaja for your permanent desire to help, Christine Gendarme for the training in SEM, Sylvie Migot and Hiba Kabbara for their TEM lamella preparation, Denis Mangin for the SIMS analysis, and Sandrine Mathieu for IV the training in the SEM and EBSD images. I need to address how much I enjoy the time with Sylvie Migot and Sandrine Mathieu, their smiles, and their honest wish to help always made me want to come back to the -1 floor.

I also want to thank Pascal Boulet for the XRD formation, Arnaud Voignier for the hardness measurements, and Sébastien Hupont for the Raman and Reflectance measurements and our entertaining discussions about music.

I would like to thank all the other members of team 202: Silvère Barrat for letting me have a first experience teaching in French, Brigitte Vigolo for your encouragement and support, Alexandre Nominé for always good pieces of advice, and the funny music links. A special thanks to David Horwat for your interest in my research, ideas, and suggestions, and for allowing me to be part of a research project where I learned and enjoyed it a lot. I also want to acknowledge members of different teams who made valuable contributions to this work: Michel Vilasi for your ideas, proposals, discussions, and interest in the nitrides oxidation; Lionel Aranda for the TGA measurements, discussions and the readiness to help; Ludovic Pasquier for the assistance with the sample transfers in the Tube and the RHEED measurements. Finally, to Stéphane Andrieu whose support was fundamental for the growth of the VN epitaxial layers, for the RHEED measurements, and staying with me after work hours (together with Ludovic), even when I was not your student.

Thanks to Thierry Czerwiec, Grégory Marcos and Thomas Gries for giving me the opportunity to continue working in the IJL and for the patience during these last months. I am grateful to the French PIA project "Lorraine Université d'Excellence", reference

Introduction

Chromogenic materials display changes in their optical properties thanks to an external stimulus. There are different types of chromogenic materials, the most common are electrochromic, photochromic, and thermochromic, where the stimuli are the variations in the applied electrical field, irradiation by light, and in the applied temperature, respectively. The market for these smart materials is increasing over the years and covers aircraft, smart windows, automotive, and many others [START_REF] Lampert | Chromogenic smart materials[END_REF]. Numerous chromogenic materials have been the subject of research in the last decades. Among the electrochromic ones, the most relevant are NiO [2],

NixW1-x oxides [3] and WO3 [4], while TiO2 and MoO3 are the most studied for photochromic ones [5]. For the thermochromic materials, the temperature variation leads to a modification of their optical and electrical properties. Several materials present this property, such as rare earth perovskites such as cobaltites [6,7], nickelates [START_REF] Boileau | Mechanisms of Oxidation of NdNiO3-δ Thermochromic Thin Films Synthesized by a Two-Step Method in Soft Conditions[END_REF], manganites [START_REF] Boileau | Thermochromic effect at room temperature of Sm0.5Ca0.5MnO3 thin films[END_REF][START_REF] Shen | Influence of structure on infrared emissivity of lanthanum manganites[END_REF] and vanadium dioxide [START_REF] Warwick | Advances in thermochromic vanadium dioxide films[END_REF].

Among the thermochromic materials, vanadium dioxide (VO2) is the most studied compound. The VO2 undergoes a reversible metal-insulator transition (MIT) approximately at TMIT = 68°C. At low temperature, this oxide presents a semiconductor behavior with high transmittance in the infrared (IR) region and high electrical resistivity. On the contrary, at high temperature, VO2 exhibits a low transmittance in the IR region and low electrical resistivity.

The dramatic change in its physical properties offers a path to diverse applications, such as novel electronic and optical devices, thermal solar collectors, smart windows, passive radiator, among others. Nevertheless, the elaboration of pure thermochromic VO2 films on large surfaces remains considered a challenge. The reason behind this defiance is the difficult task of avoiding other phases belonging to the vanadium-oxygen system. This thesis aims to achieve thermochromic VO2 films from a new process based on the controlled oxidation of sputter-deposited VN and V2N films. The manuscript is composed of six chapters, one appendix, and a general conclusions section:

In chapter 1, we depict the principal crystallographic structures of VO2 as a function of the temperature. A short comment on the different VO2 polymorphs is presented. Furthermore, the physical properties, the band structures, and the transition phase mechanism are also described. Finally, possible application and synthesis methods of VO2, with an emphasis on magnetron sputtering are discussed.

In Chapter 2, technical and fundamental aspects of the magnetron sputtering and characterization techniques used throughout this thesis are presented. Moreover, we develop a simple approach to obtain analogous samples independently of the sputtering machines used in the present work.

Chapter 3 provides the first results on the controlled oxidation of VN films deposited on Si in a semi-industrial sputtering reactor. Depending on the oxidation duration, several phases can be identified by XRD and Raman spectrometry. Four-point probe and emissivity measurements evidence an optimized oxidation duration to form thermochromic VO2. From these results, a VN oxidation phase diagram is plotted.

In chapter 4, thermochromic VO2 films are synthesized by air oxidation of VN samples on Al performed at three temperatures (450, 525, and 550°C) at various durations (lower than 50 min). Once again, the air-annealed duration (at each temperature) determines the formed phases in the oxidized films. We study the relationship between the maximum emissivity switch with the oxidation temperature and the surface morphology of the samples.

In chapter 5, a standard sputtering reactor is employed to deposit single crystallized VN thin films on MgO substrates. The air-annealing process of the epitaxial VN films is investigated. Furthermore, we explore the possibility to transpose the deposition conditions between the two sputtering machines employed in the thesis.

In chapter 6, continue working with the standard sputtering reactor, V2N thin films are growth on Si substrates. The films are oxidized at two temperatures, once again, the annealing duration controls the detected phases. TEM analysis allows us to differentiate the oxidation mechanisms of VN and V2N. Finally, we explore the effect of the initial precursor thickness on the optical modulation properties of the oxidized films.

General conclusions and perspectives to the main findings of the thesis are given.

Finally, an appendix is dedicated to preliminary results on the thermogravimetric analysis of the VN and V2N oxidation. The preliminary results indicate a possible crucial role of water vapor on the oxidation of the films.

Vanadium and its compounds

Vanadium is a transition metal with the symbol V and atomic number 23. Its molecular weight is 50.9 g/mol and its electronic configuration is [Ar] 3d 3 4s 2 . At room temperature vanadium is solid and crystallizes in the body-centered cubic (bcc) structure. It presents several oxidation states, between 0 and 5. It reacts easily with non-metallic atoms to form borides, carbides, nitrides, oxides… Details on the V-N and V-O binary systems are presented in the two following paragraphs.

The vanadium-nitrogen binary system

The V-N binary system contains two principal compounds: a face-centered cubic δ VNx and a hexagonal close packed β V2Nx have been well documented [START_REF] Carlson | The vanadium-nitrogen system: a review[END_REF] (Figure 1.1).

The δ VNx phase exhibits the NaCl-type B1 structure (space group Fm3 ̅ m) and occurs over a wide range of compositions (from VN0.72 and VN1.00) [START_REF] Carlson | The vanadium-nitrogen system: a review[END_REF]. The stoichiometric VN phase has a lattice constant aVN = 4.139 Å [START_REF] Toth | Superconducting critical temperatures of nonstoichiometric transition metal carbides and nitrides[END_REF]. The composition variation of the δ VNx is caused almost entirely for vacancies on the N sites [START_REF] Carlson | The vanadium-nitrogen system: a review[END_REF]. VN thin films have been studied for several applications, for instance, for their self-lubricious abilities reaching low friction coefficients when converted into vanadium oxides during high-temperature tribology tests [START_REF] Fateh | Influence of high-temperature oxide formation on the tribological behaviour of TiN and VN coatings[END_REF][START_REF] Qiu | Improvement of tribological performance of CrN coating via multilayering with VN[END_REF], and as electrode materials for electrochemical supercapacitors [START_REF] Lucio-Porto | VN thin films as electrode materials for electrochemical capacitors[END_REF][START_REF] Durai | Influence of chromium content on microstructural and electrochemical supercapacitive properties of vanadium nitride thin films developed by reactive magnetron co-sputtering process[END_REF].

The β V2Nx metallic phase displays a hexagonal structure with space group P3 ̅ 1m and a narrow range of homogeneity (see Figure 1.1) [START_REF] Carlson | The vanadium-nitrogen system: a review[END_REF]. According to Christiansen and Lebech, the lattice constants are aV2N = 4.917 Å and cV2N = 4.568 Å [START_REF] Christensen | The structure of β-vanadium nitride[END_REF]. Throughout the present work, we use the notation VN and V2N to denote the δ VNx and the β V2Nx phases. To our knowledge, there is not abundant literature on possible applications of V2N and on the synthesis of such nitride in thin film form. Nitride materials being sensitive of oxidation, VN and V2N will be considered in the present work as precursor to obtain thermochromic VO2 using a controlled oxidation process.

The vanadium-oxygen binary system

Contrary to the V-N system, the V-O one contains a high amount of phases (Figure 1.2). Among these oxides, VO2 and V2O5 are the most used compounds for optical or energy applications. The properties of these vanadium oxides are strongly dependent on their stoichiometry. V7O13 exhibits metallic properties while V2O5 is insulating. Between these two compounds, the vanadium oxides exhibit properties that are dependent on the temperature. At low temperature, they are semiconducting or even insulating while at high temperatures they exhibit metallic-like properties. The change of the properties occurs at a temperature called TMIT that refers to the metal to insulating transition. The value of TMIT is strongly dependent on stoichiometry; it varies from -203 to 157°C (Table 1.1) [START_REF] Yang | Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions[END_REF][START_REF] Chudnovskii | Insulator-metal transition in V3O5[END_REF]. Figure 1.2 highlights the need to control with great accuracy the stoichiometry of the vanadium oxides to obtain a single compound. For instance, for VO2, the amount of oxygen must be approx. between 1.98 and 2.02 [START_REF] Wriedt | The O-V (Oxygen-Vanadium) system[END_REF]. However, even for a single compound, a small change in the stoichiometry could affect the physical properties and transition temperature. In temperatures) with the decrease of the oxygen amount in the films [START_REF] Griffiths | Influence of stoichiometry on the metalsemiconductor transition in vanadium dioxide[END_REF]. One year later, Brückner et al. showed two orders of magnitude of resistivity ratio changes in VO2 monocrystalline samples with a stoichiometry difference of 0.3% [START_REF] Brückner | The range of homogeneity of VO2 and the influence of the composition on the physical properties. II. The change of the physical properties in the range of homogeneity[END_REF]. More recently, in 2013, Liu et al. studied the dynamic behaviors of photoinduced insulator-metal phase transition in VO2 thin films with different oxygen stoichiometry using optical pump-terahertz probe spectroscopy. They observed a correlation between photoinduced phase transition and oxygen stoichiometry. The excessive oxygen content in vanadium dioxide reduces the magnitude of phase transition [START_REF] Liu | Effect of oxygen stoichiometry on the insulator-metal phase transition in vanadium oxide thin films studied using optical pump-terahertz probe spectroscopy[END_REF]. Therefore, for industrial applications on large surfaces, it is even harder to control the stoichiometry.

Crystallographic structures of VO2

Among the oxides of the V-O system, VO2 is a compound with several polymorphs, including VO2(R) [START_REF] Cao | Pure Single-Crystal Rutile Vanadium Dioxide Powders: Synthesis, Mechanism and Phase-Transformation Property[END_REF], VO2(M) [START_REF] Caruthers | Energy Bands of Semiconducting VO2[END_REF], VO2(B) [START_REF] Popuri | Rapid Hydrothermal Synthesis of VO2 (B) and Its Conversion to Thermochromic VO2 (M1)[END_REF], VO2(A) [START_REF] Oka | Crystal Structures and Transition Mechanism of VO2(A)[END_REF], VO2(C) [START_REF] Hagrman | A New Polymorph of VO2Prepared by Soft Chemical Methods[END_REF], VO2(D) [START_REF] Liu | Newphase VO2 micro/nanostructures: investigation of phase transformation and magnetic property[END_REF],

and VO2(P) [START_REF] Wu | Synthetic paramontroseite VO2 with good aqueous lithium-ion battery performance[END_REF]. These phases transform into each other under certain specific conditions [START_REF] Liu | Newphase VO2 micro/nanostructures: investigation of phase transformation and magnetic property[END_REF][START_REF] Wu | Synthetic paramontroseite VO2 with good aqueous lithium-ion battery performance[END_REF][START_REF] Horrocks | Scalable Hydrothermal Synthesis of Free-Standing VO2 Nanowires in the M1 Phase[END_REF][START_REF] Oka | Structural phase transition of VO2(B) to VO2(A)[END_REF][START_REF] Zhang | VO2(B) conversion to VO2(A) and VO2(M) and their oxidation resistance and optical switching properties[END_REF][START_REF] Valmalette | High efficiency thermochromic VO2(R) resulting from the irreversible transformation of VO2(B)[END_REF][START_REF] Liu | Infrared chameleon-like behavior from VO2(M) thin films prepared by transformation of metastable VO2(B) for adaptive camouflage in both thermal atmospheric windows[END_REF]. Only the couple monoclinic VO2(M) and rutile-type VO2 (R) undergoes a fully reversible metal-insulator transition as a function of the temperature. For this reason, they are the main phases in the scope of this thesis and also in the literature.

The VO2 at high temperature (T > TMIT), noted VO2(R), presents a rutile structure (Figure 1.3) composed of a tetragonal lattice with space group P42/mnm [START_REF] Mcwhan | X-ray diffraction study of metallic VO2[END_REF][START_REF] Westman | Note on a Phase Transition in VO2[END_REF]. The lattice constant are aR = bR = 4.5546 Å and cR = 2.8514 Å [START_REF] Mcwhan | X-ray diffraction study of metallic VO2[END_REF]. The basic structural unit is a VO6 octahedron (vanadium atoms of oxidation state +4 are surrounded by six oxygen atoms of oxidation state -2). The VO6 octahedra share edges along the c-axis direction and corners in the a-b planes (Figure 1.3). VO2(R) is opaque to infrared radiation and exhibits a low electrical resistivity [START_REF] Choi | Mid-infrared properties of a VO2 film near the metal-insulator transition[END_REF][START_REF] Ordonez-Miranda | Modeling of the electrical conductivity, thermal conductivity, and specific heat capacity of VO2[END_REF] typical of metal-like materials.

The VO2 at temperatures lower than TMIT exhibits a monoclinic structure (VO2(M))

characterized by a monoclinic lattice with space group P21/c [START_REF] Longo | A Refinement of the Structure of VO2[END_REF][START_REF] Andersson | Studies on Vanadium Oxides. II. The Crystal Structure of Vanadium Dioxide[END_REF] (Figure 1.4). This phase is described as a distortion resulting from the stretching along the cR axis of two superimposed lattices of the VO2(R), followed by a torsion. Anderson reported lattice constants aM = 5.743 Å, bM = 4.517 Å, cM = 5.375 Å, and an angle βM = 122.6° [START_REF] Andersson | Studies on Vanadium Oxides. II. The Crystal Structure of Vanadium Dioxide[END_REF]. There is a close connection between the rutile and monoclinic structures (compare Figure 1.3 and Figure 1.4). The relation existing between the rutile-type cell and the monoclinic unit cell of VO2 can be approx. written as: 𝑎 ⃗ 𝑀 = 2𝑐 ⃗ 𝑅 , 𝑏 ⃗⃗ 𝑀 = 𝑎 ⃗ 𝑅 and 𝑐 ⃗ 𝑀 = 𝑎 ⃗ 𝑅 -𝑐 ⃗ 𝑅 [START_REF] Andersson | Studies on Vanadium Oxides. II. The Crystal Structure of Vanadium Dioxide[END_REF]. Figure 1.4 displays the characteristic metalmetal zigzag pairing along aM axis, with two alternate V-V distances of 2.619 and 3.164 Å. Furthermore, there are two different vanadium-oxygen interatomic distances noted V-O1 and V-O2, with respective lengths of 1.77 and 2.01 Å [START_REF] Eyert | The metal-insulator transitions of VO2: A band theoretical approach[END_REF]. VO2(M) behaves as a semiconductor with high IR transmittance and high electrical resistivity [START_REF] Hoshino | Infrared-light switching in highly oriented VO2 films on ZnO-buffered glasses with controlled phase transition temperatures[END_REF][START_REF] Luo | Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering[END_REF].

Moreover, at low temperature, a metastable modification of the M phase, noted VO2(M2), crystallizes in a centered monoclinic lattice with space group C2/m [START_REF] Marezio | Structural Aspects of the Metal-Insulator Transitions in Cr-Doped VO2[END_REF]. This phase is usually obtained when stress is applied during film growth [START_REF] Pouget | Electron Localization Induced by Uniaxial Stress in Pure V${\mathrm{O}}_{2}[END_REF][START_REF] Gu | Thermodynamics of strained vanadium dioxide single crystals[END_REF] or when VO2 is doped with certain elements such as titanium [START_REF]Switching properties of V1-xTixO2 thin films deposited from alkoxides[END_REF], aluminum [START_REF] Strelcov | Doping-Based Stabilization of the M2 Phase in Free-Standing VO2 Nanostructures at Room Temperature[END_REF][START_REF] Didelot | Films d'oxydes de vanadium thermochromes dopés aluminium obtenus après un recuit d'oxydation-cristallisation pour applications dans le solaire thermique[END_REF] or chromium [START_REF] Guo | Analysis of metal insulator transitions in VO2 and V2O3 for RRAMs[END_REF]. However, the VO2(M2) is not part of the scope of this thesis.

VO2(A) and VO2(B) are the other two most commonly studied VO2 polymorphs. The metastable VO2(A) undergoes an irreversible first-order phase transition at 162°C, in which the resistance can change one order of magnitude [START_REF] Wang | Structural transition and temperature-driven conductivity switching of single crystalline VO2(A) nanowires[END_REF]. At low temperature LTP-VO2(A) adopts the tetragonal system P4/ncc with aA = bA = 8.440 Å and cA= 7.666 Å, while at high temperature HTP-VO2(A) depicts the body-centered tetragonal system I4/m with aA = bA = 8.476 Å and cA = 3.824 Å [START_REF] Oka | Crystal Structures and Transition Mechanism of VO2(A)[END_REF]. On the other hand, distinct from monoclinic VO2(M) and VO2(A), VO2(B) has no MIT characteristic or drastic variations in electric and optical properties. The metastable VO2(B) displays a monoclinic structure similar to the layered V6O13 structure [START_REF] Théobald | Essai sur la Structure de VO2(B)[END_REF]. The parameters of the VO2(B) cell are aB = 12.03 Å, bB = 3.693 Å, cB = 6.42 Å, and βB = 106.6°, and the space group is C2/m [START_REF] Théobald | Essai sur la Structure de VO2(B)[END_REF][START_REF] Ch | From VO2(B) to VO2(R) : Theoretical structures of VO2 polymorphs and in situ electron microscopy[END_REF]. Thanks to its layered structure and outstanding electrochemical performance, VO2(B) is considered a promising cathode material for lithiumion batteries [START_REF] Li | High-surface vanadium oxides with large capacities for lithium-ion batteries: from hydrated aerogel to nanocrystalline VO2(B), V6O13 and V2O5[END_REF][START_REF] Liu | Design and synthesis of a novel nanothorn VO2(B) hollow microsphere and their application in lithium-ion batteries[END_REF].

Band structure of VO2

To understand the VO2 band structure, we must describe the atomic d levels. From molecular orbital theory, we know that the V 4+ ion in the VO2 is a d 1 system, which means that there is one electron in the d band.

In the spherical crystal field the d band is composed of fivefold degenerate levels. Once we consider the V 4+ ion inside the VO2 structure, the d levels are split by the crystal field (Figure 1.5 a)). The octahedral field (of the six oxygens) provides a splitting into a triply degenerate lower state of t2g symmetry and a doubly degenerate upper state of eg symmetry (Figure 1.5 a)).

On the figure, each level is labeled according to the symmetry of the corresponding atomic d orbital, following the rectangular coordinate system in Figure 1.5 b). These levels give rise in the crystal to the partially occupied lowest d bands. According to a band model proposed by

Goodenough [START_REF] Goodenough | The two components of the crystallographic transition in VO2[END_REF], the dx 2 -y 2 orbital gives rise to a band which he labels d║ whereas the other two t2g orbitals (dxz and dyz) mix with the anion 2p orbital forming a wider π* band [START_REF] Zylbersztejn | Metal-insulator transition in vanadium dioxide[END_REF].

In VO2(R), the band structure consists of oxygen 2p bands well below the Fermi level, as well as vanadium 3d t2g and eg bands. The band near the Fermi energy (EF) contains the d║ (modified from [START_REF] Eyert | The metal-insulator transitions of VO2: A band theoretical approach[END_REF]).

states parallel to the characteristic V chains and the π* bands (Figure 1.6). The vanadium atoms form straight lines with a uniform V-V distance of 2.88 Å. In this case, the V-V distance is shorter than the critical interaction distance (2.94 Å) [START_REF] Wu | Design of vanadium oxide structures with controllable electrical properties for energy applications[END_REF]. Hence, the overlapping of orbitals is possible (electrons flow freely between the atoms) giving conductive characteristics to this phase. During the MIT, the distortion of the rutile structure to form the monoclinic one comes with the formation of metal-metal zigzag pairing along aM (cR) axis, with two alternate V-V distances of 2.619 and 3.164 Å. The displacement of the cations lifts the π* bands above the EF, and the V-V pairing splits the d║ band in two (Figure 1.6), as a consequence, appears a bandgap of about 0.7 eV. In this case, one of the V-V distance is higher than the critical interaction distance, resulting in the localization of d-orbital electrons (electrons are trapped and cannot move inside the crystal), which leads to the insulating behavior of VO2(M).

The VO2 phase transition

The origin of the VO2 phase transition is an object of debate in the scientific community.

The two models applied to explain this transition are: the Mott-Hubbard transition and the Peierls transition.

The Mott-Hubbard transition

This model proposes that at temperatures lower than TMIT the electron-electron interactions are strong enough to localize the electrons in the atoms [START_REF] Zylbersztejn | Metal-insulator transition in vanadium dioxide[END_REF][START_REF] Mott | The Basis of the Electron Theory of Metals, with Special Reference to the Transition Metals[END_REF]. Hence, there are not movements of electrons from one site to another. These electronic correlations, related to the carrier density metal-like mid-IR optical properties and agree with the Mott-Hubbard transition [START_REF] Morrison | A photoinduced metal-like phase of monoclinic VO revealed by ultrafast electron diffraction[END_REF]. This model also describes the transition from VO2(M2) to the rutile phase [START_REF] Rice | Comment on VO2: Peierls or Mott-Hubbard? A View from Band Theory[END_REF].

The Peierls transition

In 1955, Peierls published a solid-state physics textbook in which he pointed out that a onedimensional metal coupled to the underlying lattice is not stable at low temperatures [START_REF] Peierls | Quantum Theory of Solids[END_REF]. The ground state of the coupled electron-phonon system is characterized by a gap, thus behaving as an insulator. Later the metal-to-insulator transition was verified in one-dimensional conductors or layered structures [START_REF] Kagoshima | Peierls Phase Transition[END_REF][START_REF] Grüner | The dynamics of charge-density waves[END_REF]. This transition is now called Peierls transition. In these systems, one-dimension chains are responsible for the electrical conductivity. At high temperature, the atoms are equidistant, and it is possible to transfer the thermal energy of electrons from one atom to another (metallic state). As the temperature drops below a critical value, the structure is distorted. The distance between the atoms changes, the electron localization occurs due to the decrease in energy, leading to an insulating state. The Peierls transition is a thermodynamic transition in which the gap disappears by a distortion applied to the lattice that facilitates orbitals overlapping.

In this case, numerous theoretical and experimental investigations agree with the idea that VO2 follows a Peierls transition. driven by the electron-phonon interaction [START_REF] Okazaki | Temperature-Dependent Electronic Structure of VO2 in the Insulating Phase[END_REF].

Debate about the phase transition

There are numerous theoretical and experimental works devoted to unraveling the details of the mechanism responsible for the MIT of VO2. Yet, there is not consensual approval whether the driving force causing the MIT is a Mott-Hubbard or a Peierls transition. However, a relatively new possibility advocates that the VO2 phase transition has the contribution from both models. For instance, Baum et al. reported a femtosecond electron diffraction study on the phase transition of VO2 single crystals. The results displayed different time scales. It begins with a fs motion along the aM axis (direction of the V-V bond in the VO2(M), see Figure 1.4).

Afterward, a local rearrangement of the VO6 octahedra changing to a more rutile-like geometry occurs on a ps scale [START_REF] Baum | 4D Visualization of Transitional Structures in Phase Transformations by Electron Diffraction[END_REF]. These two steps caused by electron interactions agree with the Mott-Hubbard model. It ends with a long-range shear motion on the scale of hundreds of picoseconds (distortions supported by Peierls transition). Therefore, in the description of the structural pathway, the two mechanisms were involved [START_REF] Baum | 4D Visualization of Transitional Structures in Phase Transformations by Electron Diffraction[END_REF]. On the other hand, Weber et al. studied the VO2(M) phase using first principle calculations as a function of the disorder. Their results show that a large 3d-3d Coulomb interaction alone is insufficient to generate the bandgap for VO2.

Hence, the disorder due to imperfections of the crystal, or self-trapping caused by the strong electron-phonon coupling could be relevant. They propose a new mechanism for the VO2 phase transition based on an orbital selective Mott transition, assisted by the Peierls distortion: the Peierls instability involves an orbital selection, and bonds the dxy and dxz orbitals along the rutile axis, filling each orbital with one electron, and in turn generates a Mott instability [START_REF] Weber | Vanadium Dioxide: A Peierls-Mott Insulator Stable against Disorder[END_REF]. These research studies may suggest that the MIT problem in VO2 is closer to resolution.

Physical properties of VO2

The metal-insulator transition of VO2, described in the section before, is accompanied by an abrupt change in the electrical and optical properties of the material as a function of the temperature. In 1959, Morin revealed the first order MIT in single crystals of VO2 by measuring the dependence of the electrical conductivity with temperature [START_REF] Morin | Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature[END_REF]. As already mentioned, at low temperatures the VO2 behaves as a semiconductor with a monoclinic structure, while at temperatures higher than TMIT it changes to a metallic state with a rutile-type structure. This change may result in a resistivity variation of several orders of magnitude [START_REF] Qazilbash | Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging[END_REF][START_REF] Kumar | Electrical Switching in Semiconductor-Metal Self-Assembled VO2 Disordered Metamaterial Coatings[END_REF][START_REF] Givernaud | Microwave Power Limiting Devices Based on the Semiconductor-Metal Transition in Vanadium-Dioxide Thin Films[END_REF]. However, the magnitude, sharpness, and transition temperature have a dependence on different parameters like substrate temperature during the deposition [START_REF] Jin | Dependence of microstructure and thermochromism on substrate temperature for sputter-deposited VO2 epitaxial films[END_REF][START_REF] Ruzmetov | Structure-functional property relationships in rf-sputtered vanadium dioxide thin films[END_REF] and average grain size (grain boundaries density) [START_REF] Jian | Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films[END_REF].

Nevertheless, these are not the only significant parameters. In 2017, Yu et al. performed the oxidation of sputter-deposited 100 nm V films varying the oxygen flow rate from 1 sccm to 20 substrate bias during deposition [START_REF] Hoshino | Infrared-light switching in highly oriented VO2 films on ZnO-buffered glasses with controlled phase transition temperatures[END_REF], film thickness [START_REF] Xu | Thickness dependence of optical properties of VO2 thin films epitaxially grown on sapphire[END_REF], and the IR optical properties of the substrate [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF]. The study carried out by Benkahoul et al. in thermochromic VO2 thin films deposited on quartz, Si, and Al was critical to understand the influence of the IR optical properties of the substrate in the response of the VO2/substrate system. They demonstrate that emissivity dependence on temperature for the VO2 film deposited on IR reflective substrate (Al) is opposite to that of VO2 deposited on an IR transparent substrate (quartz and Si), as shown in Figure 1.9. The difference is attributed to the higher IR reflective properties of Al in comparison to quartz and Si [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF]. These results suggest that the VO2/Al system could be employed as a smart radiator device while the VO2/Si and VO2/quartz systems are interesting for furtivity applications.

Possible applications of VO2

In the previous section, it was illustrated how increasing (decreasing) the temperature above (below) the transition temperature of VO2 drastically changes its physical properties. Moreover, this transition can be triggered by diverse external stimuli including, photons, electrochemical, magnetic field, temperature, stress and electric field (Figure 1.10) [START_REF] Ke | Vanadium Dioxide: The Multistimuli Responsive Material and Its Applications[END_REF]. All the stimuli and their corresponding responses provide broad possibilities for industrial applications, like smart radiator devices [START_REF] Beaini | Thermochromic VO2-based smart radiator devices with ultralow refractive index cavities for increased performance[END_REF], thermal rectification [START_REF] Forero-Sandoval | VO2 Substrate Effect on the Thermal Rectification of a Far-Field Radiative Diode[END_REF], smart windows [START_REF] Sun | A novel multifunctional thermochromic structure with skin comfort design for smart window application[END_REF], memory devices [START_REF] Lee | Two Series Oxide Resistors Applicable to High Speed and High Density Nonvolatile Memory[END_REF],

thermal solar collectors [START_REF] Mercs | Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors[END_REF], among others. Since temperature is the stimulus used in this thesis to activate the MIT in the VO2 films, we will briefly describe one possible application related to this stimulus, the smart radiator devices (SRDs) in spacecraft or satellites.

Spacecraft and satellites in orbit run into large temperature fluctuations depending on their exposure to solar energy (it could be from about -150 to 150°C). Nevertheless, the internal temperature needs to be regulated to an optimal operating range (generally ranging from 0 to 40°C). This is a crucial issue to keep the electronic structures working under stabilized conditions and avoid impacts on the performances and life endurance of the systems. The unique way to heat regulation is radiative heat transfer into deep space [START_REF] Kim | VO2-based switchable radiator for spacecraft thermal control[END_REF]. One possibility is to use different types of electromechanical louvers (macro and micro-scale louvers) that are opened and closed to regulate the radiation to dark space [START_REF] Osiander | Microelectromechanical devices for satellite thermal control[END_REF]. The correct control of these louvers involves complex, heavy and expensive structures for the spacecraft and satellites [START_REF] Hendaoui | Enhancement of the positive emittance-switching performance of thermochromic VO2 films deposited on Al substrate for an efficient passive thermal control of spacecrafts[END_REF].

Another interesting possibility is the use of the so-called smart radiator devices (SRD), which can control the temperature inside the satellites or spacecraft without complementary systems. The principal advantages of the SRD temperature-control system are its simplicity, low cost and low weight. The operating principle of an SRD is supported by the fact that the heat exchange between the satellite and its environment is primarily controlled by radiation. A SRD must have low emissivity at low temperatures (to preserve the desired temperature) and a high emissivity at high temperatures (to dissipate the extra unnecessary heat), therefore keeping the system working under the optimal operating range [START_REF] Hendaoui | Enhancement of the positive emittance-switching performance of thermochromic VO2 films deposited on Al substrate for an efficient passive thermal control of spacecrafts[END_REF]. VO2 thanks to the emissivity changes during the MIT transition is an ideal candidate for thermal regulation in spacecraft and satellites.

Yet, there are tasks to address before the real use of the VO2 based SRD system. First, the ∆𝜀 = 𝜀 𝐻𝑇 -𝜀 𝐿𝑇 (emissivity switch at high and low temperatures) must be positive. The higher the ∆𝜀 the better the SRD performance. Second, it is crucial to achieving high values of 𝜀 𝐻𝑇 to enhance the thermal power emitted by the radiator, thus reducing the size and weight of the satellite [START_REF] Hendaoui | Enhancement of the positive emittance-switching performance of thermochromic VO2 films deposited on Al substrate for an efficient passive thermal control of spacecrafts[END_REF][START_REF] Hendaoui | Highly tunable-emittance radiator based on semiconductor-metal transition of VO2 thin films[END_REF].

Hendaoui et al. studied the capabilities for SRD application of a 260 nm VO2 deposited on mirror-like polished Al substrates using of reactive pulsed laser deposition (RPLD). The best values of 𝜀 𝐻𝑇 and ∆𝜀 for the VO2/Al system were found in the vicinity of the MIT. They explained this phenomenon thanks to the high absorption generated by the coexistence of both insulating and metallic phases of VO2. They also demonstrated, theoretically and experimentally, that it is possible to increase 𝜀 𝐻𝑇 values, above the upper limit allowed for the VO2/Al system, by adding an appropriate top a-Si:H/SiO2 λ/4 stack layer on the VO2/Al system.

The thickness of these additional layers (approx. 450 nm for the hydrogenated amorphous silicon (a-Si:H) layer and approx. 100 nm for the SiO2) were chosen to produce a broadband destructive interference of the reflectance close to the maximum of the blackbody spectrum at 100°C when the VO2 is in the metallic state. The 𝜀 𝐻𝑇 and ∆𝜀 values enhanced from 0.29 and 0.21, respectively for the VO2/Al system to 0.62 and 0.38, respectively, for the a-Si:H/SiO2/VO2/Al structure.

In the same year, the same group of authors published two more papers about the use of VO2 as SRD in spacecraft thermal control. In the second paper, the multilayer structure was different. First, a 350 nm thick Au IR-high reflecting layer was deposited on a quartz substrate by RF magnetron sputtering. Second, an 850 nm SiO2 layer was grown by plasma-enhanced chemical vapor deposition (PECVD). Third, the VO2 30 nm layer was deposited by RPLD.

Note that in this case, the SiO2 layer has the same function as the a-Si:H/SiO2 λ/4 stack layer in the previous study, resulting in a higher value of 𝜀 𝐻𝑇 thanks to the broadband destructive interference for temperatures above TMIT. The 𝜀 𝐻𝑇 and ∆𝜀 values are 0.71 and 0.49 [START_REF] Hendaoui | Highly tunable-emittance radiator based on semiconductor-metal transition of VO2 thin films[END_REF]. Both values are higher than the previously reported for the a-Si:H/SiO2/VO2/Al structure suggesting that this new VO2 multilayer system is better for the passive control of the internal temperature of the spacecraft. However, it will be interesting to include a top antireflective layer to the VO2/SiO2/Au/ quartz structure to avoid further oxidation of VO2 and consequently degradation of the thermochromic performance.

Finally, in the third paper, [START_REF] Chae | Effects of W doping on the metal-insulator transition in vanadium dioxide film[END_REF][START_REF] Soltani | Effects of Ti-W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition[END_REF]. The best compromise between emissivity performance and transition temperature was detected for the V0.979W0.021O2-based SRD, characterized by TMIT = 31.5°C, 𝜀 𝐻𝑇 = 0.81 and ∆𝜀 = 0.43 [START_REF] Hendaoui | VO2-based smart coatings with improved emittance-switching properties for an energy-efficient near roomtemperature thermal control of spacecrafts[END_REF].

In conclusion, these three works exhibit the potentialities of VO2 based multilayer structure as an SRD for energy-efficient thermal control of spacecraft. The use of VO2 in thermal solar collectors is another interesting application in which the MIT is triggered by temperature. In the last eight years, two PhD theses were developed around this topic in our group, one of them with a successful industrial transfer. More details can be found in references [START_REF] Didelot | Films d'oxydes de vanadium thermochromes dopés aluminium obtenus après un recuit d'oxydation-cristallisation pour applications dans le solaire thermique[END_REF][START_REF] Corvisier | Élaboration par pulvérisation magnétron réactive d'une couche thermochrome à base de dioxyde de vanadium[END_REF].

VO2 thin films growth methods

Driven by potential industrial applications coupled with scientific research on the VO2 phase transition, the scientific literature on this material has increased significantly in the last decades. Different deposition techniques have been employed to deposit VO2 with excellent thermochromic behavior, such as sol-gel [START_REF] Wu | Effect of annealing temperature on thermochromic properties of vanadium dioxide thin films deposited by organic sol-gel method[END_REF], chemical vapor deposition (CVD) [START_REF] Binions | Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties[END_REF], molecular beam epitaxy (MBE) [START_REF] Fan | Growth and phase transition characteristics of pure M-phase VO2 epitaxial film prepared by oxide molecular beam epitaxy[END_REF][START_REF] Paik | Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy[END_REF], atomic layer deposition (ALD) [START_REF] Prasadam | Study of VO2 thin film synthesis by atomic layer deposition[END_REF], pulsed laser deposition (PLD) [START_REF] Hamaoui | Thermophysical characterisation of VO2 thin films hysteresis and its application in thermal rectification[END_REF][START_REF] Dumas-Bouchiat | rfmicrowave switches based on reversible semiconductor-metal transition of VO2 thin films synthesized by pulsed-laser deposition[END_REF][START_REF] Rougier | Electrochromism of Mixed Tungsten-Vanadium Oxide Thin Films Grown by Pulsed Laser Deposition[END_REF], magnetron sputtering [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF][START_REF] Yoon | Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method[END_REF][START_REF] Yuce | Investigation of electron beam lithography effects on metal-insulator transition behavior of vanadium dioxide[END_REF], among others. However, it is still a defiance to grow pure VO2 on large surfaces caused by the large amounts of oxides in the V-O system. reported the MBE growth of VO2 films as thin as 1.5 nm with abrupt and reproducible MIT on (001) TiO2 substrate [START_REF] Paik | Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy[END_REF]. Although the excellent control of VO2 stoichiometry, the low deposition rate of MBE is a major drawback for the use of this technique for large-surface industrial applications.

PLD is another deposition technique suitable for the growth of epitaxial VO2 with excellent thermochromic properties. This method enables precise control of the VO2 stoichiometry. For instance, Kim and Kwok disclosed the deposition of VO2 thin films on (0001) and (101 ̅ 0) sapphire substrates. The films present outstanding electrical modulation properties with electrical resistance ratio between the low and high temperature as large as 4.0 10 4 and 10 5 for the films on (0001) and (101 ̅ 0), respectively [START_REF] Kim | Pulsed laser deposition of VO2 thin films[END_REF]. PLD has higher deposition rates than MBE and guarantees exceptional thermochromic performance. However, it is not suitable for deposition on large surfaces as required for industrial applications since the laser beam is focused on a small area.

On the other hand, magnetron sputtering is a deposition technique that facilitates the growth of films on large surfaces with a high deposition rate. There are several approaches to obtain VO2, within them, reactive sputtering in Ar and O2 gas mixture without a post-annealing procedure is extensively applied [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF][START_REF] Yuce | Investigation of electron beam lithography effects on metal-insulator transition behavior of vanadium dioxide[END_REF][START_REF] Zhao | Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications[END_REF][START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF]. For example, Yuce et al. deposited VO2 thin films on c-cut sapphire (0001). The substrate temperature and the oxygen-argon gas mixture pressure during deposition were fixed at 550°C and 0.85 Pa, respectively. They studied the effect of oxygen concentration during the deposition and thickness (deposition duration) on the properties of the obtained films [START_REF] Yuce | Investigation of electron beam lithography effects on metal-insulator transition behavior of vanadium dioxide[END_REF]. The Raman spectra display that only sample A presents pure VO2, while the other films show a mixture of VO2 and V2O5 (Figure 1. [START_REF] Warwick | Advances in thermochromic vanadium dioxide films[END_REF]. The only difference between films A and B is the deposition time (Table 1.2). The authors claimed that decreasing the time leads to reduce the reactions of scattered V atoms with oxygen atoms, forming a mixture of vanadium oxide phases, instead of pure VO2 [START_REF] Yuce | Investigation of electron beam lithography effects on metal-insulator transition behavior of vanadium dioxide[END_REF]. Samples A and C are the only ones to present thermochromic performance (Figure 1.12), illustrating the significance to control these parameters during deposition. V vacancies leading to hole doping and V 5+ valence state formation. Therefore, the resistivity increases in both the insulating and the metallic phases. The higher growth temperatures introduce a relatively large density of O vacancies leading to V 3+ valence state formation. Thus, the resistivity decreases in the monoclinic phase and increases in the metallic phase [START_REF] Zhao | Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications[END_REF].

In general, the VO2 crystallinity requires high deposition temperatures during the deposition. This constitutes a disadvantage for the large-scale production of the VO2 films because of the high energy consumption and the difficulty to guarantee temperature homogeneity over large substrate surfaces. Moreover, it could cause the diffusion of substrate atoms into the films. Therefore, it eliminates the use of some temperature-sensitive substrates.

A possibility to overcome this drawback is to employ reactive high-power impulse magnetron sputtering (HiPIMS) with pulsed O2 flow rate control to deposited the VO2 films. The highly ionized fluxes of particles with high fractions of ionized sputtered metal atoms onto the substrate enable to reduce the deposition temperature to 300°C and still obtain VO2 films with good thermochromic performance [START_REF] Vlček | Controlled reactive HiPIMSeffective technique for low-temperature (300°C) synthesis of VO2 films with semiconductor-to-metal transition[END_REF][START_REF] Vlček | Kos, Ion-flux characteristics during low-temperature (300°C) deposition of thermochromic VO2 films using controlled reactive HiPIMS[END_REF][START_REF] Fortier | Thermochromic VO2 thin films deposited by HiPIMS[END_REF]. Furthermore, Loquai et al. demonstrated that VO2 films deposited by HiPIMS exhibit better environmental stability than conventional magnetron sputtering-prepared VO2 samples (approx. three times longer) thanks to the higher density and larger average grain size of the HiPIMS films [START_REF] Loquai | HiPIMS-deposited thermochromic VO2 films with high environmental stability[END_REF].

Another approach is to apply a post-annealing process to enhance the thermochromic performance of the deposited films [START_REF] Melnik | Lowtemperature method for thermochromic high ordered VO2 phase formation[END_REF][START_REF] Yu | Oxygen pressure manipulations on the metal-insulator transition characteristics of highly (0 1 1)-oriented vanadium dioxide films grown by magnetron sputtering[END_REF][START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF]. Melnik et al. reported the growth of 100 nm VO2 thin films on glass substrates. The deposition parameters were 5% of O2 in Ar at a total gas pressure of 0.4 Pa, and the substrate temperature close to 200 °C. The deposited films were further annealed at 300°C in air for their crystallization [START_REF] Melnik | Lowtemperature method for thermochromic high ordered VO2 phase formation[END_REF]. This second step is essential for the crystallization of the films (Figure 1.13), changing from amorphous (before annealing) to well crystalline films (after annealing) with a change in resistance of about four orders of magnitude [START_REF] Melnik | Lowtemperature method for thermochromic high ordered VO2 phase formation[END_REF].

Nevertheless, it is critical to choose proper conditions for the post-annealing procedure. Dou temperature. Then the films were annealed to crystallize them at 500°C for 400 s with a pressure of 220 Pa atmosphere by a rapid thermal annealing system. After this procedure, all of the samples exhibit resistance change of more than four orders of magnitude, and good IR optical modulation efficiencies at the transition temperature. Thereafter, the samples were heated up to 450°C in 8 s and kept in air at 450°C for 200, 400, 600, 800 and 1000 s under atmospheric pressure, respectively [START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF]. After this last annealing, all the samples suffered a decrease in the resistance change and in the IR optical modulation properties with prolonging the oxidation time. These findings suggest progress in the annealing evolution of the films from VO2 to V2O5 [START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF].

An alternative two-step proposal to produce VO2 is the reduction of previously obtained V2O5 in ammonia [START_REF] Qi | Synthesis, characterization, and thermodynamic parameters of vanadium dioxide[END_REF], vacuum [START_REF] Ningyi | Valence reduction process from sol-gel V2O5 to VO2 thin films[END_REF], or hydrogen [START_REF] Sun | Anisotropic vanadium dioxide sculptured thin films with superior thermochromic properties[END_REF]. The reduction of the V2O5 + C mixture in a dynamic vacuum system is also reported [START_REF] Guan | Carbonreduction as an easy route for the synthesis of VO2(M1) and further Al, Ti doping[END_REF]. Furthermore, there is an additional two-step procedure, in which a vanadium film is deposited and then annealed in an air or oxygen-controlled atmosphere to form thermochromic VO2. This process could be found in the literature under the name of thermal oxidation method [START_REF] Yoon | Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method[END_REF][START_REF] Ji | Thermochromic VO2 films by thermal oxidation of vanadium in SO2[END_REF][START_REF] Jiang | Evolution of thermochromism during oxidation of evaporated vanadium films[END_REF] or sputtering oxidation coupling (SOC) method [START_REF] Xu | A novel sputtering oxidation coupling (SOC) method to fabricate VO2 thin film[END_REF][START_REF] Xu | The study of optimal oxidation time and different temperatures for high quality VO2 thin film based on the sputtering oxidation coupling method[END_REF]. Xu et al. sputter-deposited V films (85 nm) on c-plane sapphire substrates. Thereafter, the samples were oxidized in a furnace under air atmosphere at 470°C with a different duration between 250 and 325 s [START_REF] Xu | A novel sputtering oxidation coupling (SOC) method to fabricate VO2 thin film[END_REF]. Their results indicate a transformation of the amorphous vanadium into crystalline VO2 in the 250 -285 s range. The optimum oxidation time was 285 s with a resistance change of almost three orders of magnitude.

Further oxidation of the films for more than 285 s led to the formation of V2O5 affecting the thermochromic behavior of the samples [START_REF] Xu | A novel sputtering oxidation coupling (SOC) method to fabricate VO2 thin film[END_REF]. One year later, the same group found that the logarithm of the optimal oxidation time (ln(t)) as a function of the reverse temperature (1/T) of vanadium films (100 nm) under air atmosphere satisfies a linear relationship when the temperature ranges from 430 to 510°C [START_REF] Xu | The study of optimal oxidation time and different temperatures for high quality VO2 thin film based on the sputtering oxidation coupling method[END_REF]. Using this two-step method, Yoon et al. grew 100 nm vanadium films on (112 ̅ 0) sapphire substrates. Their samples were annealed for 10 min at different oxygen pressures ranging from 200 to 600 Pa and temperatures from 510 to 600 °C [START_REF] Yoon | Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method[END_REF]. The results display a strong dependence of the electrical properties of the VO2 films (resistivity ratio and sharpness of the MIT) on the annealing temperature and oxidation pressure (Figure 1.14).

These reports illustrate that the possibility to regulate the temperature and duration along the oxidation process makes the two-step procedure suitable for achieving high-quality VO2 films over large surfaces. In this thesis we use this method to obtain thermochromic VO2, yet, we change the precursor. First, we deposit VN or V2N films by reactive magnetron sputtering.

Second, the as-deposited samples are air annealed in a preheated furnace under atmospheric pressure. It is relevant to stress the freedom in the choice of parameters when using a two-stage procedure. The thickness of the precursor film is determined during the deposition process.

Afterward, the temperature and duration of the oxidation step depend on the initial thickness of the as-deposited films. Hence, these three parameters are the key to the successful oxidation of VN or V2N into thermochromic VO2. Failure to optimize these parameters may lead to the formation of other oxides, such as the Magnéli phases VnO2n-1 (V4O7, V5O9…) [START_REF] Marwitz | Correlation of adhesion force and electrical conductivity in Magnéli-type vanadium oxides and highly oriented pyrolytic graphite[END_REF][START_REF] Schwingenschlögl | The vanadium Magnéli phases VnO2n-1[END_REF][START_REF] Bahlawane | Vanadium Oxide Compounds: Structure, Properties, and Growth from the Gas Phase[END_REF] and Wadsley phases VnO2n+1 (V4O9, V6O13…) [START_REF] Bahlawane | Vanadium Oxide Compounds: Structure, Properties, and Growth from the Gas Phase[END_REF][START_REF] Zeng | Growth dynamics and photoresponse of the Wadsley phase V6O13 crystals[END_REF][START_REF] Yamazaki | Synthesis, structure and magnetic properties of V4O9-A missing link in binary vanadium oxides[END_REF][START_REF] Choi | High infrared transparency up to 8-μmwavelength in correlated vanadium Wadsley conductors[END_REF].

Chapter conclusions

In this chapter, the different oxides of the V-O system have been exposed with an emphasis on thermochromic VO2. The crystallographic and band structure of thermochromic VO2 has been described, together with comments on the mechanism of the MIT of this material. The change in the physical properties of VO2 and some possible applications were discussed.

Finally, a brief illustration of different methods to obtain a high-quality thermochromic VO2 was displayed. A more detailed discussion on the magnetron sputtering technique was depicted, with particular attention to smart two-step procedures. The difficult task of obtaining a pure VO2 phase on large surfaces has been presented. The major influence of the deposition parameters (deposition temperature, oxygen partial pressure…) and oxidation conditions (temperature, oxygen pressure and duration) on the properties of VO2 was also highlighted.

The following chapter will present the oxidation process carried out to produce the VO2 thermochromic films. The basic principles of the main characterization techniques and the particular parameters used for the analysis of the samples will be specified.

Introduction

In this thesis, VN and V2N films were grown by reactive magnetron sputtering. We discussed the principal features of the two different sputtering machines employed: the in-line semi-industrial machine and the standard laboratory one. We present a simple approach that allows us to obtain analogous films on both machines. We also described the oxidation protocol followed to achieve the VO2 thermochromic films. X-ray diffraction, Raman spectrometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and secondary ions mass spectrometry (SIMS) were performed for structural, microstructural, physicochemical characterizations and qualitative elemental depth profiles of the oxidized films. The thermally-induced properties of the oxidized VN and V2N films were analyzed, in the 30-120°C temperature range, in terms of their electrical resistance through the four-point probe method and their emissivity modulation properties by infrared camera. Thermogravimetry analysis was employed to understand the impact of water vapor on the oxidation process. In this chapter, we present the general characteristics of the experimental techniques used throughout this thesis.

In addition, we comment on each technique, the particular parameters employed for the analysis of the samples.

Magnetron sputtering deposition process 2.2.1 Sputtering: Basic principles

Physical vapor deposition (PVD) methods define several vacuum deposition techniques employed to produce thin films and coatings. Each of them is characterized by the generation of a vapor from a source material (generally denominated as target), which is condensed into a thin film on a substrate. All PVD processes differ from the others in the way the vapor is generated from the target. Throughout this thesis, we used sputtering (more specific magnetron sputtering) to synthesize the desired materials.

Sputtering is a process in which the atoms of a solid target are evaporated by bombarding it with energetic ions present in the plasma. This is due to collisions between the ions and the atoms of the target. To achieve this process, in a rarefied atmosphere of inert gas, generally Ar, a potential difference is applied between a cathode (the target) and an anode (the reactor's grounded chamber walls). Hence, an electrical discharge is originated between the target and the reactor chamber walls resulting in the ionization of the inert gas. Thanks to the electrical potential difference, the positive ions are accelerated towards the target. Figure 2.1 schematically represents the different processes that could occur during the ion bombardment of the target: (1) ejection of target atoms, (2) reflection of the incident ion by the target surface which is neutralized after the charge transfer, (3) stimulate the emission of secondary electrons that helps to sustain the plasma discharge, and (4) implantation of the incident ion into the target. It should be pointed out that the sputtering phenomenon requires that the energy of the incident ions needs to be higher than a threshold energy for each studied target. It must be higher than the binding energy of the source material. This threshold energy is between 15-30 eV for most of the metals. An important parameter in the sputtering process is the sputtering yield 𝑌(𝐸), which is the mean number of ejected atoms per incident ion. In the case of low energetic incident ions (𝐸 < 1 𝑘𝑒𝑉) and perpendicular bombardment, 𝑌(𝐸) could be written as [START_REF] Sigmund | Theory of Sputtering. I. Sputtering Yield of Amorphous and Polycrystalline Targets[END_REF][START_REF] Depla | Magnetrons, reactive gases and sputtering[END_REF]:

𝑌(𝐸) = 3𝛼 𝜋 2 𝑚 1 𝑚 2 (𝑚 1 + 𝑚 2 ) 2 𝐸 𝑈 𝑆 2.1
where 𝛼 is a dimensionless function of the mass ratio between the incoming particle and the target atom, 𝑚 1 and 𝑚 2 are the atomic masses of the ion and the target, 𝐸 is the energy of the incident ion and 𝑈 𝑆 the surface binding energy.

Magnetron sputtering

In the previous section we described the classical sputtering process (diode sputtering). The main problem with this method remains the low ionization rate of the discharge leading to work at high pressures to sustain the plasma. As a consequence, the sputtered atoms suffer several collisions before arriving at the substrate, forming porous films with a low deposition rate. To avoid these drawbacks the target is provided with a magnetron device [START_REF] Arnell | Recent advances in magnetron sputtering[END_REF][START_REF] Kelly | Magnetron sputtering: a review of recent developments and applications[END_REF]. It consists of two concentric magnets of opposite polarities, as shown in Figure 2.2. This results in a generation of a magnetic field in the vicinity of the target, trapping secondary electrons, thus increasing the probability of a collision with an argon atom in an ionizing interaction. Hence a dense plasma is created near the target, increasing the ionization rate and the ion bombardment of the target, and subsequently the deposition rate. Moreover, under these conditions, it is possible to decrease the working pressures achieving the deposition of denser films.

Reactive magnetron sputtering

In the previous section, we have described how the sputtering process could be used to synthesize metallic films from metallic targets. However, it is also possible to deposit more complex compounds such as oxides, nitrides, carbides... The use of ceramic targets is one approach to synthesize these compounds [START_REF] Minami | Highly Conductive and Transparent Aluminum Doped Zinc Oxide Thin Films Prepared by RF Magnetron Sputtering[END_REF][START_REF] Yabuta | High-mobility thin-film transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering[END_REF][START_REF] Singh | Structural and optical properties of RF magnetron sputtered aluminum nitride films without external substrate heating[END_REF][START_REF] Shigesato | Thermochromic VO2 Films Deposited by RF Magnetron Sputtering Using V2O3 or V2O5 Targets[END_REF][START_REF] Ho | High quality thermochromic VO2 films prepared by magnetron sputtering using V2O5 target with in situ annealing[END_REF]. Nevertheless, this is a more challenging process due to the charge accumulation on the surface of the insulating targets, and the consequently arcs formation. These obstacles are overcome by employing the radio-frequency (RF) magnetron sputtering mode. Yet, the alternate voltage applied during the RF mode decreased the sputtering rate compared to direct current magnetron sputtering (working with the same power). Besides, ceramic targets are more expensive than metallic ones. Since ceramic targets are brittle, the heating of the target due to the ion bombardment induces a thermal gradient in the target and its cracking. Thus, another approach has to be used to deposit ceramics thin films.

Reactive magnetron sputtering is another procedure to synthesize compound materials. In this mode, a reactive gas is introduced in the chamber, forming a mixture with the Ar gas. The addition of a reactive gas has a significant influence on the sputtering process. For instance, the deposition rate and the composition of the films are dependent on the reactive gas flow [START_REF] Safi | Recent aspects concerning DC reactive magnetron sputtering of thin films: a review[END_REF][START_REF] Berg | Modeling of reactive sputtering of compound materials[END_REF]. as a function of the reactive gas flow rate for a reactive sputtering process (carried out with a constant target current during processing) [START_REF] Berg | Fundamental understanding and modeling of reactive sputtering processes[END_REF]. It is important to note that the curve does not follow the same path for the increase of the reactive gas flow rate than for the decrease, displaying a hysteresis behavior. In the beginning, for small values of the reactive gas flow rate, the partial pressure is nearly zero. Almost all the reactive gas is absorbed by the sputtered atoms that condensate on the chamber walls and at the substrate surface (getter effect of the metallic flux). From zero to position A, the target operates in the metallic mode called elemental sputtering mode (ESM). After the critical point A, the chamber walls are saturated (a film is formed on the whole wall), a small increase of the reactive gas flow rate drives to a considerable increase in the partial pressure (range A-B in Figure 2.3). This new value of partial pressure (point B) is high enough to form a compound at the entire surface target. From this position, the target is in poisoned or compound mode. Generally, the binding energy increases when metal is transformed into a compound. As a result, the sputtering yield of the target declines when compared to the elemental target material, and the deposition rate decreases. Since less metal is sputtered after point B, the excess of reactive gas will increase the partial reactive pressure. The process is then working in the compound sputtering mode (CSM). In CSM, when the reactive gas flow rate begins to decrease, the partial reactive pressure becomes different than during the increase of the flow rate, due to the remaining saturation of the walls. This process happens until the compound layer on the surface of the target is removed (decontamination of the target), and metal is sputtered once more (region C-D). In this stage, the reactive gas consumption increases, and the deposited film is in the metallic mode again.

The hysteresis effect is one of the central problems in experimental reactive sputtering systems. It can be avoided in some systems with low reactivity for gas/target material combination. For instance, Schulte et al. showed that the small hysteresis effect and not avalanche-like transitions are observed in the Al/N system [START_REF] Schulte | Magnetron sputtering of aluminium using oxygen or nitrogen as reactive gas[END_REF]. On the other hand, the pumping speed of the vacuum system influences the hysteresis effect. The higher the pumping value, the less pronounced the hysteresis. For high enough pumping values, it could even disappear [START_REF] Kadlec | Influence of the pumping speed on the hysteresis effect in the reactive sputtering of thin films[END_REF][START_REF] Kadlec | Hysteresis effect in reactive sputtering: a problem of system stability[END_REF]. Methods such as plasma emission monitoring (PEM) can be used to work in the instability zone to deposit ceramic films with high deposition rate. PEM method is mainly used for the deposition of oxides. For the deposition of transition metal nitrides, such equipment is not mandatory due to the quite high deposition rate of such materials.

Magnetron sputtering depositions machines

During this work, all the VN and V2N samples were deposited using two different sputtering machines. Both deposition chambers are connected to the IJL "TUBE" equipment.

This unique tool combines 25 thin film deposition/growth analysis and post-deposition treatment techniques, all connected under Ultra High Vacuum (10 -9 Pa), see Figure 2.4. In this configuration, as-deposited samples are transported to an analysis chamber, without exposing them to the air, protecting the films from undesired effects (like oxidation). In the following subsections, we will describe the principal characteristics of each machine.

Description of the semi-industrial sputtering machine

This "in-line" prototyping reactor has two chambers, the load-lock, and the deposition chamber (Figure 2.5). Both of them provide with a turbo-molecular pump to achieve a pressure up to 10 -5 Pa. The load-lock chamber allows a fast transfer of samples to the deposition chamber without venting every time a new film is synthesized. Substrate cleaning is performed in the load-lock chamber using a microwave plasma combined with a negative bias of the substrate in DC-pulsed mode. This machine is coupled to the standard sputtering chamber (see section 2.3.2) and to the Tube via a UHV load-lock chamber.

The sputtering chamber has two Flecher Plasma Technich rectangular cathodes placed side by side (with the possibility to install a third cylindrical cathode). The cathodes (330 x 50 x 6 The substrate holder does not rotate in this machine. In this case, it moves back and forth on a rail in front of the cathode that is used. In co-sputtering mode, the substrate holder crosses in front of each of the cathodes. The raster scanning speed is adjustable from 0 to 300 cm/min.

The target-substrate distance is fixed to 5 cm. A Brooks mass flow meter controls each of the three available gas: argon (0-98 sccm), nitrogen (0-18 sccm), and oxygen (0-18 sccm). This sputtering machine allows neither the synthesis of biased films nor in-situ heating of the substrates. A PC-controlled regulation VAT valve allows precise control of the working pressure in opening or pressure mode.

VN thin films deposition in the semi-industrial sputtering machine

The first step before the deposition was to clean the substrates with ethanol. Afterward, the cleaning process continued in the load-lock chamber with the introduction of Ar gas, then the application of 500 Watts to the Magdrive microwave generator to form a plasma. The Ar + ions bombarded the negatively biased substrate holder (-280 Volts applied to the pulsed-DC Pinnacle+ generator) for 90 seconds. After this process, the substrate holder was transferred to the deposition chamber. VN thin films were deposited on Al and undoped (100) silicon substrates by reactive magnetron sputtering of a metallic vanadium target (99.95% purity) in an Ar-N2 mixture with flow rates of 30 and 5 sccm, respectively. The deposition pressure was fixed to 1 Pa. A pulsed-DC power supply, working on a set current of 1.5 A, provided power to the V target. The frequency of DC-pulses and their off-time were kept at 50 kHz and 4 μs, respectively. The substrate is composed of a Si wafer (four inches diameter) fixed on an Al foil (29 x 19 cm 2 ). It is important to remark that thanks to the size of the substrate holder, in the same deposition, VN thin films were synthesized on Al and Si substrates. The substrate holder moved back and forth in front of the V target (to ensure film homogeneity) at a speed of 50 cm/min for 1 h. The films deposited with these parameters have a thickness of approx. 175 nm.

All the results detailed in chapters 3 and 4 have been obtained from the oxidation of these samples.

Description of the standard sputtering machine

The standard laboratory sputtering reactor (Figure 2.6) has two chambers, the load-lock, and the deposition chamber. The load-lock chamber, equipped with a primary pump allows a fast transfer of samples to the deposition chamber without venting every time a new film is synthesized. A turbo-molecular pump in the deposition chamber guarantees a pressure up to 10 - 6 Pa. The volume of the deposition chamber is around 70 L. The machine is attached to the sputtering chamber of semi-industrial size and to the Tube via a UHV load-lock chamber, pumped by an ion/sublimation pump.

There are 3 Meivac cathodes (two inches diameter) in confocal "sputter-up" configuration.

Each cathode is equipped with a different power supply: RF (Seren R300 of 300 W), DC Maxim1000 (1 kV), and pulse DC Advanced Energy Pinnacle+ (5 kW, 5-350 kHz, τON from 60 to 100%). The chamber wall, as well as the three cathodes, are water-cooled. The substrate holder is a two inches diameter Inconel molybloc. The device used to manipulate the molybloc allows at the same time, the rotation of the substrate (0-50 rpm), its RF bias, as well as its heating by Joule effect (up to 800°C). The homogeneous deposition zone is the entire surface of the molybloc. The target-substrate distance is adjustable from 9 to 19 cm. A Brooks mass flow meter controls each of the three available gas: argon (0-98 sccm), nitrogen (0-18 sccm), and oxygen (0-18 sccm).

VN, V2N and epitaxial VN thin films deposition in the standard sputtering machine

In this section, we described the conditions to deposited VN, V2N, and the epitaxial growth of VN in the standard sputtering machine. There are significant differences between these depositions. However, let's first discuss the mutual parameters.

All the films were deposited by reactive magnetron sputtering of a metallic vanadium target (99.95% purity). A pulsed-DC power supply, working with discharge current regulation, provided power to the V target. The frequency of DC-pulses and their off-time were kept at 50 kHz and 4 μs, respectively. The distance between the target and substrate was 9 cm. During the deposition, the substrate holder was in rotation mode (5 rpm) to guarantee a good homogeneity of the films.

In Table 2.1 we present the parameters that differed in the growth of VN, V2N and epitaxial VN films in the standard sputtering machine. In the case of VN and V2N samples, the substrates were cleaned with ethanol before the introduction in the load-lock chamber. Once inside the deposition chamber, the substrate's surface was etched by Ar + ions using a RF Seren R300 power supply at 50 W for 3 minutes at 1 Pa.

On the other hand, the MgO wafers with orientation (100), polished, and dimensions 20 x 20 x 0.5 mm 3 followed a completely different process. Before using the MgO substrate, it was kept inside a glove box under an Ar atmosphere and individually sealed in a sack to reduce the exposure to moisture while storing. Then it was mounted in the substrate holder and introduced into the TUBE to be cleaned it before the deposition. To achieve epitaxial growth the substratecleaning process is crucial. Annealing at high temperatures removes the organic contaminants and changes the surface by atomic reconstruction of the surface [START_REF] Le Febvrier | Wet-cleaning of MgO(001): Modification of surface chemistry and effects on thin film growth investigated by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy[END_REF]. The thermally degassed process usually lasts one hour and is performed at different temperatures, for instance, 800°C [START_REF] Mei | Elastic constants, Poisson ratios, and the elastic anisotropy of VN(001), (011), and (111) epitaxial layers grown by reactive magnetron sputter deposition[END_REF][START_REF] Mei | Growth, nanostructure, and optical properties of epitaxial VNx/MgO(001) (0.80 ≤ x ≤ 1.00) layers deposited by reactive magnetron sputtering[END_REF] and 900°C [START_REF] Calamba | Dislocation structure and microstrain evolution during spinodal decomposition of reactive magnetron sputtered heteroepixatial c[END_REF]. In our case, the process is carried out for about 45 min at 1000°C.

As mentioned before, the standard sputtering machine allows the heating of the substrate holder up to 800°C. Hence, the MgO substrate was thermally degassed in the MBE machine connected to the TUBE. This chamber is equipped with a reflection high-energy electron diffraction (RHEED) device to study the surface state of the samples. We will discuss in more details the RHEED technique in section 2.6.2. Afterward, the cleaned substrate was transferred from the MBE to the sputtering chamber where the deposition process begins using the conditions described in Table 2.1.

First approach to compare the deposition conditions used in the two sputtering chambers

The goal of this section is to obtain a simple criterion that allows us to compare the films synthesized on one machine with the ones obtained on the other. In our model, strong assumptions have been made. We will not consider:

-the difference in volume between the deposition chambers, -the different pumping speeds in each machine, -the size of the magnetron, -the difference in the flow of sputtered atoms to reach the substrate in each deposition chamber.

We will assume that the fundamental parameter that governs the growth of thin films is the energy at which the sputtered atoms arrive on the substrate. Hence, the criterion to have analogous films from both machines is that the sputtered atoms coming from the target arrive with the same energy at the substrate.

During the sputtering process, the atoms leaving the surface of the target will lose energy due to collision with the Ar atoms in the plasma. The energy of the atoms arriving at the film surface (𝐸 𝐹 ) is strongly dependent on the pressure and the target-substrate distance, and can be estimated from [START_REF] Meyer | Thermalization of sputtered atoms[END_REF]:

𝐸 𝐹 = (𝐸 -𝑘 𝐵 𝑇 𝑔 )𝑒 [𝑛 𝑙𝑛〈 𝐸 𝑓 𝐸 𝑖 〉] + 𝑘 𝐵 𝑇 𝑔 , 2.2
where 𝐸 is the energy of the sputtered atoms as it leaves the target, 𝑘 𝐵 is Boltzmann constant, 𝑇 𝑔 is the sputtering gas temperature and 𝑛 is the number of collisions that take place before reaching the substrate. When considering a hard-sphere interaction between a particle with energy 𝐸, mass 𝑚 𝑠 and stationary Ar atoms of mass 𝑚 𝑔 , the average ratio of energy of the sputtered particle after a collision is given by [START_REF] Gras-Marti | Slowing down and thermalization of sputtered particle fluxes: Energy distributions[END_REF]:

〈 𝐸 𝑓 𝐸 𝑖 〉 = 1 - 𝛾 2 , 2.3
where 𝐸 𝑖 , 𝐸 𝑓 are the energies before and after the collision and 𝛾 is the kinetic energy mass transfer defined as:

𝛾 = 4 𝑚 𝑔 𝑚 𝑠 (𝑚 𝑔 +𝑚 𝑠 ) 2 . 2.4
The number of collisions 𝑛 = 𝑑 𝜆 ⁄ depends on the target-substrate distance 𝑑 and the mean free path of the sputtered particle 𝜆. The latter can be written as:

𝜆 = 𝑘 𝐵 𝑇 𝑔 √2𝜋𝑑 𝑠 2 𝑃 , 2.5
where 𝑃 is the sputtering gas pressure and 𝑑 𝑠 is the diameter of the sputtered particle. In our case, we use a vanadium target, where 𝑑 𝑠 = 0.264 nm [START_REF] Takeuchi | Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element[END_REF].

To calculate the energy of the V atoms arriving at the film surface 𝐸 𝐹 , a Maxwell-Boltzmann (MB) distribution at 𝑇 𝑔 = 300 𝐾 is assumed for the Ar gas. Moreover, following the Sigmund-Thompson theory for sputtering [START_REF] Sigmund | Theory of Sputtering. I. Sputtering Yield of Amorphous and Polycrystalline Targets[END_REF][START_REF] Thompson | The energy spectrum of ejected atoms during the high energy sputtering of gold[END_REF], the most-likely energy of the sputtered atoms is in the peak of the energy distribution, which corresponds to 𝐸 𝑆𝐵 2 ⁄ , where 𝐸 𝑆𝐵 is the surface binding energy. We will consider that 𝐸 = 𝐸 𝑆𝐵 2 ⁄ . In the case of vanadium, 𝐸 𝑆𝐵 = 5.31 𝑒𝑉 [START_REF] Yamamura | Energy dependence of ion-induced sputtering yields from monatomic solids at normal incidence[END_REF].

It is important to note that we did not change the parameters for the VN deposition in the semi-industrial machine (see section 2.3.1.1). Moreover, we fixed the target-substrate distance in the standard deposition reactor to 9 cm (see section 2.3.2.1). Therefore, using equations (2.2)-(2.5) we determined the value of pressure (0.56 Pa) to obtain analogous films in the standard sputtering machine. We will use these results in Chapter 5 for the oxidation of different VN films.

As our principal goal is to compare energy values for both machines, working with 𝐸 = 𝐸 𝑆𝐵 2 ⁄ seems relevant. This assumption allows us to calculate the most-likely energy with which the sputtered atoms reach the surface of the substrate. However, equation (2.2) could be used for most general and accurate predictions. In this case, the calculus is repeated for each 𝐸 in the distribution of sputtered atoms and weighted by the collision probability. This probability is the product of the sputtering distribution 𝑓(𝐸) and the value of the MB distribution at 𝑇 𝑔 [START_REF] Meyer | Thermalization of sputtered atoms[END_REF]. This results in the energy distribution of the sputtered particles arriving at the substrate, and most crucial, it provides the average energy value of the atoms reaching the surface's substrate. Therefore, one can compare this adatom's average energy values with the energy order of different surface processes (chemisorption, physisorption, and diffusion) [START_REF] Gall | Pathways of atomistic processes on TiN(001) and (111) surfaces during film growth: an ab initio study[END_REF].

This criterion has a significant drawback, related to the non-rotation of the substrate holder of the semi-industrial machine. When using one target, as in our case, the as-deposited films are homogeneous. Nevertheless, while working with two targets (La and Co) in the cosputtering mode a multilayers structure was obtained [6]. This multilayer structure affects the properties of the films. On the other hand, no multilayer structure was formed with the standard sputtering reactor [6]. To avoid the formation of a multilayer structure during depositions that involved two targets one should increase the raster scanning speed of the substrate holder and decrease the flux of particles from both targets.

Thin film oxidation

Once the VN and V2N films are obtained, the second step is their oxidation to form VO2.

In this thesis, we used two equipments to oxidize the VN thin films: a furnace under atmospheric pressure and a thermogravimetry analysis chamber. During this work, the furnace was the most used equipment to oxidize the samples. On the other hand, the thermogravimetry was used to analyze the role of water vapor in the oxidation.

Carbolite HRF 7/45 furnace

The modified Carbolite HRF 7/45 furnace is equipped with a quick sample tray loader to prevent a huge drop in the temperature when the sample is introduced ( In this thesis, we used the furnace working under atmospheric pressure at a preheated temperature (450, 525 and 550°C) and various durations (from a few minutes up to 50 min).

The as-deposited films were labeled VN or V2N, and "VN_temperature_time" or "V2N_temperature_time" for the oxidized films at various temperatures and annealing duration. The most common instruments operate at atmospheric pressure or under vacuum, in isothermal mode, or programmed temperature mode. Therefore, TGA can be applied to any samples that will undergo a mass variation over time due to the effect of temperature in a given atmosphere. Evaporation, sublimation, oxidation are among the transformations that will be detected by thermogravimetry [START_REF] Wirth | Thermogravimétrie (Technique de l'ingénieur)[END_REF].

In our work, we study the oxidation of VN and V2N on the isothermal mode. Moreover, for the oxidation of the films, we used "dry air" (AlphaGaz 1) or "humid air" (AlphaGaz 1 + 10% absolute humidity). In both cases, the sample was heated at a rate of 20 °C/min up to 450°C in Ar atmosphere. Once the desired temperature was achieved, the Ar gas valve was closed, and simultaneously the dry air (or humid air) valve was opened. After the oxidation time finished, the sample was cooled at a rate of 20 °C/min until reached room temperature. We used a 1750 Simultaneous symmetrical thermoanalyser (SETARAM) for the oxidation of the films in dry air. On the contrary, a Setsys Evolution 1750 SETARAM coupled with a humidity Wetsys (SETARAM) equipment was employed for the oxidation of the films in the humid air.

Thin film characterization 2.6.1 X-ray diffraction (XRD)

Over the years, X-ray diffraction (XRD) has become an essential technique for the structural characterization of materials. The phenomenon of diffraction occurs when a wave deviates from its trajectory upon encountering an obstacle. It only takes places when the length of the wave is of the same order of magnitude of the scattering object. Therefore, when studying crystalline structures with interatomic distances between 0.5 and 2.5 Å, X-rays are used. When a monochromatic X-ray beam interacts with a crystalline solid, it is scattered by the atoms forming the solid. The resulting scattered waves (with the same wavelength) interfere each other by adding their amplitudes, producing a diffraction pattern. If the scattered waves are completely out-of-phase, they will extinguish one another (destructive interference). On the contrary, if the scattered waves are completely in-phase, the resulting wave has its amplitude doubled (constructive interference). Bragg's law describes the condition where constructive interference is achieved (see Figure 2.8 a)) [START_REF] Bragg | The reflection of X-rays by crystals[END_REF]:

𝑛𝜆 = 2𝑑 ℎ𝑘𝑙 sin 𝜃, 2.6
where 𝑛 is an integer determining the order of reflection, 𝜆 is the wavelength of the incident radiation, 𝑑 ℎ𝑘𝑙 is the distance between crystallographic planes with Miller indices ℎ, 𝑘 and 𝑙, and 𝜃 is the diffracting angle.

For the most commonly used Bragg-Brentano (or 𝜃 -2𝜃) geometry, the X-ray source is fixed, while the sample and detector change their positions at a rate 𝜃 ̇ and 2𝜃 ̇, respectively. where 𝛽 is full width at half maximum (FWHM), and 𝐾 is a constant that depends on the particle geometry (0.829 for spherical particles) [START_REF] Langford | Scherrer after sixty years: A survey and some new results in the determination of crystallite size[END_REF]. Furthermore, micro-strain induces a broadening of the peak. However, equation 2.7 does not consider this contribution to the FWHM of the peak. As a consequence, Scherrer's equation gives a lower limit for the crystallite size.

In this thesis, an AXS Bruker D8 Advance diffractometer in the Bragg-Brentano configuration with Cu Kα1 radiation (λ = 1.5406 Å) was used to perform all the XRD measurements.

Reflection high-energy electron diffraction (RHEED)

Reflection high-energy electron diffraction (RHEED) is a technique employed to characterize the surface of crystalline materials during and after the growth. The principle is the same as in X-ray diffraction. In this case, a monochromatic beam of electrons (0 to 40 keV) is sent onto the sample. Incident electrons diffract from atoms at the surface of the sample, at specific angles, a small fraction of the diffracted electrons interfere constructively and form regular patterns on the detector. Figure 2.9 depicts two RHEED patterns acquired on a epitaxially-growth VN on MgO substrate from two different angles. To ensure the proper study of the surface's sample is mandatory to fulfill the following conditions [START_REF] Andrieu | Les Surfaces solides : concepts et méthodes[END_REF]:

-the chamber must be in UHV (to avoid contamination of the studied surface),

-the electron beam hits the sample at an extremely low angle (approx. 1°) relative to the sample surface (to guarantee that diffraction occurs just at the surface of the sample).

RHEED's principal advantage over other surface techniques is the possibility to monitor the growth evolution of the sample during the deposition [START_REF] Andrieu | Les Surfaces solides : concepts et méthodes[END_REF]. The initial goal of electron diffraction was to explore the surface structures and growth process of single-crystalline films (especially in Si and GaAs epitaxial growth). However, this is a quick and powerful technique to determine the crystalline order in thin films. Indeed, very different RHEED patterns are obtained for an amorphous film (continuous background)), a polycrystalline film (rings as in X-ray powder diffraction), or a single-crystalline film (pattern with straight streaks when the beam is aligned with atomic lines). Consequently, a simple look at the RHEED pattern just after the growth process allows the experimentalist to immediately determine the crystalline order range (at least qualitatively). In this thesis, a RHEED (Riber manufacturer) set-up available in the MBE chamber close to the sputtering one (in the TTO part of the TUBE) was employed to investigate the surface of the epitaxial films.

Raman spectroscopy

Raman spectroscopy is a non-destructive analysis technique based on the detection of inelastically scattered photons as a result of the interaction between a laser and a sample. When the electromagnetic radiation interacts with a material, the energy may be transmitted, absorbed, or scattered. Commonly, the radiation frequency of the laser is lower than the electronic frequencies but is much higher than the vibrational frequencies [START_REF] Colthup | Introduction to Infrared and Raman Spectroscopy[END_REF]. The rotational and vibrational energy of the molecule does not change in an elastic collision. Hence, the frequency of the scattered photon is the same as the incident photon. This phenomenon is called Rayleigh scattering, and it is by far the highest component of the scattered radiation. On the contrary, an inelastic collision between the incident photon and the material results in a change of the vibrational or rotational energy of the bonds. In this case, this phenomenon is named Raman effect. Thanks to energy conservation, there are two cases for the frequency of the scattered photon. First, the frequency of the scattered photon is smaller than the incident frequency, giving rise to the Stokes lines in the Raman spectrum. Second, the frequency of the scattered photon is higher than the incident frequency, giving rise to the anti-Stokes lines in the Raman spectrum. At ordinary temperatures, most of the materials exist in the ground state and therefore Stokes lines have greater intensities than anti-Stokes lines which originate from an excited level with a lower population [START_REF] Colthup | Introduction to Infrared and Raman Spectroscopy[END_REF]. A Raman spectrum contains several bands, characteristics of each chemical compound that are employed as a "fingerprint" to distinguish one compound from another.

In this thesis, Raman spectra of samples were acquired on a Horiba Jobin-Yvon LabRAM HR800 spectrometer. The films were excited by a Nd:YAG laser (532 nm) with a spot size of around 1.5 µm. The laser beam is focused on the samples' surface using a microscope objective with a magnification of x100. The laser power (approx. 10 mW) was low enough to avoid any heating effect (such as oxidation from VO2 to V2O5). Each spectrum was recorded with an acquisition time of 60 seconds.

Secondary ion mass spectrometry (SIMS)

Secondary ion mass spectrometry (SIMS) is a technique employed to investigate the composition of solid surfaces and thin films. It works by sputtering the studied surface with a focused energetic ion beam (commonly between ∼0.1 and 50 keV). These ions (called primary ions) produce the ejection of atoms and molecules from the solid's surface, a small percentage of them are in the ionized state. The emitted ions (referred to as secondary ions) are then collected and transferred through a mass spectrometer. Figure 2.10 displays a well-simplified schema of the main components of SIMS.

This technique can identify all elements, including hydrogen. Moreover, it holds the highest sensitivity among the surface analysis methods, with elemental detection limits ranging from parts per million to parts per billion [START_REF] Horrocks | Scalable Hydrothermal Synthesis of Free-Standing VO2 Nanowires in the M1 Phase[END_REF]. There are two distinct modes in SIMS analysis.

Dynamic SIMS provides the elemental and (or) molecular distributions over numerous atomic layers (in-depth). In this mode, SIMS data are typically presented in three formats: mass spectra, spatial image, and depth profiles (Figure 2.10). Mass spectra display secondary ion intensity as a function of mass to charge ratio. Ion images show secondary ion lateral distribution. Depth profiles exhibit secondary ion intensity as a function of depth into the sample. On the other hand, the second mode is named Static SIMS, the difference with Dynamic SIMS is that this method is only suitable for surface analysis (depth profile is not possible).

In this work an IMS 7F Cameca secondary ions mass spectrometry system was used to obtain the compositional profiles of the VN oxidized films as a function of depth. The instrument was configured to have Cs + primary ion energy of 5 keV, secondary ion energy of 2 keV, a primary beam intensity of 10 nA, a raster size of 100 μm and an analyzed area of 33 μm 2 .

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is a widely employed technique to achieve highquality topographical images of the studied material. A focused beam of high-energy electrons (accelerated by a voltage of a few kV) scans the surface of a sample. When the electrons beam hit the atoms elastic and inelastic interactions occur. The collisions produce a variety of signals:

secondary electrons (SE), backscattered electrons (BSE), diffracted backscattered electrons (EBSD), characteristics X-rays, among others. Secondary electrons (from the conduction or valence band) have low energy and are generated close to the surface. Hence, these are the electrons use to achieve high-quality topographical images. On the other hand, BSE arises from a deeper region in the pear-shaped interaction volume (see Figure 2.11). The higher the atomic number of the element the higher the scattered signal. Therefore, the image from BSE displays composition information. EBSD is used to determine crystal structures and orientations of the studied sample. Moreover, a chemical analysis (using the X-rays emitted during the interactions) could be performed if an energy-dispersive spectroscopy (EDS) system is installed in the microscope. 6, samples were studied with a ZEISS GeminiSEM 500.

Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) is a unique characterization technique that provides structural and microstructural information about the studied sample. A high-energy [START_REF] Williams | Transmission Electron Microscopy: A Textbook for Materials Science[END_REF].

crystallographic plane in the direct space. In the case of polycrystalline and amorphous materials, the diffraction pattern yields a series of rings (wide and diffuse for amorphous films).

A fast Fourier transform (FFT) applied to a high-resolution TEM (HRTEM) micrograph gives the same crystallographic information.

On the other hand, in the image mode, the intermediate lens is readjusted so that its object plane is the image plane of the objective lens. Then the image is projected onto the screen. In Scanning TEM mode (convergent beam + scan), diffused electrons are collected to form images (see Figure 2.14): bright-field (BF) and high angle annular dark-field (HAADF). BF images are obtained from the electrons that are transmitted through the sample without much deflection (close to the direct beam). Hence, brighter areas correspond to the presence of lighter elements (the electron beam does not suffer much deviation). On the contrary, darker areas correspond to heavy elements, thicker regions of the sample. In the case of the HAADF images, the image is formed from electrons that are diffused with a high angle from the optical axis of the microscope. In such circumstances, bright regions correspond to heavy elements, and also to grains that are under the selected diffraction conditions.

Electron Energy Loss Spectroscopy (EELS) is a technique associated with STEM. In EELS an electron spectrometer directly records the energy loss experienced by the electron beam incident on the sample. These energy-loss events provide information about the chemistry and the electronic structure of the specimen atoms. These results could reveal significant sample information such as the bonding/valence state, the nearest-neighbor atomic structure, their dielectric response, the free electron density, the band gap (if there is one), and the specimen thickness [START_REF] Williams | Transmission Electron Microscopy: A Textbook for Materials Science[END_REF]. time, and the diaphragm were set to 2 cm, 1 second, and 5 mm, respectively. Finally, the resolution of the full width at half maximum (FWHM) of the zero-loss peak was 0.7 eV.

Infrared thermal camera

One approach to characterize the thermochromic behavior of the VO2 films is to study the difference of the optical properties with the temperature. The determination of the emissivity change from the semiconductor state to the metal state is a practical method to accomplish this task. We can define the emissivity (𝜀) as the ratio of the thermal radiation from a surface to the radiation from an ideal black body surface at the same temperature, therefore, 𝜀 is a value between 0 and 1.

During this thesis, we employed an infrared thermal camera1 FLIR 325sc that yields a cumulative response in the of 7.5-13 µm range together with a Prazitherm heater system to calculate the emissivity switch of the samples. The measurements were taken in the 30-120°C range, with approximately 55 cm between the camera and the sample. The camera forms an angle of around 57° relative to the sample surface. The IR camera provides the measure of the infrared temperature (TIR) of the sample while the heater gives the actual temperature of the sample (TS).

To calculate the emissivity, we follow the procedure employed by Benkahoul et al. [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF]:

𝜀 = 𝜀 𝑟𝑒𝑓 𝑇 𝐼𝑅 𝑇 𝑆 , 2.8
where 𝜀 𝑟𝑒𝑓 represents the emissivity of a blackbody reference. It is relevant to note that we do not have a proper blackbody reference in the 30-120°C range. For this reason, we will use throughout the entire thesis 𝜀 𝑟𝑒𝑓 = 1.

Four-point probe method

A four-point probe is a technique to measure the resistivity of a material. It consists of four in a line equidistant electrical probes (Figure 2.17). It operates by applying a current (𝐼) on the two outer probes and measuring the resultant voltage drop (∆𝑉) between the two inner probes.

In this configuration, the wire resistances and the contact resistances do not contribute to the measurement. This is an advantage for the precise measurement of low resistance values. The sheet resistance (SR) is determined by:

Introduction

In 1959, Morin demonstrated that vanadium dioxide (VO2) exhibits a reversible metalinsulator transition (MIT) approximately at TMIT = 68°C [START_REF] Morin | Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature[END_REF]. At low temperature, VO2 behaves as a semiconductor with a monoclinic structure, high transmittance in the infrared region and high electrical resistivity, while at temperatures higher than TMIT a transition to a metallic tetragonal rutile structure occurs and VO2 exhibits low transmittance and low electrical resistivity. The change of these properties during the phase transition of VO2 makes this material convenient for several applications, such as infrared modulation in smart windows [START_REF] Ji | Al-doped VO2 films as smart window coatings: Reduced phase transition temperature and improved thermochromic performance[END_REF][START_REF] Li | Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows[END_REF][START_REF] Liang | Active and passive modulation of solar light transmittance in a hybrid thermochromic soft-matter system for energysaving smart window applications[END_REF][START_REF] Ji | High thermochromic performance of Fe/Mg co-doped VO2 thin films for smart window applications[END_REF][START_REF] Ji | Inkjet printing of vanadium dioxide nanoparticles for smart windows[END_REF][START_REF] Kim | Highthroughput roll-to-roll fabrication of flexible thermochromic coatings for smart windows with VO2 nanoparticles[END_REF][START_REF] Vu | Physical vapour deposition of vanadium dioxide for thermochromic smart window applications[END_REF], thermal solar collectors [START_REF] Mercs | Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors[END_REF][START_REF] Paone | Thermal solar collector with VO2 absorber coating and V1-xWxO2 thermochromic glazing -Temperature matching and triggering[END_REF], dynamic thermal control of spacecraft [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF][START_REF] Hendaoui | Enhancement of the positive emittance-switching performance of thermochromic VO2 films deposited on Al substrate for an efficient passive thermal control of spacecrafts[END_REF][START_REF] Hendaoui | Highly tunable-emittance radiator based on semiconductor-metal transition of VO2 thin films[END_REF][START_REF] Hendaoui | VO2-based smart coatings with improved emittance-switching properties for an energy-efficient near roomtemperature thermal control of spacecrafts[END_REF], ultrafast electronic devices [START_REF] Pergament | Oxide Electronics and Vanadium Dioxide Perspective: A Review[END_REF][START_REF] Qiu | Electricallydriven metal-insulator transition of vanadium dioxide thin films in a metal-oxideinsulator-metal device structure[END_REF] and many others. Although the research on this material has been carried on for several decades, the synthesis of VO2 films with good thermochromic properties on large surfaces still remains a challenge. The reason behind this defiance is the imperative need to avoid several other phases in the vanadium-oxygen system [START_REF] Kang | Critical evaluation and thermodynamic optimization of the VO-VO2.5 system[END_REF].

Magnetron sputtering is one of the most used methods to produce VO2. Among the different approaches to obtain VO2, reactive sputtering in a mixture of Ar and O2 without a postannealing process is one of the most used [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF][START_REF] Yuce | Investigation of electron beam lithography effects on metal-insulator transition behavior of vanadium dioxide[END_REF][START_REF] Zhao | Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications[END_REF][START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF][START_REF] Zhu | Thermochromism of vanadium dioxide films controlled by the thickness of ZnO buffer layer under low substrate temperature[END_REF][START_REF] Kana | Thermochromic VO2 thin films synthesized by rf-inverted cylindrical magnetron sputtering[END_REF]. However, the need to properly control the O2 flow rate makes this method not suitable on large surfaces. On the other hand, a post-annealing process in air or oxygen is commonly used to improve the quality of the deposited films [START_REF] Melnik | Lowtemperature method for thermochromic high ordered VO2 phase formation[END_REF][START_REF] Yu | Oxygen pressure manipulations on the metal-insulator transition characteristics of highly (0 1 1)-oriented vanadium dioxide films grown by magnetron sputtering[END_REF][START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF]. Other interesting and simple proposals are the so-called sputtering oxidation coupling (SOC) method and the thermal oxidation method. These are a two-stage procedures, in which a vanadium film is deposited and then annealed in a furnace outside of the deposition chamber [START_REF] Ji | Thermochromic VO2 films by thermal oxidation of vanadium in SO2[END_REF][START_REF] Xu | A novel sputtering oxidation coupling (SOC) method to fabricate VO2 thin film[END_REF]. The possibility to regulate the temperature and duration during the oxidation process makes this method suitable and attractive for the industrial process.

In this work, we chose to modify these two-stage procedures using an original precursor:

vanadium nitride (VN). At room temperature, the NaCl-like structure of the VN phase is more resistant to oxidation than the body-cubic centered of the V one [START_REF] Kubel | Structural phase transition at 205 K in stoichiometric vanadium nitride[END_REF][START_REF] Mei | Dynamic and structural stability of cubic vanadium nitride[END_REF]. Besides, several studies have shown that the oxidation of VN at elevated temperatures resulted in the formation of a series of vanadium oxides, which significantly improved the tribological properties of the film [START_REF] Guo | Comparison of microstructures and properties of VN and VN/Ag nanocomposite films fabricated by pulsed laser deposition[END_REF][START_REF] Fateh | The Beneficial Effect of High-Temperature Oxidation on the Tribological Behaviour of V and VN Coatings[END_REF]. Nevertheless, to the best of our knowledge, controlled oxidation of VN has never been performed to produce thermochromic VO2.

In this chapter, VN thin films are reactively sputter-deposited on Si substrates by using an

Microstructure and morphology of oxidized samples

The X-ray diffractograms for as-deposited and air annealed VN films deposited on silicon are shown in Figure 3.1 (the deposition conditions of the VN films are described in Chapter 2).

VN films (175 nm thick) exhibit two diffraction peaks located at approx. 37.9° and 80.3° corresponding to [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] and ( 222) planes of the face-centered cubic VN structure (PDF no.00-035-0768), respectively. No other peak is detected except those of silicon substrate. Figure 3.1 also presents diffractograms for the films after air-annealing at 450°C during different durations in the 7-50 min range. For the lowest duration, the monoclinic VO2 phase is not detected.

However, the intensity of the VN peaks diminishes, which indicates that the oxidation process has effectively begun. In addition, these peaks are shifted to higher angles because of stress relaxation during the oxidation process. As the annealing time increases, the VN peaks intensity decreases even more and the main [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] peak is noticed up to 25 min, indicating the presence of a remaining layer of vanadium nitride. Monoclinic VO2 is detected for the first time after 10 min of air oxidation. The most significant peaks corresponding to m-VO2 are located at 2θ = 37.1, 42.3 and 72.1° (PDF no. 04-003-2035), showing that the oxide layer corresponds to an equiaxed structure (i.e. that it does not grow with a strong preferred orientation). The most intense peak of VO2 appears at 30 min of oxidation and the VN peak completely disappears at the same time. In this sample (VN_30), two new peaks with a low intensity corresponding to orthorhombic V2O5 phase are detected (PDF no. 00-041-1426). For oxidation times higher than 30 min, the peaks corresponding to VO2 decrease in intensity while the opposite occurs with the peaks of V2O5. For the VN_50 sample, the VO2 vanishes, leading to the V2O5 being the only crystalline phase present. These results show oxidation from V 4+ state to V 5+ state as the oxidation time increases and are consistent with the findings of several authors [START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF][START_REF] Rampelberg | In situ X-ray diffraction study of the controlled oxidation and reduction in the V-O system for the synthesis of VO2 and V2O3 thin films[END_REF].

Due to the overlapping of the diffraction peaks of the various vanadium oxide phases, Xray diffraction is not the most powerful method to evince the formation of a pure VO2 phase.

To verify the formation of VO2, cross-examination with Raman spectrometry is carried out for all oxidized films. Figure 3.2 presents room-temperature Raman spectra of the oxidized VN films, deposited on silicon substrates.

The Si substrate signal (520.7 cm -1 ) is barely visible until 10 min of oxidation, thanks to the presence of the VN film that exhibits metallic behavior. As the oxidation time increases the Si signal becomes more perceptible indicating a decrease of the VN layer thickness. From sample VN_25, the peak at 520.7 cm -1 is intense; suggesting that the sample is almost completely oxidized, as shown by XRD. Thus, the evolution of the Si Raman band is a powerful tool to monitor the VN oxidation. VN are not perceived anymore. The oxidized VN grains coalesce into an elongated structure, increasing grain size. As oxidation continued, the morphology of the samples does not change significantly and the grain size continues to grow. In all the images, the grain boundaries are clear and well defined.

Optical and electrical properties of oxidized samples

Near the MIT transition of VO2, some physical properties present a hysteresis between the heating and cooling cycles. Within these properties, changes in electrical resistivity and emissivity at low and high temperatures are considered as indicators of VO2 quality [START_REF] Yoon | Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method[END_REF][START_REF] Xu | The study of optimal oxidation time and different temperatures for high quality VO2 thin film based on the sputtering oxidation coupling method[END_REF][START_REF] Mlyuka | Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation[END_REF]. The higher the difference, the better the thermochromic properties.

For all oxidized samples, the optical modulation properties in the infrared (IR) domain are studied using an infrared camera. proves that at 10 min, the oxidation is not complete, indicating that higher oxidation time is needed to observe a thermochromic behavior. Furthermore, a thermochromic effect is not observed for samples oxidized for a duration longer or equal to 40 min, ∆𝜀 𝑉𝑁_40 = -0.03. In this case, this is attributed to the increased formation of the V2O5 phase with oxidation time.

On the contrary, for samples oxidized in the range of 15 and 35 minutes, a thermochromic behavior with values of ∆𝜀 between -0.14 and -0.20 is evidenced, confirming the presence of m-VO2 consistent with XRD and Raman results.

Nevertheless, the emissivity contrast obtained from our samples is lower than that reported in the literature. For example, Benkahoul et al. found ∆𝜀 = -0.36 in 300 nm thick VO2 film deposited on Si [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF]. Emissivity switching properties can be improved by doping [START_REF] Mercs | Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors[END_REF][START_REF] Hendaoui | VO2-based smart coatings with improved emittance-switching properties for an energy-efficient near roomtemperature thermal control of spacecrafts[END_REF][START_REF] Ji | Al-doped VO2 films as smart window coatings: Reduced phase transition temperature and improved thermochromic performance[END_REF],

or thickness changes [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF][START_REF] Voti | Optimization of thermochromic VO2 based structures with tunable thermal emissivity[END_REF]. Moreover, modifying the temperature and duration of the airannealing process could play a significant role in the optical properties of the oxidized films. increased by 25% (45%) when compared to the highest value in the oxidized sample at 450°C.

These results confirm the influence of this parameter on the thermochromic performance. In the next chapter, we will discuss in more detail the relationship between oxidation parameters and the optical properties of oxidized samples. prolonged air open contact. To protect the films for further and undesired oxidation antireflective layers such as TiO2 [START_REF] Jin | Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings[END_REF][START_REF] Zheng | TiO2(R)/VO2(M)/TiO2(A) multilayer film as smart window: Combination of energy-saving, antifogging and self-cleaning functions[END_REF], SiO2 [START_REF] Yu | Enhanced Visible Transmittance of Thermochromic VO2 Thin Films by SiO2 Passivation Layer and Their Optical Characterization[END_REF], AZO [START_REF] Kang | Thermochromic properties and low emissivity of ZnO:Al/VO2 double-layered films with a lowered phase transition temperature[END_REF] and SiNx [START_REF] Long | Application-oriented VO2 thermochromic coatings with composite structures: Optimized optical performance and robust fatigue properties[END_REF] are used.

Phase diagram of VN oxidation at 450°C

Gathering the results of XRD and Raman, as well as the study of optical and electrical properties, it is possible to obtain a phase diagram of VN oxidation [START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF]. evident at 40 min of oxidation. Finally, as oxidation time continues to increase, there is a region where V2O5 is the only compound because of the over-oxidation of the samples.

Chapter conclusions

This chapter provides an initial insight into the air-controlled oxidation of VN films deposited on Si substrates. We reveal, for the first time, that VN is a new efficient precursor to achieve a high-quality monoclinic VO2 using a two-step procedure.

During the oxidation process at 450 °C, there are only two phases corresponding to the V-O system, VO2 and V2O5. The first oxide formed is VO2, several techniques (XRD, Raman, IR thermometry and four-point probe method) allow us to confirm that m-VO2 is present in numerous samples oxidized between 10 and 40 min. Furthermore, V2O5 is only detected after the total oxidation of VN. The amount of V2O5 presents on the samples increases with the oxidation time (after 30 min) and affects the thermochromic performance of the oxidized films.

After 50 min of oxidation, the only phase identified is V2O5 and not thermochromic conduct is perceived.

On the other hand, the best thermochromic behavior is accomplished in the oxidized samples in the range between 25-35 min. However, it is important to point out that in these samples two phases coexist:

1-VN and VO2 in the sample VN_25, 2-VO2 and V2O5 in samples VN_30 and VN_35.

In both cases, VN and V2O5 represent a small fraction compared to the VO2 content.

Therefore, we could expect to obtain a sample with only VO2 in the range between 25-30 minutes of oxidation, which could present a better thermochromic behavior.

The TEM analysis performed on the oxidized sample for 25 minutes shows that there is an abrupt interface between VN and VO2.

Finally, a phase diagram of VN oxidation is displayed showing the formation of the different phases as a function of the oxidation time at 450°C. These results are the first indication that during the VN oxidation process, several phases can coexist. Hence, the importance of optimizing the annealing duration to obtain thermochromic vanadium dioxide is verified, in correspondence with previous studies of oxidation of vanadium films.

In this chapter, the oxidation of VN films was performed at one specific temperature (450°C). Furthermore, the first findings of optical modulation properties for films oxidized at 525 and 550°C are revealed. In the next chapter, we will study the effect that could have three different oxidation temperatures on the optical properties of the oxidized films deposited on Al substrates.

Introduction

Vanadium dioxide (as we mentioned before) has been one of the most studied thermochromic materials since its discovery by Morin in 1959 [START_REF] Morin | Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature[END_REF]. The significant change in optical transmittance and reflectance (during the metal-insulator transition), particularly in the infrared (IR) region makes this material suitable for numerous applications, for instance as, smart radiator devices for spacecraft [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF][START_REF] Kim | VO2-based switchable radiator for spacecraft thermal control[END_REF][START_REF] Hendaoui | VO2-based smart coatings with improved emittance-switching properties for an energy-efficient near roomtemperature thermal control of spacecrafts[END_REF][START_REF] Sun | VO2 Thermochromic Metamaterial-Based Smart Optical Solar Reflector[END_REF][START_REF] Madiba | Effects of gamma irradiations on reactive pulsed laser deposited vanadium dioxide thin films[END_REF], IR bolometers detectors [START_REF] Gurvitch | Nonhysteretic behavior inside the hysteresis loop of VO2 and its possible application in infrared imaging[END_REF][START_REF] Son | Characteristics of Vanadium Oxide Thin Films Prepared by Metal-Organic Decomposition for Bolometer Detectors[END_REF],

infrared camouflage [START_REF] Xiao | Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/CNT Thin Films[END_REF][START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF][START_REF] Ji | Vanadium dioxide nanopowders with tunable emissivity for adaptive infrared camouflage in both thermal atmospheric windows[END_REF][START_REF] Ji | VO2/ZnS core-shell nanoparticle for the adaptive infrared camouflage application with modified color and enhanced oxidation resistance[END_REF]... However, to employ VO2 as smart coating some challenges need to be overcome. For example, in the case of smart windows: the luminous transmittance (Tlum) and solar transmittance modulation (ΔTsol) have to be high. On the other hand, the transition temperature of the material should be close to room temperature, therefore, the TMIT of the VO2 should be lowered [START_REF] Ji | Al-doped VO2 films as smart window coatings: Reduced phase transition temperature and improved thermochromic performance[END_REF]. There are different strategies to improve the thermochromic performance of VO2 thin films. The selection of the substrate can influence the switching performance of the material [START_REF] Sakai | Impact of thermal expansion of substrates on phase transition temperature of VO2 films[END_REF][START_REF] Radue | Substrate-induced microstructure effects on the dynamics of the photoinduced metal-insulator transition in VO2 thin films[END_REF]. Nevertheless, the substrate choice is linked directly to the desired application. Elemental doping is one of the most practical and feasible methods to tune the optical properties of VO2 films as well as change TMIT. It has been shown that W is one of the most successful elements to reduce TMIT of VO2 films [START_REF] Wang | Fabrication of VO2-based multilayer structure with variable emittance[END_REF][START_REF] Paone | Influence of doping in thermochromic V1-xWxO2 and V1-xAlxO2 thin films: Twice improved doping efficiency in V1-xWxO2[END_REF]. On the contrary, Krammer et al. proved that TMIT could be increased to approx. 94°C by Ge doping [START_REF] Krammer | Elevated transition temperature in Ge doped VO2 thin films[END_REF]. Several dopants (Mg, Ca, Sr, Ba, Al, Sn, among others) have been studied as an effective way to enhance the optical properties of VO2 films [START_REF] Mercs | Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors[END_REF][START_REF] Ji | Al-doped VO2 films as smart window coatings: Reduced phase transition temperature and improved thermochromic performance[END_REF][START_REF] Li | Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows[END_REF][START_REF] Mlyuka | Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature[END_REF][START_REF] Zhao | Sn dopants improve the visible transmittance of VO2 films achieving excellent thermochromic performance for smart window[END_REF][START_REF] Dietrich | Influence of doping with alkaline earth metals on the optical properties of thermochromic VO2[END_REF]. Moreover, it has been proven that some anti-reflective coatings can improve the optical properties of VO2 films (ZrO2 [START_REF] Xu | Optimization of antireflection coating for VO2based energy efficient window[END_REF], TiO2 [START_REF] Voti | Optimization of thermochromic VO2 based structures with tunable thermal emissivity[END_REF][START_REF] Jin | Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings[END_REF]). Furthermore, anti-reflective layers such as TiO2 [START_REF] Jin | Design, formation and characterization of a novel multifunctional window with VO2 and TiO2 coatings[END_REF][START_REF] Zheng | TiO2(R)/VO2(M)/TiO2(A) multilayer film as smart window: Combination of energy-saving, antifogging and self-cleaning functions[END_REF], SiO2 [START_REF] Yu | Enhanced Visible Transmittance of Thermochromic VO2 Thin Films by SiO2 Passivation Layer and Their Optical Characterization[END_REF], AZO [START_REF] Kang | Thermochromic properties and low emissivity of ZnO:Al/VO2 double-layered films with a lowered phase transition temperature[END_REF] and SiNx [START_REF] Long | Application-oriented VO2 thermochromic coatings with composite structures: Optimized optical performance and robust fatigue properties[END_REF] have been used to prevent further oxidation (in case the films continue to be in air-direct contact).

Although great advances have been made in the thermochromic properties of VO2, the synthesis of this material on large surfaces is still an issue. In chapter 3, we proposed a new strategy to obtain high-quality VO2 films by the controlled oxidation of vanadium nitride films (VN) deposited on Si substrate [START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF].

In the present chapter, VN thin films were reactively sputter-deposited on Al substrates by using an in-line semi-industrial machine. The air oxidation of VN samples is performed at three temperatures (450, 525 and 550°C) and several durations up to 50 min. The films were labeled "VN" for as-deposited vanadium nitride films (as in chapter 3) and "VNtemperature_time" for the oxidized films at various temperatures and annealing duration. We study the effect of the temperature and the oxidation duration on the formation of VO2 films. We aim to determine the influence of the oxidation parameters on the functional properties of the obtained VO2 films.

Thermochromic behavior of oxidized VN samples at 450°C

VN precursor thin films (175 nm thick) have been deposited by the sputtering of a vanadium target in a reactive Ar-N2 atmosphere on Al substrate (the deposition conditions of the VN films are described in Chapter 2). The films have been air oxidized at 450°C during different durations. As-deposited VN films exhibit two peaks corresponding to ( 111) and ( 222) planes of the face-centered cubic VN structure (PDF no.00-035-0768), located at approx. 37.9° and 80.3°, respectively (Figure 4.1). No other peaks are detected except those coming from the aluminum substrate.

The X-ray results for films annealed at 450°C during various durations in the 7-50 min range are also presented in Figure 4.1. After 7 min of annealing, the VO2 phase is not detected.

Nevertheless, the oxidation process started, as evidenced by the decline of the VN peak intensity. Besides, these peaks are shifted to higher angles due to stress relaxation during the oxidation process. VN peaks intensity continues to decrease as the annealing time increases, and the main [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] along the [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] direction and because the m-VO2 grains present several diffraction peaks, it may be concluded that there is no structural relationship between the VN and the m-VO2 grains.

Results between 10 and 25 min suggest that within this period m-VO2 is formed from the VN precursor oxidation. The most intense peak of VO2 appears at 30 min of oxidation when there is no VN peak anymore (see Figure 4.2).

SIMS is a well-known surface analysis technique suitable to obtain composition depth

profiles. The SIMS capability of depth profiling with high resolution turns out useful to check the presence of a possible residual layer of VN between the substrate and the formed VO2 layer. Back to the Figure 4.1, all samples oxidized more than 35 min show three peaks that correspond to the orthorhombic V2O5 phase (PDF no. 00-041-1426). Once again there is a competition between phases, in this case, the over-oxidation of VO2 leads to the formation of the most stable phase of the V-O system i.e., V2O5. For the VN450_50 sample, the VO2 diffraction peaks almost disappeared, and V2O5 becomes the predominant crystalline phase.

These results are in agreement with our previous work on silicon substrate [START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF] and also with several authors who already proved the transition from V 4+ state to V 5+ state as the oxidation time increases [START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF][START_REF] Rampelberg | In situ X-ray diffraction study of the controlled oxidation and reduction in the V-O system for the synthesis of VO2 and V2O3 thin films[END_REF].

Among the exciting changes in properties during the phase transition on VO2, the emissivity switch provides an excellent idea of the thermochromic response of the films. The higher the difference in emissivity between the monoclinic and the tetragonal phase of VO2, the better the thermochromic properties. and the growth of VO2. However, with higher oxidation duration the formation of V2O5 starts and affects the thermochromic effect. Following the same line of thought due to the increasing (decreasing ) presence of V2O5 (VO2) samples with 40 and 50 min of oxidation do not exhibit thermochromic response anymore. Moreover, samples oxidized less than 15 min with an important residual VN layer (see Figure 4.2 a)) present a poor thermochromic performance.

These findings are in agreement with XRD and also with our previous results (chapter 3). The difference between the present chapter and the previous one corresponds to the nature of the substrate: Al vs. Si. The type of substrate does not modify the vanadium oxides phases found in the films. Thermochromic behavior is obtained for both substrates for very close annealing durations. The main difference is related to the sign of the emissivity switch that is positive using a reflective substrate (Al) and negative using an IR-transparent substrate (Si).

Thermochromism performance on oxidized VN samples at 525°C and 550°C

To study the effect of air oxidation at higher temperatures on the thermochromic behavior, VN precursor thin films were annealed at 525°C and 550°C during distinct durations. Compared to section 4.2, the annealing temperature has been increased, as a consequence, the annealing duration has been reduced to take into account the increase of the oxidation kinetics. min is m-VO2. For higher durations, peaks corresponding to the monoclinic and orthorhombic V2O5 phase arise due to the over-oxidation of VO2. During this period the intensity of the peaks belonging to m-VO2 starts to decrease inducing a vanishing of this phase after 5 min of oxidation. For the films oxidized at 550°C, the V2O5 phase appears after 3.5 min of annealing, while 6 min at 525°C are required to overoxidize the VO2.

The X-ray results for the three studied temperatures suggest that the VN oxidation occurs on the same stages. The first phase of the V-O system detected is m-VO2 that could coexist with VN. The second and third phases are orthorhombic and monoclinic V2O5, which always appeared only when there is no signal of VN anymore. Yet again, these results illustrate the change from the V 4+ state into the V 5+ state with the oxidation time increase and are consistent with the findings of previous studies [START_REF] Dou | Oxidizing annealing effects on VO2 films with different microstructures[END_REF][START_REF] Rampelberg | In situ X-ray diffraction study of the controlled oxidation and reduction in the V-O system for the synthesis of VO2 and V2O3 thin films[END_REF][START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF].

Oxidation kinetics seems to be the main difference in the annealing process between 525°C and 550°C, according to X-ray results. The phases belonging to the binary V-O system crystallize in structures with low symmetries giving a high amount of diffraction peaks. The application of the sole XRD method to characterize the oxide layer may induce some mistakes in the film structure determination. Thus, the use of a second characterization method is mandatory. Within this objective, we have employed Raman spectrometry. Figure 4.6 presents room-temperature Raman spectra of oxidized VN films between 3 to 5 min at 525°C and 550°C.

The samples annealed at 525°C and the VN550_3 one show similar vibration bands (located at approx. 194, 224, 263, 389 and 624 cm -1 ) that can be assigned to m-VO2 phase, in agreement with the literature [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Schilbe | Raman scattering in VO2[END_REF]. In the case of the oxidized films at 550°C during 4 and 5 min, the shape of the spectra is different from the other samples. Some vibration bands still correspond to VO2, yet new bands are detected at approx. 195, 282, 301, 405 and 532 cm -1 that are readily ascribed to the orthorhombic V2O5 phase [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF]. These findings confirm the simultaneous presence of VO2 and V2O5 in VN550_4 and VN550_5 samples as well as the only existence of preferred orientation evidenced by XRD (as the VN films on Si substrate [START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF]). Along the oxidation process, the morphology of the oxidized samples considerably changes when compared to the VN films. After 15 min of oxidation at 450°C ( The optical modulation properties for samples oxidized at 525°C in a range of 3 to 6 min are studied with an infrared camera (Figure 4.9 a)). Air-annealed samples between 3 and 5 min display hysteresis loops near the MIT region, a characteristic thermochromic signal. The corresponding values for the emissivity switch for these samples slightly increase with oxidation time. This is a consequence of the continuous growth of VO2 from the oxidation of nm thick VO2 film deposited on Al using an infrared camera in the 8-14 µm range [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF].

It is well reported in the literature the close correlation between electrical properties with the surface morphology of VO2 [START_REF] Yu | A metal-insulator transition study of VO2 thin films grown on sapphire substrates[END_REF][START_REF] Yoon | Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method[END_REF][START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF]. However, the morphology of the samples with the highest emissivity switch (VN450_30, VN525_5 and VN550_3) in the different studied systems is remarkably different (Fig. 4.8 c), e) and f) respectively). These results suggest that the optical properties are independent of the VO2 surface morphology, at least in the 7.5-13 µm IR range.

One more interesting result is related to the best-oxidation time for each studied temperature. Xu et al. [START_REF] Xu | The study of optimal oxidation time and different temperatures for high quality VO2 thin film based on the sputtering oxidation coupling method[END_REF] reported an optimal oxidation time at different temperatures to obtain high-quality VO2 films by oxidation of 100 nm thick vanadium films (using the Sputtering Oxidation Coupling method [START_REF] Xu | A novel sputtering oxidation coupling (SOC) method to fabricate VO2 thin film[END_REF]).

Introduction

In the previous chapters, we have shown that VN can be considered as a new precursor to produce thermochromic VO2 thin films. The VN films were oxidized under certain specific conditions to achieve VO2 as the predominant compound on the sample. We have illustrated that the choice of proper oxidation parameters (temperature and oxidation duration for a given thickness) is crucial to reach the desired final phase [START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF][START_REF] García-Wong | Surface morphology-optical properties relationship in thermochromic VO2 thin films obtained by air oxidation of vanadium nitride[END_REF].

Doping can be considered as an alternative strategy to improve the properties of VO2 resulting from the oxidation of vanadium nitride. It has been reported a successful addition of Cr and Al into the VN structure. In both cases, the sputtered-deposited doped films form a solid solution and retain the rocksalt-like structure of VN [START_REF] Durai | Influence of chromium content on microstructural and electrochemical supercapacitive properties of vanadium nitride thin films developed by reactive magnetron co-sputtering process[END_REF][START_REF] Ju | The effects of adding aluminum on crystal structure, mechanical, oxidation resistance, friction and wear properties of nanocomposite vanadium nitride hard films by reactive magnetron sputtering[END_REF]. Based on the above results, we decided to enhance the quality of the VN precursor. The idea is to obtain epitaxial VN, then oxidize these high-quality films to synthesize improved thermochromic VO2 (first and second sections of the chapter). Several substrates are used to deposit epitaxial VN, such as SrTiO3 [START_REF] Luo | A Review of Epitaxial Metal-Nitride Films by Polymer-Assisted Deposition[END_REF][START_REF] Liu | Epitaxial growth of vanadium nitride thin films by laser molecule beam epitaxy[END_REF], Pt [START_REF] Glaser | The growth of epitaxial VN(111) nanolayer surfaces[END_REF] and MgO [START_REF] Mei | Elastic constants, Poisson ratios, and the elastic anisotropy of VN(001), (011), and (111) epitaxial layers grown by reactive magnetron sputter deposition[END_REF][START_REF] Zheng | Phonon and electron contributions to the thermal conductivity of VNx epitaxial layers[END_REF][START_REF] Zhang | Unveiling the atomic and electronic structure of the VN/MgO interface[END_REF][START_REF] Lazar | Interface structure of epitaxial (111) VN films on (111) MgO substrates[END_REF]. MgO has a small lattice mismatch of less than 2% to that of VN. Furthermore, the thermal expansion coefficient of MgO and VN are similar (i.e., αMgO ≈13 × 10 -6 K -1 [START_REF] Holleck | Material selection for hard coatings[END_REF], αVN ≈ 9.7 × 10 -6 K -1 [START_REF] Lengauer | Lattice parameters and thermal expansion of δ-VN1-x from 298-1000 K[END_REF]). Hence MgO is a suitable substrate for the epitaxial growth of VN. In our case, we operated with MgO (100), the parameters of the deposition are listed in section 2.3.2.1 (Chapter

2).

All the results depicted in chapters 3 and 4 were from the oxidation of polycrystalline VN deposited on the semi-industrial machine. In this chapter, we will use a lab-scale sputtering machine with 2" target diameter. The change of reactor and the VN deposition conditions should lead to a variation of the film microstructure and then to the oxidation parameters to get high-performance thermochromic VO2.

Furthermore, the deposition of similar films using different sputtering machines must be an even more difficult task. In the final section of the chapter, we will discuss about a simple criterion to obtain similar VN films between the two sputtering reactors employed throughout the thesis. Moreover, three series of samples with different working pressures were deposited.

We study the oxidation of the three series of VN films. The results display that the only oxidized films that present thermochromic performance are the ones with the parameters obtained from our criterion.

Deposition and characterization of epitaxial growth VN thin films

All the studies performed in chapters 3 and 4 employed the semi-industrial machine to deposit the VN films. As mentioned in chapter 2 (section 2.3.1), this machine is not equipped to heat the substrate holder. The growth of epitaxial VN films has been reported in high-energy deposition conditions, with deposition temperatures higher than 430°C to form stoichiometry epitaxial VN on MgO substrates [START_REF] Mei | Elastic constants, Poisson ratios, and the elastic anisotropy of VN(001), (011), and (111) epitaxial layers grown by reactive magnetron sputter deposition[END_REF][START_REF] Zheng | Phonon and electron contributions to the thermal conductivity of VNx epitaxial layers[END_REF]. Thus, we have to use another sputtering chamber.

The lab-scale machine connected to the TUBE allows the deposition at temperatures as high as 800°C. Using the TUBE it is possible to transfer the samples (raw and VN-coated MgO substrates) to the RHEED setup to check the surface structure. We will exploit this possibility as a benefit to obtain VN films with higher quality. corresponds to [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] and ( 222) planes of the fcc VN structure (PDF no.00-035-0768)).

Nevertheless, once the substrate holder is heated (at 450°C), the (111) peak intensity decreases and the preferred orientation changes; the [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] peak becomes more intense than the (111) one.

When heating the substrate holder, the adatom's energy increases, thus the adatom's mobility enhances stimulating the formation of the (100) orientation [START_REF] Huang | Texture evolution of vanadium nitride thin films[END_REF]. This change in the texture is employed as an advantage for the growth of high-quality epitaxial VN thin films. The increase of the deposition temperature also comes with an enlargement of the diffraction peak, suggesting a grain size decrease. This phenomenon could be due to an increase in the nucleation site density with increasing deposition temperature. Huang et al. have also reported a grain size decrease when changing the texture from ( 111) to ( 200) [START_REF] Huang | Texture evolution of vanadium nitride thin films[END_REF].

The X-ray diffractograms of the VN films deposited on MgO substrates are shown in Figure 5.2 (details about the preparation of the MgO substrates are given in section 2.3.2.1). The films were grown with the same parameters as samples in Figure 5.1. There are four diffraction peaks, two corresponds to the [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] and the (400) planes of the MgO substrates located at 42.9 and 94.0°, respectively (PDF no.00-004-0829). The other two peaks are assigned to the [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] and the (400) planes of the fcc VN structure (PDF no.00-035-0768). Once the deposition finished, every sample went again to the MBE machine to study the surface by RHEED (Figure 5.3).

The samples characterized in Figure 5.2 differ from a thermal treatment following the PVD deposition: one was submitted to a thermal treatment at 990 °C during 45 minutes (blue curve) and the other one was not submitted to thermal treatment (black curve). The thermal treatment may induce some changes in the film due to the stress relaxation or to a change of the nitrogen content. Since this treatment is performed in the MBE reactor at high temperature under ultrahigh vacuum, no oxidation of the film surface has been noticed. The use of such a high temperature (990 °C) may also induce some diffusional steps generating a flattening of the film surface. The sample that did not follow the thermal treatment present peaks displaced at angles lower than the theoretical VN values. On the contrary, the sample that was thermally treated

shows diffraction peaks at 43.7 and 96.2° in agreement with theoretical values for the [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] and the (400) planes of the fcc VN structure (PDF no.00-035-0768), respectively. From now on, all the epitaxial VN films presented in the thesis underwent a thermal treatment. To corroborate the RHEED results, we performed an electron backscatter diffraction (EBSD) study of an epitaxial growth VN film. Figure 5.4 exhibits two EBSD maps from the same sample collected at two different angles, in a) at the [START_REF] Binions | Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties[END_REF] direction and in b) at the [START_REF] Zhao | Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications[END_REF] direction. A first result to point out is that in the analyzed volume there are no grain boundaries, this states that we are in the presence of a single-crystalline film. Besides, in both cases, the Xaxis indicates the [START_REF] Binions | Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties[END_REF] direction, which is consistent with the texture deduced from the XRD results. On the other hand, the results of the Y and Z axis change with the orientation of the sample in front of the detector. Yet, the results in Figure 5.4 a) and Figure 5.4 b) for the Y and Z axis show the same color in the whole analyzed surface (either all red or all green). Therefore, the RHEED and EBSD results confirm that the epitaxial VN film is a single crystal with [START_REF] Binions | Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties[END_REF] growth direction.

A high-resolution cross-sectional transmission electron image displaying the film-substrate interface is shown in Figure 5.5 a). The higher average atomic number of VN compared to MgO enables to distinguish between the film and the substrate due to the contrast differences in the lattice images. No grain boundary is evidenced in the VN film. This result differs from that obtained on VN grown on Al substrates at room temperature (Figure 4.7 b)).

Oxidation of epitaxially growth VN thin films as a new possible route to achieve thermochromic VO2

Once the deposition of epitaxial VN on MgO is demonstrated, we can focus our work on the controlled oxidation of such highly crystallized nitride. Thus, the second step consists of the air-oxidation of these films to obtain VO2. We have chosen the best oxidation conditions found in chapters 3 and 4 for polycrystalline VN films deposited on Si and Al substrates (25, 5, and 3 minutes at 450, 525, and 550°C, respectively). Compared to the films elaborated at room temperature on Si or Al substrates, the epitaxial VN layer is still evidenced after the oxidation test. Hence, the oxidation of epitaxial VN films exhibits a slower kinetics than that observed for the polycrystalline films (with similar thickness) studied in Chapters 3 and 4.

For the sake of clarity in the notation throughout the thesis, from now on, we will use VO2(M) when we refer to m-VO2 that we have been studying so far. On the other hand, VO2(B) will be adopted to designate the phase detected after oxidation of epitaxial VN films.

One approach to continue studying the oxidation of VN epitaxial films is to decrease the deposition time. In this way, there will be less matter to convert into VO2. Therefore, we will continue the analysis with single-crystalline VN thin films of about 55 nm. Once again, the film is oxidized for 25 min at 450°C. the [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] VN peak is still intense, which indicates that a significant residual VN layer is still present. This result confirms that the single-crystalline VN oxidizes in a completely different route than the polycrystalline VN films. A characteristic feature in a single crystal is the absence of grain boundaries, which play a crucial role in the oxidation of the VN samples. We believe that the nonexistence of grain boundaries on these films is the principal reason for the distinctive oxidation pathway. Indeed, the diffusion of oxygen species at the grain boundaries is not possible anymore. orthorhombic V2O5, whereas the bands detected at 133 and 416 cm -1 correspond to V6O13 [START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF][START_REF] Julien | Raman Scattering Studies of Microcrystalline V6O13[END_REF]. Yet, the V6O13 phase was not detected by X-ray diffraction, suggesting that the amount of this phase is too low or that this oxide is amorphous. Moreover, the vibration bands located at 188, 265 and the enlarged one at 666 cm -1 are attributed to VO2(B) [START_REF] Lee | Epitaxial stabilization and phase instability of VO2 polymorphs[END_REF][START_REF] Miao | Phase transition induced Raman enhancement on vanadium dioxide (VO2) nanosheets[END_REF]. Finally, the band at 500 cm -1 corresponds to VO2(M) [START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Shvets | A review of Raman spectroscopy of vanadium oxides[END_REF]. As already pointed out (Figure 3.2), several bands of different compounds may overlap, as could be the case, for instance, in the regions between 220 to 360 cm -1 and 360 to 480 cm -1 . To clarify a possible conflict, Figure 5.8 The morphology of the oxidized epitaxial VN film is different from all that we have studied until now (Figure 5.9 a)). The surface resembles a grid of nanorods that form a right angle between them. In the left zone, the rods are bigger and in certain places form clusters that lose the order of the grid (not shown). However, in general, the sample has a typical surface as shown in Figure 5.9 b) (zoom in the center of Figure 5.9 a)). On the other hand, scattered throughout the sample appear structures without a well-defined shape that can be distinguished in the SEM image by its light color. Figure 5.9 c) depicts an AFM image on the same sample with a roughness of (2.60 ± 1.00) nm. The "light" colored structures from the SEM images also appear in the AFM, this is highlighted in Figure 5.9 d), where a height profile performed in one of these structures provides a value of 31 nm height with a 145 nm length. The AFM image indicates that the "light" colored structures are nanorods that grow perpendicular to the surface.

In summary, in the vast majority of the sample, the surface is formed by a grid of nanorods where two distinct layers are well defined. The first one corresponds to residual VN (as detected by XRD) with a thickness of around 35 nm. The second layer (approx. 55 nm thick) seems to be epitaxially grown on VN and presents the same growth direction. Nevertheless, there are defects throughout this layer, which could be dislocations and/or stacking faults, specific analysis (STEM images in a zone axis) is needed to corroborate this hypothesis. Furthermore, enclosed in white-dashed rectangles are found the nanorods perpendicular to the surface. Figure 5.10 b) depicts an image with a higher magnification that enables to distinguished the perpendicular nanorods. Since most of the peaks in XRD and vibration bands in Raman belong to VO2, we believe that this oxide layer may match with VO2(M) or VO2(B). To clarify which VO2 polymorph corresponds to the oxide layer an FFT pattern was performed on a HRTEM image in each of the layers and the interface, as shown in Figure 5.11.

The FFT pattern of the blue square has an excellent match with the [001 ̅ ] zone axis of VN (Figure 5.11 c)). For the FFT pattern in the red square, corresponding to the oxide, the (001),

(2 ̅ 00) and (2 ̅ 01) planes of VO2(B) were identified. The measured interplanar distances are 0.619, 0.580 and 0.494 nm for the (001), (2 ̅ 00) and (2 ̅ 01) planes, in agreement with those reported for VO2(B) (PDF no. 04-007-0514). One interesting result is that both remaining VN layer and the oxide one exhibit a preferred growth orientation. On the other hand, Figure 5 However, there is still the inquiry about the phase that corresponds with the perpendicular nanorods. To better elucidate this, an EELS characterization is performed in two regions of the sample, one without and one with the presence of the perpendicular nanorods, corresponding to Figure 5.12 a) and Figure 5.12 b), respectively. In both images, there are four distinct regions equivalent to MgO, VN, VO2(B) and Pt. EELS spectra recorded in six zones (labeled from 1 to 6) between the two images cover the VN zone (1 and 4), the VO2(B) region (2, 3 and 5) and the Pt (6). In Figure 5.12 b), the white-dashed rectangles represent the zones with perpendicular nanorods, in particular, the EELS spectra no. 6 is conducted in one of these regions. Figure 5.12 c) and Figure 5.12 d) exhibit the results for the EELS characterization in each of the six regions.

There are two differences between the EELS spectra concerning the VO2(B) and VO2(M) phases (see Figure 5.12 c) and Figure 5.12 d) for the VO2(B), and Figure 3.5 b) for the VO2(M) phase). The first is a shift of approximately 5 eV towards higher energies of the first O peak in VO2(B) concerning the equivalent one in VO2(M). The other difference is that the intensity of the second O peak (located approx. at 545 eV) is higher in the VO2(M) phase. Furthermore, it is relevant to mention, that in the EELS spectra, the chemical environment of the atom determines the shape, position, and intensity of the peaks related to each element. For example, VL3 = 513 eV and VL2= 521 eV refer to metallic V, NK = 401 eV and OK= 532 eV indicate nitrogen and molecular oxygen. Yet, these values change when the elements form a compound.

To our knowledge, there is no literature reporting the difference between the EELS spectra of VO2(B) and VO2(M). Therefore, it may be relevant to continue the study on this topic as a possible way to distinguish between these two VO2 polymorphs.

Figure 5.12 c) and d) display that in zone 1 and 4 there are only peaks of V (VL3 and VL2)

and N, as expected for the remaining vanadium nitride layer. In zones 2, 3, 5 and 6, the spectra are equivalent (there are no peaks of N). The only difference between the spectrum of zone 6

with the spectra recorded in zones 2, 3 and 5 is the intensity of V and O peaks. This could be explained due to the presence of a sole nanorod in the Pt analyzed volume. Since the V and O peaks position, as well as the V/O peaks-intensity ratio, are constant, the labeled zones 2, 3, 5 and 6 corresponds to the same phase, VO2(B). This result indicates that the perpendicular nanorods are also VO2(B).

Once that VO2(B) is confirmed as the predominant compound in the VN_450_25 (MgO) sample, it is crucial to check the optical properties. Figure 5.13 displays apparent infrared temperature (TIR) of the VN_450_25(MgO) sample versus the sample temperature (TS) during heating and cooling for the studied films. There is no thermochromic behavior since the slope does not change. The results of the thermal camera are in agreement with all the evidence presents so far, since the VO2(B) is the predominant phase in the sample VN_450_25(MgO).

Although VO2(B) is not thermochromic, it is possible to convert this phase into VO2(M) by thermal treatment in Ar [START_REF] Zhang | VO2(B) conversion to VO2(A) and VO2(M) and their oxidation resistance and optical switching properties[END_REF][START_REF] Valmalette | High efficiency thermochromic VO2(R) resulting from the irreversible transformation of VO2(B)[END_REF] or N2 [START_REF] Liu | Infrared chameleon-like behavior from VO2(M) thin films prepared by transformation of metastable VO2(B) for adaptive camouflage in both thermal atmospheric windows[END_REF] atmospheres. We believe that conducting this process in our VO2(B) films will lead to VO2(M) with bigger grains than the polycrystalline VO2(M) obtained in chapters 3 and 4, hence with better thermochromic performance. In the future, further investigation should be focused on this topic.

All the results concerning the annealing of epitaxial VN thin films display a distinct oxidation route compared with the findings in polycrystalline VN thin films from the semiindustrial machine. As previously mentioned, we consider that the nonexistence of grain boundaries on the epitaxial films is the principal reason for the distinctive oxidation pathway.

However, it is vital to address that the deposition of the epitaxial VN films was conducted on a lab-scale sputtering machine under different conditions than those obtained from the semiindustrial reactor. For these reasons, VN films are deposited on Si substrate with the equal thickness (about 55 nm) on the lab-scale machine. The samples were grown with the same conditions as the epitaxial ones (applied current to the target 0.5 A, working pressure 0.22 Pa and Ar and N2 flow rate of 30 and 5 sccm, respectively) except the deposition temperature (no intentional heating in the case of Si and 450°C for the MgO). Furthermore, it is worthy to note that both VN samples (on Si and MgO substrate) were oxidized simultaneously. always shown near the film-substrate interface a polycrystalline layer of around 20-40 nm thick composed of small VN grains with preferential orientation along the [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] direction. Vanadium crystallizes in the bcc structure, in which ( 110) is one of the densest planes with lower surface energy than other planes. Thus, at the beginning of the deposition, the vanadium adatoms tend to form clusters with (110) plane configuration on the substrate. Then, the nitrogen atoms occupy the four interstitial sites between the V atoms and settle the (200) plane of the fcc VN structure [START_REF] Huang | Texture evolution of vanadium nitride thin films[END_REF]. It has been suggested that low adatom mobility (low substrate bias and deposition temperature) is the key to obtain [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] texture [START_REF] Huang | Texture evolution of vanadium nitride thin films[END_REF]. In our case, the depositions are carried out at room temperature and with no substrate bias. Hence, the first stage [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] texture is followed by a (111) texture with increasing deposition time for VN films under these growing conditions. TEM analysis for films with higher thickness proves that the growth is columnar with large grains exhibiting a [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] preferred orientation (not shown). On the other hand, the oxidized VN film of Figure 5.14 a) displays that all diffraction peaks correspond to orthorhombic V2O5 (PDF no. 00-041-1426) with no sign of VN or VO2, indicating that the sample is completely oxidized. 701 and 995 cm -1 [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF][START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF]. On the other hand, Figure 5.14 c) shows the IR thermal camera measurement for the VN_450_25 (Si) sample in the 30-120°C range. There is no change in the slope of the curve, therefore, not thermochromic behavior. The results from XRD, Raman and IR camera are in agreement and illustrate that V2O5 is the only phase present in the VN_450_25 (Si) sample.

The findings on the oxidized sample on Si substrate are dissimilar that the ones obtained in chapter 3 for samples oxidized under the same conditions. One possible reason is the distinct precursor initial thickness, around 175 nm for the films from the semi-industrial machine and 55 nm for those deposited on the lab-scale chamber. Nevertheless, we believe that the main difference comes from the deposition in two distinct sputtering chambers under dissimilar conditions. Hence, the next section will be dedicated to the possible relation between the deposition in different sputtering machines.

Impact of VN deposition conditions on its oxidation to obtain thermochromic VO2: theoretical approach and experimental verification

In the previous section, we have exposed that even by applying the "best oxidation conditions" (found in chapters 3 and 4) to polycrystalline VN films (grown in the lab-scale sputtering machine) we are not able to reach the formation of VO2(M). In this section, we will employ the criterion developed in section 2.4 to transpose the results found from one sputtering machine to another. The purpose is to determine the deposition conditions used in the lab-scale sputtering chamber to synthesize similar polycrystalline VN films to those deposited in the semi-industrial chamber. Within this objective, we should be able to oxidize these films with the already well-known oxidation conditions to form thermochromic VO2.

It is worth to mention that the criterion established in section 2.4 to grow analogous films from different sputtering chambers is that the sputtered atoms coming from the target arrive with the same energy on the substrate surface. In our case, the deposition conditions of the semiindustrial machine are fixed. From equations (2.2)-(2.5), it is possible to calculate the values of working pressure (P) and target-substrate distance (𝑑) to obtain the conditions to deposit similar VN in the standard sputtering chamber. In the lab-scale machine, 𝑑 was fixed to 9 cm, which represents the closest possible distance. In the semi-industrial chamber, the P.d value corresponds to a pressure of 0.56 Pa in the lab-scale chamber (d being 9 cm). Thus, we chose to conduct three sets of depositions in the lab-scale machine with different working pressure.

Series I, with 0.22 Pa, the same value as the applied to obtain the epitaxial VN. The series II, with the corresponding working pressure (0.56 Pa). Finally, 0.83 Pa (series III), to also have a higher value than the expected (less dense films). Thereby the criterion will be evaluated. Moreover, under these conditions, the VN was completely oxidized. There are several differences in the oxidation of the samples coming from the three series. For the annealed films of series I (Figure 5.15 a)) and II (Figure 5.15 b)), the VN peaks intensity decreases but it is still evidenced even after 3.5 minutes of oxidation. This suggest that there is still a residual layer of VN nitride in all of these samples. On the contrary, none of the films belonging to the c), the Si band is more intense than all the other vibration bands, which is associated with the complete oxidation of the VN layer.

All the spectra in Figure 5.16 a) are similar, with most of the vibration band located at 222, 260, 308, 337, 388, 500 and 613 cm -1 assigned to VO2(M) [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Shvets | A review of Raman spectroscopy of vanadium oxides[END_REF]. Besides, there is a band in 133 cm -1 that corresponds to V6O13 [START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF][START_REF] Julien | Raman Scattering Studies of Microcrystalline V6O13[END_REF]. This vibration band slightly change for the samples oxidized 3 and 3.5 minutes, attributed to a new band at 143 cm -1 ascribed to VO2(M)

or orthorhombic V2O5 [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF][START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF]. On the three samples, there is also a band at 195 cm - 1 that matches either with VO2(M) or orthorhombic V2O5 [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF][START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF]. The spectra of (possibly VO2(M)). Figure 5.17 b) displays enclosed in a dashed-white rectangle a discontinuous VN layer. It is important to mention that the presence of a VN layer is irregular.

On top of the VN layer, there is a film with a distinct morphology that should correspond to VO2(M)). On the contrary, Figure 5.17 c) depicts that there is no VN, the films characterized by big grains with different shapes correspond to orthorhombic V2O5. Figure 5.17 To verify the thermochromic performance of the films, the optical modulation properties in the infrared domain are studied using an infrared camera. These results evidence the influence of the VN deposition pressure on its oxidation behavior to form a thermochromic VO2(M) phase. As predicted, the only samples that display a thermochromic behavior are the ones deposited at 0.56 Pa, even with the presence of a residual VN layer. Samples deposited with lower pressure (0.22 Pa) are denser and more difficult to oxidize. On the contrary, films deposited with higher pressure (0.83 Pa) are less dense and easy to oxidize. These findings reveal the relevance of our criterion to produce analogous films through different sputtering machines. The conditions obtained could be employed as initialization parameters to transpose deposition from one sputtering chamber to another (even to industrial scale). Moreover, it could be used to reproduce results from other articles.

Although the encouraging results, the final films (series II) were not completely oxidized.

Therefore, it is necessary to improve the criterion. 𝑗 𝑠𝑒𝑚𝑖-𝑖𝑛𝑑 = 1.8. Therefore, the V flux is higher for the films deposited in the lab-scale machine, while the N2 flow rate is the same for both depositions (see Table 2.1). Hence, the concentration of V is more important in the films deposited in the lab-scale machine than the ones produced in the semi-industrial chamber, as a consequence, the oxidation step is different. In future works, we will include the equality of the current density to validate this assumption. Such a conclusion may also be useful to study the oxidation of epitaxial VN films but keeping in mind that the absence of grain boundaries strongly influences the oxidation kinetics.

Chapter conclusions

In In the last section of the chapter, we apply a criterion to compare the deposition conditions between the two sputtering machines used in this thesis. Three sets of VN films were deposited, series II with the equivalent conditions of the semi-industrial chamber, series I with lower working pressure and series III with higher working pressure. All films were oxidized at 550°C

for up to 3.5 minutes. The XRD, Raman and SEM results display that:

-The oxidized VN samples of series I (0.22 Pa) present a significant VN residual layer, with VO2(M) phase as the predominant oxide.

-The air-annealed VN films of series III (0.83 Pa) are completely oxidized, with V2O5 as the only phase present.

-The oxidized VN samples of series II are not completely oxidized. However, there is less VN than in the air-annealed films of series I. The principal phase of these films is the VO2(M) phase.

None of the VN oxidized samples deposited at 0.22 and 0.83 Pa evidence thermochromic properties. For the series I films, it is a consequence of the residual VN layer found in the samples. For series III, it is due to the complete conversion of the VN into V2O5. Only the oxidized films deposited with the analogous conditions (series II) show thermochromic performance, even with a residual VN layer. Our criterion evinces that it is possible to transpose deposition from one sputtering chamber to another (even to an industrial scale).

Introduction

The attractive physical properties of transition metal nitrides (TMN), such as high hardness and high melting points, are of great interest in numerous industrial applications. When used as protective coatings on cutting tools and other mechanical components, they provide resistance to wear and extended life. The crystallographic phase, morphology, and chemical composition of the TMN determine their mechanical and electronic properties.

It is reported that most of the equiatomic TMN crystallize in the well-known NaCl-type structure. However, there is also a stable hexagonal phase for some nitrides, such as Cr2N, Ta2N, V2N, δ-MoN [START_REF] Sanjinés | Hexagonal nitride coatings: electronic and mechanical properties of V2N, Cr2N and δ-MoN[END_REF][START_REF] Lavigne | The effect of nitrogen on the passivation mechanisms and electronic properties of chromium oxide layers[END_REF][START_REF] Hones | Characterization of sputter-deposited chromium nitride thin films for hard coatings[END_REF]. In the particular case of V2N, there are not as numerous studies on its properties and possible applications as for VN. One of the first studies carried out by Christensen and Lebech in 1979 described the formation of V2N by melting single crystals of the cubic phase VN0.74 in nitrogen at 1 MPa [START_REF] Christensen | The structure of β-vanadium nitride[END_REF]. X-ray diffractograms indicate that the center of the sample was almost pure V2N and that the surface contained a minor fraction of VN [START_REF] Christensen | The structure of β-vanadium nitride[END_REF].

In 1996 Chu et al. deposited polycrystalline VN films onto M2 steel substrates by sputtering a vanadium target in an Ar + N2 atmosphere. The dominant parameter for the formation of different compounds (crystalline V metal, hexagonal V2Nx, cubic VNx, amorphous V-N solid solution, and their mixtures) was the nitrogen partial pressure. Among these phases, V2N exhibits the highest hardness. Thus, these authors studied the effect of substrate bias by fixing the nitrogen partial pressure. The XRD results depicted that the crystallinity was modified with higher substrate bias, although the crystal structure remained the same. They also detected that increasing the target power (keeping the nitrogen flow rate and the substrate bias constant) changed the phases from VN to V2N, maybe due to the decreasing nitrogen partial pressure in front of the target with increasing target power [START_REF] Chu | Reactive magnetron sputter deposition of polycrystalline vanadium nitride films[END_REF].

Recently, Wu et al. performed an optimization of deposition conditions to obtain highquality VN thin films [START_REF] Wu | Optimization of deposition processing of VN thin films using design of experiment and single-variable (nitrogen flow rate) methods[END_REF]. The samples were deposited on Si (100) substrates by magnetron sputtering. The selected processing parameters were substrate bias, nitrogen flow rate, substrate temperature and substrate rotational speed. They found V and V2N as the main phases when the N2 flow rate was at the minimum value (1 sccm), substrate bias was -65 or -90 V, and the deposition temperature was either 300 or 450°C. Once again, V2N exhibited the highest values of hardness than VN films.

Based on our success to obtain VO2 films from the oxidation of VN and since another nitride is known in the binary V-N system, the objective of this chapter is first to deposit pure V2N phase on Si (100) substrates (see chapter 2 for more details in the deposition conditions).

Second, to study the ability of V2N to become a precursor for VO2 synthesis. We also compare the structure, microstructure, optical and electrical properties of VN and V2N films. Finally, the last section of this chapter is devoted to the study of the effect of the initial V2N thickness in the optical modulation properties of the VO2.

The films are labeled "V2N" for as-deposited films and "V2N_temperature_time" for the films oxidized at various temperatures and annealing durations.

Comparison of structure, microstructure, optical and electrical properties of VN and V2N thin films

The X-ray diffractograms of VN and V2N thin films prepared by reactive sputtering in the lab-scale chamber are depicted in Figure 6.1. The deposition parameters of this series of three samples are shown in Table 6.1. The total amount of flow rate gases (Ar + N2) remains unchanged for each of the three depositions. Sample S1 presents peaks located at approx. 37.6, 43.5 and 80.2° corresponding to [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF], [START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] and ( 222) planes of the face-centered cubic VN structure (PDF no.00-035-0768), respectively. Note that S1 was deposited with the same parameters as non-epitaxial VN films (see first section of chapter 5). The first step towards the deposition of V2N was to increase the V target current and decrease the amount of nitrogen flow rate (see sample S2 in Table 6.1). However, the diffractogram of S2 (Figure 6.1) exhibits the VN peaks shifted to higher angles indicating that changing these parameters is not enough to form V2N. The peak position shift may come from the variation of the film stoichiometry or to the stress level. The only relevant parameter that can be modified is the deposition temperature (compared to the work of Wu et al. [START_REF] Wu | Optimization of deposition processing of VN thin films using design of experiment and single-variable (nitrogen flow rate) methods[END_REF]). The sample S3 deposited at 450 °C displays new peaks detected at approx. 41.7 and 54.9° ascribed to the (21 ̅ 1) and ( 112) planes of the hexagonal V2N phase (PDF no.00-033-1439), respectively.

These peaks cannot be assigned to VN diffraction peaks. No other peak is noticed except those of silicon substrate, which indicates that the sample S3 only contains the crystalline V2N phase. position of the absorption band of V2N cannot be estimated. The S3 (V2N) specular reflectance is higher than the S1 (VN) in the 250-815 nm range. Above 815 nm, the VN sample becomes more reflective than the V2N one. The specular reflectance for the VN (V2N) is in the scope of 13% at 250 nm to 34% at 2500 nm (17% at 250 nm to 32% at 2500 nm). The diffuse reflectance exhibits the same behavior for both films, suggesting that the samples' roughness is quite similar. This indicates that the difference in the total reflectance for both compounds is given entirely by the behavior of the specular reflectance. is dissimilar for both films 𝑇𝐶𝑅 𝑉𝑁 = 3.7 10 -4 K -1 and 𝑇𝐶𝑅 𝑉 2 𝑁 = 2.0 10 -4 K -1 . Note that the sample with higher resistivity (S3) shows lower total reflectance in the near infrared region, in agreement with theory and literature [START_REF] Hu | Correlation between reflectivity and resistivity in multi-component metallic systems[END_REF][START_REF] Canulescu | Optical and electrical properties of polycrystalline and amorphous Al-Ti thin films[END_REF].

In this section, we have illustrated the required changes in the films deposition conditions to obtain V2N. It is mandatory to decrease the amount of nitrogen, increase the V target current and the deposition temperature. However, the application of a negative bias to the substrate is not necessary. Besides, the principal differences between VN and V2N are shown. The next section is dedicated to the study of the V2N oxidation.

Oxidation of V2N thin films as an original precursor to form thermochromic VO2

At the end of the previous chapter, we discussed the effect of the VN deposition conditions on the film oxidation behavior. The V2N phase has a higher amount of vanadium than the VN films, which makes its oxidation different. Thus, to form thermochromic VO2 from the V2N oxidation with the same conditions as before, it should be necessary to decrease the film thickness.

The V2N samples (approx. 100 nm) were oxidized at 450 and 550°C during different durations. For the V2N_450_30 sample, the XRD results display a peak that could correspond either to the V2O5 or the V6O13 compound. The Raman spectrum depicts one band ascribed to the V6O13 phase. To further investigate this topic, a 2D Raman mapping was carried out in a nonhomogeneous zone of the film (Figure 6.9 a)). The studied area has dimensions of approx. 42 µm x 62 µm. The inhomogeneities present different shapes with average diameters from 3.5 to 16 µm. The applied laser has a diameter of about 1.5 µm. Each spectrum is registered from 95 to 1100 cm -1 , with an X-Y scanning step motion of 0.75 µm between them. To discern between each phase a characteristic band was chosen and associated with a color. The band at 845 cm -1 of the V6O13 phase (correlated with the green color) appears only in the prominent inhomogeneous regions (Figure 6.9 b)). The 132 cm -1 band of the V6O13 phase was not selected for potential interference from the band at 138 cm -1 of the VO2(M) phase. On the contrary, the band at 283 cm -1 of the V2O5 (associated with the red color) is detected only in the small inhomogeneities (Figure 6.9 c)). Finally, the vibration band located at 222 cm -1 of the VO2(M) was associated with the blue color, and it is present in the whole studied zone (Figure 6.9 d)).

The merge of the three 2D maps (Figure 6.9 b), c) and d)) in Figure 6.9 e) reveals the growth of the V6O13 and V2O5 phases in a VO2(M) matrix. These results are in agreement with the XRD findings and give extra information to the Raman spectrum shown in Figure 6.8 for the V2N_450_30 sample.

The Raman mapping could be used as a powerful tool to distinguish which phases are present in each film and to study with more details the oxidation of VO2. However, a careful analysis must be conducted before reaching important conclusions. For example, depending on the homogeneity of the sample, several Raman mappings in different zones may be necessary.

In some cases, it might be essential to complement this study with other characterization techniques, such as XRD, TEM, or even optical and electrical measurements. Another attractive application of 2D Raman mapping is the study of the MIT of VO2(M) [START_REF] Zhang | Structural mapping of single-crystal VO2 microrods through metal-to-insulator phase transition[END_REF][START_REF] Zhang | Synthesis, structure and optical properties of high-quality VO2 thin films grown on silicon, quartz and sapphire substrates by high temperature magnetron sputtering: Properties through the transition temperature[END_REF]. To evaluate the thermochromic performance of the oxidized films, optical measurements were performed in the IR domain with an infrared camera. These four-point probe results confirm the XRD, Raman, and IR thermometry finding for the studied samples. However, the ratio between the SR values at low and high temperatures is small compared to the results obtained in Chapter 3 for the VN oxidation and the literature data,

On the precursor thickness impact in the optical modulation properties of the formed thermochromic VO2

In the present manuscript, it has been demonstrated that thermochromic VO2 can be formed after air oxidation of new precursors: VN and V2N films. Whatever the oxidation temperature, the performance of the VO2 layer shows an optimum as a function of the annealing duration.

When the duration is lower than the optimized value a remaining VN or V2N layer has been evidence due to an incomplete oxidation of the precursor. All these results have been obtained using a quite low thickness of the nitride layer (in the 100 -175 nm range), but we do not study the properties of a VO2 thin film formed by air oxidation of a thick nitride film. In other words, does the initial nitride thickness layer play a role on the performance of the resulting VO2 film?

The idea is to oxidize a thick film of as-deposited VN or V2N film on Si substrate. The oxidation should be incomplete and a certain thickness of the precursor should be remaining while the VO2 phase should be detected on the sample surface. We expect that the combination of the thick non-oxidized film with Si substrate should behave like a highly IR-reflective substrate such as Al. Hence, the emissivity switch should change to a positive value, rather than a negative one, characteristic of a VO2 layer deposited on an IR-transparent substrate such as Si or quartz [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF][START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF]. The VN thickness value chosen (350 nm) is twice than that used in chapters 3 and 4. The VN films are grown with two different working pressures: 0.22 and 0.56 Pa (the other parameters remains the same as in chapter 5). For the V2N, the selected thickness is higher, about 445 nm. To test our hypothesis, both nitride films deposited on Si substrate are oxidized at 550°C for 2.5, 3, and 3.5 minutes. Figure 6.13 depicts the X-ray diffractograms for both precursors (VN and V2N) and the airannealed films at 550°C for 2.5, 3, and 3.5 min. As expected, the VN films present the usual [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] texture. On the other hand, the V2N sample exhibits a [21 ̅ 1] texture. The oxidation of both precursors is incomplete. There is a VN intense peak in Figure 6.13 a) and Figure 6.13 b), and a V2N strong peak in Figure 6.13 c). Besides, these peaks are shifted to higher angles due to stress relaxation during the oxidation step. Although the VO2(M) peaks are not intense, this is the only detected phase of the V-O system in all the oxidized films. These results differ from the one related to the oxidation of thinner films (Figure 5.15). A possible explanation is that in the thinner films the oxygen diffusion is limited, 3.5 min at 550°C are sufficient to entirely oxidize the precursor forming VO2 and continue the overoxidation reaching V2O5. On the other hand, for thicker films, 3.5 minutes at 550°C is not enough to transform all the VN precursor into VO2. In this case, oxygen continues to diffuse inside the precursor before begins to overoxidize the already formed VO2.

To check the VO2(M) formation, Raman scattering was carried out for all the oxidized films (Figure 6.14). As expected, none of the samples in Figure 6.14 exhibits the Si band. Hence, a significant layer of VN or V2N must be still present in the films. All the spectra in Figure 6.14 a) are similar, with most of the vibration bands located at 138, 223, 262, 308, 337, 388, 441, 500 and 615 cm -1 assigned to VO2(M) [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Schilbe | Raman scattering in VO2[END_REF]. On all the samples, there is also a band around 195 cm -1 that matches either with VO2(M) or orthorhombic V2O5 [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF][START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF]. The spectrum of the VN_550_2.5 film, corresponding to oxidation treatments made for a thick VN synthesized at a higher working pressure of 0.56 Pa, are shown in Figure 6.14 b) and, resembles the ones of Figure 6.14 a), with VO2(M) as the predominant phase. There are some variations in the spectra of VN_550_3 and VN_550_3.5 samples compared to the one oxidized during 2.5 min (Figure 6.14 b)). First of all, the vibration band at 138 cm -1 disappears and a new band rises at 131 cm -1 ascribed to V6O13 [START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF][START_REF] Julien | Raman Scattering Studies of Microcrystalline V6O13[END_REF]. The bands characteristic of VO2(M) at 222 and 613 cm -1 decrease in intensity with the oxidation time increase. The intensity ratio between the bands at 262 at 308 cm -1 also changes. The band at 388 cm -1 also decreases in intensity and becomes broader with a contribution of a new band located at 416 cm -1 , which corresponds to V6O13 [START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF]. On the contrary, the band at 500 cm -1 grows in intensity and emerges a shoulder attributed to V2O5. Finally, two feeble bands arise, one at 700 cm -1 and the other one around hysteresis and an almost irrelevant slope change (with the highest value  of -0.06 for the VN_550_3 sample). The main drawback is not the small value of Δε, but its negative sign. The results confirm that the oxidation of 350 nm-thick VN cannot change the sign of Δε. Figure 6.15 c) related to oxidized V2N films evidences a thermochromic behavior for all samples.

Besides, an appealing result is obtained on the sign of Δε that is positive (for 100 nm thick V2N films oxidized at 550 °C, Δε is negative (Figure 6. are comparable, 550°C for 2.5 min for the V2N films and 3 min for the VN one. Gathering all this information, one can "presume" a conclusion: the temperature is the driven parameter for the VO2(M) surface morphology of the oxidized films. These results also confirm that the surface morphology does not play any role in the emissivity switch as previously discussed in chapter 4.

First attempt to describe the oxidation mechanism of V2N films

At this stage, all the results reported indicate that the oxidation of V2N occurs with the same steps as reported for VN. The first oxide to appear is VO2(M), which can coexist with V2N. As the oxidation process continues, VO2(M) is oxidized into V6O13 and finally into V2O5. A significant feature of VN oxidation is the absence of an intermediate layer between VN and VO2.

To check if this conclusion can also be drawn for the oxidation of V2N, STEM and EELS analyses have been performed on two oxidized V2N samples, which present good thermochromic behavior. Figure 6.17 depicts a high angular annular dark-field image of films oxidized for 2.5 min at 550°C from a 100 nm V2N precursor in Figure 6.17 a) and from a 445 nm V2N precursor in Figure 6.17 b). There are four distinct regions in Figure 6.17 a) equivalent to Si, a thin layer of less than 5 nm, VO2(M) and Pt. The 5 nm-thick layer is almost undetectable in Figure 6.10, and it is expected to correspond to a remaining V2N layer. On the other hand, Figure 6.17 b) exhibits four regions ascribed to Si, V2N (about 335 nm), VO2(M) (about 160 nm) and Pt. On these two samples, EELS spectra were recorded on nine zones (labeled from 1 to 9). Spectra from 1 to 4 were registered in Figure 6.17 a), while the ones from 5 to 9 correspond to Figure 6.17 b). The spectra from 2 to 4 are similar and correspond to the VO2(M) EELS spectra. This result is in agreement with XRD, Raman, TEM and IR camera that evidence the formation of a vanadium dioxide layer. Interestingly, spectrum 1 conducted in the thin layer of Figure 6.17 a) matches the characteristic VN-EELS spectrum. Indeed, it presents two clear NK peaks and the VL3 intensity peak higher than the VL2 one.

correspond to VO2(M) phase. The four spectra validate the XRD, Raman and IR camera results previously shown. However, there is a region of about 25 nm thick, highlighted with black dashed lines in Figure 6.17 b), where the EELS spectrum is different from the features of V2N and VO2(M) phases. Indeed, spectrum 7 (registered in this transition zone) matches with the VN-EELS spectrum. These results clearly evidence that during the oxidation of V2N a VN layer is formed at the interface between the remaining V2N layer and the growing VO2 one. During the oxidation of V2N, oxygen diffuses into the growing VO2 layer and reacts with V2N at the V2N-VO2 interface. This oxidation reaction leads to the release of nitrogen. Up to now, we were considering that nitrogen atoms diffuse outwards of the sample into the VO2 film and form molecular nitrogen on its surface, contributing to the denitriding of the sample. However, the occurrence of a VN layer at the V2N-VO2 interface suggests that a certain fraction of nitrogen diffuses towards the substrate and nitride the V2N layer to form a thin VN intermediate layer.

Thus, from a microscopic point of view, the oxidation of a V2N layer comes with the formation of an intermediate VN phase, and the growth of the VO2 layer occurs on this intermediate layer.

Similar behavior has been reported for the air oxidation of Cr2N films in a temperature range between 500 and 1000°C. In the interface between the non-oxidized Cr2N and Cr2O3 oxide, an intermediate CrN layer is detected [START_REF] Kacsich | Oxidation of thin CrN and Cr2N films analyzed via nuclear reaction analysis and Rutherford backscattering spectrometry[END_REF][START_REF] Qi | A comparative study of the oxidation behavior of Cr2N and CrN coatings[END_REF].

Chapter conclusions

This chapter offers an overview of the air-controlled oxidation of V2N films deposited on Si substrates. In the first section, we compare the structure, mechanical, electrical and optical properties of VN and V2N compounds. The results depict that the EELS spectra could be used as a fingerprint to distinguish between both phases. The hardness value of V2N is more than twice that of VN. Both phases have metallic properties, the resistivity being increased with temperature. Nevertheless, VN (𝑇𝐶𝑅 𝑉𝑁 = 3.7 10 -4 K -1 ) has a higher temperature coefficient of resistance than V2N (𝑇𝐶𝑅 𝑉 2 𝑁 = 2.0 10 -4 K -1 ).

We study the oxidation of the V2N films at two temperatures (450 and 550°C). For the first time, our work shows that V2N can also be considered as a precursor for the synthesis of Finally, we provide a first insight into the influence of the V2N thickness on the optical modulation properties of the formed VO2(M). We managed to change the sign of the Δε for the oxidized films by changing the initial V2N thickness. For the VO2(M) samples obtained from the oxidation of a 100 nm V2N film, the Δε is negative, with the direct effect of the Si substrate.

However, for the VO2(M) films achieved after the air-annealing of a 445 nm V2N layer, the Δε is positive. This means that a combination of a thick V2N layer (at least 335 nm) with the Si substrate behaves as a highly IR-reflective substrate such as Al. We believed that these results may pave the way for future investigations on the oxidation of V2N as a new precursor to form VO2(M).

General conclusions

The development of thin film processes for the synthesis of thermochromic vanadium dioxide is crucial for the applications of such material at the industrial scale. The main issue comes from the complex phase diagram of the V-O binary system as presented in chapter 1. In this thesis, we obtained for the first time thermochromic VO2 from the controlled oxidation of sputter-deposited VN and V2N polycrystalline films. It was discussed the importance of optimizing the annealing duration in each of the studied oxidation temperatures to achieve highquality thermochromic VO2.

During the oxidation process at different temperatures (450, 525 and 550°C) of the polycrystalline VN and V2N films, we found, in general, only two phases belonging to the V-O system: VO2 and V2O5. As a function of the oxidation duration, the first oxide formed was VO2. This oxide can coexist with the remaining precursor for a long period at 450°C or suddenly disappeared at 525 and 550°C. The best thermochromic performance was achieved after the almost complete or complete oxidation of the precursor. It was show that a remaining precursor layer affects the thermochromic performance of the films. Above the optimal annealing duration, V2O5 appears due to the continued oxidation process, affecting the thermochromic behavior of the films. TEM analyses displayed that the interface between the polycrystalline VN films and the formed VO2 is abrupt. On the contrary, EELS analyses evidenced that V2N oxidation occurs in two stages. First, the V2N turns into VN (approx. 25 nm layer) before its oxidation into VO2(M).

In the matter of the oxidized polycrystalline VN films on Al substrate at different temperatures, the best emissivity switch in each system is quite similar (between 0.20 and 0.22), whatever the oxidation temperature. On the other hand, the film morphology is strongly dependent on the annealing temperature. The results displayed an independence of the optical modulation properties of VO2 in the 7.5-13 µm range with the surface morphology.

The oxidation of VN epitaxial films followed a completely different path than polycrystalline VN thin films, which could be caused by the absence of grain boundaries in the Furthermore, we applied a simple criterion to transpose the deposition conditions between the two sputtering machines employed in the thesis. Three sets of VN films were deposited using the lab-scale machine: series II with the equivalent conditions of the semi-industrial chamber, series I and III with lower and higher working pressure, respectively. All films were oxidized at 550°C up to 3.5 minutes. The results displayed that: none of the VN oxidized samples of series I and III evidenced thermochromic behavior. For the annealed films of series I, this is due to the residual VN layer found in the samples and for the complete conversion of the VN into V2O5 in the annealed samples of series III. Only the oxidized films deposited with the analogous conditions (series II) showed thermochromic performance. Therefore, we assume that it is possible to transpose deposition from one sputtering chamber to another keeping constant the pressure x distance conditions.

Finally, we provided a first insight into the influence of the V2N thickness on the optical modulation properties of the formed VO2(M). For the VO2(M) samples obtained from the oxidation of a 100 nm V2N film, the Δε is negative, caused by the direct effect of the Si substrate. Yet, for the VO2(M) films achieved after the air-annealing of a 445 nm V2N layer, the Δε is positive. These findings indicate that a combination of a thick V2N layer (at least 335 nm) with the Si substrate behaves as a highly IR-reflective substrate such as Al.

This thesis may pave the way for future investigations on the oxidation of VN and V2N as a new precursors to form high-quality thermochromic VO2(M). min in the total) the mass gain for the sample in humid air is higher than for the that in dry air, suggesting an effect of water vapor in the oxidation process. This trend is maintained until the end of the process where both curves merge, which indicates that both samples present the same final compound. In this configuration, the possible influence of water vapor at the beginning of the oxidation is disguised because the annealing atmosphere is composed of a gas mixture with different densities. Hence, we change the procedure: the sample is introduced in the SETARAM Setsys thermobalance chamber with an air atmosphere from the very beginning. Therefore, the heating process is carried out in a dry air atmosphere, avoiding the influence of different gases density during the first minutes of the oxidation process. In the new configuration, VN films (around 1 cm 2 of surface and 176 nm thick) deposited on Si substrate at 0.56 Pa are oxidized at 450°C for 90 min in dry air (AlphaGaz 1) or humid air (AlphaGaz 1 + 10% absolute humidity) which means that we must use thicker or larger samples. Also, it will be interesting to perform studies with various amounts of absolute humidity.

Figure 1 . 1

 11 Figure 1.1 Phase diagram of the V-N system. Modified from [12].

Figure 1 . 2

 12 Figure 1.2 Phase diagram of the V-O system [19].

Figure 1 . 3

 13 Figure 1.3 The VO2 rutile structure. Large red and blue small spheres denote V and O atoms, respectively. The VO6 octahedron is also shown. Modified figure from [41].

Figure 1 . 4

 14 Figure 1.4 The VO2 monoclinic structure. Modified figure from [41].

Figure 1 .

 1 Figure 1.5 a) Crystal-field splitting of the V 4+ (3d 1 ) energy levels. b) Angular part of the d orbitals

Figure 1 . 6

 16 Figure 1.6 Schematic energy diagram of the 3d bands around the Fermi level for VO2. The hybridization of the V 3d and O 2p levels reflects the symmetries of the atomic arrangement in the crystal lattice [62].

(a function of

  temperature), create a gap, and VO2 is in an insulator state. The temperature increase gives the required energy to overcome the strong Coulomb interaction, becoming a free conducting electron. Continue increasing the temperature leads to a critical charge-density value where the energy gap reduces to zero, and therefore an insulator-to-metal transition occurs. Several theoretical and experimental investigations indicate the Mott-Hubbard transition as the mechanism for the VO2 MIT. For instance, Zayed et al. performed first-principle DFT calculations to investigate the electronic structure and optical properties of the VO2 (M) phase. The need to include the Hubbard potential to reproduce the VO2 properties indicates that the transition is of the Mott-Hubbard type [64]. On the other hand, Morrison et al. reported on combined ultrafast electron diffraction and infrared transmissivity experiments in PLD-grown VO2 samples. The results demonstrate a photoinduced transformation to a long-lived state with

Figure 1 . 7

 17 Figure 1.7 Resistivity as a function of temperature for films oxidized at a) 425 °C with different oxygen flow rate and b) at different oxidation temperatures from 390 to 475 °C at fixed oxygen 2 sccm flow rate. Modified figure from [81].

Figure 1 .

 1 Figure 1.8 a) Transmittance temperature dependence (from -30 to 90°C) in the 300-2500 nm wavelength range of sputtered VO2 film on ZnO-buffered glass (modified figure from [44]). b) Reflectivity spectrum at temperatures from 24 to 86°C in the 2-15 µm wavelength range of sputtered VO2 film on c-plane sapphire substrate . At 70°C, the reflectivity drops to ~0.0025 at λ =11.6 µm (modified figure from [82]).

Figure 1 . 9

 19 Figure 1.9 Temperature determined from IR camera (TIR), as a function of actual temperature (Tact) of VO2 films deposited on Si, quartz, and Al substrates [84].

Figure 1 . 10

 110 Figure 1.10 Scheme for the VO2 based devices, its respective input stimuli, and output response. The color represents the category of input stimuli and output response of the device [85].

  MBE is a well-known technique used for the growth of high-quality homogeneous epitaxial crystal layers. Fan et al. prepared VO2 epitaxial film by MBE on two-inches Al2O3 (0001) substrate. The stoichiometry film displays a change of the resistance of more than four orders of magnitude. They tuned the MIT temperature along with the resistance change by the modification of oxygen vacancies content during the deposition [101]. Moreover, Paik et al.

Figure 1 . 11

 111 Figure 1.11 Raman spectra of thin films deposited at different O2/Ar flow rate ratios. Note that sample A is thicker than the rest of the samples [108].

Figure 1 . 12

 112 Figure 1.12 Thickness dependence of resistivity change across the MIT for sample A (310 nm) and C (125 nm) [108].

  et al. have deposited high-quality VO2 films on m-, a-, and r-plane sapphire substrates under identical conditions by DC magnetron sputter deposition. The deposition was performed in an Ar-O2 mixture containing 2 % of O2 at a total deposition pressure of 1 Pa and 550°C of substrate

Figure 1 . 13

 113 Figure 1.13 Temperature dependence of resistivity for synthesized VO2 films before (dashed lines) and after annealing. Modified figure from [116].

Figure 1 . 14

 114 Figure 1.14 Temperature dependence of the resistivity of the VO2 films oxidized at (a) various oxidation annealing temperatures ranging from 510 to 600°C at fixed oxygen pressure (Poxidation) of 333 Pa and b) various oxidation annealing pressures ranging from 200 to 600 Pa at a fixed oxidation annealing temperature of 580°C. Modified figure from [107].

Figure 2 .

 2 Figure 2.1 a) Scheme of the sputtering process. b) Principal mechanisms resulting from the interaction between the Ar + ions and the target atoms.

Figure 2 . 2

 22 Figure 2.2 Scheme of the magnetic configuration and plasma confinement in a magnetron sputtering source.

Figure 2 . 3

 23 Figure 2.3 Typical experimental curve for the reactive sputtering of a highly reactive metal. The partial pressure, P, of the reactive gas as a function of the reactive gas flow. Qtot is expressed in standard cubic centimeters per minute (sccm). 1 mTorr is approx 0.133 Pa. Modified figure from[START_REF] Berg | Fundamental understanding and modeling of reactive sputtering processes[END_REF].

Figure 2 .

 2 Figure 2.3 shows a typical experimental processing curve for the partial reactive pressure

Figure 2 .

 2 Figure 2.4 a) Transfer Technology Office (TTO) part of the TUBE. b) Semi-industrial and standard sputtering machine connected to the TUBE.

mm 3 )

 3 are powered with a pulsed-DC Pinnacle+ generator from Advanced Energy (5kW, 5-350 kHz, τON from 60 to 100%) allowing process regulation in current, power or voltage. The two cathodes are water-cooled. The volume of the deposition chamber is around 350 L. The substrate holder offers the possibility to coat samples with a 29 x 19 cm² surface.

Figure 2 .

 2 Figure 2.5 a) Front view and b) rear view of the semi-industrial sputtering reactor connected to the TUBE.

Figure 2 . 6

 26 Figure 2.6 Standard sputtering chamber connected to the TUBE.

Figure 2 .

 2 7 a)). It has a maximum operating temperature of 750°C and a volume of 45 liters. Figure 2.7 b) schematically represents a top-view scheme of the furnace. It is equipped with a motor/fan unit mounted at the rear of the chamber (1) and heating elements located on the side of the furnace (2). The fan mixes the heated air, equalizing its temperature, and providing a high uniform volume within the chamber.

2. 5 . 2

 52 Thermogravimetry analysis (TGA) Thermogravimetric analysis (TGA) is a technique which measures the mass change of a sample as a function of temperature or time, under a programmed temperature regime, in a controlled atmosphere. Besides, it can be coupled with other methods of analysis to perform simultaneous measurements. The most frequently used complementary techniques are -differential scanning calorimetry, -differential thermal analysis, -separation techniques: mass spectrometry, chromatography.

Figure 2 .

 2 Figure 2.7 a) Front-view picture of the HRF 7/45 furnace. b) Top-view scheme of the HRF 7/45 furnace.

Figure 2 .

 2 Figure 2.8 a) presents a schema of this configuration. Under this configuration, only the crystallographic planes parallel to the sample surface are in constructive interference condition. Hence, these are the only peaks observed in the diffractogram.

Figure 2 .

 2 Figure 2.8 b) displays X-ray diffractograms for VN and V2N deposited by magnetron sputtering on Si substrate. Structural and microstructural information is extracted from the diffractograms. The peak position enables us to calculate the interplanar distance 𝑑 ℎ𝑘𝑙 using Bragg's law (equation 2.6). These results, coupled with the Powder Diffraction File (PDF) database are used to identify the crystalline structure of the samples. The peak intensity helps to determine if there is a preferred crystallographic orientation. In the case of single crystalline or textured films, the only peaks detected corresponds to the same family of planes. In opposition, in non-textured-polycrystalline films, several peaks are identified since the grains are oriented in different directions in the sample. The diffractograms displayed in Figure 2.8 b) are polycrystalline showing (111) and (200) Bragg reflection for the VN sample, and (21 ̅ 1) and (112) for the V2N one. Yet, the (111) intense peak in the VN sample and the (21 ̅ 1) in the V2N one indicate a preferred crystallographic orientation. Furthermore, peak shape provides information about the microstructure. It is correlated with defects present in the structure, nonuniform deformations, and crystallite size (coherently scattering domains).

Figure 2 .

 2 Figure 2.8 a) Schematic representation of Bragg's law and the Bragg-Brentano configuration. b) X-ray diffractogram of VN and V2N films deposited on Si substrate.

Figure 2 . 9

 29 Figure 2.9 RHEED patterns recorded on an epitaxially-growth VN on MgO substrate (after a thermally degassed process at 1000°C) from two different azimuths.

Figure 2 . 10

 210 Figure 2.10 Schematic of the major components found in a SIMS instrument along with the common data output noted in Dynamic SIMS. For the sake of simplicity, the additional primary ion beams, lenses, deflectors, apertures, and secondary ion energy filters, which are present in all SIMS instruments, are not shown [162].

Figure 2 .

 2 Figure 2.12 a) presents top-view SEM images of a V2N thin film. The thickness of the films could be determined by the cross-sectional SEM images shown in Figure 2.12 b).Two microscopes were used in this work to investigate the morphology of the samples. A Philips XL-30 S-FEG scanning electron microscope was employed in chapter 4. In chapters 3, 5 and

Figure 2 . 11

 211 Figure 2.11 Schematic presenting components of the signals generated in pear-shaped interaction volume between the electron beam and the sample surface.

Figure 2 .

 2 Figure 2.12 a) Top-view and b) cross-sectional images of a V2N thin film.

Figure 2 . 13

 213 Figure 2.13 Schematic of the two basic operation modes in TEM. In diffraction mode (left) the diffraction pattern (DP) is projected onto the screen and in image mode (right) the image is projected onto the screen. In each case the intermediate lens selects either the back focal plane (left) or the image plane (right) of the objective lens as its object. Image taken from[START_REF] Williams | Transmission Electron Microscopy: A Textbook for Materials Science[END_REF].

Figure 2 . 14

 214 Figure 2.14 Schematic of the STEM configuration detectors, on-axis detector for BF images and offaxis annular detectors for DF images.

Figure 2 .

 2 15 a) displays a high angle annular DF image of a VN sample deposited on Si substrate. Figure 2.15 b) depicts the EELS spectra recorded in four zones (labeled from 1 to 4) of Figure 2.15 a). In the four zones there are only peaks corresponding to V (VL3 =513 eV and VL2= 521 eV) and N (NK =401 eV), suggesting that the sample is homogeneous.In this thesis, TEM studies were carried out with a JEOL ARM 200-Cold FEG (point resolution 0.19 nm). The cross-section TEM samples were prepared by a focused ion beam (FIB)-SEM dual beam system (FEI Helios Nanolab 600i). The lift-out technique was used to obtain the samples. An acceleration voltage of 30 kV was applied during the preparation and thinning process. However, during the last step of the thinning, the acceleration voltage was reduced to 5kV, to decrease the possibility of ion implantation and amorphization. EELS experiments were performed in scanning electron transmission microscopy mode (STEM). The spectrometer was set to an energy dispersion of 0.1 eV/channel. The camera length, the pixel

Figure 2 .

 2 Figure 2.15 a) High angle annular dark field image of a VN sample deposited on Si substrate, EELS characterization was performed in the four zones (labeled from 1 to 4). b) EELS spectra of the four different zones of Figure 2.15 a). The color of each curve corresponds to the area of the same color where the spectrum was conducted.

Figure 2 .

 2 Figure 2.16 displays TIR versus TS during heating and cooling for a VN film on Si substrate oxidized 25 minutes at 450 °C. The emissivity at low temperature (𝜀 𝐿𝑇 ) and high temperature (𝜀 𝐻𝑇 ) was calculated from the slope of the temperature measured using the IR camera as a function of the real temperature of the sample (equation 2.8). These values serve to calculate the emissivity change (∆𝜀) and thus to evaluate the thermochromic performance. The highest the ∆𝜀 the better the thermochromic properties.

Figure 2 . 16 9 where

 2169 Figure 2.16 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for a VN film on Si substrate oxidized 25 minutes at 450°C. Up and down triangles represent the profiles for temperature ramping up and down, respectively. The red lines obtained by a linear fitting in the low temperature (LT) and high temperature region (HT) enabled the determination of emissivity values before and after MIT.

Figure 2 . 17

 217 Figure 2.17 Schematic of the four-point probe method.

Figure 2 . 18

 218 Figure 2.18 Sheet resistance (SR) as a function of the temperature for an oxidized VN films deposited on silicon substrate at 450 °C during 25 minutes.

  in-line semi-industrial machine. A second step consists of air-oxidation of the VN films to form vanadium oxide films. The as-deposited VN films are annealed under atmospheric pressure at 450°C during different durations 7, 10, 15, 20, 25, 30, 35, 40 and 50 minutes. The films are labeled "VN" for as-deposited vanadium nitride films and "VN_time" for the oxidized films for various annealing durations. The air-oxidation conditions of these films have been optimized. The results will indicate that VN is an interesting new precursor to synthesize highquality thermochromic VO2 films.

Figure 3 . 1 X

 31 Figure 3.1 X-ray diffractograms of as-deposited VN film on silicon substrate and the resulting oxide films after air-oxidation at 450°C for various durations.

Figure 3 .

 3 Figure 3.3 shows the top view SEM images of six relevant samples: the as-deposited VN films and samples oxidized during 10, 20, 25, 30 and 50 min, respectively. The VN film exhibits a surface morphology with small pyramidal-shaped grains in agreement with the [111] preferred orientation observed by XRD. Throughout the oxidation process, a drastic change in the morphology of the samples is noticed when compared to as-deposited VN films. After 10 minutes of oxidation (Figure 3.3 b)), the surface changes and the pyramidal-shaped grains of

Figure 3 . 2

 32 Figure 3.2 Raman spectra of air-oxidized VN films at 450°C for various durations.

Figure 3 .

 3 Figure 3.4 a) shows transmission electron microscopy micrograph for the VN_25 sample.The figure displayed that there is no well-defined shape for the "supposed" VO2 grains (two

Figure 3 . 3

 33 Figure 3.3 Top-view SEM images corresponding to a) the as-deposited VN films and sample oxidized during 10, 20, 25, 30 and 50 min for b), c), d), e) and f) respectively.

Figure 3 .

 3 Figure 3.4 a) TEM cross-sectional image of sample VN_25. b) HRTEM image of the highlighted red area in Figure 3.4 a). c) FFT from the region of Figure 3.4 b)

Figure 3 .

 3 6 represents apparent infrared temperature (TIR) of samples monitored versus the sample temperature (TS) during both, heating and cooling for the studied films. All the films in Figure3.6 exhibit hysteresis loops near the MIT region. Samples oxidized for 10 minutes or less show no representative thermochromic behavior (∆𝜀 𝑉𝑁 10 = -0.03). This is due to the presence of a considerable residual layer of VN. Figure3.7 presents a cross-section SEM image of the VN_10 sample. Two different layers are evidenced. On the silicon surface substrate, a thick residual layer of VN is covered by the VO2 one. This image

Figure 3 .

 3 Figure 3.5 a) High angle annular dark field image of the sample VN_25, EELS characterization was performed in the four zones (labeled from 1 to 4). b) EELS spectra of the four different zones of Figure.

3. 5

 5 a). The color of each curve corresponds to the area of the same color where the spectrum was conducted.

Figure 3 .

 3 Figure 3.8 a) (Figure 3.8 b)) displays the optical modulation properties for identical raw VN films oxidized at 550°C between 2.5 and 4.5 minutes (at 525°C between 4 and 7 minutes). The experiment was performed under the same conditions that Figure 3.6 and the films were labeled "VNtemperature_time" for the oxidized films for various temperature and annealing durations.

Figure 3 . 6

 36 Figure 3.6 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for VN films oxidized between 15 and 35 minutes. Measurements are carried out by considering ε = 1 irrespective to the sample. Up and down triangles represent the profiles for temperature ramping up and down, respectively. The red lines obtained by a linear fitting in the low temperature (LT) and high temperature region (HT) enabled the determination of emissivity values before and after MIT.

Figure 3 .

 3 Figure3.9 shows the evolution of the sheet resistance (SR) as a function of the temperature for oxidized samples at 450°C during both heating and cooling. In this study, sheet resistance is calculated rather than the resistivity, due to the presence of the remaining metallic VN layer for films oxidized for less than 25 minutes. Up and down triangles in Figure3.9 represent the profiles for temperature ramping up and down, respectively. The samples oxidized during 7 and 50 min do not evidence any hysteresis. For the low time oxidation duration (7 minutes) there is no remarkable change between low and high temperatures sheet resistance values, this is due to the presence of a remaining layer of VN. On the other hand, the sample oxidized for a long duration (50 minutes) displays no sign of VO2, only V2O5.

Figure 3 . 7

 37 Figure 3.7 SEM cross-section image for the sample VN_10. Two different layers are evidenced. On the silicon surface substrate, the remaining VN layer is covered by the VO2 one. This image shows an thick residual layer of VN, proving that at 10 min the oxidation is not complete.

Figure 3 . 10

 310 Figure 3.10 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for VN film oxidized during 25 minutes at 450°C. The experiment was conducted on the same sample 10 months later.

Figure 3 . 11

 311 Figure 3.11 Logarithm of the ratio of sheet resistance at low and high temperature (left axis, blue squares), emissivity switch (right axis, red circles) as a function of oxidation time. We used a color gradient to illustrate the different five regions for the structure of oxidized films at 450 °C.

Figure 4 .

 4 Figure 4.2 shows the SIMS depth profiles of samples VN450_15 and VN450_30. Starting from the surface the signal intensities for V, N, O and Al elements are displayed as a function of etching time in each of the two samples. The twenty first seconds of the analyses (until V and O lines cross each other) can be omitted, it corresponds to the depth needed to reach an equilibrium state. In Figure 4.2 a), the intensity of V and O is quite constant during the first 400 s, suggesting the presence of vanadium oxide. On the contrary, in the same range, the amount of N is negligible due to the oxidation process. After 400 s intensity of these three elements changes, the amount of O decreases while an increase is noticed for V and N. This indicates the presence of a residual layer of VN on the VN450_15 sample. In the case of sample VN450_30 (Figure 4.2 b)), V and O signals are almost constant throughout all the depth profile. Besides,

Figure 4 . 1 X

 41 Figure 4.1 X-ray diffractograms of as-deposited VN film on aluminum substrate and the resulting oxidized films after air annealing at 450°C for various durations.

Figure 4 .

 4 3 a) shows the apparent infrared temperature as a function

Figure 4 . 2

 42 Figure 4.2 SIMS depth profile of samples oxidized at 450°C during a) 15 and b) 30 minutes.

Figure 4 . 4

 44 Figure 4.4 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for samples oxidized during 15, 20, 25 and 35 minutes at 450°C. Measurements were carried out by considering an emissivity equal to 1 regardless of the sample. The red lines obtained by a linear fitting in the low temperature (LT) and high temperature region (HT) enabled the determination of emissivity values before and after MIT.

Figure 4 . 5 a

 45 Figure 4.1) are included as a guideline for a better comparison of the oxidation process. Sample VN525_3 displays several peaks corresponding to m-VO2 and the signal of VN does not disappear, indicating that the oxidation is not complete. VO2 is the only oxide presents on the samples oxidized between 3-5 min, besides VN peaks completely vanished at 5 min. The maximum intensity of VO2 peaks is detected after 5 min; following the oxidation process, these peaks rapidly decrease in intensity and almost fade away after 7 min. In contrast, after 5 min of oxidation, intense peaks of V2O5 emerge caused by the over-oxidation of m-VO2. Interestingly, a monoclinic V2O5 phase (PDF no. 00-061-0027) is revealed together with the already detected orthorhombic V2O5 phase (PDF no. 00-041-1426). The peaks of the m-V2O5 are situated around 2θ = 17.8, 26.8 and 61.4° and identified with an asterisk (*) in the figure. The X-ray diffractograms of samples oxidized at 550°C, Figure 4.5 b) are close to those reported at 525°C (Figure 4.5 a)). The only phase of the V-O system detected between 2 and 3

Figure 4 . 5 X

 45 Figure 4.5 X-rays diffractograms of as-deposited VN film on aluminum substrate and the resulting oxide films after air oxidation at a) 525°C and b) 550°C for various durations.

Figure 4 .

 4 Figure 4.7 a), c) and d) show the SIMS depth profile of samples oxidized at 525°C for 3, 4 and 5 min respectively. Starting from the sample surface the signal intensities for V, N, O and Al elements are displayed as a function of etching time in each of the three samples. Once again; the twenty first seconds of the analyses (until V and O lines cross each other) can be omitted, it corresponds to the depth needed to reach an equilibrium state. The signal corresponding to V and O in Figure 4.7 a) is relatively constant during the first 400 s, suggesting the presence of vanadium oxide with a constant composition. During this period the amount of N is negligible thanks to VN oxidation. On the contrary, after 400 s the intensity of these three elements changes, the signal of O drops while the opposite occurs for V and N. The latter evince the presence of a remaining VN layer in the VN525_3 sample.

Figure 4 .

 4 Figure 4.7 b) shows a transmission electron microscopy cross-section of the VN525_3 sample. The figure depicts close to the Al substrate an uninterrupted columnar VN layer of approx. 90 nm thick and above a VO2 layer of approx. 140 nm thick (corresponding to the oxidation of a 85 nm thick VN layer). This TEM cross-section image confirms the results obtained by XRD and SIMS. From Figure 4.7 a) and b) it can be concluded that the sample

Figure 4 . 6

 46 Figure 4.6 Raman spectra of air-oxidized VN films during 3 to 5 min at 525°C and 550°C.

Figure 4 .

 4 Figure 4.8 depicts the top view SEM images of six noteworthy samples: the as-depositedVN films and samples oxidized during 15, 30 min at 450°C, 4, 5 min at 525°C and 3 min at 550°C. Note that all the images have not the same magnification, but all of them except the asdeposited VN were acquired with identical acceleration voltage. The as-deposited VN film exhibits a surface morphology with small pyramidal-shaped grains in agreement with the[START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF]

Figure 4 . 7

 47 Figure 4.7 SIMS depth profiles of samples oxidized at 525°C during 3, 4 and 5 min for a), c) and d), respectively. The etching time indicates the depth of the sample from the surface. b) TEM cross-section image of sample VN525_3.

Figure 4 .

 4 8 b)), the oxidized VN grains merge into an extended structure, enlarging grain size. Increasing the annealing time at the same temperature leads to the growth of the grains (Figure 4.8 c)). Samples VN525_4 (Figure 4.8 d)) and VN525_5 (Figure 4.8 e)) present different morphology than samples oxidized at 450°C, the grains are bigger and more rounded. In the case of Figure 4.8 f) (VN550_3) the grains are equiaxed and are the largest among all the images. It is noteworthy that despite drastic different morphology and grain size the images of Figure 4.8 c), e) and f) correspond to samples completely oxidized, in which the only phase detected is m-VO2.

Figure 4 . 8

 48 Figure 4.8 Top-view SEM images corresponding to a) the as-deposited VN films and samples VN450_15, VN450_30, VN525_4, VN525_5 and VN550_3 for b), c), d), e) and f), respectively. Note that the magnification is different, as indicated in each image, besides the image of the as-deposited VN was acquired with a lower acceleration voltage.

Figure 4 . 10

 410 Figure 4.10 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for VN film oxidized during 2.5, 3, 3.5 and 4 minutes at 550°C. Measurements were carried out by considering an emissivity equal to 1 regardless of the sample. The red lines obtained by a linear fitting in the low temperature (LT) and high temperature region (HT) enabled the determination of emissivity values before and after MIT.

Figure 4 .

 4 11 displays the optimal oxidation time (ln t) expressed as a linear function of the reversed temperature (1/T). In red circles the results of our work and in blue square the results of Xu et al. In the latter, the fit serves to obtain the diffusion activation energy and also to predict theoretical optimal oxidation time at other temperatures. A direct evaluation of both works shows that the values of activation energy are similar. Yet, the most important result from the comparison is that the time required to obtain the best VO2 is larger in VN than in V films, indicating that it is easier to control the oxidation of VN.

Figure 4 . 11

 411 Figure 4.11 Relationship between optimal oxidation time and temperature. In red circles the corresponding values for this work and blue squares for the results of Xu et al.[126].

  On the other hand, Cr and Al are well-known doping elements for VO2.Brown et al. and Zou et al. showed that doping with Cr ions gradually increased the metal-insulator transition temperature of the films, meanwhile, the electrical and optical modulation properties decreased with the amount of Cr[START_REF] Brown | Electrical and optical characterization of the metalinsulator transition temperature in Cr-doped VO2 thin films[END_REF][START_REF] Zou | Thermochromic, threshold switching, and optical properties of Cr-doped VO2 thin films[END_REF]. On the contrary, Al doping increases luminous transmittance, improves solar modulation ability, and efficiently reduces the phase transition temperature to 45°C[START_REF] Ji | Al-doped VO2 films as smart window coatings: Reduced phase transition temperature and improved thermochromic performance[END_REF]. Moreover, Mercs et al.reported a new generation of thermochromic thermal solar collectors based on the oxidation of VOx/SiO2 layers on aluminum. The air oxidation of Al-doped film (8 at. %) during 3.5 minutes at 550°C gives a thermochromic selective absorber (VO2/VnO2n±1/Al2O3/SiO2) that guarantees a lifetime of at least 25 years without significant change of its optical properties[START_REF] Mercs | Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors[END_REF]. These results illustrate that doping the VN could be a compelling strategy to enhance the thermochromic performance of the conceived VO2.The change of the VN deposition parameters (deposition temperature, operating pressure, nitrogen partial pressure, substrate bias, among others) represents another attractive approach.For instance, Chu et al. displayed that the change of nitrogen partial pressure on the sputterdeposited VN films controlled the formation of different phases (from V metal to cubic VNx with other compounds in between)[START_REF] Chu | Reactive magnetron sputter deposition of polycrystalline vanadium nitride films[END_REF]. In 2019, Wu et al. optimized the deposition conditions to sputter-deposit VN films. The optimum conditions were: -90 V for the substrate bias, 2.5 sccm nitrogen flow rate, a substrate temperature of 300°C and a substrate rotational speed of 20 rpm. The achieved VN film presented an exceptional combination of properties, including high hardness, low residual stress and low electrical resistivity[START_REF] Wu | Optimization of deposition processing of VN thin films using design of experiment and single-variable (nitrogen flow rate) methods[END_REF].

Figure 5 .

 5 Figure 5.1 displays the diffractograms of VN films deposited on Si substrates with the same Ar and N2 flow rates as in previous chapters (30 and 5 sccm, respectively) during 30 minutes at 0.22 Pa. The films grown at room temperature (RT) present the same preferred orientation that in preceding chapters as illustrated in Figure 5.1 (peaks located at approx. 37.9° and 80.3°

Figure 5 . 1 X

 51 Figure 5.1 X-ray diffractograms of as-deposited VN thin films on Si substrate at room temperature and at 450°C. Both samples were deposited with the same parameters (current applied to the target of 0.5 A, working pressure 0.22 Pa, deposition time 30 min and Ar and N2 flow rate of 30 and 5 sccm, respectively) except the substrate holder temperature.

Figure 5 . 2 X

 52 Figure 5.2 X-ray diffractograms of epitaxial growth VN thin films on MgO substrate at 450°C in the 2θ region from a) 15 to 120 and b) zoom in the region with peaks. The blue and black curves represents the diffractograms for a sample that follow a thermal treatment right after the deposition and for the one who doesn't follow it, respectively.

Figure 5 .

 5 Figure 5.3 illustrates the RHEED patterns for an epitaxial VN growth on MgO substrate along two different azimuths. The straight lines in both patterns indicate that the surface of the epitaxial VN corresponds to a single-crystalline film.

Figure 5 . 3

 53 Figure 5.3 RHEED patterns recorded on an epitaxially-growth VN on MgO substrate after thermal treatment at 990°C under ultra-high vacuum along two different azimuths.

Figure 5 .

 5 5 b) and c) represents the FFT pattern performed in the enclosed areas of Figure 5.5 a). It was possible to identify the (200), (220) and (020) planes of the VN and MgO. The obtained interplanar distances are 0.207, 0.147 and 0.206 nm for (200), (220) and (020) planes, respectively, in agreement with those reported for the fcc VN (PDF no. 00-035-0768). Both substrate and VN film exhibit the same FFT pattern. The combination of XRD, RHEED, EBSD and HRTEM microscopy analyses prove that VN films grow with film/substrate epitaxial relationships: VN (100) || MgO (100) and VN [010] || MgO [010].

Figure 5 . 4

 54 Figure 5.4 Electron backscatter diffraction images of an epitaxially-growth VN on MgO substrate when the sample was placed in front of the detector in a) the [100] direction and b) the [110] direction.

Figure 5 .

 5 6 shows the X-ray diffractograms for as-deposited epitaxial VN (around 150 nm thickness) and air-annealed epitaxial VN films deposited on MgO substrate. The three oxidized films present numerous peaks, two of them correspond to the (110) and (2 ̅ 22) planes of the m-VO2(M) (PDF no. 04-003-2035) located at approx. 26.8° and 55.2°. Besides, one peak at 45.4° corresponds to the (411) plane of orthorhombic V2O5 (PDF no. 00-041-1426). For the first time, the oxidation of VN films reveals several peaks assigned to another oxide phase, namely m-VO2(B) (PDF no. 04-007-0514) located at 2θ =17.7, 28.9, 35.9, 44.1 (shoulder of the (200) peak of VN, see Figure 5.6 b)), 49.5, 50.4 and 59.9°. Figure 5.6 b) displays a zoom in the region around the (200) VN peak, the most striking result is that the intensity of the VN peaks slightly decreases after this oxidation test.

Figure 5 .

 5 Figure 5.5 a) HRTEM micrograph acquired near the VN/MgO(100) film-substrate interface. FFT from the region highlighted in b) red (VN) and c) blue (MgO) of Figure 5.5 a).

Figure 5 .Figure 5 .

 55 Figure 5.6 a) X-ray diffractograms of as-deposited epitaxial VN films (approx. 150 nm) on MgO substrate and the resulting oxide films after air-oxidation at three different temperatures for various durations. b) Zoom in the region where the Mg(200) and VN(200) are located.

Figure 5 .

 5 Figure 5.8 depicts the room temperature Raman spectrum of the 55 nm VN epitaxial oxidized sample recorded to corroborate the XRD results shown in Figure 5.7. The spectrum features a mixture of phases; vibration bands located at 102 and 303 cm -1 are assigned to

Figure 5 . 7 X

 57 Figure 5.7 X-ray diffractograms of as-deposited and oxidized (450°C during 25 minutes) VN films (55 nm thickness) on MgO substrates.

Figure 5 .

 5 Figure 5.8 a) Raman spectra of air-oxidized (during 25 min at 450°C) VN film deposited on MgO; b) and c) depict the deconvolution of the vibration bands located between 220 to 360 cm -1 and 360 to 480 cm -1 , respectively.

Figure 5 .

 5 Figure 5.10 exhibits a TEM cross-sectional micrograph for the VN_450_25 (MgO) film,

Figure 5 .

 5 Figure 5.10 a) TEM cross-sectional image of the epitaxial VN oxidized at 450°C during 25 minutes. b) TEM cross-sectional image of the same film in an area where a nanorod perpendicular to the sample surface is clearly depicted. In both images the white-dashed rectangles highlight the nanorods perpendicular to the surface.

. 11 d

 11 ) represents the FFT pattern performed in the interface between the VN and the oxide (green square). The third point from the center to the growth direction matches the (003) plane for the VO2(B) and the[START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] plane for the VN. The interplanar distance obtained is 0.204 nm, in agreement with the theoretical values of 0.205 for the (003) plane of the VO2(B) and 0.207 nm for the (200) plane of the VN. The results indicate that the predominant oxide layer on the film corresponds to VO2(B) that grows with epitaxial relationships: VO2(B) (001) || VN (200) outof-plane and VO2(B) (200) || VN (020) in-plane orientation.

Figure 5 .

 5 Figure 5.11 a) High-resolution TEM (HRTEM) micrograph for the VN_450_25 (MgO) film. FFT pattern of the region enclosed in the b) red square, c) blue square and d) green square.

Figure 5 . 12

 512 Figure 5.12 High angle annular dark field image of the sample VN_450_25 (MgO) in two different regions: a) without and b) with the presence of the perpendicular nanorods. EELS characterization was performed in three zones in each image labeled from 1 to 3 (Figure 5.12 a)) and from 4 to 6 (Figure 5.12 b)). c) and d) illustrates the EELS spectra of the three different zones of Figure 5.12 a) and b), respectively. The color of each curve corresponds to the area of the same color where the spectrum was conducted.

Figure 5 .

 5 Figure 5.14 a) exhibits the X-ray diffractograms of the as-deposited VN on Si substrate and the oxidized film at 450°C for 25 minutes. The VN film features the (200) peak stronger than the (111) one. All the TEM studies performed in VN samples deposited on Si substrate have

Figure 5 . 13

 513 Figure 5.13 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for the VN_450_25(MgO) films. Measurements are carried out by considering ε = 1 irrespective to the sample. Up and down triangles represent the profiles for temperature ramping up and down, respectively.

Figure 5 .

 5 Figure 5.14 b) depicts the Raman spectra recorded at room temperature on the VN_450_25(Si) sample. The spectrum for the sample deposited on Si shows that the only phase identified is orthorhombic V2O5, with several bands detected at approx.102, 145, 195, 282, 303, 405, 483,

Figure 5 .

 5 Figure 5.14 a) X-ray diffractograms of a VN film deposited on Si substrate at 0.22 Pa and the resulting oxide film after air-oxidation for 25 minutes at 450°C. b) Raman spectra for then oxidized film at 450°C for 25 minutes. c) Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for the VN_450_25 (Si) film. Up and down triangles represent the profiles for temperature ramping up and down, respectively.

Figure 5 .

 5 Figure 5.15 depicts the X-ray diffractograms for as-deposited VN samples and air annealed films at 550°C during 2.5, 3 and 3.5 minutes deposited at three working pressures: 0.22, 0.56 and 0.83 Pa (Figure 5.15 a), b) and c), respectively). The thickness of the VN films are about 140, 150 and 170 nm for the different set of samples I, II and III, respectively. The oxidation conditions (time and duration) were based on those giving VO2 with high thermochromic behavior from the oxidation of VN films (175 nm) obtained in the semi-industrial chamber .

Figure 5 .

 5 Figure 5.15 X-ray diffractograms of as-deposited VN films and the resulting oxide films after airoxidation at 550°C during 2.5, 3 and 3.5 minutes for the three sets deposited with different working pressure a) 0.22 Pa (series I), b) 0.56 Pa (series II) and c) 0.83 Pa (series III).

Figure 5 . 16

 516 Figure 5.16 Raman spectra of air-annealed VN films at 550°C during 2.5, 3 and 3.5 minutes deposited with different working pressure a) 0.22 Pa, b) 0.56 Pa and c) 0.83 Pa.

Figure 5 .

 5 Figure 5.16 b) and Figure 5.16 a) are alike (except for the band at 143 cm -1 ), yet with different intensities. The predominant phase is also VO2(M). On the contrary, the films of Figure 5.16 c) depict equivalent spectra among them, yet completely different from the ones of Figure 5.16 a) and Figure 5.16 b). The only phase identified is orthorhombic V2O5, with several bands detected at approx. 102, 145, 195, 282, 303, 405, 481, 700 and 995 cm -1 [182,183,186,228]. The XRD and Raman results evidence the difference in the VN oxidation for each working pressure. When the films deposited at 0.22 Pa are oxidized, there is a coexistence of phases: an important residual layer of VN and VO2(M). The samples grown at 0.56 Pa and oxidized, must have a thinner layer of VN and VO2(M) appears as the predominant phase. Finally, the samples deposited at 0.83 Pa and then air-annealed, are entirely oxidized, with orthorhombic V2O5 as the principal compound.

Figure 5 .

 5 Figure 5.17 depicts a SEM cross-section image performed on each of the samples oxidized during 3.5 minutes at 550°C for the three working pressure. Figure5.17 a) exhibits on top of the Si substrate a continuous columnar VN layer and above a coating with different morphology

Figure 5 .

 5 Figure 5.17 SEM cross-section images for VN samples deposited at different working pressure a) 0.22, b) 0.56 Pa and c) 0.83 Pa and afterwards oxidized at 550°C during 3.5 minutes.

  validates the results obtained by XRD and Raman spectrometry suggesting that the remaining VN layer is only observed with a deposition pressure of 0.22 and 0.56 Pa.

Figure 5 . 18

 518 Figure 5.18 Thermal camera measurements of apparent infrared temperatures (TIR), as a function of sample temperature (TS) for films of the three different series a) I, b) II and c) III oxidized between 2.5 and 3.5 min at 550°C. Up and down triangles represent the profiles for temperature ramping up and down, respectively. Measurements were carried out by considering an emissivity equal to 1 irrespective to the sample.

Figure 5 .

 5 18 represents the apparent infrared temperature (TIR) of samples monitored versus the sample temperature (TS) during both, heating and cooling for the three series of oxidized films. None of the oxidized samples illustrated in Figure 5.18 a) and Figure 5.18 c) exhibits neither hysteresis loop characteristic of a thermochromic behavior nor change in the slope. In Figure 5.18 a), this is the result of the residual VN layer found in the samples. For the films presented Figure 5.18 c), it is due to the complete conversion of the VN into V2O5. On the other hand, all samples displayed in Figure 5.18 b) show thermochromic performance, even with a residual VN layer.

  this chapter, we deposit epitaxial VN on MgO substrate in the lab-scale sputtering machine. The epitaxial films have been submitted to a thermal treatment after their deposition to obtain stoichiometric VN films. The results from XRD, RHEED, EBSD and HRTEM microscopy analyses prove that VN films grow with film/substrate epitaxial relationships: VN (100) || MgO (100) and VN [010] || MgO [010]. The VN epitaxial films oxidize within the conditions determined in chapters 3 and 4, follow a completely different oxidation pathway. Even a thin VN layer (about 55 nm) is very difficult to oxidize, which could be caused by the absence of grain boundaries in the epitaxial layers. The XRD and Raman results display a mixture of phases, with VO2(B) as the predominant compound. Remarkably, oxidation leads to epitaxial growth between VO2(B) and VN with the following relationships: VO2(B) (001) || VN (200) out-of-plane and VO2(B) (200) || VN (020) in-plane orientation, as shown by the TEM results. Moreover, the SEM images and the AFM results depict a completely different surface morphology for the VO2(B) phase, with nanorods perpendicular to the substrate. As expected, the VO2(B) phase does not exhibit a thermochromic behavior, corroborated by the IR camera findings.

Figure 6 . 1 X

 61 Figure 6.1 X-ray diffractograms of as-deposited VN and V2N films on silicon substrate at room temperature and at 450°C. The deposition parameters of the three samples are shown in Table 6.1.

Figure 6 . 2

 62 Figure 6.2 Top-view SEM images corresponding to the as-deposited a) VN film ( sample S1) and b) V2N film (sample S3).

Figure 6 .

 6 Figure 6.6 depicts the dependence of resistivity with temperature for the S1 and S3 films in the 30-120°C range. The resistivity for both samples increases linearly with temperature as expected for metallic conductors. The values at 30°C are 𝜌 𝑉𝑁 = 178.4 µΩ cm and 𝜌 𝑉 2 𝑁 = 185.7 µΩ cm, indicating that both materials exhibit quite the same electrical properties. The

Figure 6 .

 6 Figure 6.5 a) Evolution of a) total, b) specular and c) diffuse reflectance spectra at normal incidence as a function of the wavelength for samples S1 and S3.

Figure 6 .

 6 7 a) (Figure6.7 b)) depicts the X-ray diffractograms for the air-annealed films at 450°C (550°C) between 20-30 min (2.5-3.5 min). The XRD results for the as-deposited V2N film show a broad peak detected at approx. 42.1°, between the theoretical values of the[START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] and the (21 ̅ 1) planes of the V2N phase (PDF no.00-033-1439). Therefore, it is ascribed to the (21 ̅ 1) plane to be consistent with the diffractogram in Figure6.1. No other peak is noticed except those of silicon substrate. Sample V2N_450_20 displays several peaks corresponding to VO2(M), and the broad peak of V2N disappeared (Figure6.7 a)). The peak at approx. 28.5° attributed to the (101) plane of the V2N phase indicates that the oxidation is not complete. This

Figure 6 . 6

 66 Figure 6.6 Resistivity evolution of samples S1 and S3 with the temperature in the 30-120°C range.

Figure 6 . 7 X

 67 Figure 6.7 X-rays diffractograms of as-deposited V2N film on Si substrate and the resulting oxide films after air oxidation at a) 450°C and b) 550°C for various durations.

Figure 6 .

 6 Figure 6.9 a) Image of sample V2N_450_30 in the area where the 2D Raman mapping was conducted. The image was taken with the CCD camera coupled to the Raman spectrometer. 2D Raman mapping on the b) green, c) red, and d) blue color area indicates the presence of the bands at 845 cm -1 band (V6O13 phase), 283 cm -1 (V2O5 phase) and 222 cm -1 (VO2 phase), respectively. e) Merge of the three 2D Raman mappings in b), c) and d) and superposition on the image of Figure 6.9 a).

Figure 6 .

 6 Figure 6.10 illustrates a TEM cross-sectional micrograph for the V2N_550_2.5 sample. The figure displayed an almost complete oxidized film with a small zone of V2N. The other layer

Figure 6 .

 6 Figure 6.10 a) TEM cross-sectional image of sample V2N_550_2.5. b) HRTEM image of the highlighted red area in Figure 6.10 a). c) FFT from the region of Figure 6.10 b).

Figure 6 .

 6 11 displays the evolution of the apparent infrared temperature (TIR) of samples as a function of the actual sample temperature (TS) during heating and cooling for the oxidized films. The three air-annealed samples at 450°C (Figure 6.11a)) exhibit hysteresis loops and a change in the slope, characteristics of a thermochromic behavior. The values of Δε are ranging from -0.13 to -0.21. The highest value of emissivity switch (∆𝜀 = -0.21) corresponds to the sample oxidized 25 minutes, in which VO2(M) is the predominant phase. The V2N_450_20 sample presents a residual V2N layer, and the V2N_450_30 one shows signal of over-oxidation of the VO2(M), with both phases V6O13 and V2O5 affecting the thermochromic behavior of the films. For the

Figure 6 . 11

 611 Figure 6.11 Thermal camera measurements of apparent infrared temperatures (TIR) as a function of the sample temperature (TS) for films oxidized at a) 450 and b) 550°C. Up and down triangles represent the profiles for temperature ramping up and down, respectively. Measurements were carried out by considering an emissivity equal to 1 irrespective to the sample.

Figure 6 .

 6 Figure 6.12 displays the evolution of the sheet resistance (SR) as a function of the temperature during both heating and cooling for the four films that present a thermochromic effect. For these electrical measurements, we prefer to plot the SR values rather than the resistivity ones. This choice results from the presence of the remaining conductive V2N layer or the V6O13 and V2O5 phases in some samples. For the V2N_450_20 sample, there is almost no change between low and high temperatures SR values, due to the presence of a V2N layer.A similar response is observed for the V2N_450_30 film, caused by the over-oxidation of the VO2(M). The V2N_450_25 and the V2N_550_2.5 films exhibit hysteresis loops in the near MIT region: with a maximum difference in SR values for the V2N_550_2.5 sample.

Figure 6 . 12

 612 Figure 6.12 Sheet resistance (SR) as a function of the temperature for the V2N films oxidized at 450°C and during 2.5 min at 550°C. Up and down triangles represent the profiles for temperature ramping up and down, respectively.

Figure 6 .

 6 Figure 6.13 X-ray diffractograms of as-deposited VN (350 nm) and the resulting oxide films after airoxidation at 550°C during 2.5, 3 and 3.5 minutes deposited with different working pressure a) 0.22 Pa and b) 0.56 Pa. c) X-ray diffractograms of as-deposited V2N (445 nm) and the resulting oxide films after air-oxidation under the same conditions as films in Figure 6.13 a) and b).

Figure 6 . 14

 614 Figure 6.14 Raman spectra of air-annealed VN films at 550°C during 2.5, 3 and 3.5 minutes deposited with different working pressure a) 0.22 Pa and b) 0.56 Pa. c) Raman spectra of air-annealed V2N films under the same conditions as films in Figure 6.14 a) and b).

  11 b)). The positive value of Δε means that the combination of the remaining V2N with the Si substrate behaves as a highly IR-reflective substrate such as Al. The V2N_550_2.5 film displays the maximum emissivity switch (0.15).Surprisingly, the Δε value for the V2N_550_3 sample decrease before increasing again for the V2N_550_3.5 film. Our results clearly evidence that the sign of the emissivity switch can be tuned according to the thickness of the as-deposited V2N film. This stimulating new result opens the possibility for applications on tunable furtivity (Δε < 0) or smart radiator devices (Δε > 0).

Figure 6 .

 6 Figure 6.16 displays the surface morphology of samples oxidized during 2.5 min at 550°C for a 100 nm V2N precursor in Figure 6.16 a) and for a 445 nm V2N precursor in Figure 6.16 b). Both images are very similar, almost identical. In the two images, the grains have analogous shape and size distribution. More fascinating is the resemblance with Figure 4.8 f), in which also VO2(M) is the predominant phase on the surface. There are several differences between the three films: the substrate (Si for the images of Figure 6.16 and aluminum for the Figure 4.8 f)), the precursor (V2N for Figure 6.16 and VN for Figure 4.8 f)). Yet, there are similarities, for instance, all the precursor films present columnar microstructure, and the oxidation conditions

Figure 6 .

 6 Figure 6.16 Top-view SEM images of films oxidized for 2.5 min at 550°C. The thickness of the V2N initial films are a) 100 nm and b) 445 nm.

  thermochromic VO2. During the oxidation step, VO2(M) is the first oxide formed, it coexists with V2N for a long time at 450°C or swiftly vanished at 550°C. Further increase of the oxidation duration leads to the formation of V6O13 or V2O5 phases that do not exhibit thermochromic properties. EELS characterization of thin and thick oxidized V2N films evidences the formation of a very thin VN intermediate layer (about 25 nm) is formed at the V2N-VO2(M) interface. These findings evidence that the oxidation of V2N to VO2(M) occurs in two stages. The oxidation of V2N leads to the release of nitrogen that diffuse in two opposite directions: towards the surface and towards the substrate and contribute to form the intermediate VN layer. For high oxidation duration, the VO2(M) becomes V2O5, which dramatically affects the thermochromic response of the films. The values for the Δε of the oxidized V2N films are similar to those obtained from the VN oxidation. However, the films present a feeble change in the sheet resistance values, which is not well understood.

  epitaxial layers. Remarkably, oxidation led to the epitaxial growth of the VO2(B) phase on the single crystalline VN with the following relationships: VO2(B) (001) || VN[START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] out-of-plane and VO2(B)[START_REF] Kats | Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance[END_REF] || VN (020) in-plane orientation, as shown by the TEM results. Moreover, the SEM images and the AFM results depicted a completely different surface morphology for the VO2(B) phase, with nanorods perpendicular to the substrate.

Figure A. 2

 2 Figure A.2 a) displays the X-ray diffractograms of as-deposited V2N films and the resulting oxides. As expected, after 15 hours of annealing the V2N is transformed into orthorhombic V2O5. The results indicate the formation of a polycrystalline V2O5 phase with no preferred orientation. Raman spectra of the oxidized films (Figure A.2 b)) corroborate the XRD resultswith all the bands ascribed to V2O5. This result indicates that the structure of the oxidized films is not influenced by the composition of the oxidizing atmosphere.Moreover, the surface morphology of both oxidized samples are similar, they may even be considered indistinguishable(Figure A.3). Both films show large grains of approx. 1 µm and a plateau-like growth. This surface morphology is different from that of the films analyzed in the previous chapters.

Figure A. 1

 1 Figure A.1 Mass gain curves as a function of time of the V2N oxidized samples for 15 hours at 450°C in dry (green curve) and humid (blue curve) atmosphere. The figure depicts the first 130 min of the oxidation process, where the beginning (21.5 min) corresponds to the heating procedure. The inset illustrates the whole process.

Figure A. 3

 3 Figure A.3 Top-view SEM images corresponding to the oxidized V2N films for 15 hours at 450°C in a) humid atmosphere and b) in dry atmosphere.

Figure A. 2

 2 Figure A.2 a) X-rays diffractograms of as-deposited V2N film on Si substrate and the resulting oxide films after oxidation in dry (green curve) and humid (blue curve) atmosphere at 450°C for 15 hours. b) Raman spectra of the V2N oxidized films in dry and humid atmosphere at 450°C for 15 hours.

Figure A. 5 X

 5 Figure A.5 X-rays diffractograms of as-deposited VN film on Si substrate and the resulting oxide films after oxidation in dry (green curve) and humid (blue curve) atmosphere at 450°C for 90 min.

Table 1 . 1

 11 Transition temperatures of different oxydes of the V-O system. Griffiths et al. sputter-deposited VO2 films on quartz and polished sapphire. The samples displayed a drop in the transition temperature and the resistivity ratio (between low and high

	Oxyde	V8O15 VO V5O9 V6O13 V2O3 V6O11 V4O7	VO2	V3O5
	TMIT (°C) -203 -147 -138	-123	-108	-96	-23	68	157

  Wentzcovitch et al. carried out a theoretical study about the electronic and structural properties of VO2 across its MIT. They conclude that there is enough energy gain to account for the metal-insulator transition through strengthening the vanadium dd bonds (reorganizing states near the Fermi level) [70]. Furthermore, Okazaki et al. made a detailed temperature-dependent photoemission study of VO2 single crystals in the insulating

phase. The results revealed a shift of the top of the O 2p and the V 3d bands toward the Fermi energy as the temperature increases. They suggest that the metal-insulator transition of VO2 is

  Hendaoui et al. optimized the SiO2 thickness to achieve the best possible value of 𝜀 𝐻𝑇 and ∆𝜀 in the same VO2 multilayer structure studied in the previous paper. For the first sets of samples, the variation of SiO2 layer thickness enables to increase the 𝜀 𝐻𝑇 , however, 𝜀 𝐿𝑇 also increases due to the presence of SiO2 absorption bands. The higher values of 𝜀 𝐻𝑇 and ∆𝜀 were 0.81 and 0.46. These values correspond to the sample with a 1360 SiO2 intermediate layer. For the second sets of samples, the undoped VO2-based SRD displayed a sharp transition at 68°C, 𝜀 𝐻𝑇 = 0.8 and ∆𝜀 = 0.48, almost identical values to the best of series one. For the doped films, the emissivity switching properties were strongly affected. The transition temperature decreased with the W content at a rate of 16.5°C per at.%, achieving a minimum at 19.5°C for the 2.9% W content film[START_REF] Hendaoui | VO2-based smart coatings with improved emittance-switching properties for an energy-efficient near roomtemperature thermal control of spacecrafts[END_REF]. On the other hand, the 𝜀 𝐻𝑇 was not affected by the doping, however, 𝜀 𝐿𝑇 increase with W content, consequently decreasing

	Moreover, they doped the VO2 with W to decrease the TMIT, which is critical since the spacecraft
	generally work around room temperature. Two sets of samples were prepared: the first one
	comprises a top 30 nm of VO2 with numerous intermediate SiO2 layer thicknesses (460, 720,

1030, 1360 and 1570 nm) employing Au as an infrared-reflective layer on top of the quartz substrate. The second one contains a top approx. 30 nm thick undoped and W-doped VO2 layer deposited on a SiO2 1340 nm-thick layer and Al film as infrared reflecting layer growth on quartz substrate.

∆𝜀. Doping with W donors enhanced the carrier concentration causing a rise of the absorption in the semiconductor state and therefore 𝜀 𝐿𝑇

Table 1 .2

 1 Deposition parameters of samples in[START_REF] Yuce | Investigation of electron beam lithography effects on metal-insulator transition behavior of vanadium dioxide[END_REF].

	Sample Oxygen to argon ratio (%)	Deposition time (min)	Thickness (nm)
	A	2.00	45	310
	B	2.00	22.5	120
	C	2.25	22.5	125
	D	2.50	22.5	124
	E	3.00	22.5	127

Table 2 .

 2

		VN	V2N	Epitaxial VN
	Substrate	Si, Al	Si	MgO (100) single crystal
	Pressure (Pa)	0.22, 0.56, 0.83	0.22	0.22
	Deposition temperature (°C)	No intentional	450	450
		heating (T< 55°C)
	V target curent (A)	0.5	0.7	0.5
	Ar flow rate (sccm)	30	33.5	30
	N flow rate (sccm)	5	1.5	5

1 Growth conditions of the VN, V2N and epitaxial VN thin films obtained in the standard sputtering machine.

  peak disappears at 25 min, indicating that the sample is completely oxidized.

	On the other hand, after 10 min of air oxidation, a first peak corresponding to the (2 ̅ 12)
	monoclinic(m)-VO2 is detected close to 42.0°for the first time. As oxidation continues, other
	peaks located at approx. 2θ = 27.8, 33.3, 36.7, 60.3 and 71.4° (PDF no. 04-003-2035) emerge
	belonging to m-VO2. The intensities of these different peaks indicate that the oxide layer does
	not grow with a strong preferred orientation. Since the as-deposited VN films exhibit a texture

  The idea is to include a term without excessively increase the criterion complexity. Under this condition, we should first check the current density (𝑗) applied to each target. The racetrack surface in each target is different, approx. 58.6 cm 2 for the semi-industrial target and 10.7 cm 2 for the 2" target diameter. The calculated values for each target (semi-industrial and lab-scale) are 𝑗 𝑠𝑒𝑚𝑖-𝑖𝑛𝑑 = 25.6 𝑚𝐴 𝑐𝑚2 ⁄

	and 𝑗 𝑠𝑡𝑑 = 46.7 𝑚𝐴 𝑐𝑚 2 ⁄	, with a ratio of	𝑗 𝑠𝑡𝑑

Table 6 . 1

 61 Growth conditions of samples S1, S2 and S3 shown in Figure6.1 obtained in the lab-scale

	sputtering machine.
	Sample Thickness	V target	Pressure	Ar flow rate	N2 flow rate	Deposition
		(nm)	current (A)	(Pa)	(sccm)	(sccm)	temperature (°C)
	S1	520	0.5	0.22	30	5	RT
	S2	430	0.7	0.22	33.5	1.5	RT
	S3	450	0.7	0.22	33.5	1.5	450

An infrared camera is a device that creates an image using infrared radiation, similar to a common camera that forms an image using visible light. However, infrared cameras are sensitive to wavelengths from 1-14 μm. A major difference with optical cameras is that the focusing lenses cannot be made of glass, as glass blocks long-wave infrared light. The main components are a lens that focuses IR onto a detector, plus electronics and software for processing and displaying the signals and images.

Acknowledgements

Chapter conclusions

In this chapter, we have depicted the essential features of the magnetron sputtering technique. We have also displayed the principal characteristics of both sputtering reactors employed and the growth conditions of the deposited films. Furthermore, we have described the characterization techniques utilized throughout the thesis and detailed the parameters used for each procedure. In the coming chapters, we will present the main results of the work, for instance, chapter 3 will be dedicated to the effect of the oxidation duration on the thermochromic performance of VO2 obtained by the oxidation of VN thin films in the semiindustrial machine.

Chapter 3 3 Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO 2 thin films

Abstract

In this chapter, we propose vanadium nitride (VN) as a new precursor to obtain thermochromic VO2. VN films were reactively sputter-deposited on Si substrates by using an in-line semiindustrial machine. The films of 175 nm thickness were submitted to an air-annealing process implemented at different durations at 450°C. X-ray diffraction and Raman spectrometry were performed for structural characterizations of the oxidized films showing the presence of monoclinic VO2 in a wide range of oxidation time. As the annealing time increased, V2O5 appeared to affect the performance of the oxidized films. The TEM analysis carried out on the oxidized sample for 25 minutes showed that there is an abrupt interface between VN and VO2.

The thermal-induced properties of the studied films were analyzed in terms of their electrical resistance employing a four-point probe method and their emissivity modulation properties by infrared camera. The results showed a thermochromic behavior for the samples oxidized in the range of 15-35 min. On the contrary, the presence of residual VN layers in samples oxidized for less than 15 min and V2O5 in the ones annealed for more than [START_REF] Valmalette | High efficiency thermochromic VO2(R) resulting from the irreversible transformation of VO2(B)[END_REF] opposite behavior to VO2 deposited on IR-transparent substrates such as Si and quartz [START_REF] Benkahoul | Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications[END_REF].

Hence, the choice of the substrate is directly related to the desired application (applications in the solar thermal domain required a highly IR-reflectivity substrate). Second, the significant difference in the hysteresis width of both systems is mainly driven by the distinct substrates [START_REF] Gomez-Heredia | Thermal hysteresis measurement of the VO2 emissivity and its application in thermal rectification[END_REF], which could be connected with the VO2 film grow orientation (compare Figure 3.1 and

Chapter conclusions

In this chapter, we study the effect of three oxidation temperatures (450, 525 and 550°C) on the optical properties of the VN oxidized films. The results indicate that for all studied systems the VN oxidation occurs on the same stages. The first oxide formed is VO2, it coexists with VN in a long period at 450°C or suddenly disappeared at 525 and 550°C. Furthermore, V2O5 is detected after the total oxidation of VN. These findings confirm the oxidation from the V 4+ state into the V 5+ state with the annealing time increase and correspond with the results of previous studies.

Concerning the optical properties of the oxidized films, the best emissivity switch in each system is quite similar (between 0.20 and 0.22), whatever the oxidation temperature. On the other hand, the film morphology is strongly dependent on the oxidation temperature.

Nevertheless, the change of the surface morphology does not influence the optical properties of the films. This indicates that the optical properties of VO2 in the 7.5-13 µm range are independent of the surface morphology.

In chapters 3 and 4, we have shown a simple method to deposit VN on large surfaces and different substrates. The VN oxidized films display good optical modulation properties at different temperatures and durations. These reasons make the VN oxidation process encouraging for the production of high-quality thermochromic VO2 films for several applications as furtivity and smart radiator devices.

In the next two chapters, we will use a standard sputtering machine. In chapter 5, we will deposit epitaxial VN films. The oxidation of these films will be performed with the best oxidation conditions found in chapters 3 and 4. Furthermore, we will compare the samples obtained in the two different sputtering reactors. X-ray diffraction and Raman results manifest a mixture of compounds in the oxidized VN epitaxial sample at 450°C for 25 minutes. From one side, the diffractogram displays a higher number of peaks corresponding to the VO2(B) phase. Meanwhile, the Raman spectrum is different from the typical spectrum of the VO2(M) and V2O5 phases shown so far. As previously stated, the morphology of this sample is unusual from that depicted so far. For all these reasons, we decided to perform a TEM analysis in the predominant area of the film (similar to the one shown in Figure 5.9 b)). We will continue the analysis with samples S1 and S3 since they only feature the VN and V2N phases, respectively. Figure 6.2 depicts the surface morphology of sample S1 and S3. As already presented, the VN film (Figure 6.2 a)) shows small pyramidal-shaped grains in agreement with the [START_REF] Brassard | Grain size effect on the semiconductor-metal phase transition characteristics of magnetronsputtered VO2 thin films[END_REF] preferred orientation observed by XRD. On the other hand, V2N displayed in Figure 6.2 b) exhibits an elongated structure, with bigger grains than the one of VN. In both images, the grain boundaries are clear and well defined. only one peak is evidenced for the V2N phase. Besides, the VN displays a less intense and broad peak around 430 eV, which is not present in the V2N spectrum (Figure 6.4 d)). The presence of one or two peaks for the nitrogen element has been already found in CrN and Cr2N [START_REF] Lavigne | The effect of nitrogen on the passivation mechanisms and electronic properties of chromium oxide layers[END_REF][START_REF] Aguas | Chromium nitrides (CrN, Cr2N) from solid state metathesis reactions: effects of dilution and nitriding reagent[END_REF][START_REF] Mitterbauer | Electron energy loss-near edge structure as a fingerprint for identifying chromium nitrides[END_REF], and also in VN and V2N [START_REF] Kothleitner | A combined SNMS and EFTEM/EELS study on focused ion beam prepared vanadium nitride thin films[END_REF][START_REF] Hofer | Electron energy loss near edge structure on the nitrogen K-edge in vanadium nitrides[END_REF]. The differences concerning the V peaks (VL3 and VL2) lie in the position and the ratio between the intensities of both peaks. For the VN, the VL3 peak intensity is always higher than the VL2 one. For the V2N, the intensity ratio between the V peaks is the opposite of the VN results, which is different from the reported results by Hofer et al. [START_REF] Hofer | Electron energy loss near edge structure on the nitrogen K-edge in vanadium nitrides[END_REF]. These features in the VN and V2N spectra could be used as a fingerprint to distinguish both compounds at the TEM scale.

Once we illustrate how the structure and microstructure of VN and V2N differ, we perform complementary measurements to compare the mechanical, optical and electrical properties of both compounds. The hardness and Young's modulus of the studied films were measured by an Anton Paar Ultra Nano-Hardness Tester (UNHT 3 ) equipped with a Berkovich diamond indenter head. To control the penetration depth to less than one-tenth of the film thickness, which avoids the substrate effect, the applied loads were 0.8 mN (VN) and 1.4 mN (V2N). At least 15 indentations were conducted for both samples, and the average values are shown in Table 6.2. The Young's modulus and hardness of the V2N film are higher than the VN, in agreement with the literature [START_REF] Chu | Reactive magnetron sputter deposition of polycrystalline vanadium nitride films[END_REF][START_REF] Wu | Optimization of deposition processing of VN thin films using design of experiment and single-variable (nitrogen flow rate) methods[END_REF][START_REF] Farges | Preparation and characterization of V-N films deposited by reactive triode magnetron sputtering[END_REF]. An attractive topic for future works is to improve the hardness of V2N by applying a substrate bias during deposition.

To compare the optical properties of both compounds, reflectance measurements were carried out in the 250-2500 nm range using a Varian Cary 7000 UV-vis-NIR spectrophotometer with a coupled integrating sphere. Figure 6.5 a) illustrates the total reflectance for the S1 and S3 samples. Moreover, the specular and diffuse reflectance are depicted in Figure 6.5 b) and c), respectively. The sample S1 exhibit a minimum of the total reflectance at approx. 250 nm that is related to the plasmon absorption of VN [START_REF] Patsalas | Conductive nitrides: Growth principles, optical and electronic properties, and their perspectives in photonics and plasmonics[END_REF]. Within the studied wavelength range, the Raman spectrometry (Figure 6.8) was performed on the oxidized samples to compare with the XRD results. The films annealed at 450°C and the V2N_550_2.5 exhibits similar spectra, with vibration bands (located at approx. [START_REF] Safi | Recent aspects concerning DC reactive magnetron sputtering of thin films: a review[END_REF][START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF][START_REF] Zheng | Phonon and electron contributions to the thermal conductivity of VNx epitaxial layers[END_REF]262,308,388, 500 and 613 cm -1 ) assigned to VO2(M), in agreement with the literature [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Parker | Raman scattering from VO2 single crystals: A study of the effects of surface oxidation[END_REF][START_REF] Schilbe | Raman scattering in VO2[END_REF]. The V2N_450_30 film displays a vibration band at 132 cm -1 , which corresponds to the V6O13 phase [START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF][START_REF] Julien | Raman Scattering Studies of Microcrystalline V6O13[END_REF], caused by the ongoing oxidation of VO2(M). The presence and intensity of the Si vibration band (520.7 cm - 1 ) have been used as an indicator of VN oxidation (chapters 3 and 4), and it will be employed likewise for the V2N oxidation. From Figure 6.8, one can notice that the Si band intensity on the V2N_450_20 film is lower than the other samples. Hence, this sample is the only one not completely oxidized, in agreement with the XRD results. On the other hand, the V2N_550_3 and V2N_550_3.5 films display similar spectra but different from the samples oxidized at 450°C and the V2N_550_2.5. As depicted in Figure 6.8, some vibration bands corresponding to VO2(M) are still evidenced, yet new bands are detected at approx. [START_REF] Paik | Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy[END_REF][START_REF] Kadlec | Influence of the pumping speed on the hysteresis effect in the reactive sputtering of thin films[END_REF][START_REF] García-Wong | Oxidation of sputter-deposited vanadium nitride as a new precursor to achieve thermochromic VO2 thin films[END_REF]283,303,404, 529, 701 and 995 cm -1 that are ascribed to the orthorhombic V2O5 phase [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF][START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF]. 845 cm -1 , corresponding to V2O5 [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF][START_REF] Ureña-Begara | Raman and XPS characterization of vanadium oxide thin films with temperature[END_REF][START_REF] Baddour-Hadjean | Raman Microspectrometry Study of Electrochemical Lithium Intercalation into Sputtered Crystalline V2O5 Thin Films[END_REF][START_REF] Wang | XRD and Raman study of vanadium oxide thin films deposited on fused silica substrates by RF magnetron sputtering[END_REF][START_REF] Hardcastle | Determination of vanadium-oxygen bond distances and bond orders by Raman spectroscopy[END_REF] and V6O13 [START_REF] Zhang | Characterization of vanadium oxide thin films with different stoichiometry using Raman spectroscopy[END_REF], respectively. These differences between the samples annealed at 2.5 min and the ones at 3 and 3.5 min (of Figure At this stage, two hypotheses can be considered to explain the occurrence of a thin layer of VN at the film-substrate interface. The first one corresponds to the deposition of a thin VN layer before the growth of the V2N film due for instance to bad control of the deposition process.

However, in TEM analysis we have never found such a VN layer. The second hypothesis suggests that this thin VN layer is formed during the oxidation of the V2N film. To distinguish these two hypotheses, EELS spectra have been also recorded on a thick V2N oxidized at 550°C for 2.5 minutes (Figure 6.17 d). Spectra 5 and 6 in Figure 6.17 b) corroborate that V2N is still present in this region. Spectra 8 and 9 are recorded in the second layer of Figure 6.17 b) and

Perspectives

• Our work was not dedicated to tune the metal insulating transition temperature.

However, such an objective could be reached by doping the VN or V2N films before the oxidation. Indeed, elements such as chromium or aluminum are highly miscible in the vanadium nitrides. The oxidation of such doped nitride films could change the TMIT and improve the optical modulation properties of the obtained VO2 samples.

• In the literature, the effect of the oxygen pressure during the annealing of V films to achieve thermochromic VO2 is well established. The same kind of work could be done for the oxidation of vanadium nitrides. The simplest way to achieve it would be to use the heating substrate holder of our deposition chamber to oxidize the films at various oxygen partial pressures.

• The oxidation of epitaxial VN leads to the formation of a VO2(B) film that is not thermochromic. However the literature reports some works that evidence a possible transformation of VO2(B) into VO2(M). Such kind of processes could be studied to get epitaxial VO2(M) films. An increase of the thermochromic performance is expected.

• Finally, the first experiments of thermogravimetry analyses on the VN and V2N oxidation suggest that the humidity plays a role on the oxidation kinetics. Further works should be devoted to the study of the water partial pressure on the oxidation step and on the thermochromic performance of the obtained VO2 films.

Appendix

Oxidation of VN and V 2 N films by thermogravimetry analysis

During the thesis, all characterizations have focused on as-deposited and oxidized vanadium nitride films; there is no in situ analysis of the oxidation process itself. The annealing of the samples was performed in the air under atmospheric pressure, with no control of the annealing atmosphere. In the literature, several authors have reported the influence of the oxidation atmosphere in the thermochromic performance of the obtained VO2 films [START_REF] Yu | A metal-insulator transition study of VO2 thin films grown on sapphire substrates[END_REF][START_REF] Yoon | Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method[END_REF]. The goal of the appendix is to start a first study on the oxidation of VN and V2N polycrystalline films with two different atmospheres by thermogravimetric (TGA) analysis.

The oxidation procedure in this study is different than the one used throughout the thesis.

First, the sample is introduced in the SETARAM Setsys thermobalance. Afterward, a flow rate of Ar cleans the chamber during six hours before the oxidation. The heating process up to the desired temperature (450°C) is carried out in an Ar atmosphere (AlphaGaz 1) to avoid possible oxidation. This procedure takes approx. 21.5 minutes with a heating rate of 20°C/min. As soon as 450°C is reached, the Ar valve is closed, simultaneously the dry air (or humid air) valve is opened. At the end of the annealing process, the temperature drops to room temperature at the same rate as the heating.