Delphine Marris-Morini

Carlos Alonso Ramos

Eric Cassan

Dorian Oser

Skylar Deckoff-Jones

Thi Thuy Duong

Em Yêu

Miguel Montesinos Ballesteros

Lucas Deniel

Vincent Pelgrin

Jonathan Peltier

Misha Dyatlov

Zhengrui Tu

Natnicha Koompai

Qiankun Liu

Daniele Melati

Jerôme Michon

Sylvain Guerber

Elena Durán-Valdeiglesias

Samuel Serna

Jianhao Zhang

Daniel Benedikovic

Joan Manel

Ramírez Ramírez

Thomas Maroutian

Philippe Lecoeur

Stéphane Gable

Komalika Rani

Abdelnour Benamar

Loïc Guillemot

Etienne Thiebault

Ngoc Linh

Camille Barbier

David Bouville

Nathalie Isac

Stéphane Collin

Ludovic Largeau

Marta De Frutos

Laura Bocher

Michael Walls

Nathalie Brun

Mathieu Kociak

Odile Stéphan

Je

Paqui Peiró

Gracies Pels Anims I L'empenta !)

Sonia Estradé

Josep Rebled

Sònia Chambrier

Lluís Conesa-Boj

Lluís López

Alberto Yedra

Pau Eljarrad

Gemma Martin Torruella

Teresa Castan

Julià López-Vidrier

Laura Rodríguez

Marc Torres

Aswathi K To

Alejandro Sivan

Galán

Adrien Teurtrie

Geoffrey Monet

Eloi Haltz

Ewen Belloq

Anaïs Molineri

Antoine Laudrain

Cécile Carcy

Irene Medina

Matthieu Sun

Irina Piazza

Vanessa Sánchez García De La Torre

Marta De

Frutos Mis

Itziar Navarro

Kike Lope

Gonzalo Cascante

Núria Espada

Elena Aragüés

Albito Cristina Alonso

Eli Dicente

Sheila Lope

Ana Fernández

v ACKNOWLEDGMENTS I would like to thank my two advisors Laurent Vivien and Sylvia Matzen for the opportunity they gave me to do this PhD in their groups, as well for all the advices, humanity, kindness, trust and support to develop my thesis work (courses, conferences, facilities, discussions, PhD funding and extensions, corrections,…). Because of all the help, patience, comprehension and care they showed during this time I feel very grateful and I could not think of better advisors.

I am very thankful to whole group of MINAPhot and OXIDE.

També voldria agraïr a Joan Manel Ramírez Ramírez per posar sempre el seu coneixement a la meva disposició, per ajudar-me tant en aquest doctorat i a trovar solucions en la vida, i que amb 'pit i collons' s'arriba arreu.

Quería agradecer a mis padres, Conchita y Esteban sobretodo su amor incondicional, el dejarme equivocarme, el eterno apoyo y la infinita paciencia y comprensión. Os quiero muchísimo. A mi familia, a mis abuelos, a quienes echo de menos y en quienes pienso muy a menudo. I a la Sakura, l'Ume i l'Akira, per compartir amb mí, les seves vides i fer la meua mes feliç.

Palaiseau, 14 th February 2021

Introduction

The presence of a large silicon industry since the early 1980s has allowed the fast development of silicon photonics in expenses of other photonic materials [ 1 ]. In this context, it was possible to develop photonic devices with a high level of functionalities at low-cost, low-power consumption and high-scalability.

Silicon is one of the most abundant elements on earth but is a poor photonic material due to its indirect bandgap leading to weak light emission efficiency. In this regard, during the last decade efforts to overcome these limitations have been done in the fields of materials engineering, device design and fabrication techniques. One of the promising approaches is related to implementation of new optical materials and structures for hybrid integration on silicon-based platforms. Highly-compact efficient photonic components including III-V laser on Si, optical amplifiers and fast electrooptic (EO) modulators have been demonstrated [ 2 , 3 , 4 ]. In optical amplification and lasing, lot of efforts have been done in hybridization of rare-earth (RE)-doped oxides such as Al2O3 [ 5 ], Gd2O3 [ 6 ], Y3Al5O12 [ 7 ] or Zr2O3 [ 8 ] on silicon. RE elements are photo luminescent materials emitting from the visible to the far-IR wavelength range. Hence, RE ions are used for optical amplification and lasing applications. Besides, oxides are good host candidates for RE-doped materials due to the multiple deposition techniques which renders easy the implementation of these materials in silicon. The wide range of oxides and their properties make oxide materials very interesting for photonic applications. Crystalline YSZ is an understudied photonic material, wellknown as a silicon buffer for other functional oxides. YSZ is transparent from UV to IR wavelength range and has a lattice parameter compatible with that of silicon.

The aim of this thesis is to grow crystalline RE-doped YSZ thin films by PLD on both crystalline and amorphous substrates. Thus, the thin film characterization includes a structural analysis of the samples after growth and the doping of RE ions on the oxide host matrix (YSZ) and an optical characterization for emissions in the visible and near-IR for telecom and photonic applications. After such characterization, we studied implementation of the active thin film on an amorphous silicon-based device and determined optical gain.

In chapter 1, I grew and studied the epitaxial growth by PLD of Er 3+ -and Pr 3+ -doped YSZ thin films on a crystalline (c-cut sapphire) and on an amorphous (SiNx waveguide) substrate. Regarding materials deposition, I explored two different structures: monolayers of RE-doped YSZ and multilayers at different RE ion concentration.

In chapter 2, I structurally characterized and analyzed the thin films by X-Ray diffraction, AFM. STEM characterization was performed in collaboration of Ludovic Largeau at C2N and I analyzed the obtained data.

In chapter 3, I characterized optically the thin films by means of PL.CL-SEM in collaboration of Stéphane Collin at C2N and CL-TEM in collaboration of Laura Bocher at LPS were used to determine the best approach (monolayer vs multilayer) to implement RE-doped YSZ on a silicon-based photonics platform Besides, I treat data from all optical characterization techniques.

In chapter 4, we focus on multistacks of Er:YSZ grown on a planarized SiNx waveguide buried on SiO2. After optical characterization by PL, I simulated and measured optical amplification in waveguides.

This thesis ends with a conclusion in which we sum up the most relevant results obtained during the last 3 years and we conclude by perspectives of future work. This work has received the funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Program (ERC POPSTAR -grant agreement No 647342) and ANR FOIST project.

On chip light emission on silicon

Nowadays leadership of silicon photonics as the platform for telecommunications is based on the fast developments in the 1990s. Efficient modulation [ 9 ] and photodetection [ 10 ] have been already demonstrated in silicon photonics as well as compact optical passive devices of multiple channels with low-loss characteristics [ 11 ] leading to high bandwidth chips [ 12 ].

Complex photonics circuits have been developed with many devices. However, all of the devices induce some losses which reduces the power budget of the link. Thus lasing and optical amplification in silicon remain one of the last missing elementary component to develop in silicon technology. Hybrid integration permits to overcome some limitations on silicon-photonics platforms by conceiving new architectures implementing silicon-based materials such as SiOx or SiNx providing not only the implementation of on-chip lasing structures [ 13 ] but as well other optical properties as higher electro-optic effects, strong nonlinearities, among others [ 14 , 15 ]. In this regard, most efforts to implement hybrid integration in silicon platforms lies in materials science and new fabrication techniques of photonic materials of the III-V groups, implementation of rare-earth doping on Si-based substrates, Si quantum dots structures or by tuning Ge bandgap [ 16 , 17 , 18 , 19 ]. Alternatively, dielectric materials such as rare-earth-doped functional oxides have also been explored due to their optical gain characteristics in intra-chip optical communications [ 20 ].

Laser in silicon photonics

An integrated on-chip light source would allow higher integration density of devices on silicon platforms. Despite silicon low emission efficiency hinders progress of perfect monolithic integration, multiple solutions have been explored on silicon based platforms including the use of different materials and techniques. Other important parameters to consider for integrated on-chip lasers and amplifiers are the operating wavelength, power consumption, thermal stability and fabrication complexity to develop integrated circuits.

In the past years, optical gain and lasing has been achieved in silicon platforms by the engineering of materials such as Si nanocrystals, Germanium (Ge)-on-Si lasers, porous Si, Erbium (Er)-related light sources, III-V-based Si lasers and Si Raman lasers[ 21 , 22 , 23 , 24 ].

Although research explores a wide spectra of options, just a few can develop integrated lasers in the nowadays technology for optical communications. The basic requirements for such a source is to emit in the C-band telecommunications range of wavelength, to lase under electrical pumping, to display high power efficiency and to be compatible with silicon fabrication techniques in a CMOS pilot line. Accordingly, some of the materials and techniques such as porous Si, Si-nanocrystals, and Si Raman lasers do not meet these requirements either because of emission wavelength mismatch or low efficiency. Therefore, the most effective and competitive lasers for monolithic integration are: (i) III-V-based Si lasers, (ii) Germanium (Ge)-on-Si lasers and (iii) Er-doped Si based lasers. III-V-based Si lasers are nowadays the only commercial integrated lasers in silicon photonics industry due to its direct band gap which allows high optical gain. Despite lattice parameter mismatch and different thermal expansion coefficients between III-V and Si which reduces the possibility to epitaxially grow III-V materials on silicon, different proposals to overcome these restrains have been studied such as different bonding techniques (figure 1.1) [28] or hetero-epitaxial growth of III-V materials on Si [ 25 ]. As a result, III-V on Si lasing has been demonstrated within the telecom Oand C-band range of wavelengths at 1.3 µm [ 26 ], and 1.55 µm [ 27 ], respectively. Finally, etching and deposition give a vertical p-i-n diode laser structure in image (f) [ 28 ].

Recently a lot of efforts have been carried out using Germanium (Ge). Indeed, Ge-on-Si laser material and fabrication are highly compatible with Si technology. Ge is an indirect bandgap semiconductor with a lattice parameter of 5.6 Å sharing the same crystalline structure than Si. It has been demonstrated that Ge shows direct bandgap behavior which lacks to Si when applying a tensile strain and high doping with electrons (n-type doping) (figure 1.2) [ 29 , 30 ]. Such a technique allows Ge to operate in the telecom range of wavelengths [ 31 ]. The use of Ge alloys such as germanium-tin alloy allows a band structure modification, resulting in germanium becoming a direct bandgap material in mid-IR [ 32 ]. In 2012 studies from MIT proved electrically pumped Ge-on-Si laser with laser emissions in the telecom range by heavy doping and applying a biaxial tensile strain [ 33 ]. Recently in 2015, SnGe lasing at low temperature in the mid-IR has been reported [ 34 ]. Rare-earth elements emit in a range of wavelengths from the visible to the mid-IR range. For the telecom range of wavelengths, erbium (Er 3+ ) ions are a promising doping element because erbium ions emit at 1.5 µm, and Er-doped fiber amplifiers (EDFA) are one of the major successes in rare-earth doping [ 36 ].

However, difficulties in Er 3+ -based lasers are the energy transfer efficiencies that lead to poor optical gain and impossibility of lasing. Moreover, electrically pumped Er 3+ laser has never been demonstrated yet although optically pumped laser have been successfully reported [ 37 ]. The efforts in achieving Er-based Si lasers have been reported by engineering materials to overcome Si limitations. Therefore, several approaches have been proposed: to enhance energy transfer efficiencies. Er-doped Si-rich materials such as Er-doped SiO2 or Er-doped SiNx [ 38 , 39 ] have been proposed in which the main handicap is the Er 3+ clustering which quenches luminescence [ 40 ]. To achieve a greater solubility Er 3+ compound materials such as erbium silicates [ 41 ] or erbium chloride silicate (ECS) [ 42 ] have been studied.

Materials for integrated optical amplification 1.3.1. Rare-earth doping

There are 17 rare-earth elements: 15 of group of lanthanides and 2 of the column 3 of the periodic table (scandium and yttrium). Electron configuration for lanthanides can be [Xe] 4f n 6s 2 or [Xe] 4f n-1 5d 1 6s 2 where n is the number of electrons in the 4f shell or inner shell and 5d 6s the configuration for the outer shell or valence electrons. For the column 3 elements, the electronic configuration is [Ar] 3d 1 4s 2 and [Kr] 4d 1 5s 2 for yttrium and scandium, respectively. All rare-earths exist as trivalent (3+) ions. However, other configurations as divalent (2+) for samarium, neodymium, europium, gadolinium, dysprosium, thulium and ytterbium or tetragonal (4+) for cerium, praseodymium, terbium and dysprosium are observed when incorporated in a host material matrix. Mainly, trivalent ions exhibit a high efficient narrow-band intra-4f luminescence. Two of the bond electrons between a rare-earth (among the lanthanides) and the host matrix have 6s configuration and a third one takes either 5d or 4f configuration [ 43 ]. These ions are characterized by a partially filled 4f shell shielded by 5s 2 and 5p 6 electrons in the outer shell. The screening of the inner shell allows to compare radiative transitions of rare-earth ions in a host matrix with those of free ions. As a result, the positions of rare-earth electronic levels are influenced much more by spin-orbit interactions than by the applied crystal field. By Laporte rule [ 44 ], intra-4f transitions are parity forbidden as a non-centrosymmetric bonding with the host material is required. Therefore, transitions between energy levels, which are partially allowed by crystal field interactions combine opposite parity wavefunctions and radiative de-excitation happens with lifetime in the range of milliseconds. Emission in rare-earths ranges from the ultraviolet (UV) to the infrared (IR) wavelengths. Rare-earth energy levels are typically represented by the Russell-Saunders notation ( 2S+1 LJ) in which S is the total spin quantum number, J the total angular momentum quantum number and L, the orbital quantum number [ 45 ].

The percentage of RE-doping in a host matrix has to be considered carefully. In one hand the concentration of rare-earth cannot overcome the solubility in the hosts as it tends to either form cluster of rare-earth ions like in silica quenching luminescence or it tends to form non-optical compounds or alloys with elements of the matrix like in gallium arsenide hosts [ 46 , 47 ]. RE ion-ion interaction leads to an encumbering of luminescence by different means (figure 1.3): co-operative up-conversion (figure 1.3 (a)), where ions in a metastable state decaying to the ground state can couple in such a way that the decay of ion 1 promotes nearby ion 2 into a higher excited state; energy migration (figure 1.3 (b)), ion 1 in the metastable state can interact with a nearby ground state ion 2, promoting it to the excitation state and one photon can be emitted or cross relaxation (figure 1.3 (c)), process by excitation energy from an ion decaying from a highly excited state which promotes a nearby ion from the ground state to the metastable level. In the other hand a low phonon host can lead to a non-radiative de-excitation by phonon interaction (figure 1.3 (d)), that occurs when in a host, a low amount of phonons is required to de-excite the upper excited state to the low electronic state of the rare-earth ion [ 48 ]. While energy transitions are RE dependent, the widening of emission spectra relies on host structure. Large emission spectra are obtained for amorphous materials, in contrast with the sharp emission lines of RE doped crystalline materials (figure 1.4). While in crystalline host materials the emission is narrower, the emission can be more efficient due to higher ordering [ 50 ]. Crystalline materials are studied as amplifiers or on-chip lasers. In the other hand, the broad emission provided by amorphous materials makes them suitable for EDFAs and tunable lasers [ 51 ].

Semiconductor materials

Among all the possible materials, semiconductors are of great interest as hosts for rare-earth ions. In these hosts, electro-optic properties can be exploited, allowing integration of Er-doped devices. The high refractive index contrast in such materials also allows high integration density. However, due to the lack of suitable RE-ion bonding sites, it is generally difficult to achieve practical RE concentrations in semiconductors [ 53 ]. Incorporation of rare-earth ions in a wide range of semiconductors remains thus challenging. Main disadvantages of RE doping semiconductors is their solubility in the host matrix, formation of aggregates and compounds with the host and quenching effects. Nevertheless, same demonstrations have been performed on different semiconductor matrix.

(a) Silicon (Si)

Silicon is the most popular semiconductor in microelectronics. Integrated optical emission on a silicon platform would allow to produce low-cost, high-speed communication devices. However, the indirect bandgap of silicon, its transparency window, low solubility as a host of rare-earths and being a high-phononic material has precluded such integration. Current high emission photonic devices rely on III-V semiconductor heterostructures despite the complexity in the processing and the high cost. Among the RE, the one of interest in the field of telecommunications is erbium with an emission around 1.5µm. One of the advantages of erbium in crystalline silicon is its large absorption cross-section, although a high percentage of its excitation is de-excited non-radiatively at room temperature due to the lattice phonons. Moreover, erbium has low solubility in silicon due to the mismatch between ionic radii, creating new aggregates or clustering and hindering luminescence [ 54 ]. There is thus intensive research in this field in order to find solutions to these problems and better hosts materials for Er 3+ doping.

(b) Amorphous silicon (a-Si) and silicon-germanium (SiGe) One proposed solution to increase the solubility of RE ions consists in using a-Si as a host matrix. One handicap in a-Si is that despite its high percentage of non-radiative de-excitation paths, it shows low electrical conductivity, thus weaker efficiency for optoelectronic devices. The use of hydrogenated Si has been proposed in an attempt to increase electrical properties in a-Si [ 55 ]. These studies aimed for the crystallization of amorphous Er 3+ -doped silicon at high temperatures but caused the undesired segregation and trap of erbium into the amorphous silicon leading to quenching effect of the luminescence. Further research [ 56 ] in this direction proved that the Er 3+ and surrounding oxygen ions placement within the amorphous silicon is determinant to obtain active erbium ions and increase the luminescence efficiency. As a result, oxygen-doped hydrogenated amorphous silicon have demonstrated that luminescent ions are found within Er-O quantum dots [ 57 ]. Besides oxygen, other elements have been studied to co-dope with RE to increase luminescence using quantum efficiency. In this regard, it has been demonstrated moderate emission of erbium ions and electroluminescence in erbium-implanted SiC in PN junction at room temperature showing low temperature depending quenching effect [ 58 ]. SiGe can also be considered as a matrix for RE. Co-doping this structures with RE allows luminescence within SiGe waveguides for optical amplification although quenching of luminescence is still temperature dependent [ 59 ]. A solution to reduce quenching due to heat effects pointed out a multilayer approach of RE doped Si/SiGe systems in which RE is doping exclusively SiGe layers and luminescence is obtained at room temperature [ 60 ]. In this regards, Si/SiGe:Er/Si structures have been proposed to implement lasers on silicon substrates [ 61 ].

(c) Porous silicon

Porous silicon has been thought to overcome the low carrier current on silicon as the carriers' quantum confinement created during silicon etching process increases the efficiency of electroluminescence from silicon [ 62 ]. By changing etching conditions, the size of the pores in the material changes, and allows to control the bandgap of the host matrix. In RE-doped porous silicon, it is possible to activate luminescence from rare earths with luminescence energies greater than the bandgap of bulk silicon. Annealing is required to activate the rare-earth luminescence, and moreover, luminescence is dependent on the annealing atmosphere [ 63 ]. Despite the efforts, efficiency in photoluminescence is still low , although luminescence from erbiumdoped porous silicon has been demonstrated at room temperature integrated in planar waveguides [ 64 ].

(d) Nanocrystalline silicon

As electron quantum confinement has proved interesting for electronic and optical properties, nanocrystalline silicon has been studied as a quantum confined structure of two or three dimensions. Doping these confined systems with rare-earth ions can help to overcome some of the non-radiative de-excitation paths, reducing quenching effects [ 65 ]. In the beginning of the 2000s, modal and net material optical gain has been proved by introducing silicon nanoclusters (Si-nc) on a SiO2 hosts. With this approach, the group of Pavesi in Trento demonstrated net optical gain of 100dBcm -1 at 800 nm [ 66 ]. Comparison net modal gain with III-V low dimensional nanostructures were referenced in this work (Table 1) proving as much as an order of magnitude higher. In this approach, the energy structure of the interface between the silicon dioxide and the Si-nc increases lifetime allowing population inversion. Moreover, energy structure of this configuration diminishes free carrier absorption and Auger saturation of the luminescence which hinders emission in silicon structures.

These outstanding results lead to the use of Si-nc as sensitizers of active materials such as rare-earths. Si-nc have a higher pumping absorption cross-section than REdoped silica systems. As result, by placing Si-nc nearby RE ions, absorption crosssection of this material is increased. Therefore, optical amplification has been studied in RE-doped SiO2 with Si nanoclusters as sensitizers. Among rare-earths the most studied were Er 3+ ions with their emission wavelength at 1530 nm for telecommunication applications. Internal optical gain of 1dBcm -1 by indirect transfer was observed for an Er 3+ ion doping content of 23 at. % by Navarro-Urrios et al. [ 67 ]. Two different strategies have been carried out to decrease the effects of two mechanisms hindering luminescence in these systems: carrier absorption losses and Auger energy transfer. For the first one, a reduction on the Si-nc size has been proposed. Regarding the second one, to diminish energy transfer, an increase in the concentration of Er 3+ ions rises the probability of having ions close enough to the Sinc to be excited indirectly. Another efficient sensitizing effect of Si-nc has been proved also in neodymium. This RE is of interest in the near-IR technologies due to its two transitions at 0.9 and 1.06 m. In this study has been stated an effective size of 4 nm for the Si-nc and an optimal distance of 0.4 nm between the Si-nc sensitizers and the Er 3+ ions to achieve luminescence enhancement [ 68 ].

Further development of Si-nc structures as RE sensitizer were studied on MOS devices to improve electrical and electroluminescence (EL) properties. In these studies, Er-doped host materials such as silicon-rich oxide (SRO), SiNx and siliconrich nitride (SRN) on a SiO2 layer have been proposed by B. Garrido and L. Pavesi [ 69 , 70 ].

A different approach on two dimensions' quantum confined structure is multilayer structures [ 71 ]. The increase of luminescence and the short lifetimes make Si resonating structure suitable for fast optical devices as modulators integrated on silicon platforms. Finally, three-dimensionally confined silicon system can be created with silicon-rich silica consisting of silica doped with excess of silicon forming nanocrystallites, the confinement system in this case is in the boundary between silica and silicon. Low efficiency emission from this interface is created by the quantum confinement effect and emits in wavelengths in the visible and near IR region [ 72 , 73 ,]. The increase of photoluminescence comes from the emission of the rare-earth doping. Emission of erbium is achieved when absorption bands of the silicon nanoclusters are coupled to the erbium excited states. Moreover, it has been demonstrated that adding silicon nanoclusters in a silica matrix increase the effective absorption cross-section of erbium [ 74 ]. Finally, optical gain in an Er-doped siliconrich silica waveguide has been proved [ 75 ].

(e) III-V semiconductors

Other studied options in hosting RE in semiconductor materials are the use of III-V semiconductors, although RE luminescence is low in these hosts due to strong quenching effects and the integration of these materials in silicon is more expensive and requires multiple fabrication processes. At the beginning of the 1980s, RE emission in the near-IR wavelengths has been studied in III-V materials such as GaAs, InP, GaP and AlGaAs [ 76 , 77 ], and electroluminescent devices have been produced from erbium and neodymium-doped GaAs [ 78 , 79 ] and neodymium-doped GaP [ 79 ]. RE excitation mechanism in doped III-V materials is produced when RE ions are excited by the recombination of an electron-hole pair [ 80 ]. Regarding RE ions doping III-V semiconductors, while erbium is the RE of reference for telecom wavelengths , there are other convenient RE with emission in the near-IR such as ytterbium doping in InP due to the similar size between Yb ion and In [ 81 ]. With a lattice parameter similar to that of silicon, co-doping is a technique already used in other host materials to increase luminescence. Thereupon, to avoid quenching luminescence related to temperature, Er:GaAs has been co-doped with other elements as O and N [ 82 , 83 ]. Nevertheless, high doping percentage of N in GaAs creates phase separation of GaAs and GaN and precipitates of ErAs, hindering photoluminescence [ 84 ].

(f) Wide bandgap materials

Wide bandgap semiconductors with bandgap range below 2.2 eV which composition includes compounds of the II group elements and oxygen, sulphur or selenium, compounds of the II group elements and nitrogen, silicon carbide and diamond [ 85 ].

First, gallium nitride (GaN) possesses a number of properties that make it a suitable host for rare-earth ions, including a wide and direct bandgap, good chemical and thermal stability, excellent high field transport properties, and solubility of RE ions [ 86 ]. Generally, gallium nitride is grown on either silicon or sapphire substrates. Even if rare-earth ions can form aggregates at high temperature in a similar fashion to that seen in other hosts, the solubility of rare-earth ions in GaN is much higher than in silicon and other narrow gap semiconductors due to GaN structure. The first report of visible emission from erbium in gallium nitride was performed by Steckl and Birkhahn in 1998 [ 87 ]. More recent work has demonstrated visible emission in GaN from other rare-earth ions, including praseodymium, europium, dysprosium, and thulium [ 88 , 89 , 90 ]. Electroluminescence in GaN diodes have been demonstrated [ 91 ], and mixed color emission from material doped with more than one rare earth has been reported [ 92 ]. Samples prepared using ion implantation must be annealed to remove implantation damage, typically at temperatures in excess of 1000°C, though, as in silicon and silica, annealing at too high temperature reduces rare-earth luminescence due to clustering. In addition, high temperature annealing can change the composition and structure of the GaN matrix. Co-doping erbium-doped GaN with elements such as oxygen or carbon increases the intensity of 1.5 m emission, as well as decreases the temperature quenching effects [ 93 , 94 ].

Other wide bandgap materials including silicon carbide, silicon nitride, aluminum nitride, zinc sulphide, zinc oxide, and diamond are also considered to host RE materials. Semiconductor nitrides are specifically of interest thanks to their compatibility with existing semiconductor technologies [ 95 ]. The temperature quenching effects affecting rare-earth ions emission are found to be inversely proportional to the bandgap of the host [ 96 ], making wide bandgap materials attractive choices for rare-earth incorporation. Wide bandgap materials have higher excitonic ionization energies, and therefore are more efficient hosts for electrically activated rare-earth emission. In contrast to silicon, research on rare-earth doped wide bandgap materials has covered both visible and infrared emission from the rareearth ions [ 97 ].

Dielectric materials

Dielectric materials have a wide band gap energy, thus are highly transparent over a wide wavelength range. Due to their properties, these materials present higher RE solubility than semiconductors. All these materials are used either as low phonon materials or in fibers. However, by definition, they do not permit efficient electrical excitation, limiting their use to all-optical applications.

(a)

Polymers Certain deposition surfaces can be challenging for thin films or nanostructures to grow. However, RE can be integrated in a wide variety of substrates using polymers. Polymer doping is interesting as not only it can be implemented in a high range of substrates but as well it can be done at low cost fabrication and it allows high scalability. Some studies have successfully reported doped polymers with erbium, ytterbium and neodymium in the near-IR proving optical gain [ 98 , 99 , 100 ]. However, the thermal damage is low, about 100°C, which is not compatible with their integration in Si technology.

(b)

Silicon dioxide (SiO2) Silicon oxide (SiO2) is studied as a host for rare-earth as it is the material used in fiber and waveguide technology. Rare earths ions are not very soluble in silica, thus it is limited to low concentrations as it shows tendency to cluster and quench luminescence due to ion-ion interaction. RE doping at low concentrations allows low absorption cross section which is interesting for long fibers or waveguides but also induces low emission cross-section of the system. In contrast to erbium-doped silicon, in which short luminescent lifetimes are required in order to produce highly rated modulation sources, the emphasis in erbium-doped silica is on long luminescent lifetimes. This effect is required to create population inversion to obtain optical amplification and lasing applications.

In the field of optical amplifiers based on rare-earths, the most important achievement is the erbium-doped fiber amplifier (EDFA) [ 101 ] in which the RE host is an aluminosilicate. These fibers present low noise and high optical gain in the 1.5 µm wavelength range of telecommunications. High optical gain is due to the RE doping at low concentrations to avoid clustering effects. Moreover, absorption and emission cross-sections are small in silica and non-radiative transfer losses are nonexistent. As a result, luminescence lifetimes are long. A similar approach of EDFA are integrated erbium-doped waveguides on silicon-based substrates [ 102 ]. This structure and material are of special interest for waveguide-division multiplexing (WDM) technology. Despite the low solubility of erbium, silica allows annealing in an inert atmosphere without erbium diffusion, thus, aggregations are not created and luminescence intensity can be increased. In order to increase solubility of erbium other materials as silicate glasses have been studied to reach high optical gain [ 103 ].

(c)

Low phonon materials It is known that hosts with low phonon energies reduce the probability of multiphonon non-radiative decay from rare-earth excited states [ 104 ], and also allow near-and mid-infrared transitions that are not seen in other hosts. Moreover, these hosts are also required to enable efficient emission from some rare-earths as thulium, holmium, europium, which is not possible in highly non-radiative materials. Additionally, photoluminescence efficiency is extremely dependent on the host matrix: low phonon energies are required for high emission efficiency. Dielectric materials as fluoride, chalcogenide and tellurite glasses are the choice for low nonradiative decay.

(i)

Fluoride glasses are promising low phonon materials as they show high RE solubility. While in these glasses up to a 19.8% doping in RE has been proved [133], in other hosts such as III-V, silicon compounds or oxides to name the most popular RE-doped materials doping percentage is lower. Typical values for GaN would be of 2% [ 105 ]; in III-V materials as GaAs as low as 410 8 cm -3 [ 106 ] ; for silicon nanocrystals on silica, of 1%[ 107 ] or porous silicon up to 2.6% [ 108 ] , and for oxides as ZrO2, 310 20 cm -3 [ 109 ] or Al2O3, 710 20 cm -3 [ 110 ]. In these low phonon materials, luminescence has been reported for transitions in the near IR and Mid IR at 1.3, 1.4 and 2.7 µm for praseodymium, erbium, ytterbium, and thulium [ 111 , 112 , 113 , 114 , 115 ]. However, these glasses are chemically and mechanically unstable and they require a multicompound structure to stabilize them. A well-known fluoride glass in optoelectronic devices is ZBLAN (ZrF -BaF -LaF -AlF -NaF). Praseodymium (Pr)-doped fluoride glasses are also used to produce up-conversion fiber lasers [ 116 ]. Although, neodymium with a larger emission cross-section reveals as promising RE for gain in the same near-IR range of wavelengths than praseodymium.

(ii)

Chalcogenide glasses are compounds containing an anion of the 16th group of the periodic table: S, Se, and Te. They are interesting materials for photonic applications as the window transparency ranges in the visible, near-and mid-IR, they have a high refractive index, are low-phonon materials, etching can be easily performed for fabrication and they show nonlinear optical effects [ 117 ]. Emission has been proved for Er-[ 118 ] and Pr-doped chalcogenides [ 119 ] and Pr-, Dy-and Tb-doped chalcogenide fibres [ 120 ] in the mid-IR. Moreover, amplification has been studied in the near-IR for neodymium, praseodymium, dysprosium, terbium and holmium doping fibers [ 121 , 122 , 123 , 124 , 125 ].

(iii) Other low phonon materials are tellurite glasses composed of mainly tellurite oxide. These materials have a wider transparency window than chalcogenides, going from ultraviolet to mid-IR and show nonlinear effects [ 126 ]. Moreover, unlike fluorides they show mechanical and chemical stability. Tellurite EDFA have been reported as fiber amplifier combined with a Tm -doped fiber [ 127 ]. As well, these glasses are proved to be efficient hosts for erbium, europium, neodymium, ytterbium and thulium [ 128 , 129 , 130 , 131 ].

(d) Oxide materials

Oxide growth is a mature technology with a wide range of deposition techniques: plasma enhanced chemical vapor deposition (PE-CVD), electron beam physical vapor deposition (EB-PVD), sputtering, molecular beam epitaxy (MBE) or laser ablation.

One of the main handicaps in photonic devices is the transmitted light scattering losses at the interface between the grown material and the substrate. In that regard, the epitaxial growth of the oxide material on the silicon substrate appears very promising. Crystalline oxides present an additional challenge as lattice mismatch introduces lattice constraints and defects hindering hybrid integration of materials. To overcome these constraints a usual procedure is to use buffer layers to enable epitaxial growth of high-quality crystalline functional oxide films onto silicon while avoiding severe interface intermixing. Some of the most used buffer layers for functional oxides on silicon are: CeO2 for ITO or YBa2Cu3O7 [ 132 , 133 ]; yttria-stabilized zirconia (YSZ) which is a buffer layer for functional oxides such as LiNbO3, (Ba,Sr)TiO3, La0.7Sr0.3MnO3, YBa2Cu3O7 or SrTiO3 [ 134 , 135 , 136 , 137 , 138 ] or SrTiO3 used as buffer for LaAlO3 thin films [ 139 ], with lattice parameters compatible with the one of silicon. Functional oxides are widely studied due to their superconductivity, thermoelectrical and optical properties [ 140 ]. These materials have been doped with rare-earths for multiple applications as photocatalysts, sensors, detectors, display screens, light emitting-diodes, solar cells, among others.

Oxides can be active materials for on-chip light sources with the incorporation of RE ions as optical dopants in the oxide matrix. In order to attain optical amplification at a minimum power level a high percentage of RE is required. Such increase normally leads to high ion-ion interaction and ultimately to RE clustering quenching effects, hindering optical amplification. Moreover, highly doped oxides with large difference among ionic radii are also prone to reveal high defects density caused by RE ion incorporation to the host decreasing photoluminescence [ 141 ]. In addition, low phononic materials are required to avoid non-radiative pathways for rare-earth ions. Despite these constraints, excellent optical properties have been already demonstrated for a few materials, and light amplification on hybrid photonic integrated circuits containing LiNbO3 [ 142 ], Al2O3 [ 143 ] or Y2O3 [ 144 ]active layers have been achieved.

The multiple properties of LiNbO3 include ferroelectricity, piezoelectricity, electrooptic effect and nonlinearity [ 145 , 146 ]. Lithium niobate is a functional oxide used in photonic industry for modulators and lasers when doped with RE [ 147 ]. As in other photonic applications based on RE doping which are operative in the range of telecom, Er 3+ ions are the RE of choice for LiNbO3 for optical amplification and lasing. Erbium implantation on a LiNbO3 host requires a post-implantation annealing to maintain the crystallinity of the oxide. However, high temperature annealing leads to erbium clustering, quenching luminescence, thus a compromise between temperature activation of the RE ions luminescence and crystallinity is required. During the last decade, lasing in Er-doped LiNbO3 waveguides has been proved in tunable wavelengths in the near-IR. And recently, optical amplification and lasing with a total internal gain of 14 dB have been achieved at a wavelength of 1561 nm on Ti:Erco-doped LiNbO3 ridge waveguides [ 148 ].

Alumina (Al2O3) is a ceramic which has been also studied as a photonic material and used for sensing applications as host of RE with light amplification purposes and in photonic crystals in the form of nanoporous anodic alumina (NAA) [ 149 , 150 ]. Aluminum oxide is of great interest in photonics due to its properties as it is a mechanically, chemically and thermally stable material with a transparency window from the ultraviolet to the mid-IR [ 151 ]. As well, this oxide can be deposited or grown by multiple techniques on large scale. Moreover, it has been observed that its mechanical properties improve with RE doping [ 152 ] being this oxide very attractive for optical amplification. Multiple studies have been performed regarding optical amplification in Al2O3 and Al2O3-based materials during the last 30 years on a wide range of RE doping such as terbium, erbium, ytterbium and thulium [ 153 , 154 , 155 ]. In addition, different structures such as nanoparticles and different compositions of alumina have been investigated in the fabrication of erbium-and neodymium-doped fibers [ 156 , 157 ]. Studies of D.B. Bradley and M. Pollnau, proved in 2009 a net gain of 2.0 dB and a total gain of 9.2 dB in an alumina amplifier at 1533nm [ 158 ]. The same group in 2014, explored rare-earth-doped microring lasers, erbium-and ytterbiumdoped microrings, with emissions at 1.5-µm and 1.0-µm with efficiencies of 0.3 and 8.4%, respectively.

In the review by Pollnau and Bradley in 2010 on Er 3+ -doped amplifiers and lasers [ 159 ], they report net gain of Er-doped waveguide amplifiers EDWAs on oxide materials and Er 3+ ion concentration, with lengths of more than 1 cm. In these studies, [ 159 ] highest internal net gain of 2.0 dB/cm for EDWA lengths of 2.1 and 5.7 cm are reported for Al2O3 and LiNbO3, respectively and internal net gain of 1.3 dB/cm for length of 4.3 cm for EDWA is obtained for Y2O3. Moreover, such oxide hosts as Al2O3 or Y2O3 allow high Er 3+ ion concentration (2.1210 20 and 1.310 20 cm -3 , respectively) without introducing significant quenching effects. Low phonon oxides such as Y2O3 or materials containing Y2O3 such as YSZ (Y2O3-ZrO2) with a cationic radii similar to Er 3+ are good candidates as Er 3+ hosts in constructing active waveguides for telecommunication applications operating at 1.55 µm. As commented, LiNbO3 and Al2O3 are two of the most used oxides for optical gain. With a phonon energy lower than other oxides as Al2O3 (870 cm -1 ), LiNbO3 (685 cm -1 ) or Y2O3 (591 cm -1 ), YSZ (550 cm -1 ) is a good candidate for RE doping. Previous studies in YSZ probed luminescence in the visible range of wavelengths for RE such as 0.75 at. % Ho 3+ or 0.5 at. % Pr 3+ in multiple phases (tetragonal, cubic or monoclinic) [ 163 , 164 ]. For Pr 3+ -doped YSZ, red emission after annealing has been proved. In addition, green emission from up-conversion mechanisms for Ho 3+ -and Er 3+ -doped YSZ has been reported. Erbium efficient up conversion has been found at 20K for tetragonal crystals as result of erbium placement within the YSZ structure [ 165 ].

There are many host approaches for RE-doping materials but the use of oxides and specially YSZ is very promising as new photonic material. YSZ is a chemically and mechanically stable oxide, with refractive index of 2.15, a bandgap of 5 eV which prevent two photon absorption (TPA) and transparent from the UV to the mid-IR and its lattice parameter compatible with Si. Moreover, its atomic structure which includes Y 3+ with a similar radius than Er 3+ and its oxygen distribution which is known to play a crucial role in RE emission, makes YSZ a perfect candidate for optical amplification purposes.

Outline of the thesis

YSZ has been proved an outstanding crystalline photonic material by its optical properties, with loss transmission losses of 2 dB/cm at 1380 nm and a high refractive index [ 166 ]. Moreover, structurally it is a chemically and mechanically stable material, with a lattice parameter of a=5.14 Å similar to that of Si (a=5.43 Å).

In YSZ, substitution of Zr 4+ with ions radius of 0.8 Å by Y 3+ atoms (ions radius of 0.9 Å), has as effect the stabilization in the cubic structure for 8 mol % of Y 3+ . Being YSZ a low phonon material, we considered the substitution of photoluminescence materials such as RE, with trivalent (+3) and tetravalent (4) ions and similar ionic radii in the YSZ matrix. Despite only trivalent ions are photoluminescent, we wanted to implement in our YSZ matrix two RE ions: erbium (exclusively trivalent) and praseodymium (present in trivalent and tetravalent configuration). The possibility of substitution of Zr 4+ and Y 3+ by RE will result on a different oxygen and oxygen vacant configuration, modifying Er 3+ emission. Crystalline hosts show higher and sharper emission and absorption cross-sections than amorphous materials. Although deposition of amorphous materials can be performed in all kinds of substrates regardless of mismatch in lattice parameter, being YSZ a buffer layer of other functional oxides and silicon, we aimed to grow crystalline YSZ to reach higher and sharper emission by RE doping and further functionalize this photonic material.

The aims of this thesis are to obtain a highly monocrystalline and efficient active material for optical amplification. Host doping is performed in thin film deposition by PLD in crystalline and amorphous substrates. Our active material is designed to be integrated in an erbium-doped waveguide amplifier (EDWA) as a cladding as done by other research groups for optical amplification.

Conclusions

In this first chapter we just described the thesis context and we explored the stateof-the-art to target efficient photoluminescent materials for optical amplification purposes in the near-IR range of wavelengths. After studies in the literature proving outstanding results in optical gain and amplification using RE-doping in low phononic oxides on EDWA, we wanted to further explore optical amplification on YSZ. In that purpose, Er-and Pr-doped YSZ in monolayer and multilayer thin films have been grown by PLD on crystalline and amorphous substrates. Their growth and structural characterization results will be developed in chapter 2.

Epitaxial growth of RE-doped thin film oxides using pulsed laser deposition (PLD)

In a first step, we have epitaxially grown by PLD RE-doped YSZ thin films on a crystalline substrate (sapphire) for photonic purposes. Samples were structurally characterized by means of spectroscopic ellipsometry, X-ray diffraction, AFM and STEM microscopy. In a second step, we have grown RE-doped thin films under the same PLD conditions than on sapphire, on an amorphous substrate integrated on a silicon-based chip.

We have considered two different thin film approaches for RE-doped YSZ thin films grown by PLD: (i) RE-doped 8 mol% YSZ monolayer from a sintered Er 3+ -or Pr 3+doped YSZ target and (ii) multistack structure of Er-doped YSZ from two different targets, one metallic Er 3+ and a second target of sintered RE-doped or undoped YSZ. Detailed description of samples and its structure schematics are found in table 2.4 (page 57).

Thin films growth processes and techniques

A thin film is defined as a material layer of thickness between  0.1 nm to 500 nm deposited on a substrate. When materials are composed by a stack of thin films, their structure is called a multilayer structure. Monolayers and multilayers are widely used for surface coating to change properties of solids for telecommunications, integrated circuits, photovoltaic or medical applications to name a few [ 1 ].

Deposition or growth processes of thin film materials depend on environmental parameters such as temperature or pressure and substrate's surface. In this subject, material growth can be achieved by multiple deposition techniques based on physical or chemical methods.

(i) Growth processes

Thin film growth is a well-known technique in solid state electronics and photonics industry, for example to integrate hybrid devices in silicon-based platform. This technology allows high precision deposition in the nanometric scale of a wide range of crystalline, polycrystalline or amorphous materials. Properties of thin films can be different than the bulk ones, due to possible differences in chemical composition and crystallographic structure. Normally the process of growth is produced by surface phenomena in which free bonding of the surface atoms in a bulk can adsorb evaporated atoms. In a first step shown in figure 2.1 (a,b), atoms are adsorbed by substrate surface, they migrate in a Brownian movement until they are removed, trapped or collided with other atoms and cluster, forming a stable condensate. When it is trapped surrounded of several atoms, it nucleates and starts the film growth. FIGURE 2.1 -Schematic of epitaxial growth process. In image (a) is described how an atom is adsorbed in the surface of the substrate and after Brownian motion in the surface is trapped, causing nucleation. In image (b) the same atom is absorbed but in this case, instead of nucleating, it is desorbed.

When the lattice constants of the substrate and the material to deposit match, epitaxial growth might happen. To obtain low density of crystal defects and large grains a low deposition rate and high substrate temperatures are preferred. The arrangement of atoms can be done in three different ways depending if atoms are strongly bonded to each other or to the substrate. Other variables to determine the growth path are: energy of the particles arriving to the surface, its binding energy, deposition rate, temperature, pressure of the deposition chamber and kind of substrate [ 2 ] (a)

Layer or Frank-van der Merwe model: in this model adsorbed atoms are more strongly bind by electrostatic binding to substrate than to each other (figure 2.2). The first atoms to condense form a monolayer on the surface. Subsequent atoms form top layers following the same process. This model has been observed in gases adsorbed by many substrates and metal-metal systems [ 3 ].

(b)

Island or Volmer-Weber model: this type of growth occurs when absorbed atoms are privileged to bind to each other than to the bonds on the substrate. In this model, trapped atoms create small clusters which nucleate on the substrate surface and grow into islands (Figure 2.3). This growth is known to be followed by metals growing on insulators.

(c)

Layer and island or Stranski-Krastanov mode: it is an intermediate case between the previous two. After the first layers are formed, subsequent layers are grown in islands due to strains occurring at a critical thin film thickness during growth (figure 2.4). When increasing thin film thicknesses, the energies involved in the chemical bonding between substrate and thin film atoms diminish and strains appear. These phenomena produce a mismatch in lattice parameter or symmetry with substrate in intermediate layers helping the formation of islands. This model is mostly followed by metal-metal, metal-semiconductor, gas-metal and gas-layer

(ii) Growth techniques

The growth of thin films can be performed by different techniques based of chemical or physical vapor deposition, liquid phase processes or printing techniques, being chemical and physical vapor deposition the most common techniques for oxides growth. Chemical vapor deposition (CVD) processes rely on chemical reactions in which the reactors are deposited as a thin film on the substrate. Among the most used technologies in this regarding are plasma-enhanced CVD (PE-CVD), lowpressure CVD (LP-CVD), molecular vapor deposition (MVD) or atomic layer deposition (ALD). The need for chemical reactions for deposition limits the range of materials to grow a thin film and specific precursors are needed. However, physical vapor deposition (PVD) is based on physical processes to transport material and to deposit a thin film on a substrate while chemical reactions do not take place. In one hand in comparison with CVD, this technique does not have restrains for chemical reactions and allows stoichiometric deposition of almost any kind of complex materials and in a wider range of substrates than CVD. Moreover, PVD techniques are often faster and allows growing thicker films than CVP respecting the morphology of the substrate with thicknesses from the nanometer to hundreds of microns at a wide range of deposition temperatures. PVD requires lower temperatures for deposition than CVD. In the other hand, PVD are expensive and complex techniques which requires specific infrastructures [ 4 ]. Most popular PVD techniques in photonic integration includes electron beam evaporation, plasma sputter deposition, molecular beam epitaxy (MBE) or laser ablation.

(a) Electron beam physical vapor deposition (EB-PVD)

High energy electron beam is directed to a solid target in a vacuum atmosphere as shown in figure 2.5. The source material is heated and evaporated and then transferred to the substrate in where it condenses to form a thin film. At the same time, the substrate is externally heated to enhance the bonding between materials deposited and substrate. This technique is not suitable for engineered complex surfaces or with highly rough topography as it provides low control of local deposition for materials composed by different types of atoms. Another drawback of this type of deposition is its inhomogeneity [ 5 ].

(b) Sputtering deposition

In sputtering deposition (figure 2.6.), electrons are accelerated with the use of a magnetron on the cathode which impacts a neutrally charged gas, ionizing its atoms and creating a plasma. The plasma ions are directed to the cathode and bombard the target. The ejected material is transported up to the substrate forming thin films. One of the main advantages of this deposition technique is the uniform deposition, low level of impurity in comparison with other PVD techniques, high rate of scalability. Besides, it presents low directionality and costs are high [ 6 ].

(c) Molecular beam epitaxy (MBE)

This technique is based on beams of atoms or molecules ejected from guns under ultrahigh vacuum conditions (UHV) called effusion cells directed to a heated substrate. The material to deposit reaches the substrate, condenses and forms thin layers (figure 2.7.). This deposition allows the growth of high quality crystals, with a high atomic control of deposition in very thin films. This high deposition precision does not permit to implement this technique in systems which require high scalability. Moreover, it is a slow method which requires expensive equipment [ 7 ].

(d) Laser ablation

This technique consists on a solid rotating target placed at a certain distance of a heated substrate in a high vacuum chamber. A pulsed laser beam enters to the chamber through a transparent window and hits the target, pulling off material directed to the substrate by a plasma plume made of the target ionized atoms (figure 2.8). This deposition technique keeps the deposition stoichiometry, allows the growth of high quality epitaxial thin films, is suitable for small area deposition but gives not uniform films over large areas. Laser ablation is preferred in thin film deposition of metals, semiconductors, insulators, and in multilayer and superlattices structures [ 8 ]. Pulsed laser deposition (PLD) based in laser ablation is the deposition technique chose in this thesis due to its uniform and fast deposition in small areas and the possibility of deposit multistack structure.

Materials investigated in this work

(i) Yttria-stabilised zirconia (YSZ) Y2O3-stabilized ZrO2 (YSZ) or (ZrO2)1-x(Y2O3)x is a ceramic with the CaF2 structure for a wide range of stoichiometric x values. It is known for its thermal and chemical stability, hardness, mechanical durability and ionic conductivity [ 9 , 10 , 11 ]. Hence, it has been used as electrolyte in solid oxide fuel cells (SOFC) and as oxygen sensors [ 12 ], as well as a buffer layer for the growth of functional oxides such as LiNbO3, PbTiO3, La0.7Sr0.3MnO3 or YBa2Cu3O7 [ 13 , 14 , 15 , 16 ]. This oxide remains an understudied material in other fields such as photonics despite its optical properties which includes: (i) low waveguide propagation losses [ 17 ]; (ii) absence of two photon absorption (TPA) in the near-and mid-infrared wavelengths (near-IR and mid-IR); (iii) large transparency window from ultraviolet (UV) to mid-IR spectral region; (iv) nonlinear optical properties [ 17 ] or (v) mid-refractive index contrast material (MiDex) avoiding current incompatibilities of high refractive index contrast materials (HiDex). Interestingly, yttria-stabilized zirconia (YSZ) as yttrium aluminum garnet (YAG) presents oxygen vacancies due to yttria placements in its structure. Rare-earth doped YAG are very efficient commercial lasers in which low percentage of yttrium ions are substituted by rare-earth ions such as erbium or neodymium ions due to their similar ionic radii (232 pm for erbium, 229 pm for neodymium and 240 pm for yttrium).

In YSZ structure yttrium substitutes a percentage of zirconium. This percentage determines YSZ crystalline structure which can be tetragonal or cubic depending on the molecular percentage of cubic Y2O3 in its composition. [ 18 ]. Originally, zirconia is a ceramic material that shows different phases, monoclinic, tetragonal and cubic at atmospheric pressure depending on the temperature [ 19 ]. To structurally stabilize its cubic structure, yttria has been added to zirconia (figure 2.9). The addition of yttria is known to increase oxygen ion conductivity in YSZ due to the oxygen vacancies in the YSZ structure created by the mismatch of ionic radii between yttrium and zirconium [ 20 ]. Moreover, yttria is a good host for rare earths not only for being a low phononic materials but as well for the sites rare-earths can be placed at the structure [ 21 ]. Low phononic crystals are desirable hosts for RE doping as high presence of phonons in the crystalline matrix contributes to nonradiative de-excitation of the RE ion, hindering photoluminescence emission. Yttrium can be placed in the symmetry sites C2 (2-fold rotation) or C3i (3-fold rotation with inversion) in the YSZ structure. In RE-doped YSZ, these sites will be taken by rare-earth due to ionic radii compatibility. Rare-earth ions at C2 sites show no inversion symmetry and will absorb radiation through electric-dipole transitions, for those at C3i sites which present inversion symmetry only magnetic-dipole transitions will be allowed. Moreover, energy transition between the two sites have been observed by Omi et al. and other authors [ 22 , 23 , 24 , 25 ].

(ii) Erbium

Erbium is a trivalent ion of the rare-earth group which has 68 as atomic number with an electronic configuration of [Xe] 4f 12 6s 2 . Erbium photon absorption and emission are only allowed for certain transitions. The specific structure of rareearths with the electronic shielding of outer electrons protecting the crystalline electric field prevents major differences on energy transitions maintaining them independent on the host. Several photoluminescence studies on Er 3+ absorption and emission transitions in the near-IR and visible range of wavelengths are represented in figure 2.10 and 2.11. The aim of this thesis is the study of RE-doped YSZ as photonic material for telecom applications in the near-IR range of wavelengths.

(a) Erbium absorption and emission transitions

We are interested in three Er 3+ ion absorption wavelengths corresponding to the 800, 980 and 1480nm as observed in figure 2.10 which populates via non-radiative decay the 4 I13/2 4 I15/2 transition at 1530nm (figure 2.11 (a)). Besides, to prove upconversion we complementary characterized Er-doped YSZ emission in the visible range of wavelengths. Toward that end, Er 3+ is excited under 980 nm wavelength populating the 2 H11/2 energy level by upconversion, relaxing at three different energy levels ( 2 H11/2, 4 S3/2 and 4 F9/2) corresponding to the wavelength emissions at 520, 550, and 675 nm as observed in figure 2.11 (b) [ 26 ]. Er 3+ absorption and emission transitions and wavelengths corresponding to the samples of this thesis can be found in table 2.5 (page 58). A laser pumping source is required to excite rare-earth ions to an upper state. The probability of absorbing a photon of certain wavelength per unity of surface (cm -2 ) or absorption cross-section is characteristic of each material and can be expressed as the crossing unit area of the coefficient between the total energy absorbed by one atom per unit time and the total incident intensity, In which I and Io are the outgoing and incident beam intensities, respectively. L the cross-section thickness and NEr, the erbium ion density. Once the ion is excited to a higher state, de-excitation to lower levels can be performed by radiative or nonradiative decay. The time required for an excited ion to decay to the ground state is called lifetime. These dynamics can be modeled by equation 2.1.,

(eq. 2.1)

Where Ni is the population of active rare-earth ions in state i, being state i=0 the ground state and i=1 the excited state (per unit area /cm3), 10 is the lifetime of the transition from excited to ground state.  the pump flux,  the absorption crosssection of the materials and c represents the cooperative upconversion coefficient between states. In upconversion two ions placed at a metastable lever interact with each other by de-exciting radiativelly one ion and promoting a second ion to higher energy levels. FIGURE 2.12 -Schematics of inversion population in rare-earths in which the number of electrons in the excited state has to be higher than in the ground state.

Decay rate (probability per unit time) from one transition to another is the sum of radiative and non-radiative decay and thus is inversely proportional to lifetime (s) (equation 2.2.), then (eq. 2.2) If Förster energy transfer (FET) between two nearby ions is produced during the time needed for radiative decay, non-radiative de-excitation takes place [ 28 ]. To obtain optical gain or lasing, population inversion must be achieved (figure 2.12). In this situation, the electron population in the excited state must be higher than in ground state (N1>N0). Thus, long lifetimes and a stimulated photon in phase with the incoming photon are main requirements to achieve laser effect. Then, radiative decay and for extension, lifetime, is of great importance for optical gain in which population inversion is required and lifetimes have to be long enough (typically on the ms scale of time) to achieve this condition [ 29 ].

There are two main mechanisms that explain excitation energy transfer and energy migration in rare-earths called Förster and Dexter transfer. In the first one, two ions absorbing at different energies are placed at a distance in which the ion absorbing at higher energies donate this energy to the one absorbing at lower energies. Förster energy transfer presents three conditions: (i) this interaction is a dipole-dipole interaction in which the distance between dipoles has to be low, (ii) this distance produces an overlap between emission band of the donor and the absorption band of the acceptor; and iii) donor and acceptor dipoles are constraint to an adequate orientation to perform energy transfer. The second energy transfer mechanism is called electronic exchange or Dexter transfer. In this energy transfer, the two ions, donor and acceptor are almost in contact with each other. An electron in the excited level is transfer to a higher energy level of the acceptor. Simultaneously, an electron in the ground state is excited in the donor.

(b) Erbium-doped YSZ

In this thesis, we have studied the 4 I13/2 4 I15/2 erbium ion transition in YSZ structure. Numerous studies have been carried out on the 4f 11 ion Er 3+ , which is responsible for this transition corresponding to the telecommunication wavelength emission at 1.54 μm. This transition emission lies in the C-band range of wavelengths used on telecommunication applications.

One of the aims of this thesis was to dope cubic-YSZ with erbium ions for optical active materials. It is known in the literature that Er 3+ ions will substitute Y 3+ in the YSZ structure, as previously described. Luminescence in rare-earths is linked to the trivalent (+3) state and to the symmetry sites on the host structure [ 30 ]. Moreover, Er2O3 and Y2O3 have the same crystal structure and similar lattice constant. This similarity in crystal structure allows the incorporation of high concentrations of erbium ions in Y2O3 maintaining high crystallinity. When replacing an Er 3+ ion within the YSZ structure, it is done by an Y 3+ ion substitution as zirconium ion presents Zr +4 oxidation states, thus, Er 3+ and Y 3+ ions have similar ionic radii (1.00 Å and 1.02 Å, respectively) in comparison with Zr 4+ ionic radii (0.84 Å) [ 31 ]. It has been widely reported that cubic Y2O3 lattice has two different pointgroup sites for Er 3+ ions substitution. The unit cell has three sites with point-group symmetry C2 and one with point-group symmetry C3i. For Er 3+ ions in C2 sites, electric dipole transitions are allowed between the split of energy level lines by an external electric field called Stark levels. However, since C3i is an inversion center, the electric-dipole transitions are not theoretically allowed for Er 3+ ions occupying these sites. Despite, vibrations on Y2O3 lattice locally breaks C3i symmetry allowing forced electric-dipole transitions. In consequence, there are more photoluminescence intensity peaks from ions in C2 than in C3i sites. Besides, contribution from both C2 and C3i sites of erbium emission on the Y2O3 structure has only been evidenced in the infrared spectrum from the 4 I13/2 4 I15/2 transition. For the rest of the spectrum only emission at C2 site has been revealed [ 32 ]. It is known that excitation energy transfer among distanced ions provides enhancement of luminescence emission [29 33 ]. C3i centers are not isolated from the C2 ones since the two types are interconnected via a relative efficient energy transfer. It is demonstrated in the literature that the excitation energy flows from higher energy located metastable level of C3i that works as a donor to lower energy level of C2 which acts as acceptor. This phonon-assisted energy transfer based on Dexter's theory is reported in [ 34 , 35 , 36 , 37 ] Finally, there is a report between Er 3+ ion placement in the YSZ structure and its PL emission. Er 3+ placement in the YSZ structure and the oxygen vacancy distribution characteristic of this oxide, determines the emission spectra of these RE ions [ 38 ]. These RE ions in YSZ can be located at cation sites in eightfold (CN8), sevenfold (CN7) and sixfold (CN6) coordination. Moreover, the oxygen coordination around active dopants, in our case, Er 3+ , places the ideal site symmetry as Oh for ions in CN8 sites, C3v symmetry for ions in CN7 sites, and C3i /S6 or C2 in CN6 sites.

(c) Praseodymium

Praseodymium has 59 as atomic number with an electronic configuration of [Xe] 4f 3 6s 2 and belongs to the rare-earth elements in the periodic table. Praseodymium ions adopt both 3+ and 4+ oxidation states. Praseodymium has two emissions in the near-IR interesting for telecom applications: one emission transition at 1.3um in the O-band of telecommunications corresponding to the 1 G4 3 H5 transition, and a second one at 1.49µm in the E-band of telecommunications corresponding to the 1 D2 1 G4 transition. Pr 3+ absorption and emission transitions and wavelengths corresponding to the samples of this thesis can be found in table 2.5 (page 58). The dominant emissions of Pr 3+ come from the Pr-doped YSZ fluorescence 1 D2 level even if other emission lines from 3 P0 and 1 G4 states are also observed. In contrast to Erbium which present one state configuration Er 3+ , Praseodymium with two different states (Pr 3+ and Pr 4+ ) can substitute both Y 3+ and Zr 4+ cations of the YSZ structure. Besides, due to its smaller ionic radius closer to Y 3+ than to Zr 4+ , praseodymium ions which are found in Pr 3+ oxidation state, compensate the local oxygen charge in the non-centrosymmetric sites.

Regarding radiative emission of these ions, only trivalent state praseodymium ions (Pr 3+ ) in two different coordination oxygen-deficient sites (C2 and C3i) contribute to photoluminescence emission [ 41 ]. The 1/3 and 4/7 of the praseodymium in Pr6O11 and Pr7O12 in the fluorite like structure are in the trivalent state, respectively. Thus, as only non-centrosymmetric Pr 3+ ions contribute to luminescence, only the 40% of the total concentration contribute to optical emission. To understand the dynamics of excitation and de-excitation (figure 2.13 and 2.14), the 1 G4 level whose emission is at 1.3m can be populated via 1 D2 radiative transition or via cross relaxation within pairs of Pr 3+ . The population of the 1 D2 level comes from relaxation of higher energy levels 2 P1,2,0 and can then give either radiative emission to 1.5m or non-radiative decay to the 1 G4 level. The fluorescence emission starting at the 1 D2 state has very long lifetimes, up to 180 ms depending on the radiative path [ 42 ].

Growth of RE-doped thin films (i) Growth by PLD

Pulsed laser deposition is based on laser ablation of a target (figure 2.8) (page 10). Thin films in this thesis were grown using this deposition technique in order to obtain highly crystalline RE-doped oxides. Detailed description of samples and their structure schematics are found in table 2.4 (page 57). We used a KrF excimer laser of 248 nm wavelength with a fluence of 3 J/cm² at an incident 5 Hz repetition rate. When the fluence is high enough to ablate material from the commercial target (Toshima Manufacturing Co.), an ionized cloud is created and directed perpendicularly to the substrate surface. Typical deposition rates lie in the range of 0.001 to 1nm per pulse or count. In our samples, the oxide of choice is yttriastabilized zirconia (YSZ), and previous studies reported deposition rate of 0.13nm/s [ 43 ]. Besides 8 mol% yttria-stabilized zirconia sintered target, two more targets have been ablated: first one, a sintered target of erbium doped 8 mol% yttria-stabilized zirconia and second one, a metallic erbium target with a purity of 99.9% showing deposition rates of 0.133 and 0.184 nm/s, respectively. Sample growth and substrate preparation for all thin films in this thesis were performed in collaboration with Guillaume Marcaud and Thomas Maroutian. Target ablation was carried out in a high-vacuum stainless-steel chamber using rotating targets with an incident angle of 45° of the laser beam (figure 2.14). At 50mm from targets, the substrate is adhered to the holder by a silver dissolution for good thermal contact and heated at a temperature from room temperature to 800°C at a rate of 10 °C/min at an initial pressure of 10 -6 Torr in an oxygen atmosphere at 30 mTorr. After deposition, the substrate with the thin film is cooled at a rate of 10 °C/min and we increase pressure up to 75 Torr under oxygen atmosphere.

Crystalline substrates of sapphire cut-c (figure 2.15) have been prepared chemically and thermally before PLD deposition by following the optimized protocol of Marcaud et al. [ 43 ]. Based on this study, we chose to use the parameters of growth listed above (temperature, frequency, atmosphere pressure and fluence). Under the scope of this study it has been proved high mono-crystallinity in YSZ deposited on ccut sapphire after thermal treatment under a gas atmosphere [ 44 ]. Therefore, on a first step, we chemically cleaned with an ethanol and acetone solution the c-cut sapphire substrates, this treatment has been performed as well for amorphous substrates such as SiNx waveguides buried on SiO2 on a silicon wafer. In a second step, thermal treatment was performed for sapphire substrates under an oxygen atmosphere at 1200° for 12h with a heating/cooling rate of 8°C/min. Doping is a method in which impurities are introduced in a host material to modify its properties. As example, this technique is widely used in semiconductors to control their electrical properties. In this case, doping is carried out mainly by diffusion or ion implantation. Besides, other properties of materials as optical properties can be modified by doping the host. For this purpose, we used rare-earth (RE) photoluminescent elements to dope our photonic material. Despite diffusion and ion implantation are the most used methods for doping semiconductors to change electrical properties and can be used as well for RE doping [ 45 , 46 ], other techniques as sputtering [ 47 ], sol-gel synthesis [ 48 ], spray pyrolysis [ 49 ] or PLD [ 50 ] are commonly used for this kind of doping. While diffusion and ion implantation are techniques normally performed after the growth of the host, the rest of doping methods are carried out during growth of the host material.

(ii) Types of doped YSZ films grown for optical integration



Samples deposited by PLD in this thesis can be divided in two groups regarding the substrate in which they were deposited: sapphire substrate or siliconbased platform. Composition for each target is shown in table 2.1. The aim of the work on sapphire substrate is first to optimize and characterize doping of highly crystalline rare-earth doped YSZ thin films to study their optical and structural properties. After the growth and doping process were optimized, thin film has to be implemented on a silicon-based device to study optical amplification in the near-IR wavelengths for both (Er 3+ and Pr 3+ ) rare-earth ions.

(a)

Thin films deposited on a sapphire substrate. Three different deposition approaches for the growth of thin films on sapphire have been taken (figure 2.16):



Monolayer of rare-earth doped YSZ from a sintered target. For these thin films, sintered Er-doped YSZ targets containing 0.5, 1.5, 3, 5 and 10 at.% of Er 3+ ions (Samples 5, 6, 7, 8 and 9) and a target of sintered 0.5 at.% Pr-doped YSZ (Sample 10) have been used. Moreover, 8 %mol yttrium has been used to preserve YSZ cubic structure in all targets.



Multilayer of rare-earth doped YSZ from a metallic target and a sintered Er 3+ target (Samples 1, 2, 3, 4, 13, 14 and 15). In this approach we used an erbium metallic target with a purity >99% and either a YSZ or an Er-doped YSZ target, both sintered. Multistack deposition starts with a first layer of YSZ (doped with Er 3+ , or undoped), followed by a deposition of Er -rich layer. With this method two layers are created with the same YSZ structure: one is undoped YSZ or doped-YSZ and the second one is an Er-rich doped YSZ by using the Er 3+ metallic target (see figure 2.17). By alternating these two types of layers, we can grow thick stacks with modulation of the RE-doping distribution within the thickness.



Multilayer of Er-and Pr-doped YSZ from sintered targets (Sample 12). Multistacks of two different RE for the same host aims controlled co-doping. Praseodymium and erbium ions can be excited at similar wavelength at around 970nm and the decay of both rare-earth ions lies in wavelengths within the near-IR, opening to multiple options in the telecom range of photonics. Primarily, integrated optical amplification was the aim of the rare-earth implantation in our oxide thin film. Therefore, after optically characterizing our samples, we chose the approach with better results in luminescent properties to implement the oxide host on a passive device. (ii). Thin films deposited on an amorphous substrate. After studying the different techniques presented above to increment the doping percentage while avoiding quenching effect and minimizing absorption cross-section, we found the best approach to be the growth of multilayers doped YSZ with Er-rich alternated with layers of 0.5 at. % sintered Er:YSZ. This structure has then been implemented as active layer on a SiNx waveguide on a silicon-based platform, by growing the oxide stacks by PLD on the amorphous substrate.

Structural characterization

Once samples were grown by PLD, structural characterization is required to access the crystalline quality and control the material engineering. Specific information about thin film thickness, atomic structure, and chemical composition is crucial to understand and control deposition of such complex oxides on particular substrates for photonic applications. In this thesis, we structurally characterized our samples by means of spectroscopic ellipsometry, X-ray diffraction, AFM and STEM microscopy.

Spectroscopic ellipsometry measurements

In order to control RE-doped YSZ thin film thickness, we measured the deposition rate and thicknesses for different samples. In a first step, a counted number of 10 000 laser counts deposited a thin film of the target material by PLD. On a second step, we measured by ellipsometry the thickness of the grown thin film.

As result, we obtained very similar growth rates for all at. % doped Er 3+ -doped (samples 5  , 6 * , 7 * , 8 * and 9 * ) and 0.5 at.% Pr-doped YSZ (sample 10) thin films, grown under the same deposition conditions. Monolayer Er 3+ -doped thin films had a measured thickness of 149 nm (figure 2.20 (a)) after 5 000 counts or laser counts to the target, then we calculate a growth rate of 33.55 counts/nm. On a second deposition of thin films of 0.5 at.%. Er-doped YSZ (samples 5, 6, 7, 8 and 9), 313 nm (figure 2.20 (a)) was measured for 10 000 counts and a growth rate of 31.94 counts/nm was obtained. Very similar results under the same procedure were obtained for Pr-doped YSZ thin films. Finally, we stated the growth rate to be 32.72±0.83 and 32.7±0.79 counts/nm for all sintered targets of Er:YSZ and Pr:YSZ.

When Er is deposited under an oxygen atmosphere, Er2O3 is formed. We grew a bilayer with the same number of PLD counts of YSZ and Er2O3 on sapphire to calculate both growth rates (table 2.2).

 Same composition (Er:YSZ), doping (0.5, 1.5, 3, 5, and 10 at.%) and number of layers (1, monolayer) than samples 5, 6, 7, 8 and 9 with total thickness of 149 nm.

Thus, we grew a first thin film of YSZ on sapphire with 3 000 counts of PLD laser on a YSZ target and a second one with 2 000 counts of the metallic Er 3+ target (sample 11). Ellipsometry measured thickness was 112.74 nm and 73.94 nm, respectively. Therefore, the growth rate of YSZ and Er2O3 was calculated of 26.3±0.3 and 27.47±0.43 counts/nm, respectively. For greater clarity, growth rate for all targets are summarized in table 2.

X-ray diffraction (XRD) measurements

After deposition of the thin films by PLD, high crystallinity was expected for samples grown on a c-cut sapphire substrate. Accordingly, X-ray diffraction was performed at C2N in collaboration with Thomas Maroutian, with a step of 0.08°, counting time of 30 s/step and 2θ range 20-90° for all the samples. These structures were measured by an x-ray diffractometer model X'Pert Pro from PANalytical. C-cut sapphire (Al2O3) substrate has as lattice constants a= 4.78 Å and c=12.9 Å. Following Bragg's law for hexagonal system, it diffracts at the family of planes (0006) corresponding to 42° in the 2θ diffraction scan.

 Thin films grown on sapphire substrate (a) Monolayer Er:YSZ X-ray diffraction for 300 nm Er-doped YSZ monolayers grown on annealed sapphire substrate under oxygen atmosphere (samples 5, 6, 7, 8 and 9), reveals two diffraction peaks at 30° and 35° in the 2 scan, corresponding to the (111) and (002) YSZ family of planes, respectively. In figure 2.21, can be observed for YSZ thin film a very intense diffraction for the (002) crystal plane and a low peak intensity for the (111) plane. Diffraction peak for (111) orientation increases with the erbium percentage in the YSZ structure. Meanwhile, (002) XRD intensity peak remain invariable in intensity for all the samples independently of its Er 3+ percentage. Thus, XRD reveals a quasi-monocrystalline structure in the (002) direction for YSZ thin film (sample 0), although incorporation of increasing percentage of Er 3+ ions in the matrix, introduces a rising formation of domains in the (111) direction in doped YSZ thin films. In the XRD spectra in figure 2.21, we also observe the peak at 42° corresponding to the (0006) sapphire plane orientation. , substrate annealing under nitrogen or oxygen atmosphere determines different growth orientation of YSZ thin films. Therefore, following this study we grow alternating thin film Er-rich and Er 3+ -doped YSZ multilayers (samples 1, 2, 3, 4, 12,13, 14 and 15) by changing the ratio between counts on Er 3+ and YSZ targets (and the total number of layers) on sapphire substrates annealed under nitrogen and/or oxygen. All multilayer thin film samples studied had a total thickness of around 300nm and present two different monocrystalline orientations: (111) for thin films with substrate annealed under nitrogen atmosphere and (002) for those annealed in oxygen atmosphere. In figure 2.22 in red, it can be observed the diffraction peak at 30° corresponding to the (111) orientation, for sample 14 with 22.5 nm of sintered Er:YSZ and 1.95 nm of Er-rich multilayers and 24 total number of layers. A second study was performed on sapphire substrate annealed in oxygen in which we grow a similar multistack structure than in the previous experiment. For this multilayer (sample 15), 1.95 nm Er-rich YSZ layers alternates with 13.5 nm Er-doped YSZ layer and at total number of layers of 40. As it can be observed in black RXD profile of figure 2.22, in XRD, only one orientation at 35° corresponding to the (002) family of planes of a monocrystalline structure is revealed. It can be observed that above 40 layers for 300 nm of thin film grown on sapphire substrate annealed under oxygen atmosphere, thin films losses its monocrystallinity. Two orientations are diffracted at 30 and 35° as observed in figure 2.23 spectrum corresponding to a multilayer structure of YSZ layers with a period of 2.07 nm and 1.35 nm for layer 2 and layer 1 respectively and a total number of layers of 160 (sample 4). Multilayer crystalline structures are prone to defects in the crystalline lattice due to strain gradients arising from the different composition of the layers [ 51 ]. For this study can be concluded that samples with thicker YSZ periodic layers have a number of total layers in the thin film lower than those with thinner YSZ periodic layers. Hence an increase of strains and number of defects can be found on samples with thinner and numerous YSZ periodic layers in the thin film, which could explain the appearance of two growth orientations. FIGURE 2.23 -X-ray diffraction profile of Er:YSZ thin film grown on annealed sapphire substrate under oxygen atmosphere with multilayer structure in which the distance between two Er-rich YSZ layers is total number of 180 layers for 275 nm thickness of thin film and proves two diffraction orientations at 30° and 35°.

As seen by XRD in figure 2.2, 2.24, 2.25 and 2.26, two main YSZ orientations (111) and (002) at 30° and 35° are observed in the profiles with variations of maximum ±2°. Moreover, in all XRD spectra of thin films grown on sapphire we could not observe parasitic phases. We performed XRD from 20 to 85° for all samples but in this thesis we showed X-Ray diffraction up to 42° (in which sapphire diffraction is found) for clarification purposes. As an example of the YSZ crystal orientation, in figure 2.24, can be observed the whole X-ray diffraction of multilayer sample 1 with only the presence of cubic YSZ phase and sapphire. Hence, we can determine an average experimental lattice parameter of all the structures of a= 5.15±0.04 Å. The experimental lattice constant was calculated using eq. 2.12 and table 2.3 for cubic structure and is coherent with same results found in the literature [ 52 ].

 Annealed thin films grown on sapphire substrate Multiple studies in the literature suggested an increase of photoluminescence with the annealing of the crystalline structure. Therefore, we annealed all our Er:YSZ thin film samples for 11h at 3 temperatures: 800°, 1000° and 1200°. For samples annealed at 800°C we could not observe by XRD any significant structural difference.

Besides, before and after annealing at 1000°, for monolayer 0.5 at. % Pr 3+ -doped YSZ thin film (sample 10), two diffraction orientations have been observed, the (111) and (001) family planes of YSZ (figure 2.25 (c),(d)) without significant structural XRD changes before and after annealing. Regarding diffraction peak for the (111) orientation for annealed Pr-doped YSZ samples, a slight shift towards small angles (29.7°) could be noticed in figure 2.26 (a) XRD profile when compared with the same diffraction angle for non-annealed Er-doped YSZ samples (30°) observed in figure 2.26. This shift is often related to variations on the crystallographic lattice plane distances. Besides, for the (002) orientation, shoulder on the 35° diffraction peak has been found in non-annealed samples of Pr:YSZ (sample 10). In addition, it has been observed that the shouldering of the diffraction peak at 35° for Er-doped YSZ samples (sample 5) is oriented towards higher angles, while it is oriented, towards lower angles for Pr-doped YSZ (figure 2 We noted in figure 2.27 (red arrow) for multilayer structures (samples 1, 2, 3 and 4), a shoulder on the 35° diffraction peak. It has been largely reported in the literature that a FWHM broadening or shoulders of a diffraction peak reveals structural defaults such as microstrains or stacking faults in the crystalline structure in the diffracted family of planes. In our multilayer Er:YSZ samples, the characteristic shoulder of the diffraction peak is related to the presence of planar defaults in the crystalline structure as observed for the (002) orientation. For samples annealed at temperatures higher than 1100°, the thin film cracked and was detached from the substrate in thin pellets.

 Multilayer thin film grown on amorphous substrate

We first deposited by PLD a 150 nm multilayer structure of Er-rich YSZ and sintered Er-doped YSZ thin films (sample 13) on a SiNx waveguide under the same conditions than on sapphire substrate. Then, we structurally characterized the sample by x-ray diffraction. SiNx waveguides buried on SiO2 is an amorphous substrate, hence, polycrystalline structure is expected by X-ray diffraction. In order to have more signal than using other diffractometers, we performed x-ray diffraction in non-coplanar grazing incidence at an incidence angle of =5° (GIXRD-Grazing Incidence X-Ray Diffraction) with the diffractometer XPert with the collaboration of Olivia Mauguin and Dr. Thomas Maroutian. In figure 2.28 can be observed a highly polycrystalline profile. However, a diffraction peak at 43° and a less intense one at 80° are observed in the GIXRD profile. First diffraction peak corresponds to the (102) orientation of the tetragonal structure and the second one to the (331) cubic family plane found in the literature for 8 mol% YSZ crystalline powders. Thus, for multilayer samples on amorphous substrates two different phases of YSZ are grown: cubic and tetragonal, in which the tetragonal phase gives the strongest diffracted signal [ 53 , 54 , 55 , 56 ]. FIGURE 2.28 -Grazing incidence X-ray diffraction of an Er:YSZ multilayer sample, diffraction peaks intensity can be observed at 43° and 80°.

Atomic Force Microscopy (AFM) measurements

Images obtained during this thesis of the substates and thin films surfaces were made by an AFM model Innova from Brucker (using the program WSxM 5.0 Develop 9.0 for images analysis) in contact mode [ 57 ]. In figure 2.29 we studied AFM profiles for different surfaces of C-cut sapphire substrates after chemical and thermal treatment under oxygen atmosphere before PLD deposition. As expected, well-defined terrace-step structures characteristic of sapphire substrate annealed in an oxygen atmosphere are observed. During studies of temperature and oxygen pressure searching for well-defined sapphire substrate surface after annealing, we found out that substrate annealing at different temperature rates and oxygen pressures leads to highly inhomogeneous and overlap step structures on the sapphire substrate. This inhomogeneity can be observed also in figure 2.29 in this substrate, oxygen flux was decreased during annealing. In figure 2.30, AFM profile of c-cut sapphire clean and annealed under the described protocol in Marcaud et al. [ 43 ] is shown, in which homogeneous and clear-cut step structure is observed before PLD deposition. AFM analysis gives an average root mean square (rms) roughness of 4 Å for the surface after deposition of an erbiumdoped YSZ thin film on the substrate, which is typical of the YSZ roughness for all the thin films we grew on sapphire. In the other hand, for thin films deposited on amorphous substrate, AFM was not performed due to the complex topography of the chip which includes various photonic devices.

Scanning Transmission Electron Microscopy (STEM) measurements.

In the HAADF-STEM image in figure 2.31 a cross-section of a well-defined multilayer Er-doped YSZ (sample 4) on a sapphire substrate can be observed. In the multilayer structure we observe one wide dark (figure 2.31 (a)) and light (figure 2.31 (b)) fringe due to a double thickness of 0.5% erbium-doped YSZ for the first case and 5 at. % of Er-rich YSZ on the second one. We measured the thickness of the active layer, being 322 nm. On the bottom of the image, sapphire substrate appears in a dark contrast and on the top of the image a granulose structure corresponds to the protective layer of carbon deposited before FIB polishing. Moreover, in figure 2.31 it can be observed in the whole surface of the sample crosssection different domains, revealed by contrasts on the image. Multigrain multilayer structure with two orientations at (111) and (001) were expected from XRD analysis and widely reported in literature for this kind of oxide [ 58 ]. Horizontal periodic fringes on the host (YSZ) with light-contrast are due to the amount of dopant (Er 3+ ) in each layer. As stated before, HAADF is a technique in which contrast of the image is based on the atomic number of the elements forming the sample. Elements composing our thin layer are oxygen (Z=8), yttrium (Z=39), zirconium (Z=40) and erbium (Z=68), thus the light contrast corresponds to a higher percentage of an element with high atomic number within the structure, in our case, erbium. Can be observed in figure 2.31 (b) one layer with 0.5% Er 3+ -doped YSZ, in this layer, the percentage of erbium is not representative enough to be detected in the image, thus contrast of the image remains dark. Oppositely, in figure 2.31 (c) high contrast layer can be observed corresponding to a YSZ layer with high percentage of erbium in the structure. In augmented HAADF-STEM image figure 2.32, the periodic contrast of the fringes better appears and can be measured. A contrast profile in white line is shown, for 45 nm-thick layer analysis, in which maximum and minimum of contrast correspond to higher and lower percentage of erbium doping the YSZ structure. Errich layers of sample 4 (appearing in bright contrast) has a measured thickness of 1.35 nm by STEM, corresponding to 5 at. % of Er 3+ in the YSZ structure (percentage measured by EDX) obtained by 3 laser pulses or counts of metallic erbium target during the growth of the multilayer. On the other hand, 0.5 at. % doped YSZ layers (from laser pulses on the 0.5 at. % Er-doped YSZ target) have a measured thickness of 2.22 nm (despite theoretical thickness value in table 2.4 is 2.07 nm). EDX mapping of sample 4 in figure 2.35, shows the distribution of Er:YSZ elements along the thickness of the thin film cross-section. In violet we observe the periodic distribution of erbium in the sample, with a highly colored layer with a higher erbium at. % than in the rest of the film. The colored fringes complement the black and white HAADF image corresponding to the layers with higher amount of erbium percentage as expected. The zirconium, oxygen and yttrium EDX maps, in red, blue and green, respectively prove homogeneous distribution of these elements consistent with a YSZ crystalline thin film. For a further understanding of the elements distribution in the sample, profile of EDX map has been performed. In a first step in figure 2.36 we represent the profile for the HAADF image contrast for the multistack structure (figure 2.36). In a second step, we proceeded to compare this contrast profile with the profile of the composition of erbium, oxygen, zirconium and yttrium shown in figure 2.36 left image, violet profile corresponds to the variation on erbium in the structure, as commented, maximums in erbium corresponds to brighter contrast in the HAADF image, and minimum with low or no amounts of erbium in the darker fringes. For zirconium, in red, maximum periodic peaks in the profile are found for dark contrast layers in which erbium is present in low percentage, otherwise, low percentage of zirconium arises in highly doped stacks, hinting a substitution of erbium by zirconium. In figure 2.36 right, for both profiles of oxygen and yttrium in blue and green, respectively, homogeneity in distribution is observed. For further analysis of the chemical composition, we performed EDX profile of normalized atomic percentage for each element (erbium, oxygen, zirconium and yttrium) in the thin layer in figure 2.37 (left image) for the four elements in the Erdoped YSZ structure. This analysis reveals an oxygen atomic percentage of 67.2-71.3, 23-26.6 for zirconium, 5 for yttrium and in the range of 0.5 and 5 at.% for Er 3+ . Then, we normalized in figure 2.37 right image, the total atomic percentage of both zirconium and erbium measured at the same points, obtaining the same results in which lowest atomic percentages of zirconium corresponds to maximum doping of erbium in YSZ. This observation seems to indicate that Er 3+ substitute zirconium instead of yttrium in the Er-rich structure. To answer this question, in figure 2.38, we have analyzed the atomic percentage of the zirconium to yttrium ratio (Zr/Y) along the sample's cross-section. For Zr/Y rates higher than 1, yttrium decrease in atomic percentage and for rates lower than 1, zirconium decreases in atomic percentage. First option proves erbium substituting yttrium, as expected and observed in 0.5 at. % Er 3+ layers, having both elements the same oxidation state. For the second option, zirconium decreases because erbium is taking both Y 3+ and Zr 4+ sites despite the mismatch on oxidation states with Zr 4+ . In figure 2.38, we observe that those peaks corresponding to Zr 4+ /Y 3+ ratios higher than 1 in which yttrium is substituted by erbium, the erbium profile matches the minimum doping of 0.5 at. % corresponding to Er: YSZ. On the other hand, for Zr 4+ /Y 3+ ratios lower than 1, in which Er 3+ would be taking the Zr 4+ site, in Er-rich layers. Being oxidation states of zirconium, (4+) and of erbium and yttrium, (3+), when erbium (oxidation state +3) is taking the site of zirconium, oxygen percentage should decrease as oxygen vacancies are related to oxidation state [ 59 ]. Thus, for oxidation state 3+, oxygen vacancies should be higher than for elements with oxidation state 4+. In this regard, in figure 2.39, we indeed measured a minimum of oxygen atomic percentage for lower values of zirconium corresponding to higher doping of Er-rich taking the site of zirconium.

Conclusions

Growth orientation for multistack thin film with Er-rich YSZ layers can be controlled by substrate treatment when the total number of layers for thin films of 300 nm thickness is higher than 40. For lower thicknesses, two crystalline orientations appear corresponding to both (002) and (111) orientations.

From XRD performed on-coplanar grazing incidence (GIXRD-Grazing Incidence X-Ray Diffraction) with the diffractometer Rigaku Smartlab to thin films grown on amorphous substrate, polycrystalline x-ray diffraction profile is observed: two diffraction peaks at 43° and at 80° corresponding to the (102) orientation of the tetragonal structure and to the cubic one with (331) orientation, respectively. So when YSZ is grown on amorphous substrate, two different phases of YSZ are obtained: cubic and tetragonal.

EDX profile and mapping of multilayers introducing Er 3+ in the YSZ structure proved substitution of Er 3+ in Zr 4+ and Y 3+ sites on Er-rich layers. In photoluminescence characterization, RE ions are excited to an upper state by a laser emitting at an absorption wavelength. When electrons de-excite to the fundamental state, photons are emitted at a characteristic wavelength depending on the material energy level. By means of micro-PL we performed studies of the emission profile in a wide wavelengths range of our deposited material, including studies of optimal absorption wavelength of the material, power efficiency and lifetime which all allowed a deeper understanding of the material emission and structure. Indeed, PL characterization enabled to determine the optimum RE emission. Being the aim of this study to investigate RE-doped oxides as photonic materials, studies on lifetime were also included to comprehend the inversion population of our thin films and its adequacy for optical gain and lasing purposes.

In cathodoluminescence characterization, electrons are injected by an electron beam in the valence band of the material, those electrons transfer their energy to other electrons which then jump to the conduction band if the incident electrons have high enough energy to overcome the bandgap of the material. Therefore, RE upper states are populated with electrons which radiatively decay to fundamental states resulting in light emission at characteristic wavelengths. In this regard, RE transitions in the visible range of wavelengths have been studied by cathodoluminescence SEM and TEM, providing TEM approach high spatial resolution for emission analysis.

Photoluminescence (PL)

Following deposition by PLD and after performing structural characterization to prove high quality of the crystalline structure in the thin film, we performed optical characterization of our samples. This kind of characterization on rare-earth doped hosts aims multiple purposes: to find the optimal pumping wavelength to excite the rare-earth imbedded on the host material; to explore the most efficient RE concentration of dopants to avoid quenching effects; to achieve high photoluminescence intensity and to determine the lifetime of the rare-earth in the YSZ matrix. These measurements were performed by photoluminescence (PL) on a free-space optical bench using a normal incident scheme, as shown in figure 3.1. Continuous wave (CW) from tunable Ti:sapphire laser (600-1000 nm) in perpendicular incidence to the surface of the sample using a microscope objective with a magnification of 50x and NA of 0.55 which generates an excitation beam with a radius of ∼1.5μm. Emission and probe signals are perpendicularly collected by the same microscope objective after excitation and focused by a convergent lens of 50 cm focal length to the detector. Laser wavelength is filtered with a high-pass filter (1100 nm) and only emission in near-IR is detected on the iHR320 spectrometer provided with a nitrogen-cooled-detector. In our work we are interested in the emission range of wavelengths in the near-IR (750-2500 nm) specifically in the O-band (1250-1350 nm) and the C-band (1530-1565 nm) of the telecommunication window. In this direction, we were interested to study two RE emission transitions 1 G4→ 3 H5 and 4 I13/2→ 4 I15/2 corresponding to wavelengths of 1345 nm and 1530 nm from praseodymium and erbium ions, respectively [ 1 , 2 ]. Typical absorption wavelengths which lead to these emission transitions are 960 nm for praseodymium [ 1 ] and 800 or 980 nm for erbium ions [ 2 ]. In table 3.1 we summarize absorption and emission transitions and wavelengths measured for all our samples. TABLE -3.1. Absorption and near-IR emission transitions and wavelengths for deposited samples on sapphire. In bold wavelengths for larger absorption transitions.

Er 3+ -doped YSZ 3.1.1.1. Wavelength absorption

The first step towards rare-earth ions optical characterization is to determine the optimal excitation wavelength to obtain the highest intensity of PL emission of the desired transition. For this purpose, we used a tunable Ti: sapphire laser at a fixed pumping power. We excite our thin film at a range of wavelengths around the absorption peak. We excited Er 3+ -doped YSZ thin film for a range of wavelengths from 800 to 980 nm at 40 mW, as observed in figure 3.2. As a result, we obtained the characteristic transition intensity profiles for the rare-earth emission for each pumping wavelength as shown in figure 3.2 for Er 3+ -doped YSZ (sample 5) thin film. Black circles show maximum intensities at 1530 nm which is the wavelength emission under study. Finally, to obtain a PL absorption profile (left graph with red axes in figure 3.2), we projected all the emission intensities at a wavelength of study (here 1530 nm), for different excitation wavelengths. Results in figure 3.3 regarding absorption wavelengths for Er 3+ -doped YSZ excited from 750 to 1000 nm reveal two absorption transitions at 800 and 960 nm corresponding to the 4 I15/2→ 4 I9/2 and 4 I15/2→ 4 I11/2 transitions of Er 3+ ions, respectively. In figure 3.3, we performed wavelength excitation study for 4 samples at two different emission wavelengths (1530 and 1535 nm) corresponding to the same Er 3+ transition ( 4 I13/2→ 4 I15/2): (i) Er2O3 monolayer (sample 11) with PL emission at 1535 nm, (ii) 0.5 at. % Er 3+ -doped YSZ monolayer (sample 5) with emission at 1530 nm, (iii) a multilayer structure with Er-rich YSZ layers (sample 4) emitting at 1535 nm and (iv) a multilayer structure with Er-rich and sintered Er 3+ -doped YSZ layers with emission at 1530 nm (sample 3), all of them grown on sapphire. In this work, we wanted in particular to explore resemblances between absorption wavelengths coming from Er 3+ emission at 1535 nm in Er2O3 (sample 11) and same emission wavelength (1535 nm) from Stark effect from Er-rich oxide samples (sample 4). As we observe in Fig. 3.3., two Er 3+ ion absorption transitions are present in all the samples ( 4 I15/2→ 4 I11/2 and 4 I15/2→ 4 I11/2) as expected. For Er2O3 (sample 11 in yellow) the same transition is shifted at 980 nm with a moderate second intensity peak at ~800 nm while for Er 3+doped YSZ monolayer samples (sample 5 in red) an intensity peak at 960 is observed. Both Er-rich YSZ multilayer samples (samples 3 and 4) reveal two different absorption spectrum. A homogeneously distributed absorption profile is observed corresponding to the 4 I15/2→ 4 I11/2 transition with a broaden peak at 960-980 nm (observed everywhere on the sample, figure 3.3 in blue shown for sample 4). Another absorption profile corresponding to local enhanced intensity due to the Stark effect is also observed (figure 3.3 in green, shown for sample 3) with the main absorption 4 I15/2→ 4 I9/2 transition corresponding to a wavelength of 800 nm and a second absorption peak at 850 nm. Micro-PL spectra for each sample at a pumping power of 100 mW and at its highest absorption wavelength revealed three different profiles for the same Er 3+ ion Chapter 3 90 4 I15/2→ 4 I11/2 transition, as shown in figure 3.4. Er-doped YSZ monolayer (sample 5) PL spectrum, shows a single emission peak at 1530 nm similar to the non-Stark effect PL spectrum for sample 4, shown in blue in figure 3.4. Differently, for Er2O3 thin film (sample 11), characteristic PL profile of multiple peaks with a higher intensity at 1535nm is observed in figure 3.4, corresponding to the sites in which Er 3+ is placed within the Er2O3 structure and to oxygen distribution around Er 3+ ions [4]. For Errich multilayer samples (sample 4), we have observed two different Er 3+ emission profiles for the 4 I15/2→ 4 I11/2 transition as shown in figure 3.4. First profile corresponds to local areas of 0.25mm 2 which present an enhanced luminescence and splitting Stark effect with main emission peaks at both 1530 and 1535 nm wavelength (figure 3.4 in red) and a second spectrum homogeneously distributed on the thin film corresponding to the same Er 3+ 4 I15/2→ 4 I11/2 transition, at 1530 nm (figure 3.4 in blue).

Local PL spectra for Er-rich and sintered Er:YSZ multilayer sample (sample 4) might thus appear as PL superposition spectrum of a multilayer thin film with two layers: Er:YSZ and Er2O3. Emission at 1530 and 1535 nm could correspond to a superposition of Er:YSZ and Er2O3, respectively.

To test this hypothesis, we have grown Er-rich YSZ multilayers (sample 3). If Er2O3 is grown in the multistack structure, we will expect a PL profile with an emission peak at 1535 nm coming from the Er2O3 structure and no emission at 1530 nm wavelength. PL results from sample 3 proved the same two emission spectra as for sample 4. Analysis of figure 3.5 proves same PL spectrum for multilayer samples with Er:YSZ (in red) and either YSZ (in white) or Er-rich layers (in green) (samples 3 and 4, respectively). PL spectrum profiles, demonstrate that the enhanced Stark splitting emission, is not the result of superposition of two PL emission spectrum from Er:YSZ with the presence of Er2O3 . Moreover, wavelength absorption study (figure 3.3) for samples 3 and 4 shows same PL absorption profile for both samples and different PL spectrum for Er2O3 (sample 11). Thus, PL emission from multilayer samples 3 and 4 at 1530 and 1535 nm might have its origin in the sites where Er 3+ is placed in the YSZ structure rather than having a multilayer of two different materials (Er2O3 and Er:YSZ). Finally, structural characterization of samples by XRD and STEM in Chapter 2, confirmed this first explanation because no presence of other oxide (such as Er2O3) than YSZ was revealed. It should be noted that the growth by PLD is non-homogeneous over the whole sample surface. Using this deposition technique, our oxide material is deposited from a target to the substrate through a plasma plume. The projection of the material on the substrate surface creates a gradient of thicknesses with a maximum thickness when the plume is perfectly perpendicular to the substrate. In figure 3.6, we observe a color contrast with the form of the plume projection, each contrast is given by a different deposition thickness. Besides, the energy deposition of the material changes with the incidence direction [ 3 ], having its maximum when the incidence of deposition plume is perpendicular to the substrate. This variation in deposition conditions could contribute to inhomogeneity regarding PL profile on a same sample, but would probably not explain why very local areas on Er-doped YSZ thin films (using metallic Er target) show Stark effect PL spectrum.

Power efficiency

After determining the optimal absorption wavelength of each sample, we studied by PL the most optically efficient material. Rare-earth photoluminescence efficiency aims to reveal the RE concentration which provides the highest luminescence emission keeping quenching effects to a minimum. Thus, maximum efficiency is achieved when one photon from pump laser beam is absorbed by one RE ion and at its turn it is radiatively de-excited emitting one photon at the desired RE transition wavelength. Ideally, when increasing the pumping power, a higher number of RE ions are excited until saturation occurs. If quenching effect or other energy transfer mechanisms occur, the ratio of one input laser photon exciting one RE ion and the output of radiative emission will be lower than 1. For this study, using the described set up in figure 3.1., we excited each sample at its optimal absorption wavelength changing the pumping power parameters. Subsequently, the intensity of the studied emission peak was analyzed for each increment of pumping power for all samples. i) Er 3+ -doped YSZ monolayer (samples 5, 6, 7, 8 and 9) When exciting at 960 nm and 100mW Er-doped YSZ thin monolayer samples (samples 5, 6, 7, 8 and 9) (table 3.2) by PL, with Er 3+ ion doping percentage of 0.5, 1.5, 3, 5 and 10 at. % respectively, a characteristic Er 3+ emission spectrum can be observed in figure 3.7 with a maximum peak intensity at 1530 nm. Variations on emission peak intensity for different Er 3+ concentration depends on the number of optically active ions and the distance between Er 3+ nuclei. Thus, increasing the percentage of RE ions enhances PL intensity as it rises the number of emitting ions. However, when distance between ions is short enough to favor interaction among them, quenching effects arise, hindering PL. It can be observed in inset image in figure 3.7, that higher emission intensity is achieved for an Er 3+ concentration of 1.5 at. % (sample 6). In one hand, for RE concentrations below 1.5 at. % (sample 6), emission intensity is lower because there is a lower number of active ions. However, for concentrations higher than 1.5 at. % (sample 6), ions are close easing ion-ion interaction and reducing the emission intensity. This result is observed in inset of figure 3.7 illustrating the decrease of emission intensity for Er 3+ ion concentration higher than 1.5 at. % (samples 7, 8 and 9). Hence, 1.5 at. % (sample 6) is the most effective doping concentration of Er 3+ ions within the YSZ matrix in terms of near-IR luminescence. Efficiency studies of RE-doped thin films have been performed to measure the evolution of the collected PL intensity peak at 1530 nm as a function of the incident increasing pumping power and are reported in figure 3.8. In figure 3.8, we normalized all PL intensity at 1530 nm by the minimum value (for 1 mW pumping power) in order to compare efficiencies for all samples. Efficiency is defined in this study as the slope between input increasing pumping power and output intensity emission. Hence, grey pointed line with slope 1, reports the rate between the number of emitting Er 3+ ions at 1530 nm and the number of incident photons, denoting an ideal regime where no Er 3+ -Er 3+ interaction takes place. Figure 3.8 shows the relationship between increasing pumping power with the output emission intensity of the 1530 nm transition wavelength for samples 5, 6, 7, 8 and 9 with different doping concentrations. As it can be observed in figure 3.8, the most efficient concentration of Er 3+ doping corresponds to 0.5 at. % (sample 5) with a slope around 0.96. Efficiency decreases with increasing doping concentrations of 1.5, 3, 5 and 10 at. % (samples 6, 7, 8 and 9, respectively) due to higher concentration of RE on the YSZ matrix, thus higher ion-ion interactions leading to quenching effect. Moreover, 3, 5 and 10 at. % Er-doped YSZ thin films (samples 7, 8 and 9) show a slight saturation of the PL intensity under pumping powers around 140 mW. Saturation is observed by hindered non-linear increase of PL emission with pumping power increase which has been attributed to high population of excited state. When pumping power excites all Er 3+ nuclei, emission remains constant despite the increase of input power.

To conclude, we demonstrated in this study that doping concentration of 1.5 at. % Erdoped YSZ monolayer (sample 6) proves the highest total PL emission at 1530 nm for monolayer samples. Total PL emission is related to the output intensity coming from each de-excited Er 3+ nuclei. Hence, in absence of quenching effects, photoluminescence must increase with at. %. In this work, we proved hindering of luminescence due to quenching above 1.5 at. % in Er:YSZ monolayers. Higher efficiency was found for 0.5 at. % Er-doped YSZ monolayer (sample 5). Optical efficiency studies search to evaluate the ratio between the input photons from pumping laser and output photons from Er 3+ emission. High efficient luminescent materials are related to low quenching effects attributed to low inter-ion interaction. Quenching effects are observed with increasing Er 3+ ion concentration. Thus, despite quenching effects are higher for 1.5 at. % (sample 6) than for 0.5 at. % Er-doped YSZ (sample 5), a higher number of active ions for the 1.5 at. % concentration provides a higher emission intensity.

ii) Er-doped YSZ multilayer (samples 1, 2, 3 and 4)

As mentioned previously, Er-rich YSZ multilayer film (samples 1, 2, 3 and 4)) pumped at 960 and 800 nm provides two PL emission profiles for 4 I13/2→ 4 I15/2 Er 3+ ion transition: one with emission at 1530 nm and a second profile locally distributed on the thin film which presents enhanced multiple emission intensities due to Stark effect. In order to further understand the emission and efficiency of these multilayer samples we have investigated both PL profiles. We have grown by PLD multilayer thin films (table 3.3) with the same total thicknesses, composed by two kinds of layers using 3 different PLD targets: (i) metallic Er 3+ , (ii) sintered 0.5 at. % Er-doped YSZ and (iii) sintered YSZ. Composition, Er 3+ doping at. %, number and thickness of layers is shown in table 3.3. All samples integrate layers of Er-rich YSZ (layer1), while the second type of layer can be either undoped YSZ (sample 1 and sample 3) or sintered Er-doped YSZ (sample 2 and sample 4). After PLD deposition, we explored the role played by the thickness of the layers and their composition in the optical efficiency of the material.

a) Er 3+ ion emission at 1530 nm (e= 960 nm)

In figure 3.9, we obtained a high intensity emission peak at 1530 nm and a low broad emission at 1574 nm for each sample when exciting the thin film by pumping at a wavelength of 960 nm at a power of 40mW. These results prove the highest PL intensity at 1530 nm for multilayer sample 4, followed by sample 5 and sample 2. Both sample 2 and 4 implement layers of 0.5 at. % Er-doped YSZ in their structure. Sample 4 shows higher intensity at 1530 nm than sample 2 due to a thicker layer of 0.5 at. % Er-doped YSZ in the thin film. Sample 3 presents lower PL emission than samples 2, and 4, because its layer 2 is undoped YSZ. Finally, sample 1 with 8.5 at. % Er 3+ doping in layer 1 and undoped layer 2 presents the lowest PL intensities, lower than sample 3 (with 5 at. % Er 3+ doping in layer 1 and undoped layer 2) because quenching effects at with higher percentage of doping hinders luminescence. It is worth noting that the PL intensities of sample 5 (monolayers of 0.5 at. % Er-doped YSZ) are between the ones of sample 4 and sample 2. The efficiency analysis for emission at same excitation wavelength of 960 nm and increasing pumping power (figure 3.10) measures a higher efficiency for the 0.5 at. % Er-doped YSZ monolayer (sample 5) than for all the multilayer samples. This thin film efficiency has been taken as reference as it shows the lowest Er 3+ ion-ion interaction at 1530 nm. Following high efficiency for sample 5, multilayer thin films containing 0.5 at. % Er-doped YSZ layers (samples 2 and 4) prove low quenching effect and highest luminescent efficiency. Sample 4 has a thicker layer of Er 3+ 0.5 at%, thus proves higher efficiency than sample 2 with thinner Er 3+ 0.5 at. % YSZ layer. Finally, samples 3 and 1 have very similar low efficiency at 1530 nm. Samples 3 and 1 are doped only with metallic Er 3+ target, hence its low efficiency at 1530 nm has its origin in quenching effects produced by the high doping level. As a conclusion, when pumping the multilayer samples at 960nm, which is the preferred absorption transition for the 1530 nm emission; the highest PL emission efficiency comes from the non-metallic target Er 3+ ions.

b) Er 3+ ion emission at 1535 nm (e= 800 nm)

Areas in the thin films presenting Stark splitting and enhanced multipeak PL profile (samples 1, 2, 3, 4) reveal optimal wavelength excitation of 800 nm (figure 3.3). When exciting our samples at 800 nm and at 60 mW pumping power, a PL profile with two sharp PL emissions at 1530 and 1535 nm have been observed (figure 3.11). We observed that samples with Er-rich layers in the YSZ structure alternating 0.5% Erdoped YSZ (samples 2 and 4) present higher emission. In sample 2 and 4 configuration, higher emission at 1535 nm is found for sample 4 with 5 at % Er 3+ doping layers than in sample 2 with 8.5 at. %. For samples with larger Er 3+ doping, higher quenching effects are expected. Lower PL intensities are found for samples 1 and 3, which lacks of Er 3+ in its structure than for samples doped with Er 3+ from Erdoped YSZ sintered target. Hence, in agreement with expected low PL emissions for samples with higher Er 3+ doping, PL intensities for sample 3 with 5 at % Er 3+ doping are higher than for sample 1 with 8.5 at%.

Lastly, in figure 3 For efficiency study at 1535 nm emission, multilayer samples with presence of 0.5 at. % Er 3+ ion concentration in the layer structure (samples 2 and 4) demonstrated similar efficiency. (Fig. 3.12). Coherently, efficiency for sample 3 with 5 at. % Er 3+ doping is higher than for 8.5 at. % Er 3+ -doped sample 1 as quenching lowers efficiency for higher at. % doped samples. We noticed PL intensity saturation for 1535 nm emission with pumping power >120mW for all samples. The aim of efficiency and PL intensity studies are to find a high efficient strategy for optical amplification. For this purpose, we compared the efficiency of multilayer samples presenting Stark effect at 1535 nm with the most efficient monolayer without presence of Stark effect (sample 5) at 1530 nm. As observed in figure 3.12, sample 5 with 0.5 at % Er:YSZ monolayer has an efficiency below multilayer samples 4 and 2. Finally, we conclude that the best approach for maximum intensity and efficiency is found for multilayer structures. Among all the multilayer structures, samples 2 and 4, with 0.5 at. % layers proved higher efficiency and emission intensity at both 1530 and 1535 nm. FIGURE -3.12. Normalized PL efficiency study at 800 nm pumping wavelength of 4 I13/2 4 I15/2 Er 3+ ion emission transition presenting Stark effect at 1535 nm wavelength for 4 samples excited at 800 nm and same Er 3+ transition for 1 monolayer sample (sample 5) excited at 960 nm and emitting at 1530 nm wavelength. Inset, augment of the PL intensity in the range of 50 to 250 mW to differentiate curve overlap.

Improving photoluminescence: annealing under an oxygen atmosphere

Multiple studies in the literature suggest annealing and introducing co-doping of certain elements to enhance photoluminescence in RE materials [ 4 ]. One of the most successful processes regarding optical properties is oxygen co-doping by annealing under oxygen atmosphere [ 5 ]. In this thesis we performed PL on crystalline and amorphous Er-doped YSZ monolayer and multilayer thin films before and after annealing at 1000°C. At higher annealing temperatures (1100°C), deposited Er doped-YSZ thin films are damaged. In this study we used two samples: (i) Er-doped YSZ monolayer (sample 5) and (ii) Er-rich YSZ multilayer (sample 2) thin films. Each sample was deposited as amorphous and crystalline thin films. Amorphous thin films were deposited by PLD at room temperature and crystalline samples at 800°C. PL measurements with a pumping wavelength of 960 nm were carried out in all samples and as shown in figure 3.13, emission of Er 3+ ions at 1530 nm proved to be higher for crystalline than for amorphous samples. After annealing step for 12h under an oxygen atmosphere at 1000°C, PL measured Er 3+ emission intensity enhancements for amorphous and crystalline sample 5 and amorphous sample 2. PL emission at 1530 nm for crystalline Er-rich YSZ multilayer sample 2 remained constant (figure 3.13). Er-doped YSZ monolayer crystalline sample 5, followed by amorphous samples 2 and 5 report a higher emission enhancement after annealing. In conclusion, crystalline samples present larger Er 3+ PL intensity than amorphous samples, before and after annealing at 1000°C. Besides, we have proved mild enhanced PL emission of Er 3+ ions in an YSZ matrix for samples 2 and 5 amorphous structures after annealing. For crystalline structures deposited at 800°C, a notable PL enhancement has been found for Er-doped YSZ monolayer and mild PL enhancement for crystalline Er-rich YSZ multilayer (sample 2). PL intensity enhancement is explained by chemical environment change of Er 3+ ion and oxygen by diffusion [ 6 ]. Therefore, Er 3+ ions contained in Er-doped YSZ monolayer (sample 5) thin films and Er-rich YSZ amorphous (sample 2) multilayer migrate and change their chemical environment, enhancing luminescence. Er 3+ ions diffusivity in a host increase with temperature [ 7 ] which explains Er 3+ emission intensity increase after annealing for amorphous thin films deposited at RT. Low PL enhancement for crystalline Er-rich YSZ multilayer sample 2 in comparison with crystalline Er-doped YSZ monolayer (sample 5) might be elucidated by the time of deposition at 800°C for multilayer sample (>6h) which can be considered annealing process. Deposition time of monolayer samples of 300 nm thickness is of 30 minutes. .

Er 3+ -Er 3+ energy transfer

We excited samples 1, 2, 3 and 4 at 800 nm wavelength (Er 3+ transition 4 I15/2 4 I9/2) which are non-radiatively relaxed to the 4 I13/2 energy level. Radiative transition 4 I13/2 4 I15/2 for samples which present Stark effect can be done from different sublevels. Each sublevel emitting at a characteristic wavelength. During efficiency study of our samples, we observed a variation of the emission rate between two sublevels of 4 I13/2 4 I15/2 transition emitting at 1530 and 1536 nm with increasing pumping power (figure 3.14 (a)). This behavior hints transfer of population from sublevels emitting at 1530 and 1535 nm as shown in figure 3.14 (b). In figure 3.15 (a), we show Er 3+ emissions efficiency study for 1530 (in black) and 1535 nm (in red) wavelength for samples 1, 2, 3. We noticed efficiency studies reveal 1535 nm emission intensity as more efficient than emission at 1530 nm. We divided the efficiency study of our samples in three steps:

 For low pumping powers, in orange shade, Er 3+ emission intensity at 1530 nm is higher than at 1535 nm. Indeed, in figure 3.18 (b), we measured the ratio of 1535 and 1530 nm. We observe for samples containing layers of 0.5 at. % Er:YSZ (samples 2 and 4) dominant emission for 1530 nm is overcome by 1535 nm at a lower pumping power than samples with only Er-rich layersin its structure (samples 1 and 3).



Blue circle indicates at which pumping power both 1530 and 1535 nm emissions have the same intensity. We can observe in figure 3.18 (a) that samples with higher efficiency at 1535 nm, achieve same intensity emissions for 1530 and 1535 nm emission at lower pumping powers. This point is reached in figure 3.18 (b) by samples 2 and 4 for pumping values below 1 mW and at 25 and 30 mW for samples 3 and 1, respectively.  For pumping powers above 30mW (violet shade figure 3.18 (a,b)), Er 3+ emission for 1535 nm outcomes for the 4 samples the 1530 nm emission. Finally, for emissions above 100 mW pumping power emission intensity at 1535 nm saturates for sample 4, 1 and 2. Saturation is observed in figure 3.15, as ratio between 1535 and 1530 nm emissions decrease.

From figures 3.15 (a, b), we conclude that higher 1530 nm emissions are observed for samples with 0.5 at. %, (samples 2 and 4), as expected. For 1535 nm emission in thin films with layers of 0.5 at. % (samples 2 and 4), higher PL emission rates are related to higher emissions at 1535 nm. PL emission intensity ratio for 1535 nm/1530 nm is stronger for samples with Er 3+ at 8.5 at. % (sample 2) than at 5 at. % (sample 4), despite higher quenching effects for higher doped samples due to the energy transfer from 1530 nm Er 3+ . Besides, from 100 mW can be observed for 1535 nm wavelengths saturation which leads to a decrease on the 1535 nm /1530 nm ratio (figure 3.15 (b)).

Lowest intensities at 1530 nm are found for samples 1 and 3, these samples are doped solely with Er 3+ at 8.5 and 5 at. %, respectively. Hindering emission at 1535 nm due to quenching effects are expected for sample 1, whilst better emission at 1535 nm is expected for Er 3+ at 5 at. % sample 3. In figure 3.15, it is noticed for sample 3 has indeed a higher emission rate than sample 1.

From this study we conclude that possible energy transfer from sublevels corresponding to emissions at 1530 to 1535 nm are related to the presence of both Er 3+ ions from Er-rich and Er-doped layers. Samples with layers with 0.5 at. % Er 3+ (samples 2 and 4) would transfer energy from 1530 nm sublevel to the 1535 nm sublevel in Er-rich layers.

Lifetime

Lifetime of RE emission is studied in order to understand the population inversion of an excited state. After promotion of a RE ion to an excited state, spontaneous or induced emission to a lower state occurs, and lifetime can be measured by fitting the emission decay with an exponential curve as shown in Eq. 3.1.

(eq. 3.1)

In which A is the population constant, y, yo, t are experimental data referred the intensity and the time decay and τ is the lifetime of the system. For that purpose, continuous waves (CW) Ti:Sapphire laser beam excites the sample after passing by an optical chopper operating at a frequency of 75Hz (figure 3.16). The output signal from the excitation and decay of RE ions emission is registered by a photodiode connected to an oscilloscope. FIGURE -3.16. Lifetime measurements setup, laser beam in red excites the sample after passing through an optical chopper. Filtering of laser wavelength is carried out by a high pass filter operating at 1100 nm. Finally, RE emission is detected by a photodiode connected to an oscilloscope in which RE emission decay and optical chopper period are registered.

i) Er-doped YSZ monolayers

Er 3+ ions in the excited state ( 4 I13/2) are studied in this work to understand the material population inversion for optical amplification purposes. We have measured the decay curves of luminescence for samples with different Er 3+ concentrations of 0.5, 1.5 and 3 at. % for Er-doped YSZ monolayers (samples 5, 6 and 7). We excited the 4 I11/2 state of Er 3+ ion with a continuous wave laser at 960 nm wavelength at room temperature to obtain the resonant excitation condition. Studies on RE lifetime reveals a direct relationship with RE concentration [ 8 ]. Accordingly, in figure 3.17, we observe decrease of state 4 I13/2 lifetime with increasing Er 3+ doping concentration for the Er-doped YSZ monolayers (samples 5, 6 and 7). For Er 3+ concentrations of 0.5, 1.5 and 3.0 at. % (samples 5, 6 and 7) we obtained lifetimes of 2.0, 1.6 and 1.2 ms, respectively). FIGURE -3.17. Lifetime spectra of Er:YSZ monolayers (samples 5, 6 and 7) with three different Er 3+ concentration: 0.5, 1.5 and 3.0 at. % in blue, orange and red, respectively.

ii) Er-doped YSZ multilayers

We have also measured the decay curve of luminescence for a Er-rich YSZ multilayer (sample 2), and a longer lifetime of 3.2 ms has been found for this sample as shown in figure 3.18. Longer lifetime for multilayer sample 2 than for monolayer samples 5, 6 and 7 may be related to low quenching effects, thus higher efficient materials.

Pr-doped YSZ

Wavelength absorption

Pr 3+ emission transition ( 1 G4→ 3 H5) in the near infrared at 1350 nm can be obtained after excitation of a Pr-doped YSZ (sample 10) thin film at an excitation wavelength of about 1020-1030 nm. As done with Er-doped YSZ samples, we performed a wavelength study at a pumping power of 40mW on a range of wavelengths between 760 and 1060 nm for two types of 0.5 at % Pr-doped YSZ samples (sample 10 and 12). The first characterized thin film is a 0.5% Pr-doped YSZ monolayer (sample 10) and the second one a Er-and Pr-doped YSZ multilayers (sample 12). In the wavelength excitation study for these samples, we analyzed emission from Pr 3+ and a second one from Er 3+ ions as shown in figure 3.19. The first emission transition found at 1350 nm corresponds to the Pr 3+ emission transition 1 G4→ 3 H5, and is observed for the Pr-doped YSZ monolayer (sample 10 shown in red). Afterwards, the same Pr 3+ transition and an additional transition corresponding to Er 3+ emission at 1530 nm with a stronger intensity than Pr 3+ emission were evidenced on a Er-and Pr-doped YSZ multilayer (sample 12) and further explored. As observed in figure 3.20, an absorption peak at 1020 nm corresponding to the 3 H4→ 1 G4 transition of both samples indicates the optimal wavelength absorption for these materials for the 1350 nm emission. Moreover, for the Er 3+ transition at 1530nm in the Er-/ Pr-doped YSZ sample (sample 12), the absorption peak is found at 970 nm (in green). There are no differences between Er 3+ absorption spectra of 0.5 at. % Er:YSZ monolayer (sample 5) and Er-/ Pr-doped YSZ multilayer (sample 12). FIGURE -3.20. Normalized PL spectrum intensity (emitting wavelength at 1350 nm) for pumping wavelength range from 760 nm to 1060 nm for (from top to bottom) Er:YSZ and Pr:YSZ multilayer (sample 12) in red, and for Pr 3+ :YSZ monolayer (sample 10) in yellow. Spectrum (emitting wavelength at 1530 nm) for Er : YSZ and Pr:YSZ multilayer (sample 12) in green.

Power efficiency

In order to study efficiency at 1350 nm in the near-IR, corresponding to Pr 3+ ions emission from 3 H4→ 1 G4 transition, we have compared the two Pr-doped YSZ based samples with same thicknesses: a Pr-doped YSZ monolayer (sample 10) and a second one,.5 at. % Er-and Pr-doped YSZ multilayers (sample 12).

i) Pr 3+ ion emission at 1350 nm (e= 1020 nm)

Firstly, we excited Pr 3+ ions of each thin film, at an excitation wavelength of 1020 nm and we collected Pr 3+ ion emission at 1350 nm as a function of the pumping power. As shown in figure 3.21 efficiency for Pr 3+ -doped monolayer (sample 10) is higher than for the multilayer sample 12. Efficiency is related to the curve slope (ratio between intensity emission and pumping power). For Pr:YSZ thin film (sample 10) slope has a value of 0.4 indicating low emission efficiency at 1350 nm for Pr 3+ emission. Moreover, Er-and Pr-doped YSZ multilayer sample shows a saturation of the 1350 nm emission from 200 mW pumping power. ii) Er 3+ ion emission at 1530 nm (e= 965 nm) Secondly, efficiency is studied when pumping Er 3+ ions at 965 nm to obtain 1530 nm emission wavelength. In this work, we compared in figure 3.22, efficiency of 0.5 at. % Er-doped YSZ monolayer (sample 5) and 0.5 at. % Er-and Pr-doped YSZ multilayer (sample 12) we noticed better efficiency for Er-doped YSZ monolayer (sample 5) than for the Pr 3+ -and Er 3+ -doped multistack structure (sample 12).

Pr 3+ -Er 3+ energy transfer

Finally, to further understand if saturation of Pr 3+ emission when pumping at a wavelength of 1020 nm is due to energy transfer between Er 3+ and Pr 3+ ions in the multilayer structure, we carried out an efficiency study by pumping at 1020 nm and analyzing the Er 3+ ion emission at 1530 nm. In figure 3.23 we noticed an increase of Er 3+ emission when exciting Pr 3+ at 1020 nm. Expectedly, when exciting Er-doped YSZ samples at 1020 nm, no Er 3+ emission at 1530 nm has been detected. Therefore, the excitation of Er 3+ ions for the Er-and Pr-doped sample has its origin on the Pr 3+ ions excitation and energy transfer to Er 3+ ions. We observe in figure 3.23 a strong increment of Er 3+ emission at 1530 nm for pumping power values higher than 12 mW. This behavior hints an energy transfer in which energy transfer between Pr 3+ and Er 3+ takes place as observed in studies with rare-earths such as Eu 3+ and Tb 3+ or Yb 2+ and Tb 3+ [ 9 , 10 ].

Lifetime

After measuring our Pr-doped YSZ samples on the free-space setup, we obtained, in one hand, very low output signal at 1350 nm when exciting the Pr 3+ ions at 1020 nm. The analysis of lifetime for these RE ions revealed a non-reliable quantitative measurement for the ( 1 G4) excitation state due to the bad signal to noise ratio. We could estimate the Pr 3+ lifetime on YSZ lower than 1 ms. In table 3.4 measured lifetimes for Pr and Er-doped YSZ samples.

Cathodoluminescence (CL) excitation spectroscopy

Cathodoluminescence (CL) is a physical phenomenon of light emission under electrons pumping of the luminescence material. In this process (figure 3.24), highenergy electrons (cathode rays) from an electron beam give energy to the electrons of the valence band by inelastic scattering, promoting the electron to the empty conduction band with an energy higher than the band-gap leaving a hole in the valence band. Part of this energy is dispersed as phonons in the lattice, transferring energy from the conduction band to the luminescent material (Er 3+ in figure 3.24) which will be radiatively de-excited emitting one photon. Another phenomenon occurring in CL is the recombination of hole-electron leading to the emission of a photon with a wavelength of the band-gap energy. Besides, this technique is widely used in revealing defects and impurities in the crystalline host structure [ 11 ]. Defects can introduce unexpected energy levels in the forbidden band which is detected in the emission analysis. While pumping the Er 3+ -doped multilayer samples at 800 nm and studying its emission at 1530 nm, we observed in the visible range a white spot in figure 3.25 related to high green intensity Er 3+ up-conversion emission for multilayer samples. To further understand this behavior, we analyzed our samples in the visible by two FIGURE -3.26 -Schematic of electron beam incident to the surface of a thin film grown on a substrate in which cathodoluminescence and secondary electrons are emitted.

As shown in figure 3.26, when the electron beam is focused on the sample we obtain signals from the surface of the sample and from its volume underneath. This effect is generated by the multiple interactions of electrons with the electrons of the sample. The depth of electron interactions with the sample surface depends on the acceleration of the incident beam and the density of the material to analyze. Operational voltage of our CL-SEM is variable between 2 to 10keV. In this thesis we measured two E + -doped YSZ thin films grown on sapphire (samples 4 and 5) and a sapphire substrate with no doped layer as reference. For the two Er-doped YSZ thin films, the structure for sample 5 is 0.5 at. % Er-doped YSZ monolayer and for sample 4, 5 at. % Er-rich YSZ multilayer. CL-SEM sample preparation before measurements includes sputtering of a semi-transparent Cr/Au (1/10 nm) thin film on the sample surface to avoid charge effects on the non-conducting oxide surface of the Er-doped YSZ thin films (samples 4 and 5).

To establish the proper electron beam voltage to use, the excitation volume has to be confined between the thin film structure and the substrate. For this purpose, we simulated the electron trajectory and the reached depth for 3, 4 and 5 keV in 300 nm of YSZ with the program Casino v2.51. As shown in figure 3.27, for 3 keV voltage, electrons from the beam impact the surface and excite partially the thin film penetrating at maximum depth of 180 nm, having its maximum interaction at 90 nm depth. In reverse, for 4 keV and higher voltages, the electron beam excites a thickness of more than 280 nm with a maximum excitation of the thin film for 4 keV placed at a depth of 175 nm. At this voltage and for higher values some of the electrons of the beam reach the substrate. Therefore, for <300 nm Er-doped YSZ thin films we stated an optimal electron beam voltage at 3 keV. FIGURE -3.27. On the left, simulation electron beam trajectory on 300 nm YSZ thin film cross-section and on the right depth graphics for 3, 4 and 5 keV voltages (from top to bottom).

Experimental results

Mapping by SEM the surface for both thin film samples 4 and 5 (figure 3.28) reveals a uniform surface with granulate topography. After the oxide deposition by PLD, AFM images unveiled a flat and uniform surface. Consequently, the observed granulate surface corresponds to the sample preparation before SEM observation in which we deposited a Er-rich layer by sputtering to avoid charge effect on the sample surface. Regarding SEM images of the sample surface, we could not find significant differences between both samples. In respect of CL mapping (figure 3.28), uniform luminescence over the whole thin film structure has been observed for both samples with some negligible spots showing higher luminescence. Besides, the CL spectroscopy profile for both samples 4 and 5 in figure 3.29, demonstrates an Er 3+ transition ( 4 S3/2 4 I15/2) at 550 and 575 nm for both samples, being more intense for the Er-rich YSZ multistack (sample 4).

Multistack sample 4 shows as well a second transition ( 4 F9/2 4 I15/2) in the red range of wavelengths (between 655 and 679 nm). Thus, even though Er 3+ ions are the dopant of the same YSZ structure, there are some optical differences in the visible range among the two samples revealed in CL-SEM profiles: we observe a higher intensity emission for sample 4 and a lack of red wavelength transition for the sample with 0.5 at. % Er 3+ (sample 5). Finally, a CL intensity peak is observed at around 700 nm and it is related to the YSZ cubic structure [ 12 ].

Cathodoluminescence (CL-) TEM (Collaboration with Laura Bocher group STEM at Laboratoire de Physique des Solides (LPS))

Atomic resolution in TEM allows imaging of the sample at higher spatial resolution in the (x,y) plane than SEM. In our case, TEM has been chosen as we aim discriminating two different CL emission behavior coming from different Er 3+ placements in an YSZ multilayer structure. For TEM imaging and CL-TEM analysis, electron beam is transmitted through the sample (figure 3.30 (b)), thus higher voltages than for CL-SEM are required. For our thin films, we typically worked with voltages of 60 and 100 keV. For this reason, sample preparation is carried out by electron and ion beam polisher (FIB) of a thin film cross-section (figure 3.30 (a)) in which the specimen is polished until its thickness reaches tens of nanometers. The specimen used for this experiment is an Er-doped YSZ multilayer thin deposited by PLD with alternative layers of 0.5 at. % Er:YSZand 5 at. % Er-rich Er 3+ film (sample 4). CL-TEM spectrum images were performed in collaboration with Laboratoire de Physique des Solides (LP) by Laura Bocher on a VG lumière (VG HB501) electron microscope at 100 keV under high vacuum and nitrogen cooling. This electron microscope reaches nanometer spatial resolution and less than 0.3 eV spectral resolution, the acquisition time for our spectrum images was 150 ms.

Experimental results

We observed in the CL-TEM spectrum profile of the whole multistack cross-section corresponding to sample 4 (in grey in figure 3.31 (a)) two Er 3+ transitions in the visible ( 4 S3/2 4 I15/2 and 4 F9/2 4 I15/2). Each of these transitions in the green and red range of wavelengths have two intensity peaks. Dominant emission in the visible wavelengths corresponds to the 4 S3/2 4 I15/2 transition in all cases. In the CL spectra for the whole thin film structure (figure 3.31), highest emission peak is placed at 540 nm followed by a less intense emission at 560 nm. Meanwhile, for the 4 F9/2 4 I15/2 transition, two equally intense CL emissions are found at 650 and 673 nm. In addition a broad intensity peak is observed at 440 nm from YSZ structure in all CL profiles measured which has been reported by Constantini et al. in YSZ [ 12 ] and in other oxide studies [ 13 , 14 ] as oxygen vacancies related emission in oxides. In figure 3.31, we analyze a red and blue CL spectrum belonging to two kinds of Er 3+ -doped layers of the same cross-section: the 0.5 at. % Er-doped YSZand the 5 at. % Er-rich YSZ, respectively. Superposition of the two separately CL spectra is consistent with the CL spectrum of the whole thin film cross-section. Besides, CL signal of sample 4's layers with Er 3+ in its YSZ structure presents one transition ( 4 S3/2 4 I15/2) with two emission peaks: one dominant at 540 nm and a second emission peak at 560 nm. While for sample 4's Errich YSZ layers, two transitions ( 4 S3/2 4 I15/2 and 4 F9/2 4 I15/2) with two equally intense emissions each at 540 and 560nm for the transition in the green and 650 and 673 nm for the transition in the red wavelengths take place. These results are consistent with CL-SEM results. We observed in our study, two main CL intensity peaks at 320 and at 700 nm corresponding to the sapphire substrate [ 13 ]. By CL-TEM, one shoulder at 440 nm reveals cubic YSZ structure [ 14 ].

Conclusions

In this chapter, we have explored two approaches for Er 3+ ions implementation by PLD on a YSZ host: on a first approach, we deposit Er:YSZ monolayer thin films and on a second approach, Er-rich YSZ multilayers. Both PLD deposition approaches provide thin films emitting at an Er 3+ transition 4 I13/2→ 4 I15/2 in the near-IR when excited at ~960 and ~800 nm. Monolayer samples with more efficient absorption wavelength has been found at 960 nm. For multilayer samples, two absorption wavelengths were noticed: one at 960 nm for PL profiles without Stark effect and a second absorption wavelength at 800 nm for PL profiles with Stark splitting effect.

Efficiency study when exciting samples at 960 nm wavelength shows that Er:YSZ monolayers at 1530nm provide higher efficiency for 0.5 at.% doping (sample 5) while larger emission is achieved for 1.5 at.% (sample 6) despite quenching effects are stronger for this last percentage. For multilayer samples (samples 1, 2, 3 and 4), efficiency study at 1530 and 1535 nm proves samples 4 (multilayer with 0.5 at. % Er:YSZ and 5 at. % Er-rich layers) the most efficient and higher intensity sample. Besides, we observed energy transfer from Er:YSZto Er-rich YSZ Er 3+ ions (samples 2 and 4). Energy transfer was more effective for sample 2 with higher Er-rich number of layers and higher Er 3+ percentage (8.5 at. %) than sample 4 (5 at. %).

We proved that annealing at 1000°C in an oxygen atmosphere highly enhances luminescence for all samples, although higher PL after annealing was measured for amorphous thin films.

It has been demonstrated longer lifetimes for Er-rich YSZ multilayers (3.2ms) than for 0.5 at. % Er-doped YSZ monolayers (2 ms). Hence, for Er-doped YSZ monolayers, lifetime decreases with the increase of the Er 3+ at. %.

The goal with CL study was to better understand optically the structure showing Stark effect (multilayer approach with higher Er 3+ doping). Results by TEM-CL and SEM-CL prove two CL emissions in the visible. From TEM-CL,two different layers have been studied. The 0.5 at. % Er-doped YSZ layer exhibit two intensity peaks for 4 S3/2 4 I15/2 transition at 540 and 560 nm, with higher intensity for first peak intensity. The layer corresponding to 5 at.% Er-rich YSZ presents a two equally intense peak for a double 4 S3/2 4 I15/2 and 4 F9/2 4 I15/2 transitions for 540, 560 and 650 and 673 nm.

Other studied RE emission is Pr 3+ within the near-IR, O-band telecommunication wavelengths. We were interested in the wavelength emission at 1345 nm corresponding to the 1 G4→ 3 H5 transition.

For this purpose, we studied Pr-doped YSZ monolayer and Pr 3+ -and Er 3+ -doped YSZ multilayers absorption transitions wavelengths in the near-IR. For both samples we obtained an absorption peak at 1020 nm for Pr 3+ .

Power efficiency studies when exciting Pr 3+ ions at 1020 nm wavelength, reveals moderate high efficiency for Pr-doped YSZ monolayers and for Pr-/Er-doped YSZ multilayers. For multilayers containing Er 3+ and Pr 3+ doping, PL intensity saturates for pumping powers higher than 100 mW. Besides, for Pr-/Er-doped YSZ, we proved energy transferred from Pr 3+ ions to Er 3+ ions. To conclude, Pr 3+ ( 1 G4) excitation states lifetime measured at 1345 nm gave values below 1ms. average widths (wa) with an inclination angle () of 3°. The wafers have been then planarized by using chemical and mechanical polishing after silica deposition as schematized at figure 4.1. Silicon nitride (SiNx) was chosen as passive waveguide material as it presents negligible two-photon absorption effect at telecom wavelengths. This enables a high pumping optical power and low propagation losses (<1dB/cm). Both factors are essential to obtain strong optical signal enhancement [ 3 ]. However, one of the main handicaps of SiNx waveguides is intrinsic absorption due to the NH-bonds formed during fabrication. Deposition of SiNx waveguides by PECVD at low temperature requires in the synthesis the use of the compounds SiH4/NH3/N2 leaving H atoms trapped within the film. N-H bonds are known to have an absorption peak at 1520 nm wavelength very close to the 1530 nm Er 3+ ion emission in the near-IR. In collaboration with Sylvain Gueber and STMicroelectronics a solution was found to diminish this absorption effect, annealing the SiNx waveguides at 1050ºC and for 3h. As observed in figure 4.2 measured by Sylvain Gueber, after annealing, the transmission losses diminish more than 10 dB for all wavelengths between 1450 and 1575 nm.

Cladding simulation

Simulations are defined to study the thickness of the doped YSZ layer to maintain the mode of the pump confined in the waveguide while exciting the larger part of the cladding which contains Er 3+ ions. Planarized structure in the waveguide design presents a smooth surface to deposit crystalline Er-doped YSZ multilayers on an amorphous substrate. By growing a cladding on top of a buried strip waveguide, the result structure is an inverse rib waveguide formed by the waveguide and the cladding. Simulations were performed using a two-dimensional ( 2 We divided simulations onto two parts: in the first simulations, cladding is formed by an Er:YSZ active layer (cladding 1). In the second simulations a second top layer of 1µm PMMA film is included, forming of double cladding of Er-doped YSZ and PMMA layer (cladding 2) in order to synthesize the structure, as shown in figure 4.3.

For the Er:YSZ cladding simulations, the thickness optimization is carried out by simulating a propagation single mode at a wavelength of 1530 nm, corresponding to Er 3+ ions emission. The optimized thickness of the active layer is determined by changing cladding thickness while keeping the best trade-off between modal confinement of the propagating mode within the SiNx waveguide core and high percentage of optical power residing in the top Er:YSZ:Er thin film. The later aspect is critical to maximize the excitation of Er 3+ ions while the first allows keeping light confinement strong enough to turn the light in sharp bends.

We want to study the evolution of optical power within the active cladding material as a function of its thickness for both fundamental quasi-TE and quasi-TM modes. Particular mode field profiles of both quasi-TE and quasi-TM modes are depicted in the inset of figure 4.4. and figure 4.5. for Er:YSZ thicknesses of 50 nm, 150 nm, and 350 nm. As expected, the optical power within the cladding layer raises up, as the thickness of active layer increases and with presence of PMMA. Yet the trend is more pronounced for quasi-TE mode than for quasi-TM mode. Although reasonable modal properties are also obtained for quasi-TM mode, the passive SiNx platform is optimized for quasi-TE operation.

According to our calculations for two kinds of cladding: Er-doped YSZ (cladding 1) (figure 4.4) and Er-doped YSZ/PMMA (cladding 2) (figure 4.5), guided modes are strongly confined within a waveguide core for a 50nm thick Er:YSZ layer. In the first case cladding 1, the power within the active cladding layer is estimated to be ~3.5% while for the sample with PMMA cladding 2 this value reaches 11.46%. In contrast, for a 150 nm thick Er:YSZ layer, guided modes exhibit an improved overlap with the top cladding region which is of 21.3% for Er:YSZ cladding and 25.9% for the same cladding with PMMA. This trend is expected as the refractive index of Er:YSZ (nEr:YSZ = 2.15) is higher than that of waveguide core (nSiNx = 1.91) or that of oxide under-cladding (nSiO2 = 1.45). Moreover, by adding a PMMA cladding with a refractive index of 1.45 instead of air (nair=1), the mode is meant to propagate higher into the cladding resulting on a greater overlap. Under such modal circumstances, the initial strip SiNx waveguide behaves as an inverted rib structure. Furthermore, for a 350 nm thick Er:YSZ layer, the fundamental quasi-TE mode is highly de-confined from the SiNx waveguide core in all cladding configuration. De-confined mode strongly limits the compactness of our structure; as very large bends are needed for spiral waveguides. In order to obtain Er 3+ emission in near-IR wavelengths, we need to pump the Er:YSZ upper cladding layer at 1480 nm to excite Er 3+ ions. Moreover, since the pumping and the optical signal are not far away from each other in term of spectral separation, the geometrical optimization of waveguides becomes also easier. Therefore, a single-mode operation of the SiNx strip waveguide is envisioned for both wavelengths simultaneously, avoiding potential excitation of higher-order modes. This is also confirmed through 2-D modal simulations, as shown in figure 4.6, for both the probe (wavelength of 1530 nm) and the pump (wavelength of 1480 nm) modes. Here, we only considered quasi-TE mode and the thickness of active layer was fixed to 150 nm and cladding does not contain PMMA layer. As a result of optical power calculations, power levels of 19.5% and 24% within the active cladding layer are obtained for Er:YSZ with and without PMMA cladding (cladding 2 and 1), for 1480 nm wavelength. We also studied the modal properties of such hybrid SiNx waveguide with active Er:YSZ and YSZ:Er 3+ /PMMA claddings in a spiral bending arrangement (figure 4.7). The spiral configuration allows merging simultaneously long waveguide (i.e. long light interaction) and compactness. Indeed, spirals can be concentrated in about 100 m x 100 m square. In consistency with previous lines, we considered quasi-TE mode and various cladding thickness.

Cladding optical characterization

The fabrication process of the waveguides follows two steps: the first step by STMicroelectronics consisted on the fabrication of passive silicon nitride waveguides, the SiO2 deposition and the CMP (chemical mechanical polish) step to planarize the structure followed by a second step in which we deposited at C2N the active Er-doped YSZ multilayer on the waveguide. After a first set of characterizations with Er:YSZ cladding a second cladding of 1 m thick PMMA was added to the previous configuration. In this last step, we aimed to increase the percentage of the optical mode density in the Er:YSZ cladding. Therefore, a higher number of Er 3+ ions will be excited by the pump. Moreover, according with optical characterization of our cladding material (chapter 3), the thin films showing the highest optical efficiency was Er 3+ -doped YSZ thin films with thicknesses of 150 nm in a multilayer with metallic and non-metallic Er-doped YSZ (sample 13). This material was deposited by PLD on SiNx waveguides following the same protocol and under the same experimental conditions than those deposited on sapphire substrates as discussed in Chapter 2, section 2.1.

After the YSZ material deposition on amorphous SiNx waveguides buried on SiO2, we performed optical and structural characteristics of the thin film. Besides, we expected in one hand, high similarities in optical behavior between such Er-doped YSZ multilayer thin films grown on sapphire and on amorphous SiNx substrate because it is the same composition of material on both substrates. On the other hand, the crystalline quality of the material when grown on both substrates is really different. Indeed, it is known that YSZ grows creating multigrain which leads to polycrystalline structure. Results on this regard were discussed in chapter 2. A highly polycrystalline structure of YSZ is obtained for a growth on amorphous substrate, in contrast with the highly crystalline structure of Er:YSZ + on sapphire. Thus, to investigate the optical efficiency of the thin films when grown on amorphous substrate, we characterized by PL Er:YSZ thin films deposited on amorphous SiNx waveguides. We used the same setup and acquisition conditions as discussed for multilayer Er 3+ -doped YSZ thin films grown on sapphire. Therefore, the Er-doped active layer was excited perpendicularly to its surface by a continuous wave sapphire: Ti laser at a wavelength of 980 nm and an optical power of 5 mW using a microscope objective with a magnification of 50X and NA of 0.55. Collected emission of Er 3+ ions in near-IR is detected on a iHR320 spectrometer coupled to a nitrogencooled-detector after being focused by a convergent lens and emitted signal was filtered by a high-pass filter (1100 nm) as previously described in chapter 2.

In figure 4.8 a characteristic Er 3+ ion emission spectra at 1530 nm corresponding to 4 I13/2 4 I15/2 transition has been observed for all the thin film thicknesses. We could notice a nonlinear increase of this emission intensity peak as the cladding thickness increases. For thin films with a thickness higher than 150 nm, saturation in emission intensity has been detected. Moreover, PL profile proving Stark effect have been registered as previously observed for the same material structure grown on sapphire. Subsequent to PL studies in the near-IR and following the CL spectroscopy studies in the visible emission wavelengths (evidenced for our thin films grown on sapphire), aimed at propagating guided emission in the visible range of wavelengths for 150 nm thick cladding of Er:YSZ (sample 13). Demonstrating visible guided emission and obtaining its emission spectrum has two intentions: evidence guided emission, thus validate simulations of the hybrid photonic platform geometry with experimental results, and the study of Er 3+ ion transitions for thin films grown on amorphous substrate. Previous to the measurements, a visual inspection of waveguides already confirmed the existence of strong up-conversion processes due to the generation of strong green light emission (see figure 4.9). To further optically characterize our sample 13, we performed Er 3+ emission spectrometry in the visible. In figure 4.10, it is shown a diagram of guided emission setup consisting on continuous laser wave emitted at 1480 nm, connected by optical fiber to an optical attenuator (OA) and butt-coupled to the SiNx waveguide. Output signal from Er 3+ ions' optical emission was detected by a spectrometer, placed after the emitting waveguide covering visible and near-IR wavelength ranges. In this regard, wavelength presenting higher optical efficiency is 543 nm corresponding to upconversion in the green wavelengths for the 2 H11/2 4 I15/2 transition. Although the highest contribution in visible intensity in the spectrum over the whole waveguide is found for red wavelengths, upconversion in the green range of wavelengths could be recorded by a camera in a spiral as shown in figure 4.12. Larger efficiency related to lower quenching effects in figure. 4.11(b) has been demonstrated at 543 nm wavelength in which its normalized emission intensity slope with increasing pumping power presents the highest value among all visible wavelength emissions. High optical efficiencies are also found in the red range of wavelengths, intensity peaks at 656 and 680 nm correspond to the same 4 F9/2 4 I15/2 transition as observed in figure 4.11(a)). With similar efficiency values than for 543, 656 and 680 nm, we find green emission at 562nm. Finally, with the lowest efficiency the 525 nm emission of the 2 H11/2 4 I15/2 transition. As conclusion, in the visible, the most efficient Er 3+ transitions corresponds to the 2 H11/2 4 I15/2 transition for the green wavelength at 543 nm, while the highest intensity is observed for the 4 F9/2 4 I15/2 transition at 580 nm. In this regard, sample 13 spectroscopy gives similar emission profiles in the visible as the spectra in the visible observed for multilayer sample 4 grown on sapphire in SEM-CL and TEM-CL experiments) (chapter 3, section 3.2).

Gain measurements in waveguides

Optical characterizations and simulations of our cladded waveguides revealed that our material is suitable for photonic purposes. As a result, we carried out experimental measurements in free space as shown in figure 4.9 by exciting our material at the absorption wavelength of Er 3+ of 1480 nm and analyzing data of modal gain (gmodal) and signal enhancement (SE) for near-IR Er 3+ ion emissions. Modal gain (gmodal) (dB/cm) has been defined as the positive net output emission obtained after propagation and material losses by unit of length, while signal enhancement (SE) (dB) refers to the total Er 3+ emission output signal obtained as result of the whole waveguide cladding excitation by the pump. Calculations from experimental results were carried out by following the procedure of Rönn et al. [ 8 ]. For each probe wavelength (probe power at 313 W), the net pump power or estimated power inside the waveguide is tuned from 0.5 to 16 mW (figure 4.13). Measurements are taken with sufficient dwell time for accurate amplified spectrum emission (ASE) acquisition. For every power level, signal enhancement at pump wavelength is double checked by the difference between signal pump-on (spump_on) and pump-off state (spump_off), and evidenced by the visible up-conversion green light. Background noises, coming from the pump source is fixed in both probe-off and probeon states. Moreover, noise is acquired in the probe-off state and turned up to be no more than -55 dBm. Modal gain (gmod) and signal enhancement (SE) are calculated by subtracting the pump from the signal to the amplified output signal emission and normalized to the pumping off signal for each pumping power and waveguide length as follows in eq. 4.1, (eq.4.1)

In which Ltotal as equivalent total length of the effective waveguide, ASE refers to amplified spontaneous emission and  is defined as the absorption and propagation losses of material at Er 3+ emission (1530 nm or 1535 nm)

A second data analysis is similarly performed for signal enhancement (SE) in which the waveguide output signal of Er 3+ emission is normalized for different pumping power values to the output pump-off signal as follows in eq. 4.2, (eq.4.2)

Equivalent length measurements

The total length of waveguides includes taped structure leading to grating coupler like in figure 4.14 for fiber coupling. Taper structure has a different mode overlap than the single mode strip waveguide. Thus in order to carry out data treatment for optical gain, we had considered an equivalent total length (LT) corresponding of the pump and Er 3+ emission propagation in the waveguide as shown in eq. 4.3.

𝑳 𝑻 = 𝑳 𝒂 + 𝟐𝑳 𝒃 (  𝒃  𝒂 ) (eq.4.3.)
In this equation LT refers to the total equivalent length of the adiabatic taper structure and grating coupler (Lb) and the waveguide (La). The mode overlap () is referred as the percentage of optical power coupled in the cladding. This value has been theoretically calculated by two-dimensional (2-D) simulation (MODE Solutions from Lumerical, Inc.) of a waveguide cross-section for three waveguide widths (wa) 0.4, 0.6 and 0.8 m and average taper width (wb) of 15 m. As observed in table 4.1, for Er-doped YSZ monolayer (cladding 1) and Er 3+ -doped YSZ/PMMA bilayer (cladding 2) cladding, higher waveguide average mode overlap (a) are related to the bilayer cladding due to higher percentage of the mode in the cladding for this configuration. Values of the adiabatic taper mode overlap (b) for Er:YSZ and Er;YSZ/ PMMA cladding has been simulated to be of 19.5 and 21 %, respectively. Therefore, average mode overlap (ηa) depends on the waveguide width, decreasing for larger width values. As a result of our simulations and as shown in table 4.1, the most desirable width for all waveguide cladding configurations is 0.4 m. Moreover, it has been found following eq. 4.1. lower total equivalent length for all waveguides with smaller widths values. In table 4.2 we calculated from eq. 4.3. the total equivalent length (LT) for three waveguides width (0.4, 0.6 and 0.8 m) for Er-doped YSZ and Er-doped YSZ/PMMA cladding and we compared it with the total real length of adiabatic taper and waveguide lengths. Besides simulations for different waveguide widths (0.4, 0.6 and 0.8 m) we experimentally demonstrated higher signal enhancement for waveguides (cladding 1 and 2) with a width of 0.4 m. Experimental measurements to determine best waveguide width were performed in samples with same waveguide and cladding characteristics as sample 13. In our study, we experimentally measured the three waveguides widths. As result, in figure 4.15 can be observed signal enhancement for 1.1 mm waveguide length for all widths. The waveguide with 0.4 m width indeed proves to have higher signal enhancement (3.28 dB) which guarantees higher mode overlap with the active material. The increasing of the waveguide width leads to a decrease in signal enhancement: signal enhancement (SE) of 2.18 dB for 0.6 m wide waveguide and 0.7 dB for 0.8 m.

Absorption and material losses ()

Before achieving gain measurements, we characterized the SiNx waveguide losses with a tunable laser centered at a wavelength of 1530 nm and pumping power of 1 mW. Propagation losses of our waveguides includes losses of the grating coupler with maximum coupling efficiency of ~10 dB for wavelengths of 1530 nm, material absorption and light scattering from material interfaces and grain boundaries. Loss measurements were performed at Er 3+ emission wavelength under study (1530nm) ( ) to calculate modal gain in order to confirm if our material is suitable for optical amplification. Loss propagation is calculated by linear fitting of the transmission as a function of waveguide length (1.1, 1.5, 2 and 3 mm). First, we measured our SiNx waveguide without cladding as shown in the inset of figure 4.16 and propagation losses of -25.42 dB/cm at a wavelength 1530 nm have been obtained (figure 4.16). These losses are considered to be high due to the absorption by H-N bonds created in the SiNx deposition by PECVD. After Er:YSZ multilayer cladding deposition (inset figure 4.17(a)), propagation losses are expected to increase as result of light scattering at the interfaces between waveguide-cladding and grain boundaries of polycrystalline host and of material absorption. Results on this regard, in figure 4.17 (a), corroborate the increase of propagation losses up to -53.4 dB/cm for measurements at 1530 nm. Thereby, we added a layer of PMMA to our previous cladding as shown in inset of figure 4.17 (b). As commented, PMMA has a higher refractive index than air, pushing up the mode avoiding the mode leakage towards Si substrate through (buried oxide) BOX layer.

Measurements, reveal lower propagation losses for this double cladding configuration of -38.37 dB/cm at 1530 nm.

Single-sided pumped waveguides

Setup is configured as in figure 4.18, consisting on two input sources: one pump laser at fixed 1480 nm wavelength to excite Er 3+ ions and a second laser, a tunable laser with a wavelength range between 1500-1640 nm used as a probe. Moreover, pumping fix laser is connected by fiber to a variable optical attenuator (VOA). In this experiment we used tunable laser fixed at the Er 3+ ion emission wavelength to study the optical gain as a function of the wavelength. Both the 1480 nm pump and tunable laser are connected to a 90/10 beam splitter. Both pump and probe signals are polarized in TE mode which is the optimal polarization for the waveguide propagation (as evidenced by simulations). After a polarization control, both pump and probe signals are coupled to the device under test (DUT) using surface grating (20° incident angle). Output signal is fibered to an optical spectrum analyzer (OSA).

i) Optical gain on Er-doped YSZ cladding (cladding 1)

Modal gain (gmodal) and signal enhancement (SE) measurements for Er:YSZ multilayer cladding waveguides were performed by pumping one side of the waveguide varying pumping power from 0 to 18 mW at 1480 nm wavelength. As reported by PL measurements, Er 3+ ion gives emission at 1530 nm induced by 4 I15/2→ 4 I11/2transition, which is homogeneously distributed all over the thin film for all type of Er 3+ structures, therefore, measurements were carried out at this wavelength. Data treatment for one side pumping disposition and for Er:YSZ multilayer cladding 1 demonstrated optical modal gain of 10.64 and 10.24 dB/cm for 0.4 µm wide and 1.1 and 1.5 mm long waveguides, respectively. As defined by eq. 4.2 optical modal gain is proportional to signal enhancement per waveguide unit length. Besides, in modal gain we considered material absorption and propagation losses at 1530 nm wavelength. Thus larger modal gain values are expected for shorter waveguides as these measurements are analysed per waveguide unit length. In contrast, for signal enhancement measurements, larger signal enhancement (SE) values are ideally contemplated for longer waveguides as observed for similar studies [ 8 ]. Signal enhancement (SE) calculation takes into account total Er 3+ ion emission collected from the waveguide as the output signal, the longer the waveguide the higher number of excited Er 3+ ions. Despite signal enhacement (SE) values of 5.75 and 8.06 dB for 1.1 and 1.5 mm long waveguides, respectively (figure 4.19), we did not observe modal gain for waveguides over 1.5 mm in length, confirming high absorption and propagation losses for both pump and probe at 1480 and 1530 nm emission wavelengths. In order to diminish propagation and absorption losses due to mode leakage to Si substrate, an additional PMMA cladding has been suggested. In order to understand how emission of this field splitting effect behaves on a guided emission, we studied signal modal gain for three wavelengths: 1520, 1530 and 1560 nm. As observed in figure 4.20 (a) micro-PL emission profile for Er-doped YSZ multilayers cladding presenting Stark effect shows two maximum intensities at 1530 and 1535 nm. For the guided emission corresponding to the PL profile, we also determined modal gain at 1530 nm as well as at two wavelengths corresponding to two minimum PL on Er 3+ emission. We have chosen the wavelength of 1520 nm for being a local minimum in the PL spectra, hence, not showing a strong emission at this wavelength and being placed between the pump at 1480 nm and the main emission wavelength at 1530 nm. The second wavelength at 1560 nm shows lack of Er 3+ emission in the PL spectra. By measuring modal gain for a 1.5 mm length waveguide, we observed in figure 4.20 (b) modal gain of 10.12 dB/cm at 1530 nm. In contrast, for 1520 and 1560 nm only probe transmission and not signal enhancement is observed. As expected by simulation results for the double cladding configuration, lower propagation losses measurements, hence high modal gain at 1530 nm has been obtained for Er:YSZ/PMMA multilayer cladding (cladding 2) for three waveguide lengths (1.1, 1.5 and 2 mm) when single-sided pumping waveguides with a power range from 0 to 16 mW is considered (figure 4.21). For cladding 2, we noticed improvement of modal gain and signal enhancement in respect with Er:YSZ multilayer without PMMA cladding (cladding 1). In figure 4.21, we observed modal gain (gmodal) for 1.1, 1.5 and 2 mm long waveguides of 42.6, 30.51 and 8.57 dB/cm, respectively. As commented, the decreasing values with length for modal gain are expected. Otherwise, and as observed in previous measurements for the same cladding without PMMA layer, signal enhancement increases with the length of waveguide. We observe in figure 4.22 an increase of signal enhancement of 7.4 and 8.8 dB for 1.1 and 1.5 mm long waveguides, as expected. Although, for 2 mm long waveguide, signal enhancement is about 8.6 dB, similar to the value for 1.5 mm long waveguide. Similar signal enhancement for 1.5 and 2 mm long waveguide can be due to high absorption cross-section of pump wavelengths in long waveguides induced by highly doped RE materials. Hence, pump can only excite waveguide cladding partially, diminishing modal gain values and hindering signal enhancement values. For this reason, double pumping or two-sided pumping of the waveguide has been suggested in some studies [ 9 ]. A summary of modal gain (gmodal) and signal enhancement values are given in table 4.3.

Double-sided pumped waveguides

Previous setup configuration consisted of pumping from one side of the waveguide, but due to high absorption cross-section of the material, 1480 nm wavelength pump laser is absorbed along the waveguides resulting in a reduction of pumping efficiency with the waveguide length. Consequently, a new setup configuration similar as the one reported by Rönn et al. [ 10 ] has been proposed in figure 4.23. The pumping laser at 1480 nm wavelength is connected to a variable optical attenuator (VOA), one splitter output is connected to a circulator and one to the input of a 90 % beam splitter.

The second laser is a 1500-1640 nm tunable laser which is injected to the 10% input beam splitter. Both pump at 1480 nm and tunable laser signals are then coupled to the DUT. At the opposite edge of the DUT, the pump at 1480 nm is also injected. Therefore, waveguides are pumped at both sides, dividing input pumping power to the half for each side of the waveguide. Output signals from the waveguide are recorded an OSA. -Optical gain two-sided setup: pump laser at 1480 nm combined to a tunable optical attenuator (VOA) beam is split 50-50 % to a DUT and circulator. Tunable laser is connected to a 10% beam splitter, and coupled with the pump to the DUT in a two-sided pumping setup. Output signal from the waveguide is recorded using an optical spectrum analyzer (OSA).

i) Optical gain on Er-doped YSZ and PMMA cladding (cladding 2)

Double pumping configuration has the advantage of simultaneously pumping from both sides of waveguides. This technique is especially advantageous for long waveguides with high absorption cross-section cladding in which single-side pumping is absorbed by the cladding material, hindering excitation, thus providing only emission in a part of the waveguide. For pumping at both waveguide sides, pumping power has to be split by a 50/50 beam splitter in order to excite the whole length of the waveguide and achieve higher signal enhancement and optical gain under a power range of 0-7.5 mW at each side of the waveguide after pump power splitting. We explored modal gain and signal enhancement not only for Er 3+ ion 1530 nm emission but as well for other emission wavelengths such as 1535 nm for 0.4 µm wide waveguides and lengths of 1.1, 1.5, 2 and 3 mm.

Modal gain (gmodal) results at a wavelengths of 1530 nm (figure 4.24), reveal decreasing values (35.73, 16.57, 10.12 and 0.76 dB/cm) with increasing waveguide length 1.5, 2 and 3 mm) due to propagation and material losses. Regarding signal enhancement analysis at 1530 nm wavelength an increase of signal enhancement (SE) values (6.7, 7.9, 8.9 and 10.98 dB) were obtained with an increase of the waveguide lengths (1.1, 1.5, 2 and 3 mm) (see in figure 4.25). Modal gain and signal enhancement values for waveguide lengths of 1.1, 1.5, 2 and 3 mm can be compared with results obtained on the same waveguides with one-side pumping configuration setup. Though, input pumping power in each side of the waveguide is the half for double pump than for one-side configuration. Despite, double-pumping setup demonstrated modal gain (gmodal) of 0.76 dB/cm and signal enhancement of 10.98 dB for one additional waveguide of 3 mm long. We can notice here that such a measurement is not possible with one-side pumping configuration.

Conclusions

After optical characterizations of Er-doped YSZ thin films grown on sapphire described in chapter 3, samples with the highest photoluminescence and power efficiency are multilayer structures with metallic and non-metallic Er 3+ . We implemented by PLD multilayer structure (sample 13) on a silicon based device (strip SiNx waveguides fabricated by STMicroelectronics) to measure optical gain.

We determined by simulation the Er-doped YSZ layer thickness having the good trade-off between mode confinement and light interaction into cladding layer. Optimal Er-doped YSZ multilayer cladding thickness of 150 nm has been obtained in which maximum optical power percentage is found within the cladding for RE excitattion without losing mode confinement for TE mode and considering bending radii of 60 m. We also demonstrated with simulations that the addition of a material with refractive index moderately higher than air reduced the loss and increase light interaction. Consequently, 1 m PMMA cladding by spin coating was added to the oxide cladding.

To optically characterize sample 13, we performed spectroscopy in the visible range. Fiber-coupled spectroscopy for 150 nm thick Er:YSZ cladding (sample 13) has shown three main Er 3+ transitions: (i)

Conclusions and future perspectives

Conclusions

In my PhD research work, I strongly contributed to the development of a new hybrid silicon photonics activity based on doped crystalline oxides with rare earth ions. The main objective of the thesis was to demonstrate the capability of such platform for on-chip light amplification in the datacom and telecom wavelength range. My contribution has consisted in engineering the material, rare earth doped crystalline oxides, to obtain strong emission efficiency and optical gain and to implement such a material on a silicon-based device for light amplification.

This thesis can be divided in two main parts: (i) photonic material development including growth and its physical and optical characterizations and (ii) the hybrid integration onto a photonic device for the demonstration of light amplification.

(i) Photonic material development

In the first part, the growth and structural and optical characterizations of Er -and Pr-doped YSZ thin films were studied.

(a) Er-doped YSZ First, for Er 3+ ions implementation on a YSZ host we have explored two thin film growth approaches using two different PLD targets: metallic Er 3+ (ii) integration in photonic device First, we have grown, analyzed and demonstrated high crystallinity of multilayer Er-doped YSZ layer presenting high emission intensity and optical efficiency behavior.

In the second part of my thesis, I focus on the hybrid integration of doped YSZ on silicon nitride (SiNx) waveguides. We grow by PLD a multilayer structure of non-metallic and metallic Er-doped YSZ (sample 13) on a silicon-based device consisting on a strip SiNx waveguide buried on SiO2 (BOX) to measure emission behavior and optical gain.

When depositing multilayer thin films on amorphous substrate (SiNx waveguide) highly polycrystalline profile can be observed with two diffraction peaks corresponding to the (102) orientation tetragonal and the (331) orientation of the YSZ cubic structure. From waveguide simulations, 150 nm thick Er-doped YSZ multilayer deposited onto the SiNx waveguide demonstrates a good trade-off between mode confinement and light interaction in doped oxide. Moreover, by adding 1 m thick PMMA top cladding, it has been noticed a significant reduction of the optical loss due to light leakage towards the Si substrate through the BOX layer.

Experimental analysis on waveguide width pumped at 1480 nm proved best transmission results emission at 1530 nm wavelength for 0.4 m waveguide width among measurements for 3 different widths (0. A double-sided pumped approach has been also used to optimize the pumping of the Er:YSZ/PMMA SiNx waveguides and compensate the light cross-section absorption in long waveguides. We measured optical gain for four waveguides lengths: 1. These promising results pave the way to the development of a new hybrid platform on doped crystalline oxides for photonics.

Future perspectives

In short terms, an optimization of the material growth and a better understanding of Er 3+ substitution in the YSZ lattice will be performed: (i) Improve the crystallinity of the multilayer thin films grown on amorphous materials. A possible way is to change PLD conditions such as laser power, oxygen flux or temperature, or exploring substrate preparation and /or annealing gas atmosphere and temperature. Such an optimization was not possible during my PhD due to the move of the lab and some issues on the starting leading to some delays.

(ii)

Explore thickness and doping on Er-rich YSZ and 0.5 at. % Er:YSZ layers to enhance photoluminescence in multilayers, avoiding quenching effects.

(iii) Further study of the Er 3+ sites taken in the YSZ structure and its relationship with enhanced Stark effect emission. In this thesis we have related different PL absorption and emission profiles and CL emission profiles for layers with metallic Er 3+ and non-metallic Er 3+ . Furthermore, we might explain enhanced Stark effect emission with the substitution of Zr 4+ by non-metallic Er 3+ . Despite all the evidences found by optical and structural characterizations, oxidation study of Er 3+ by EELS and its connection to Er 3+ emission would be a next step unveiling the origin of enhanced Stark effect emission.

(iv) Low photoluminescence efficiency results on 0.5 at. % Pr-doped YSZ thin films for emissions at 1350 nm are consequence of higher presence of non-luminescent Pr 4+ than Pr 3+ luminescent ions. Higher efficiencies on Pr 3+ emission can be achieved by doping the sample at higher concentrations.

(v)

Moreover, enhancement of Er 3+ emission could be attained by co-doping the samples with other rare-earths as Y 3+ or Yb 3+ [ 3 , 4 ]. Other possibilities are doping of rare-earths with emissions in the near-IR such as Ho 3+ or Tm 3+ at 1.2 and 1.4 m, respectively [ 5 , 6 ].

In short-mid terms, the hybrid integration will develop and optimize in order to get lower linear propagation loss and high optical gain. In this regard, future work should scope:

(i) Optimization of the SiNx process is required and then the optimization of the epitaxial growth of doped YSZ. This work directly link to the material development is essential.

(ii)

Crystalline oxide deposition requires 800°C for deposition. Implementation of materials sensitive to damage at high temperatures in the Si platform requires to explore fabrication techniques to avoid material damage. (vi) By its optical properties, applications of YSZ hybrid integration on silicon-based platforms can be explored in communications by using RE ion emission in the mid-IR wavelength range, sensing or quantum optics. In the interface between two surfaces there is a ratio of light transmitted (t ,t) and the rest is reflected (r ,r). This ratio can be expressed by the Fresnel equations (equations A. Finally, it is important to remark that Cauchy model is preferred not only for fitting the experimental data accurately but as well because it is not limited by Kramers-Kroning consistency.

A.2. X-ray diffraction (XRD)

This technique has as main advantage in comparison with other structural characterization techniques such as TEM that it does not damage the sample. It is based on the elastic diffraction of X-rays with the electrons in the periodic structure of crystalline materials. This kind of diffraction is based on the direct relationship between incident and reflected beam in which momentum (p) between the two particles, (eq. A.8) is conserved. In elastic scattering, incident and reflected wavenumber (k) only changes its direction, while momentum remains constant.

Conservation of momentum (q) transfer for a general system with photons and particles is noted as shown in eq. A.9.

(eq. A.9)

In which 1 and 2 refers to the photon and particle and i and f to the initial and final momentums. For elastic scattering systems, there is no variation of momentum on the system, before and after the scattering, remaining constant. By substituting eq. A.8 in eq. A.9 applied for X-Ray diffraction we obtain (eq. A.10), (eq. A.10) For crystalline materials, constructive interferences are detected as intensity peaks after reflection. Constructive interferences come from same optical paths taken by all diffracted beams and they are a multiple of the incident wavelength, following the Braggs law, In other words, the structure factor (F) is the contribution of the waves from all atoms in the unit cell and fj is defined as the scattering factor.

(i) X-ray diffraction techniques

We have used a X′Pert XRD (PANalytical) powder diffractometer, using CuKα radiation with a wavelength of 1.5406 Å. As shown in figure A.5, the source in an X-ray diffraction system is a x-ray tube which consists in a cathode and an anode of copper under a high voltage in the vacuum. The material of the anode determines the x-ray emission wavelength, in our case it is copper, other materials are tungsten or molybdenum. X-rays emitted by the tube are focused by a parabolic mirror in a collimator. The resultant parallel x-ray beam diffracts on the sample at an angle, and the reflection is registered by a detector. The detector can be used in two different configurations: double axis or triple axis. The first configuration provides higher sensibility to detect crystalline planes and second one, higher resolution. X-ray system configuration is defined in figure A.5. Sample in our case is a thin film that can be placed in the sample holder and moved along the x, y, z coordinates, the X-ray beam and detector can be rotated in the ω,  and φ angles which allows to align the beam to the (hkl) plan in diffraction condition. Different scans (or acquisition during a rotation or translation of the sample or the detector) can be performed. For polycrystalline samples grown on an amorphous substrate, X-ray diffraction in non-coplanar grazing incidence (GIXRD-Grazing Incidence X-Ray Diffraction) with the diffractometer Rigaku Smartlab has been performed. The X-ray system is similar at the one described but this diffractometer allows diffraction of planes perpendicular to the surface. Thus, this technique is normally used to characterize the direction of epitaxy or variation of lattice parameter of the samples.

Different measurements modes can be used in the X′Pert XRD (PANalytical) powder diffractometer depending on the configuration:  The 2θ-scan or detector scan, in which the 2θ angle formed by the sample and the detector changes maintaining  fixed. If  changes (-2θ scan), this type of scan is called coupled scan. This last technique is used to measure Bragg diffraction angle and gives information of the family of planes diffracted, the growth direction of the film, d-spacing and thickness of the sample when θ is placed satisfying Bragg's law. This technique has been used in this thesis for samples grown on sapphire.

 The ω-scan, called the rocking curve allows to study the mosaicity of a plan in a thin film. Mosaicity (figure A.6) refers to the misorientation of differently oriented crystals in the same sample due to the nucleation and growth on the substrate The detector is set at a specific angle following Bragg's law diffraction, then the sample is tilted. When the sample is a perfect monocrystal, mosaicity is inexistent and a sharp diffraction peak will be obtained.

 The φ-scan can be performed in the asymmetric and symmetric condition to determine the orientation of the YSZ grains in the epitaxial relationships of the planes with the substrate.

FIGURE A.6 -From left to right, schematic of an ideal crystal with all q vectors having the same direction, this effect can be observed as a perfect circular spot in the perpendicular and parallel q diagram and is observed in the x-ray diffraction profile as a sharp lorentzian (a). From left to right, a crystal presenting mosaicity, each variation on the direction of the crystal is observed as a multiple overlap spots in the q-projection diagram. In the x-ray diffraction analysis, it is observed as a broadening of the lorentzian (b).

A.3. Atomic Force Microscopy (AFM)

Atomic Force Microscopy is a usual technique in materials science, biology, biomedicine and nanotechnology to characterize the topology, detect forces and allow mechanic measurements as viscoelasticity and hardness of a sample. As well it allows measurements in liquid medium. AFM is based in high-resolution scanning probe microscope (SPM) in which a cantilever ended by a sharp tip locally interacts with the surface of the sample. Depending on the functionability the tips and the cantilever can be made of different or the same material such as silicon, silicon nitride, glass, diamond or with metallic coatings among others. The atoms on the tip create interatomic forces of attraction or repulsion (mainly Van der Waals forces) with the sample surface, thereby, the cantilever is deflected. A laser beam incident to the surface of the cantilever, is reflected to a photodetector. Differences on the reflected laser beam due to the cantilever movement creates a difference in current in the photodetector making possible the surface mapping of the sample. Scanning of the sample surface can be performed in a reduced area in three different modes (figure A.7):

 Contact mode: is the most used mode in which mainly silicon nitride tips are used. This mode allows not only surface scan by repulsion coulomb interaction between the ion cores, achieving atomic resolution but as well force modulation measurements.



Non-contact mode: is performed on samples in which their surface can be damaged by the tip of the cantilever. In this mode the cantilever oscillates at about 1-10 nanometers of the surface. At this distance the atoms of the surface of the sample and the tip of the cantilever are bounded to Van der Waals forces. An image of the surface is obtained when the probe scans the surface of the sample and forces experienced by the tip are recorded.



Tapping mode: this mode is used in samples with difficult topography in which the tip of the cantilever oscillates at its resonance frequency and it is intermittently in contact with the sample. Cantilevers with silicon tips are mainly used for this configuration. Lower resolution than with contact mode can be achieved. The cantilevers' oscillation amplitude changes with topography, and by mapping these variations we can have an idea of the topography.

A.4. Scanning Transmission Electron Microscopy (STEM)

Scanning Transmission Electron Microscopy (STEM) is a type of transmission electron microscopy (TEM) which provides a complete characterization of nanoscale materials and devices. STEM is based on the interaction between a fast electron beam scanning through the crystalline atomic structure of a sample. The incident electrons from the beam interact with the periodic potential of the crystallized sample, then electrons' wave function modifies amplitude and phase, creating a contrast which contributes to an image. Samples in this thesis were defined by Focused Ion Beam (FIB). This technique requires ion and electron beams to polish and thin the cross-section geometry of the sample up to some tens of nanometers to be observed by STEM.

Different microscopic techniques can be used to obtain specific information of the sample; in this thesis the following techniques were used by Ludovic Largeau at C2N:



High Resolution STEM (HR-STEM) is related to phase contrast. Interaction between electrons diffracted by different families of planes and the incident undiffracted beam gives rise to a periodic contrast, parallel to a given family of planes and with a periodicity related to its spacing.



High-angle annular dark-field (HAADF) imaging is an STEM technique based on Z-contrast in which scattered electrons are detected by an annular darkfield detector placed around the beam.



Annular bright-field (ABF) STEM imaging is a technique used to observe atoms with low atomic number  Energy-dispersive X-ray spectroscopy (EDXS) technique gives chemical composition characterization of a sample related to a high resolution image. The interaction with the incident electron beam and the difference in energy between inner and outer shells electrons in the sample creates an x-ray emission with a characteristic energy for each atom. An X-ray semiconductor detector placed nearby the sample, reveals the atomic number and energy of emitted X-rays, thus chemical elements and their relative abundance can be identified.

For HR, HAADF-STEM, ABF and EDXS characterization a corrected FEI ThermoFisher TEM/STEM Titan Themis 200 microscope with a Chemistem Super-X (EDX) detector, in the C2N facilities, was used.

A.5. Equivalent length

Light propagation in gain materials follows the differential propagation equation:

Ps is referred to the probe power signal and z,t are the propagation direction and time. After the equal sign,  is the mode overlap. Then, first coefficient on the square bracket corresponds to signal enhancement in which Ce is the emission efficiency of the excited level which depends on the pump power (Pp ) which varies little along the structure for each measurement and lifetime (). Absorption coefficient (

) of the material which remains constant for pumping power measurement and doping (D) concentration of RE in the material both remains constant. o is the propagation loss of the material in which = , being  the modal propagation loss, both and  measured without pump and are negative variables. ps() are the passive propagation losses.

Thus we can approximate the material gain coefficient (Gm) as a constant value for each Pp (z,t) measurement.

+ (eq. A.20)

When propagation losses (o) are higher than emission efficiency (Ce), material gain is negative ( ). If emission efficiency (Ce) is higher than propagation losses, the material works as gain media ( Signal enhancement for the total length of taper and waveguide can be expressed as:

(eq. A. For same one-side pumping measurements considering total length 1.1, 1.5 and 2 mm, we calculated modal gains of 24±6, 20±3 and 3±2 dB/cm, respectively (figure A.9). In table A.2 we show modal gain for equivalent and total waveguide lengths, it can be observed higher values of modal gain calculated using equivalent length model. Higher modal gain is obtained for shorter (1.1 mm total length or 0.85 mm equivalent length) waveguides with cladding 2 (Er:YSZ/PMMA) pumped by one side.

A.6. Table of samples

Detailed description of samples which include layer composition, thickness, number of layers and total thickness of the thin film. Schematics of multilayer and monolayer structures.

INTRODUCTION

Au cours des dernières décennies, la recherche d'une plus faible consommation d'énergie des dispositifs photoniques a porté sur la miniaturisation et la mise en oeuvre de nouveaux matériaux pour des applications telles que la datacom, les télécommunications, la détection ou l'optique quantique. Dans ce contexte, les oxydes fonctionnels se sont révélés une classe de matériaux très intéressante en raison de leurs caractéristiques singulières comprenant la supraconductivité, le magnétisme, la ferroélectricité, l'activité catalytique, la commutation résistive, les effets électro-optiques, la piézoélectricité ou les non-linéarités optiques, entre autres [1]. L'un des principaux défis de l'ingénierie des matériaux concernant l'intégration hybride sur silicium photonique a été la croissance épitaxiale de films d'oxyde fonctionnel de haute qualité cristalline pour maintenir la diffusion de la lumière des interfaces au minimum [3]. Cette technique nécessite une couche tampon pour l'adaptation du réseau entre le silicium et l'oxyde fonctionnel, qui sert également de barrière d'interdiffusion entre le substrat et les matériaux oxydés. L'un des oxydes les plus populaires à cet égard est la zircone stabilisée à l'yttria (YSZ), largement utilisée comme couche tampon pour LiNbO3, PbTiO3, Pb (Zr, Ti) O ou YBa2Cu3O7 dans les systèmes intégrés hybrides qui sont couramment utilisés pour diverses applications telles que comme pour développer des électrolytes solides, des capteurs de gaz ou des piles à combustible. En outre, YSZ a été largement utilisé comme tampon non seulement pour son aptitude épitaxiale au silicium et aux oxydes fonctionnels, mais également pour sa stabilité thermique et chimique bien connue et sa dureté et sa durabilité mécanique. Bien qu'il reste un matériau inexploré pour l'optique intégrée, il possède des propriétés optiques très intéressantes qui pourraient être exploitées comme le fait qu'il appartient à la fascinante famille de matériaux de contraste à indice moyen (MiDex), sa large bande interdite optique qui empêche l'absorption de deux photons (TPA) dans l'IR proche et moyen; la grande fenêtre de transparence de fonctionnement de l'ultraviolet au moyen infrarouge et son bon effet Kerr, maintenant la diffusion de la lumière au minimum. En revanche, les dispositifs de guide d'ondes dopés aux terres rares sont des composants intéressants pour améliorer la lumière dans les communications optiques intra-puces, rendant ainsi le développement de nouvelles sources lumineuses intégrées et d'amplificateurs optiques très judicieux et opportun. Malgré les avantages du dopage aux terres rares, l'ingénierie des matériaux relève le défi du dopage de l'hôte à haut niveau dans des films cristallins en évitant les effets de trempe une interaction ion-ion non linéaire. À cet égard, plusieurs études fructueuses sur les hôtes matériels, les conceptions de dispositifs et les plates-formes d'intégration pour les terres rares ont été réalisées [4]. L'amplification a été démontrée dans les oxydes fonctionnels comme LiNbO3 et le gain pour les oxydes comme Al2O3 et Y2O3, et a été intégrée avec succès dans les dispositifs photoniques. Les couches minces de Y2O3 sont de bons candidats comme hôtes pour les ions de terres rares dans la construction de guides d'ondes actifs pour des applications de télécommunication fonctionnant à 1,55 µm. De plus, l'erbium est connu pour avoir des transitions énergétiques qui conduisent à des longueurs d'onde d'émission dans la bande des télécommunications, faisant de cette terre rare un candidat parfait pour les applications de télécommunications. Des études récentes prometteuses sur les guides d'ondes optiques à côtes YSZ ont démontré une perte de propagation aussi faible que 2 dB / cm à une longueur d'onde de 1380 nm [2], encourageant l'utilisation d'oxydes fonctionnels pour des applications photoniques. Dans ce travail, nous démontrons la croissance épitaxiale d'oxydes de terres rares sur un substrat cristallin, leur intégration sur des dispositifs optiques amorphes à base de silicium et leur caractérisation optique, démontrant ses remarquables propriétés de luminescence. De plus, nous explorons également la possibilité d'introduire des dopants actifs de terres rares dans YSZ pour démontrer les amplificateurs de guides d'ondes optiques sur puce basés sur YSZ. Pour cela, nous avons étudié les propriétés structurelles et optiques des ions Er 3 + dans différentes plates-formes en utilisant une approche multicouche et déposés par la technique de dépôt laser pulsé (PLD). Nous prouvons une forte luminescence pour la transition erbium qui correspond à la bande C des télécommunications et dans le visible (émission verte) sous une excitation laser à pompe à onde continue à environ 980 nm et 1480 nm. A l'ombre de ces résultats, une exploration en profondeur de l'amplification lumineuse de couches minces dopées Er 3+ déposées sur une plateforme photonique passive SiN est en jeu.

PROCÉDURE EXPÉRIMENTALE ET CARACTÉRISATION

Le dépôt de couche mince Er: YSZ a été réalisé par dépôt laser pulsé (PLD). Cette technique est basée sur l'ablation laser de deux cibles rotatives de cible céramique YSZ 8% mol et d'erbium métallique réalisée par un laser excimère KrF (longueur d'onde 248 nm, fluence 3-J / cm² et taux de répétition 5 Hz) à 800 ° C dans une atmosphère d'oxygène à différentes pressions. Une forte conversion ascendante similaire a déjà été observée dans d'autres plates-formes matérielles affichant un gain optique, offrant ainsi un scénario intéressant pour explorer davantage les caractéristiques de gain de ce nouveau matériau à gain pour la photonique intégrée.

Mesures de gain optique

La caractérisation optique et les simulations de nos guides d'ondes gainés ont révélé que notre matériau convient à des fins photoniques. En conséquence, nous avons effectué des mesures en excitant notre matériau à la longueur d'onde d'absorption de Er 3 + à 1480 nm et en analysant les données de gain modal (g) et d'amélioration du signal (SE) pour les émissions d'ions Er 3 + proches de l'IR. Le gain modal a été défini comme l'émission de sortie nette positive obtenue après propagation et les pertes de matière par unité de longueur, tandis que l'amélioration du signal fait référence au signal de sortie d'émission Er 3 + total obtenu à la suite de l'excitation de la gaine du guide d'ondes par la sonde. Pour chaque niveau de puissance, l'amélioration du signal à la longueur d'onde de la sonde est vérifiée deux fois par la différence entre le pompage du signal (spump_on) En raison de sa bande interdite indirecte, le silicium est un matériau électroluminescent très peux efficace. Pour cette raison, l'intégration hybride d'autres matériaux pour l'amplification optique a été objet d'étude. L'un des principaux handicaps de la croissance de matériaux cristallins sur des plateformes de silicium se trouve dans le décalage du paramètre de maille du silicium qui introduit des contraintes et des défauts dans le réseau du matériau. À cette fin, l'ingénierie des matériaux optiques pour l'intégration photonique est devenue un sujet de recherche intéressant visant à fournir des fonctionnalités supplémentaires.

À cet égard, les oxydes fonctionnels constituent une classe de matériaux très intéressante en raison de leurs propriétés pour la photonique. L'ingénierie des matériaux est couramment utilisée pour régler et manipuler ces propriétés à volonté, étant ainsi des oxydes fonctionnels souvent utilisés pour construire des éléments reconfigurables actifs dans des systèmes complexes. Les oxydes transparents avec des indices de réfraction modérés sont ciblés pour l'intégration hybride en raison des avantages. L'adaptation de réseau à l'aide d'une couche avec paramètre de maille entre le silicium et l'oxyde fonctionnel est une solution élégante pour faire croître par épitaxie des films d'oxyde fonctionnel cristallin de haute qualité et pour éviter l'interdiffusion entre le substrat et le matériau oxyde. Cette technique nécessite d'un buffer pour visualiser les interfaces de films minces sans défaut, la diffusion de la lumière et la densité des défauts sont maintenues au minimum. Since the beginning of the 20th century with the development of telecommunications, new designs and materials were studied for the search of low power consumption photonic devices on siliconplatforms for applications such data-com, telecom, sensing, or quantum optics. Due to its indirect bandgap, silicon is a nonefficient light emitting material. For this reason, hybrid integration of other materials for optical amplification has been studied. One of the main handicaps for crystalline materials implementation on silicon platforms is found in the lattice mismatch imposed by silicon which introduces constraints and defects in the material lattice. For this purpose, engineering optical materials for photonic integration has become a hot research topic aiming at providing additional functionalities. In this regard, functional oxides are a very interesting class of materials due to their singular properties. Material engineering is commonly employed to tune and manipulate such properties at will, thus being functional oxides often used to build active reconfigurable elements in complex systems. Transparent oxides with moderate refractive indexes are targeted for hybrid integration due to the rewarding benefits envisioned. Lattice adaptation using a buffing layer between silicon and the functional oxide is an elegant solution to epitaxially grow high-quality crystalline functional oxide films and to avoid interdiffusion between substrate and oxide material. This technique requires a buffer layer to envision defect-free thin films interfaces light scattering and defect density kept to minima.

Since the beginning of the 20th century with the development of telecommunications, new designs and materials were studied for the search of low power consumption photonic devices on siliconbased platforms for applications such data-com, telecom, sensing, or quantum optics.

Yttria-Stabilized Zirconia (YSZ) is a well-known buffer oxide for other functional oxides. Among its properties should be noted: chemical stability, transparency range from the visible to the mid-IR, a refractive index around 2.15, which makes this functional oxide interesting for the development of low-loss waveguides when grown over a low contrast substrate. While these optical properties are very interesting for various applications, including on-chip optical communications and sensing, YSZ has remained almost unexplored in photonics. After proving low propagation losses of 2 dB/cm at a wavelength of 1380 nm in highly monocrystalline YSZ waveguides, we intended to explore this material for optical amplification. In this thesis, we explore the deposition of Er 3+doped YSZ thin layers by pulsed layer deposition (PLD) on a multilayer and monolayer approach providing luminescence in correspondence with C-band of telecommunication window (λ=1.5 µm) and in the visible range of wavelengths. Structural and optical characterization lead to a highly optical efficient material showing enhanced photoluminescence at 1530 and 1535 nm wavelengths. After characterization, we integrated our material as cladding on SiNx waveguides. Propagation losses and optical amplification at 1530 and 1535 nm using different pumping techniques are discussed in this thesis. Moreover, multilayer approach used for Er 3+ has been explored for other rare-earth materials such as Pr 3+ . On this subject, structural and optical characterization results on Pr 3+ -doped YSZ emission at 1350 nm in the O-band of telecomm window (λ=1.3 µm) paves the way towards implementation and interaction of near-IR emitting rare-earth materials in YSZ.

FIGURE 1 . 1 -

 11 FIGURE 1.1 -Schematic of III-V laser integrated on a silicon substrate. Image (a) shows waveguides etched in a silicon substrate. A III-V wafer is bonded to the substrate in images (b) and (c). The removal of a sacrificial layer in image (d) allows III-V epitaxial growth in (e). Finally, etching and deposition give a vertical p-i-n diode laser structure in image (f) [ 28 ].

FIGURE 1 . 2 -

 12 FIGURE 1.2 -Band structure of intrinsic Ge, in blue and red the bandgap at the  valley and in green the direct bandgap in the L valley. Image on the left shows an indirect bandgap at the  valley for unstrained Ge structure and on the right, direct bandgap structure at both valleys in strained Ge [ 35 ].

FIGURE 1 . 3 -

 13 FIGURE 1.3 -Schematic of ion-ion interaction including (a) co-operative up-conversion, (b) energy migration,(c)cross-relaxation and (d) non-radiative de-excitation [ 49 ].

FIGURE 1 . 4 -

 14 FIGURE 1.4 -Spectrum of erbium emission in the near-IR in a crystalline host(a), showing sharp photoluminescence (PL) intensities, while broadened PL intensities are found for amorphous hosts (b) [ 52 ].

Chapter 1 10 1 . 3 . 2 .

 1132 Host matrix for RE doping for optical amplification (State-of-the-art)

  More recently in 2020, Er 3+ optical amplification at 1533 nm in Al2O3 host matrix was measured by J. Rönn et al.[ 160 ] on an integrated Si3N4 planar waveguides with Er: Al2O3 cladding (figure 1.5). In this study, they reported up to 20.1 ± 7.31 dB/cm net modal gain in 250 µm length waveguides and at least 52.4 ± 13.8 dB/cm of net material gain for 1200 µm waveguides. The same year 2020, group of Sonia Garcia-Blanco in university of Twente achieved net gain of 18.1±0.9 dB at 1532 nm for 10 cm Si3N4 waveguides with Er:Al2O3 cladding [ 161 ].

FIGURE 1 . 5 -

 15 FIGURE 1.5-Schematic of Si3N4 waveguide amplifier structure (a) and its cross-section of the Si3N4 waveguide on SiO2 and Er:Al2O3/PMMA cladding on top and simulation crosssection of the electric field distribution of the fundamental modes TE and TM (b) [ 162 ].

FIGURE 2 . 2 -

 22 FIGURE 2.2 -Schematics of the Frank-van der Merwe model or epitaxial growth layer by layer.

FIGURE 2 . 3 -

 23 FIGURE 2.3 -Schematics of the Volmer-Weber model or epitaxial growth by islands.

FIGURE 2 . 4 -

 24 FIGURE 2.4 -Schematics of the Stranski-Krastanov model or epitaxial growth by layers and islands.

FIGURE 2 . 5 -

 25 FIGURE 2.5 -Schematics of growth by electron beam physical vapor deposition on a substrat.

FIGURE 2 . 6 -

 26 FIGURE 2.6 -Schematics of sputtering of a sample surface for epitaxial growth.

FIGURE 2 . 7 -

 27 FIGURE 2.7 -Molecular beam epitaxy's schematics.

FIGURE 2 . 8 -

 28 FIGURE 2.8 -Schematics of pulsed laser deposition growth.

FIGURE 2 . 9 -

 29 FIGURE 2.9 -Schematics of zirconia (a) and yttria structure (b). In YSZ, yttria has two oxygen vacants, C3i (c) and C2 (d) depending on its site in the YSZ structure (e).

FIGURE 2 . 10 -

 210 FIGURE 2.10 -Absorption profile of erbium transitions from the visible to the near-infrared range of wavelengths [ 27 ].

FIGURE 2 .

 2 FIGURE 2.11 -Emission PL profile of Er 3+ ion transitions in the near-IR (a) and visible wavelengths. Embedded schematics represents energy transitions for absorption at 980 nm (ground state absorption (GSA) and excitation state absorption (ESA)) and PL emissions in the visible range: at 525 and 550 nm in green and 675 nm in red; in the near-IR in grey at 1530 nm.

FIGURE 2 . 13 -

 213 FIGURE 2.13 -Absorption profile of Pr 3+ from the visible to the mid-infrared wavelengths[START_REF] Heber | Energy levels and interaction between Eu ions at lattice sites of symmetry C2 and symmetry C3i in Y2O3[END_REF].

FIGURE 2 .

 2 FIGURE 2.14 -On the left profile, emission transitions of praseodymium from the visible to the near-infrared [ 39 ]. On the right image, schematics of absorption and emission energy levels in praseodymium [ 40 ].

FIGURE 2 . 14 -

 214 FIGURE 2.14 -Pulsed laser deposition chamber. Right image shows PLD chamber. Red square, top left image shows the outside of the chamber in which substrate holder with

FIGURE 2 . 15 -

 215 FIGURE 2.15 -Sapphire c-cut oriented plane in the hexagonal structure.

FIGURE 2 . 16 -

 216 FIGURE 2.16 -Monolayer of Er-doped YSZ (a), Er-rich and sintered Er-YSZ multilayer structure (b) and Multilayer structure of Er-and Pr-doped YSZ (c).

FIGURE 2 .

 2 FIGURE 2.17 -Multilayer structure deposition steps: on left image first layer type of 0.5 at.% Er-doped YSZ. Second image on the right, first and second layer. Second layer is formed by a 0.5 at.% Er-doped YSZ layer doped with metallic Er 3+ .

TABLE 2 . 1 -

 21 Values in mol percentage (mol %) of the sintered target composition: ZrO2, Y2O3, Er2O3 and Pr6O11, and the corresponding atomic percentage in rare-earth element for 6 different PLD targets.

2 .

 2 For the refractive index, measured values in this study do not differ from ellipsometry 8 %mol YSZ studies done by Marcaud et al. with values of n= 2.15±0.03[ 44 ]

FIGURE 2 . 20 -

 220 FIGURE 2.20 -Fitting profile following Cauchy model of experimental light dispersion at different wavelengths for 149 nm (samples 5  , 6 * , 7 * , 8 * and 9 * ) (a) and 313 nm (samples 5, 6, 7, 8 and 9) (b) YSZ:Er 3+ monolayers.

60 TABLE 2 . 2 -

 6022 Calculated growth rates of all PLD targets under same deposition conditions.

FIGURE 2 .

 2 FIGURE 2.21 -X-ray diffraction profile of Er-doped YSZ monolayers with different percentage of Er, showing two Er:YSZ orientations (111) and (002) on c-cut sapphire (0006) arranged from top to bottom in decreasing concentration of Er 3+ doping 3, 1.5, 0.5 and 0% in red, orange, blue and black, respectively.

FIGURE 2 .

 2 FIGURE 2.22 -X-ray diffraction profile of Er:YSZ thin film with multilayer structure in which Er-rich interlayer distance variates from one sample to another. Image on top in red (sample 14), corresponds to a total number of 24 layers for 300 nm thickness of thin film, one diffraction peak at 30° is observed. In black, bottom image, the distance between two Er-rich YSZ layers is total number of 40 layers for 300 nm thickness of thin film and diffraction peak at 35°.

FIGURE 2 .

 2 FIGURE 2.24 -X-ray diffraction profile of Er:YSZ thin film with multilayer structure consists of 1.95 nm Er-rich YSZ and 1.32 nm YSZ thickness, showing (111) and (001) orientations of YSZ on c-cut sapphire.

  .26 (b), (d)), indicating an increase of the lattice parameter for part of the film in Pr-doped YSZ and a decrease in Er-doped YSZ.

FIGURE 2 .

 2 FIGURE 2.26 -X-ray diffraction profile in red for annealed Er:YSZ and in black for Pr:YSZ monolayers, two growth orientations are evidenced at 30° and 35° (a),(c) and the 35° intensity peak presents a shoulder (b),(d).

FIGURE 2 .

 2 FIGURE 2.27 -X-ray diffraction profile of Er:YSZ. Image on top in black, thin film deposited at 800°C before annealing. Bottom image in red, after annealing.

FIGURE 2 .

 2 FIGURE 2.29 -AFM images of sapphire substrate annealed surfaces. From bottom to top, substrate annealed at high temperature rate, low oxygen flux and following Marcaud et al. protocol [ 43 ].

FIGURE 2 .

 2 FIGURE 2.30 -AFM images of (a) sapphire substrate annealed surface and (b) thin film surface after PLD deposition.

FIGURE 2 .

 2 FIGURE 2.31 -HAADF-STEM image of a cross-section multilayer Er:YSZ:Er structure (sample 4). In the multilayer structure can be observed a dark (a) and light (b) contrast fringe corresponding to thicker 0.5 at.% Er 3+ doping on Er:YSZ:Er layer and higher Er 3+ at.%, respectively.

FIGURE 2 .

 2 FIGURE 2.32 -HAADF-STEM high resolution image of a multilayer Er:YSZ structure. Superposed in white, periodic contrast profile of the fringes and their thicknesses.

FIGURE 2 .

 2 FIGURE 2.33 -HAADF-STEM image on the left, of a multilayer Er:YSZ structure in which atomic disposition can be observed. Embedded image is a magnification of local image in black square, for a better understanding of the atomic structure. On the right, top and bottom, schematic structure of YSZ in the (111) and (001) family of planes, respectively.

FIGURE 2 .

 2 FIGURE 2.34 -Left scheme of YSZ structure in which (001) and (111) family of planes in [110], [1-10], [0-11] and [100], zone axis in blue, green, orange and red squares, respectively. Right top and bottom HAADF-STEM image of the cross-section sample. Embedded in top and bottom right images, FFT and SAED, respectively with colored label of diffraction intensities.

FIGURE 2 .

 2 FIGURE 2.35 -Right image corresponds to HAADF-STEM image of multistack Er:YSZ:Er cross-section sample. From left to right and top to bottom, EDX mapping of erbium, oxygen, zirconium and yttrium in violet, blue, red and green.

FIGURE 2 .

 2 FIGURE 2.36 -HAADF-STEM images of multistack structure with variable Er-doped YSZ (sample 4). Contrast profiles in white correspond to the HAADF image. Left image profiles colored in red and violet to zirconium and erbium extracted from EDX mapping data. Right image in blue and green to EDX mapping from oxygen and yttrium, respectively.

FIGURE 2 . 37 -

 237 FIGURE 2.37 -Left graphic corresponds to normalized total EDX atomic percentage of embedded HAADF_STEM image, in white row, direction of the analysis, in blue, red, green and violet, profiles of oxygen, zirconium, yttrium and erbium. Right graphic of EDX profile, normalized percentage of zirconium and erbium separately in red and violet, respectively.

FIGURE 2 .

 2 FIGURE 2.38 -EDX profile corresponding to the ratio between the percentages of zirconium and yttrium and to the percentage of erbium, normalized separately in blue and violet lines, respectively.

FIGURE 2 .

 2 FIGURE 2.39 -From top to bottom, graphic of EDX profile corresponding to normalized percentage of erbium, oxygen and zirconium.

FIGURE3. 1 -

 1 FIGURE3.1 -Schematics (a) and image (b) of microphotoluminescence setup. Laser beam in excites the sample by means of a microscope, light emission is focused by the lens and filtered before being detected by the spectrometer.

FIGURE 3 . 2 -

 32 FIGURE 3.2 -PL spectra intensities at 40mW of YSZ: Er 3+ thin film (sample 5) in the IR range of wavelengths for pumping wavelengths from 800 to 980nm in black axis. PL emission intensity at 1530nm is shown as a function of pumping wavelengths in red axis.

FIGURE 3 . 3 -

 33 FIGURE 3.3 -Normalized PL absorption spectrum intensity for pumping wavelength range from 780 nm to 1000 nm for (from top to bottom): Er2O3 monolayer emitting at 1535 nm in yellow, Er:YSZ monolayer at 1530 nm in red, Er:YSZ multilayer thin film with emission at 1530 nm in blue (sample 4), and emission at 1535 nm in green due to local Stark effect (sample 3).

FIGURE - 3 . 4 .

 34 FIGURE -3.4. PL spectra from 4 I15/2→ 4 I11/2 Er 3+ transition from: Er2O3 (sample 11) (black) and Er-rich and Er-doped YSZ multilayer (sample 4) thin film. For multilayer thin film (sample 4): PL spectrum of local areas presenting Stark effect is shown in red and non-Stark effect in blue.

FIGURE - 3 . 6 .

 36 FIGURE -3.6. Image of Er:YSZ monolayer (sample 5) deposition by PLD, concentric colors are due to different layer deposition thicknesses.

2 .

 2 FIGURE -3.7. PL spectra in the near-IR from Er:YSZ monolayers excited at 980 nm and 100 mW for Er 3+ atomic percentages of 0.5 (sample 5 in blue), 1.5 (sample 6 in yellow), 3.0 (sample 7 in red), 5.0 (sample 8 in green) and 10.0 (sample 9 in grey). (Inset) PL intensity of 1530 nm as function of doping percentage.

FIGURE - 3 . 9 .

 39 FIGURE -3.9. PL profile of 4 I13/2 4 I15/2 Er 3+ ion emission transition in the near-IR when excited at a wavelength at 960 nm for 5 different samples.

FIGURE - 3 . 10 .

 310 FIGURE -3.10. Power study of 4 I13/2 4 I15/2 Er 3+ ion emission transition at 1530 nm wavelength for 5 samples excited at 960 nm. Inset, non-normalized PL intensity at 1530 nm for samples 1 to 5.

  .11 higher intensities are observed for emissions at 1530 and 1535 nm, although other PL intensity peaks with wavelengths of 1501, 1515, 1541, 1545, 1552 and 1574 nm are observed.

FIGURE 3 .

 3 FIGURE 3.11. -PL profile of 4 I13/2 4 I15/2 Er 3+ ion emission transition in the near-IR presenting Stark effect when excited at a wavelength at 800 nm for 4 different samples. Inset, table with numbered emission intensities at its wavelengths.

FIGURE 3 .

 3 FIGURE 3.13 -PL spectra of 1530 nm Er 3+ emission for monolayer (sample 5) and multilayer (sample 2) Er:YSZ samples deposited by PLD at 800°C (orange) and at room temperature (RT) (blue) and after annealing process at 1000°C (red). Inset graph shows schematic of Er:YSZ monolayer (sample 5 in orange) (a), Er:YSZ multilayer (sample 2 in orange and green) (b) deposited at 800°C and of Er:YSZ monolayer (sample 5 in blue) (c), Er:YSZ multilayer (sample 2 in blue and green) deposited at RT, all of them on sapphire substrate in grey and after annealing at 1000°C (in red for monolayer and red/green for multilayers).

FIGURE 3 .

 3 FIGURE 3.14 -PL emission profile for 4 I13/2 4 I15/2 Er 3+ ion transition presenting Stark effect (left) at a pumping wavelength of 800 nm at different pumping powers (2-230 mW). On the right, schematics of 4 I15/2 4 I9/2 absorption Er 3+ transition and field Stark effect at 4 I13/2 4 I15/2 transition depleted in emissions at 1530 and 1535 nm.

FIGURE 3 .

 3 FIGURE 3.15 -(a) Normalized PL efficiency study for 1530 and 1535 nm (black and red, respectively) emission wavelengths from 4 I13/2 4 I15/2 Er 3+ ion transition for multilayer samples 1, 2, 3 and 4. Colored parts in yellow show higher Er 3+ emission for 1530 nm than for 1535 nm for low pumping powers. By increasing pumping power, same intensities are found for both 1530 and 1535 nm emissions (blue circle). For high emissions, in violet, dominant wavelength emission is found at 1535 nm and it can be noticed at 100 mW saturation of the 1535 nm intensity. (b) PL Er 3+ emission intensity ratio of 1535 and 1530 nm for samples 1, 2, 3 and 4 in red, black, green and blue. Line in gray indicates pumping powers

FIGURE 3 . 18 .

 318 FIGURE 3.18. Lifetime spectra of Er:YSZ multilayer (sample 2) in green and its decay fit in red.

FIGURE - 3 .

 3 FIGURE -3.19. Normalized PL spectrum in the near-IR wavelengths for a Pr:YSZ monolayer (sample 10) excited at 1030nm and for an alternated multilayer thin film of Pr:YSZ and Er:YSZ (sample 12) excited at 960 nm in black and at 1020 nm in blue.

FIGURE - 3 . 21 .

 321 FIGURE -3.21. Normalized efficiency study of 1350 nm excitation transfer of Pr 3+ ions when excited at a wavelength of 1020 nm for multilayer Pr:YSZ/ Er:YSZ (sample 12 in black) and monolayer Pr:YSZ (sample 10 in red) thin films.

FIGURE - 3 .

 3 FIGURE -3.22. Normalized PL efficiency study of 1530 nm excitation transfer of Er 3+ ions when excited at a wavelength of 965 nm for multilayer Pr:YSZ/Er:YSZ (sample 12 in red) and monolayer Er:YSZ (sample 5 in black) thin films.

FIGURE 3 .

 3 FIGURE 3.23 -Normalized PL efficiency study of multilayer Pr:YSZ/Er:YSZ (sample 12 in red) and monolayer Er:YSZ (sample 5 in black) emission at 1530 nm from Er 3+ ions when samples are excited at a wavelength of 1020 nm in order to excite Pr 3+ ions.

FIGURE - 3 . 24 .

 324 FIGURE -3.24. Schematic of cathodoluminescence (CL) excitation of an Er 3+ ion emitting at 1530nm wavelength when an incident electron beam excites one electron of the valence band (Ev) to the conduction band (Ec).

FIGURE - 3 .

 3 FIGURE -3.28. SEM images (left) and CL-SEM intensity mapping (right) of, from top to bottom: Er-rich and 0.5 at. % Er-doped YSZ multilayer (sample 4) (a) and 0.5 at. % Erdoped YSZ monolayer (sample 5) (b) samples. Inset schema in image (a) right of emissions of Er 3+ ions in the visible range of wavelengths.

FIGURE - 3 .

 3 FIGURE -3.29. CL-SEM spectra probing Er 3+ transitions ( 2 H11/2 4 I15/2, 4 S3/2 4 I15/2 and 4 F9/2 4 I15/2) in the visible range of wavelengths, respectively for two samples: Er-rich and Er-doped YSZ multilayer (sample 4 in blue) and 0.5 at. % Er-doped YSZ monolayer (sample 5 in red).

FIGURE - 3 .

 3 FIGURE -3.30. FIB image of a sample cross-section (a) and schematic of TEM electron beam being transmitted through a polished cross-section thin film grown on a substrate (b).

FIGURE - 3 .

 3 FIGURE -3.31 Spectra (a) from CL-TEM emission in the visible range from the Er-doped YSZ multilayer structure (sample 4) thin film cross-section area observed in the HAADF image (b) in grey and from two continuous areas within the thin film in red and blue. Green square includes image of the sapphire substrate.

Finally, we

  analyzed the sapphire substrate as two main broad peaks have been observed by CL-TEM and CL-SEM (figure 3.32.) in each profile for spectra close by the substrate.

FIGURE - 3 .

 3 FIGURE -3.32. CL-TEM and SEM spectra from the sapphire substrate in the wavelength ranges of ultraviolet and visible.

FIGURE 4 . 1 -

 41 FIGURE 4.1 -Cross-section schematics of a trapezoid SiNx waveguide of variable width (wa) and 600nm height buried on 1.4m SiO2 layer on a Si substrate.

FIGURE 4 . 2 -

 42 FIGURE 4.2-Transmission profile of a SiNx waveguide before (blue). After annealing treatment (black). Reduction of N-H bonding concentration increases transmission in all wavelengths from 1400 to 1600 nm [ 4 ].

  -D) simulation tool (MODE Solutions from Lumerical, Inc.) [ 5 ]. The design of the waveguide was already fixed by previous activities in the group and the technology at STMicroelectronics. I had no possibility to change SiNx thickness nor add other widths. Waveguide design cross-section (x-y plane) is shown in figure 4.3. The waveguide structure comprises 600 nm thick and variable wide SiNx waveguides of 0.4, 0.6 and 0.8 m on a 1.4 µm thick buried oxide layer. Transversal dimensions facilitate the propagation of fundamental modes for both quasi-transverse electric (quasi-TE) and quasi-magnetic (quasi-TM) polarization states from 1.2 to 1.6 m.

FIGURE 4 . 3 -

 43 FIGURE 4.3-Schematics of a SiNx cross-section waveguide buried on SiO2 on a Si substrate with three cladding steps from left to right: without cladding, with Er:YSZ (cladding 1) and with Er:YSZ/PMMA (cladding 2) cladding.

FIGURE 4 . 4 -

 44 FIGURE 4.4-Graphic of simulated optical power percentage on the left for TM01 (red) and TE00 (black) modes related to Er:YSZ cladding thickness. Inset schematics, cross-section of SiNx waveguide buried on SiO2 with Er;YSZ cladding (cladding 1). On the right, electric field map simulations waveguide optical power density of TE (left column) and TM (right column) mode on a SiNx cross-section waveguide buried on SiO2 with Er:YSZ cladding thicknesses from top to bottom of 50, 150 and 350nm.

FIGURE 4 .

 4 FIGURE 4.5. -Graphic of simulated optical power percentage on the left for TM01 (red) and TE00 (black) modes related to Er:YSZ:Er 3+ /PMMA cladding (cladding 2) thickness. Inset schematics, cross-section of SiNx waveguide buried on SiO2 with Er:YSZ/PMMA cladding (cladding 2). On the right, electric field color map simulations of TE (left column) and TM (right column) mode waveguide optical power density on a SiNx cross-section waveguide buried on SiO2 and Er:YSZ cladding thicknesses from top to bottom of 50, 150 and 350nm and PMMA of 1m thick. Schematic cross-section simulation of TM mode for thicknesses of 350 nm is not shown as mode confinement in the waveguide was not observed.

FIGURE 4 .

 4 FIGURE 4.6.-Colour maps of the optical intensity distribution for TE-polarized modes for a cladding of 150nm thick Er:YSZ layer for wavelengths of 1530 (left image) and 1480nm (right image).

Figure 4 . 7 .

 47 shows a minimum bending radius in which TEM mode is confined in the waveguide as a function of the active layer thickness. In the inset image of figure 4.7, a spiral waveguide etched on our designed device is shown and has a minimum bending radius of 60 µm. Simulations estimate a minimum of 30 µm (respectively 35 µm) bend radius for a cladding of 150 nm-thick Er:YSZ (respectively a cladding of 150 nm-thick Er:YSZ/ 1µm PMMA). We noticed that for larger thicknesses of the top cladding, larger bends are required to keep the TE mode confined.

FIGURE 4 .

 4 FIGURE 4.7.-Simulated minimum waveguide bend radius values as a function of the thicknesses for Er:YSZ cladding with (black) or without (red) 1m PMMA cladding. Inset, SEM image of spiral and minimum radius of ~60 m.

FIGURE 4 .

 4 FIGURE 4.8.-PL profile of erbium 4 I13/24 I15/2 transition for Er:YSZ layers of 50nm (red), 150nm (blue) and 350nm (green) thickness when excited at 980nm. Inset, evolution of intensity of erbium emission peak at 1530 nm for Er:YSZ thin layer as a function of its thickness.

FIGURE 4 . 9 .

 49 FIGURE 4.9.-Top image of upconversion on waveguide excited at 1480 nm.

FIGURE 4 .

 4 FIGURE 4.10.-Butt-guided-coupled spectroscopy setup consisting on an input fixed laser at 1480nm and optical attenuator free spaced guided to a SiNx waveguide on buried oxide (BOX) and Er:YSZ thin film cladding. Output signal is free-spaced guided to a spectrometer.

FIGURE 4 . 11 .

 411 FIGURE 4.11. Visible emission intensity profile of Er:YSZ cladding for pumping powers range from 1 to 30mW excited at a wavelength of 1480nm. For the same cladding, power study (b) of wavelengths emissions at 525, 543, 562, 656 and 680 nm (black, blue, green, orange and red). Inset, scheme of Er 3+ emission transitions in the visible [ 6 ]. Profile emissions by spectroscopy in the visible (figure 4.11 (a)) correspond to two Er 3+ transitions in the green wavelengths: 2 H11/2 4 I15/2 transition for 525 and 543 nm wavelengths and 4 S3/2 4 I15/2 transition for 562 nm. And to one Er 3+ transition 4 F9/2 4 I15/2 at 656 and 680 nm in the red wavelengths, as reported in the literature [ 7 ]. As seen in figure 4.11 (a) that the most intense emission peak corresponds to the red wavelength of 680 nm. In figure 4.11. (b), we studied the efficiency and PL intensity of visible guided emission for each Er 3+ transition in the visible wavelengths.In this regard, wavelength presenting higher optical efficiency is 543 nm corresponding to upconversion in the green wavelengths for the 2 H11/2 4 I15/2 transition. Although the highest contribution in visible intensity in the spectrum over the whole waveguide is found for red wavelengths, upconversion in the green range of wavelengths could be recorded by a camera in a spiral as shown in figure4.12.

FIGURE 4 . 12 -

 412 FIGURE 4.12 -Top image of 2 mm long SiNx spiral waveguide with Er:YSZ multilayer cladding excited at 1480 nm wavelength.

FIGURE 4 .

 4 FIGURE 4.13.-Optical gain measurements taken by OSA. Pump output signal at 1480 nm increases linearly with input power while probe output signal at 1530 nm wavelength follows an exponential increase with input pumping power.

FIGURE 4 . 14 -

 414 FIGURE 4.14-Waveguide and taped grating widths (wa,wb) and lengths (La,Lb) schematics.

TABLE 4 . 1 -

 41 Waveguide width and average mode overlap simulated values depending on the cladding type.

TABLE 4 . 2 -

 42 Total equivalent length (LT) of 0.4, 0.6 and 0.8 m width waveguides (Total real length of waveguide and taper equal to 1.1, 1.5, 2 and 3 mm) with Er:YSZ (cladding 1) and Er:YSZ/PMMA (cladding 2) claddings calculated by using eq. 4.3 (LT=La+2Lb(a/b)) and a from table 4.2, ( b Er:YSZ =19.5 % and  b Er:YSZ =21 %).

FIGURE 4 .

 4 FIGURE 4.15.-Signal enhancement profiles at 1530 nm as a function of the pumping power for 1.1mm waveguide (cladding 1) length of three different widths from left to right : 0.4, 0.6 and 0.8 m at a pump wavelength of 1480 nm.

FIGURE 4 .

 4 FIGURE 4.16.-Absorption and material loss transmission at 1530 nm of four waveguide lengths in black and linear fit in red. (Inset) schematic cross-section of SiNx.

FIGURE 4 .

 4 FIGURE 4.17.-Absorption and material loss transmission at 1530 nm of four waveguide lengths in black and linear fit in red and insets of schematic SiNx waveguide for Er:YSZ cladding.

FIGURE 4 .

 4 FIGURE 4.18.-Optical gain using one-sided pumping setup: pump laser at 1480 nnm and tunable laser beams combine on a 90-10 % beam splitter, after polarization controlled pump and signals are coupled to a device under test (DUT) and output signal is measured by an optical spectrum analyzer (OSA).

FIGURE 4 .

 4 FIGURE 4.19.-Modal gain (gmodal) (a) and signal enhancement (SE) (b) experimental measurements (squares) and exponential fit (lines) for waveguide (cladding 1) lengths of 1.1 and 1.5mm in red and black, respectively. ii) Optical gain on Er-doped YSZ and PMMA cladding (cladding 2) PL spectrum of the 150 nm thick multilayer waveguide cladding reveals Stark splitting of the 4 I13/2 4 I15/2 transition over an inhomogeneous area on the thin film (figure 4.20 (a)). In order to understand how emission of this field splitting effect behaves on a guided emission, we studied signal modal gain for three wavelengths: 1520, 1530 and 1560 nm. As observed in figure 4.20 (a) micro-PL emission profile for Er-doped YSZ multilayers cladding presenting Stark effect shows two maximum intensities at 1530 and 1535 nm.For the guided emission corresponding to the PL profile, we also determined modal gain at 1530 nm as well as at two wavelengths corresponding to two minimum PL on Er 3+ emission. We have chosen the wavelength of 1520 nm for being a local minimum in the PL spectra, hence, not showing a strong emission at this wavelength and being placed between the pump at 1480 nm and the main emission wavelength at 1530 nm. The second wavelength at 1560 nm shows lack of Er 3+ emission in the PL spectra. By measuring modal gain for a 1.5 mm length waveguide, we observed in figure4.20 (b) modal gain of 10.12 dB/cm at 1530 nm. In contrast, for 1520 and 1560 nm only probe transmission and not signal enhancement is observed.

FIGURE 4 .

 4 FIGURE 4.20. -PL profile of Er:YSZ multilayer cladding on SiNx waveguides (cladding 2) showing Stark effect (a). Emission intensities at 1520, 1560 and 1530 nm in red, green and blue are analyzed and the two first wavelengths fitted linearly and the last one exponentially (b).

FIGURE 4 .

 4 FIGURE 4.21.-Modal gain (gmodal) experimental measurements and exponential fit for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelengths from left to right for waveguides of 1.1, 1.5 and 2 mm length (black, red and blue, respectively).

FIGURE 4 .

 4 FIGURE 4.22.-Signal enhancement (SE) measurements and exponential fit for Er:YSZ /PMMA (cladding 2) cladding emission at 1530 nm wavelengths from left to right for waveguides of 1.1, 1.5 and 2 mm length (black, red and blue, respectively).

FIGURE 4 .

 4 FIGURE 4.23.-Optical gain two-sided setup: pump laser at 1480 nm combined to a tunable optical attenuator (VOA) beam is split 50-50 % to a DUT and circulator. Tunable laser is connected to a 10% beam splitter, and coupled with the pump to the DUT in a two-sided pumping setup. Output signal from the waveguide is recorded using an optical spectrum analyzer (OSA).

FIGURE 4 .

 4 FIGURE 4.24.-Modal gain (gmodal) measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelength for waveguide lengths of 1.1, 1.5, 2.0 and 3.0 mm (black, red, blue and green, respectively).

FIGURE 4 .

 4 FIGURE 4.25.-Signal enhancement measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelength for waveguide lengths of 1.1, 1.5, 2.0 and 3.0 mm (black, red, blue and green, respectively).Modal gain analysis at 1535 nm wavelength emission, (figure4.26) has also been performed. Results show modal gain of 6.83 dB/cm for 1.5 mm long waveguides, increasing up to 7.2 dB/cm for 2 mm waveguides. Weak modal gain for 1.1 mm waveguide can be explained by the non-homogeneously distributed field Stark effect structure on the thin film. Actually, growth of this structure is not controlled at the nanoscale, giving irregular emissions for 1535 nm wavelength. Despite modal gain values, signal enhancement at 1535 nm (figure 4.27) for two lengths of waveguides (1.5 and 2 mm) gives increasing SE values of 5.9 and 8.19, respectively.

FIGURE 4 .

 4 FIGURE 4.26.-Modal gain (gmodal) measurements and exponential fit for Er:YSZ/PMMA cladding (cladding 2) emission at 1535 nm wavelength for waveguide lengths of 1.5 and 2 mm (red and blue, respectively).

FIGURE 4 .

 4 FIGURE 4.27.-Signal enhancement (SE) measurements and exponential fit for Er:YSZ/PMMA cladding (cladding 2) emission at 1535 nm wavelength for waveguide lengths of 1.5 and 2 mm (red and blue, respectively).

FIGURE 4 .

 4 FIGURE 4.28. -Modal gain (gmodal) (a) and signal enhancement (SE) (b) measurements for single (black) and double pump waveguides (red and blue) with Er:YSZ/PMMA cladding (cladding 2) at 1530 and 1535 nm Er 3+ emission.

(

  FIGURE -5.2. Black and white image of Er 3+ upconversion on a ring with an Er:YSZ/PMMA cladding.

  FIGURE A.2 -Schematic of Beer's law. An electromagnetic wave is attenuated exponentially when transmitted through a material.

  5) as follows, (eqs. A.5) E stands for the electric field and the symbols:  ,  for the parallel and perpendicular polarization. Finally, i, r, t refers to the incident, refracted and transmitted beam, respectively. Ellipsometry measures the complex reflectance ratio () (equation A.6) as, (eq. A.6) which expresses the amplitude ratio () and phase difference (Δ), between parallel and perpendicular polarizations. The reflection amplitude ratio () is characterized by the refractive index (n) and the absorption of the wave. The phase shift (Δ) is calculated by the dephasing of the two polarizations of the reflection. Both amplitude ratio () and phase difference (Δ) are wavelength dependent, these values are measured by the ellipsometer at multiple wavelengths. Most ellipsometric analyses involve the creation of models such as Cauchy model to obtain refractive index and thin film thickness by fitting theoretical model variables with the experimental data [ 1 ]. For a transparent sample such as insulators or glasses with low optical absorption, n() can be expressed by a Cauchy dispersion model (equation A.7) in which A is the refractive index of the material at long wavelengths; B and C are variables of the refractive index at short wavelengths (eq. A.7)

(eq. A. 11 )

 11 FIGURE A.3-Image on the left of a crystalline structure in the direct space, angles and vectors are indicated. On the right, table of the crystal systems and characteristic parameters of each unit cell.

(eq. A. 17 )

 17 This factor gives information of the electron composition of the atoms in the unit cell.

FIGURE A. 4 -

 4 FIGURE A.4 -Top image, table of all the reflections crystal must have according to the Bravais lattice and the extinct reflections. Down image, schematic of the four types of Bravais lattice: simple, body-, base-and face-centered.

FIGURE A. 5 -

 5 FIGURE A.5-Schematic of x-ray diffraction setup with typical scan angles.

FIGURE A. 7 -

 7 FIGURE A.7 -Image of AFM tip interaction force with sample surface. In orange, repulsive force in contact mode, in green attractive force in tapping mode and in blue, non-contact mode.

FIGURE A. 8 -

 8 FIGURE A.8 -Modal gain (gmodal) measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelength for waveguide equivalent lengths of 0.85, 1.25 and 1.75 mm (black, red, blue and green, respectively).

FIGURE A. 9 -

 9 FIGURE A.9 -Modal gain (gmodal) measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelength for waveguide total lengths of 1.1, 1.5 and 2.mm (black, red, blue and green, respectively).

FIGURE A. 10 -

 10 FIGURE A.10 -Modal gain (gmodal) measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelength for waveguide equivalent lengths of 0.85, 1.25 and 1.75 mm (black, red, blue and green, respectively).

FIGURE A. 11 -

 11 FIGURE A.11 -Modal gain (gmodal) measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1530 nm wavelength for waveguide total lengths of 1.1, 1.5 and 2.mm (black, red, blue and green, respectively).

FIGURE A. 12 -

 12 FIGURE A.12 -Modal gain (gmodal) measurements and exponential fits for Er:YSZ/PMMA cladding (cladding 2) emission at 1535 nm wavelength for equivalent (a) and total (b) waveguide lengths of 1.75 and 2 mm, respectively.

Figure 2 .Figure 3 .Figure 4 .

 234 Figure 2. Diagrammes XRD d'un film mince YSZ dopé à l'erbium de 150 nm sur saphir (0001) dans une configuration à trois axes.

  Afin d'éviter une absorption élevée de la sonde par la section transversale du matériau qui entrave à la fois le gain modal et l'amélioration du signal, nous avons pompé nos guides d'ondes revêtus Er 3 + : YSZ / PMMA. En utilisant cette technique, nous avons obtenu un gain optique pour les quatre longueurs de guides d'ondes (1,1, 1,5, 2,0 et 3,0 mm) pour deux longueurs d'onde d'émission Er 3 + différentes. Comme observé dans le tableau 4.3. pour l'émission Er 3 + à 1530 nm, nous obtenons des valeurs de gain modal et une amélioration du signal similaires pour les trois premières longueurs de guide d'ondes(43,66, 24,5, 18,05 dB / cm et 6,7, 7,09 et 8,9 dB, respectivement) que pour les guides d'ondes à pompe unique. De plus, nous avons obtenu un gain modal net pour le guide d'ondes le plus long de 8,49 dB / cm et SE de 10,98 dB.Le profil PL pour la gaine multicouche YSZ dopée Er 3 + a prouvé un effet de division Stark de champ, donc pour cette dernière structure nous avons fait des mesures pour un deuxième pic d'émission Er 3 + à 1535 nm. Les résultats à cet égard (Tableau 4.3 et Fig.4.26), ont prouvé un gain modal optique pour toutes les longueurs de guides d'ondes malgré la distribution non homogène de cette structure qui peut être observée dans les résultats de gain modal. Par conséquent, le gain modal pour 1535 nm présente des valeurs faibles inattendues (6,46 dB / cm) pour un guide d'ondes plus court de 1,1 mm de longueur qui augmente modérément jusqu'à 14,79 dB / cm pour des guides d'ondes de 1,5 mm et atteint des valeurs similaires à celles de 1530 nm. Comme prévu et de manière cohérente avec le gain modal de 1530 nm mesuré pour des guides d'ondes de 2,0 mm de longueur, un gain modal de 15,13 dB / cm et une amélioration du signal de 8,19 dB pour 1535 nm ont été mesurés. Enfin, nous avons analysé un faible gain modal de 3,5 dB / cm et une amélioration du signal plus grande que pour le reste des guides d'ondes pour les guides d'ondes de 3,0 mm de longueur comme observé sur la figure 4.26. Cette évolution de gain modal correspond à une faible présence de la structure de séparation Stark dans les deux guides d'ondes plus courts, diminuant l'émission à 1535 nm et la reflétant dans les valeurs de gain modal et d'amélioration du signal. Une amélioration décroissante du signal à 1535 nm correspond, comme mentionné, à une forte absorption de la pompe et des émissions du matériau malgré la conception à double pompage.CONCLUSIONSPour résumer, des films minces YSZ dopés à l'erbium cristallin de haute qualité ont été développés par épitaxie sur un substrat en saphir et des films minces polycristallins sur des guides d'ondes SiNx. La caractérisation optique par photoluminescence à 980 nm révèle une transition erbium 4 I13/2 4 I15/2 pour tous les substrats et plates-formes et il a été remarqué une émission multiple locale correspondant à l'effet Stark avec amélioration de la luminescence. Les transitions. 2 H11/2 4 I15/2 , 4 S3/2 4 I15/2 , 2 H9/2 4 I13/2 , 4 F9/2 4 I15/2 and 2 H9/2 4 I11/2 ont été mesurées par spectroscopie visible révélant une forte remontée d'erbium -conversion lors du pompage à 1480 nm. Les travaux futurs se concentreront sur l'étude du gain en Er : YSZ sur la plate-forme hybride SiNx à 1,55 µm pour démontrer les amplificateurs optiques sur puce aux longueurs d'onde des télécommunications. Titre : Oxyde cristallin dopé aux terres-rares sur plateforme photonique silicium Mots clés : Silicium, terre rares, oxydes Depuis le début du 20e siècle avec le développement des télécommunications, de nouveaux designs et matériaux ont été étudiés pour la de dispositifs photoniques à faible consommation d'énergie sur des plateformes à base de silicium pour des applications en datacom, télécommunications, la détection ou l'optique quantique.

  La zircone stabilisée à l'yttria (YSZ) est un oxyde buffer bien connu pour d'autres oxydes fonctionnels. Parmi ses propriétés, il faut noter : la stabilité chimique, la transparence du visible au moyen-IR, un indice de réfraction autour de 2,15, ce qui rend cet oxyde fonctionnel intéressant pour le développement de guides d'ondes à faibles pertes lorsqu'il est cultivé sur un substrat à faible contraste. Bien que ces propriétés optiques soient très intéressantes pour diverses applications, y compris les communications et la détection optiques sur puce, YSZ est restée presque inexplorée en photonique. Après avoir démontré faibles pertes de propagation de 2 dB / cm à une longueur d'onde de 1380 nm dans des guides d'ondes de YSZ hautement monocristallins, nous avons envisagé d'explorer ce matériau pour l'amplification optique. Dans cette thèse, nous explorons le dépôt de couches minces YSZ dopées Er 3+ par dépôt de couche pulsée (PLD) sur une approche multicouche et monocouche fournissant une luminescence en correspondance avec la bande C de la fenêtre de télécommunication (λ = 1,5 µm) et dans le domaine du visible de longueurs d'onde. La caractérisation structurelle et optique conduit à un matériau hautement optique efficace présentant une photoluminescence améliorée à des longueurs d'onde de 1530 et 1535 nm. Après la caractérisation structurel et optique, nous avons intégré notre matériau comme cladding sur des guides d'ondes SiNx. Les pertes de propagation et l'amplification optique à 1530 et 1535 nm en utilisant différentes techniques de pompage sont discutées dans cette thèse. De plus, l'approche multicouche utilisée pour Er 3 + a été explorée pour d'autres terres rares tels que le Pr 3+ . À ce sujet, les résultats de caractérisation structurale et optique de l'émission YSZ dopée Pr 3 + à 1350 nm dans la bande O de la fenêtre de télécommunication (λ = 1,3 µm) ouvrent la voie à la mise en oeuvre et à l'interaction de matériaux de terres rares-dopes YSZ émettant dans le proche-IR. Maison du doctorat de l'Université Paris-Saclay 2 ème étage aile ouest, Ecole normale supérieure Paris-Saclay 4 avenue des Sciences, 91190 Gif sur Yvette, France Title : Rare-earth doped crystalline oxide on silicon photonics platform Keywords : Silicon, rare-earth, oxides

Chapter 2 Growth of rare-earth doped oxides Summary 2.1. Epitaxial growth of RE-doped thin film oxides using pulsed laser deposition (PLD)………………………………………………………………......38 2

  .1.1. Thin films growth processes and techniques………………...………......38 2.1.2. Materials investigated in this work…………………………....................45 2.1.3. Growth of RE-doped YSZ thin films…………………………....................

	zirconia thin layer for photonic applications. Physical Review Materials, American Physical
	Society, 2018, 2 (3), pp.35202 -35202. ⟨10.1103/PhysRevMaterials.2.035202⟩.

53 2.2. Structural characterization…………………………....................................58

	2.2.1. Spectroscopic ellipsometry…………………………..................................58
	2.2.2. X-Ray Diffraction (XRD) …………………………...................................60
	2.2.3. Atomic Force Microscopy (AFM) ……………………………....................67
	2.2.4. Scanning Transmission Electron Microscopy (TEM) ……...................69

2.3. Conclusions……………………………………………………………….………...76 2.4. References………………………………………………………………………......79

TABLE 2 .

 2 4 -List of samples, detailing layer composition, thickness, number of layers and total thickness of the thin film. Schematics of multilayer and monolayer structures.

TABLE 2 .

 2 5 -Absorption and emission transitions and wavelengths of samples.

	and
	In bold transitions

Summary 3.1. Photoluminescence (PL)……………………………………….…………..…85 3.1.1. Er-doped YSZ……………………………………………………….....…..87 3.1.1.1. Wavelength absorption……………………………………..……87 3.1.1.2. Power efficiency……………………………………………….......92 3.1.1.3. Improving photoluminescence: annealing under an oxygen atmosphere…………………………….......................100 3.1.1.4. Er 3+ -Er 3+ energy transfer………………………………….........102 3.1.1.5. Lifetime……………………………............................................105 3.1.2. Pr 3+ -doped YSZ……………………………..........................................107 3.1.2.1. Wavelength absorption...………….........................................107 3.1.2.2. Power efficiency…………........................................................109 3.1.2.3. Pr 3+ -Er 3+ energy transfer.......................................................111 3.1.2.4. Lifetime……………………………...........................................112

3.2. Cathodoluminescence (CL) excitation spectroscopy……………….....113 3.2.1. Scanning electron microscope cathodoluminescence (

SEM-CL) …………………………….....................................................

114 3.2.2. Transmission electron microscope cathodoluminescence

(TEM-CL) ……………………………..................................................118 3.3. Conclusions ……………………………...........................................................

.121 3.4. References ………………………………………………………………………....123

  Optical characterization is a non-destructive technique in which matter interacts with light by emission, reflection, absorption, scattering or transmission. In this thesis we have optically characterized RE-doped oxide thin films by two different techniques: microphotoluminescence and cathodoluminescence.

TABLE - 3

 - .

TABLE - 3

 - .3. Name and description of composition, Er 3+ percentage and layer thicknesses and schematic structure of Er:YSZ multilayer thin film samples deposited by PLD.

TABLE - 3

 - .4. PLD deposited RE-doped YSZ samples atomic percentage and lifetime of Er 3+ ( 4 I13/2) and Pr 3+ ( 1 G4) excited states.

  ) which are coherent with the results obtained with the singlepumped waveguides (figure4.28). Pumping power at each side of the waveguide is half for double pumping than for one-sided configuration. Besides, we also obtained net modal gain for longest waveguide (3 mm) of 0.76 dB/cm and signal enhancement of 10.98 dB thanks to this measurement approach. PL profile of Er-doped YSZ multilayer cladding shows field Stark splitting effect, leading to strong emission peaks at 1535 nm. Then we did measurements at 1535 nm. Results (table. 4.3 and figure 4.28). Modal gain of 6.83 dB/cm for 1.5 mm waveguide which moderately increases up to 7.2 dB/cm for 2 mm long waveguides. As expected and coherently to the 1530 nm modal gain measured for 2 mm long waveguides, modal gain of 7.2 dB/cm and signal enhancement of 8.19 dB at 1535 nm have been obtained. This modal gain evolution corresponds to a low and random presence of Stark splitting structure in waveguides, decreasing emission at 1535 nm.

	2 H11/2 4 I15/2 transition at 525 nm wavelength , (ii)
	4 S3/2 4 I15/2 transition with two intensities peaks at 543 and 562 nm; and (iii) 4 F9/2
	4 I15/2 transition with wavelengths at 656 and 680 nm.
	Previous optical gain analysis, we have measured absorption and material losses for
	SiNx waveguides, without any cladding, with Er:YSZ + multilayer (cladding 1) and
	with Er:YSZ/PMMA bilayer (cladding 2). Losses of -25.42, -53.40 and -38.37 dB/cm at
	1530 nm were obtained for the three configurations: without cladding, cladding 1 and
	cladding 2, respectively. Experimental signal enhancement analysis for emission at
	1530 nm for 1.1 mm long waveguides for 3 waveguide widths (0.4, 0.6 and 0.8 m)
	shows higher signal enhancement values for 0.4 m wide waveguide. First modal gain
	measurements were performed in cladding 1 configuration by one-side pumping of
	SiNx waveguide. We obtained gain of 10.64 and 10.24 dB/cm and signal enhancement
	of 5.75 and 8.06 dB for 1.1 and 1.5 mm long waveguides, respectively. After PMMA
	deposition, we measured high modal gain (gmodal) of 42.6, 30.51 and 8.57 dB/cm for
	waveguide lengths of 1.1, 1.5 and 2.0 mm for cladding 2 structure in single-pumping
	mode configuration. This tendency is consistent with the signal enhancement
	behavior demonstrating increasing values of 7.4, 8.8 and 8.6 dB for 1.1, 1.5 and 2 mm
	long waveguides, respectively. From these results we demonstrated the addition of
	PMMA diminishes propagation losses due to mode leakage to the Si substrate
	In order to avoid high pump and probe absorption along waveguide which hinders
	both modal gain (gmodal) and signal enhancement (SE), we set-up a double-sided pump
	experiment to characterize the Er:YSZ/PMMA (cladding 2) cladded SiNx waveguides.
	We obtained optical gain for the waveguide lengths of 1.1, 1.5, 2 and 3 mm for two
	different Er 3+ emission wavelengths. As observed in table 4.3. for Er 3+ emission at
	1530 nm, we obtained modal gain values and signal enhancement for waveguide
	lengths of 1.1, 1.5 and 2 mm (35.73, 16.57 and 10.12 and dB/cm and 6.7, 7.9, and 8.9
	dB, respectively

TABLE 4 .

 4 3. -Table for modal gain (gmodal) and signal enhancement (SE) of Er:YSZ (cladding 1) and Er:YSZ/PMMA (cladding 2) Er 3+ emission at 1530 and/or 1535 nm using single or double setup for waveguides of 1.1, 1.5, 2 and 3 mm long.

TABLE 4 .

 4 4.-Table comparing type of cladding material and values of pump wavelength, Er at. %, layer thickness, propagation losses, waveguide length, modal gain and signal enhancement of state-of-the-art studies including this thesis.

  Optical absorption in RE ions have its origin in its electronic configuration, thus differences in absorption intensities suggest different electronic configuration for Er 3+ ions on the same host matrix.Efficiency studies on Er 3+ emission at 1530 nm for monolayer samples with doping concentrations of 0.5, 1.5, 3, 5 or 10 at. % (samples 5, 6, 7, 8 and 9) pumped at 960 nm proved larger optical efficiency for 0.5 at. % Er-doped YSZ monolayer thin films (sample 5). Same efficiency studies at 800 nm wavelength pumping were performed for multilayer samples showing Stark effect and higher emission Er 3+ intensity at 1536 nm. Multilayer samples 2 and 4 with non-metallic and metallic Er 3+ doping showed larger total intensity emission and longer lifetimes among multilayer samples. I15/2 only obtained from metallic Er-doped YSZ layers. Results on PL characterization, cathodoluminescence (CL) and efficiency studies on Er 3+ emission at 1530 and 1535 nm, suggest metallic Er 3+ ions take two different sites in the YSZ lattice (C3i and C2) depending on oxygen vacancies configuration. Position of Er 3+ ions in the host structure might allow energy transfer as observed in efficiency studies for 1530 and 1535 nm wavelengths. Multiple emission is related to enhance Stark effect emission. We have also grown and analyzed Pr-doped YSZ thin films. PL measurements shown absorption transitions in the near-IR with a maximum absorption wavelength of 1020 nm and Pr 3+ emission at 1350 nm. As expected, low efficiency for 0.5 at % Pr-doped YSZ thin films was obtained due to trivalent and tetravalent cations. By performing PL measurements pumping wavelength of1020 nm for Pr-and Er-doped YSZ multilayers, it has been noticed energy transmission from Pr 3+ to Er 3+ ions, when emission from Er 3+ ions at 1530 nm was detected. Thus, Pr 3+ at 1350 nm wavelength emission might be hindered by energy transfer to Er 3+ ions. Lifetime studies for Prdoped YSZ revealed measured values below 1 ms at 1350 nm.

	metallic Er-doped YSZ layers which demonstrate the presence of Stark effect
	and enhancement of Er 3+ emission.
	(b) Pr-doped YSZ
	and sintered Er-doped YSZ (called non-metallic for clarification). By photoluminescence technique (PL), we have demonstrated for the same 4 I13/2→ 4 I15/2 transition two different Er 3+ emission behaviors for metallic Er 3+ doping multilayers: (i) local enhanced Stark effect with emission at 1530, 1535 and 1545 nm and (ii) single emission at 1530 nm. While for non-metallic Er 3+ emission monolayers, only one emission intensity at 1530 nm has been obtained. The presence of Stark splitting or lack of it hints different environment of Er 3+ ions in the YSZ structure. Regarding absorption and emission wavelengths, we observed absorption at 960 nm with one emission intensity peak at 1530 nm for both non-metallic Er-doped YSZ monolayers and metallic Er-doped YSZ multilayers. Moreover, we noticed an additional local absorption at 8000 nm related to Stark splitting emission at 1530, 1535 and 1545 nm wavelength for metallic Er 3+ multilayer scanning electron microscopy cathodoluminescence (SEM-CL) performed on a multilayer samples with metallic and non-metallic Er 3+ shown two clear emission profiles in the visible wavelength range: one Er 3+ emission transition ( 4 S3/2 4 I15/2) present in both mono-and multi-layer samples and a second transition, 4 F9/2 4 From optical characterization we concluded the best thin film approach for optical amplification with large emission intensities and low quenching effect (high optical efficiency) should contain metallic Er 3+ in its structure. Besides, by adding non-metallic Er 3+ in a multilayer structure with metallic Er 3+ high optical intensities are achieved while maintaining high optical efficiency. Structurally, for Er:YSZ monolayers grown on sapphire, we observed two crystalline orientations by XRD on the thin film corresponding to (111) and (002) family of planes.. For E r -doped YSZ multilayers, mono-oriented growth can be controlled for (111) or (002) orientations depending on substrate treatment. Despite, for multistack samples with a total number of layers above 40, two crystalline orientations are observed. By HAADF and BF-STEM, crystalline multioriented and well-defined stack structure has been observed on Er-doped YSZ multilayers with metallic and non-metallic Er 3+ in the YSZ structure. FFT of HAADF analysis reveals two family of planes (001) and (111) in three different axes: one in [110] for the family of planes (001) and two, [1-10] and [0-11] for (111). SAED diffraction samples. Transmission electron microscopy cathodoluminescence (TEM-CL) and revealed an additional zone axis [100] for the (001) family plane. Finally,
	analysis of EDX profile proved substitution of Er 3+ in Zr 4+ and Y 3+ sites in

  4, 0.6 and 0.8 m). absorption and material losses of -30.44 dB/cm for Er:YSZ/PMMA (cladding 2) at 1460 nm, we measured high modal gain of 50.6±7.02 dB/cm (table 5.1.) at 1530 nm for 1.1 mm long Er:YSZ/PMMA cladded (cladding 2) waveguides, respectively, under single-sided pumping at a wavelength of 1480 nm.

	Despite

TABLE - 5

 - .1. Modal gain and signal enhancement measurements at 1530 nm of waveguides with a Er:YSZ/PMMA cladding. Waveguides of 1.1, 1.5 and 2 mm long were one-side pumped at 1480 nm.

  1, 1.5, 2 and 3 mm at two different Er 3+ emission wavelengths: 1530 and 1535 nm. Modal gain and signal enhancement at a wavelength of 1530 nm (table 5.2) have been obtained for all waveguide lengths: 43.66±2.50, 24.5±3.0, 18.05±1.75 and 8.49±2.25 dB/cm and 6.7±0.9, 7.09±0.6, 8.9±0.43 and 10.98±1.03 dB, respectively. For the sample exhibiting field Stark splitting effect, we have also performed optical gain measurements at a wavelength of 1535 nm. Results proved optical modal gain for all waveguides length (table 5.1.) despite the inhomogeneous distribution of this emission characteristics. The measured modal gain at 1535 nm presents low values (6.46±2.46 dB/cm) for shorter waveguide length (1.1 mm), which moderately increases up to 14.79±1.62 dB/cm for 1.5mm waveguides and reaches similar values than for 1530 nm for 2 mm long waveguides with a modal gain of 15.13±3.45 dB/cm and 3.5±2.5 dB/cm for longer waveguides of 3 mm. Signal enhancement for emission at 1535 nm reaches 9.45±0.93 dB for 3 mm long waveguides. This modal gain evolution can be explained by the lower presence of Stark splitting structure in the shortest waveguides, decreasing emission efficiency at 1535 nm.

TABLE - 5

 - .2. Modal gain and signal enhancement measurements at 1530 and 1535 nm of waveguides with a YSZ:Er 3+ /PMMA cladding. Waveguides of 1.1, 1.5, 2 and 3 mm long were double-side pumped at 1480 nm. Modal gain results of 50.6±7.02 dB/cm for 1.1 mm long SiNx waveguides for Er 3+ emission at 1530 nm are very similar to results published by Rönn et al.

which reported 52.4 ± 13.8 dB/cm of net material gain for 1.2 mm Si3N4 waveguides with Er:Al2O3 cladding

[ 1 ]

. Regarding signal enhancement measurements reported in the literature, is still room for improvement. We achieved 10.98±1.03 dB for waveguides of 3 mm at 1530 nm wavelength emission far from net gain of 18.1±0.9 dB at 1532 nm for 10 cm Si3N4 waveguides with Er:Al2O3 cladding obtained by the group in University of Twente

[ 2 ]

.

TABLE 5 .

 5

3.-Table comparing type of cladding material and values of pump

wavelength, Er at. %, layer thickness, propagation losses, waveguide length, modal gain and signal enhancement of state-of-the-art studies including this thesis.

Table A .

 A 1 -Table with interplanar spacing (dhkl) depending on crystal system.

25) Modal gain results: total and equivalent waveguide length.

  We compared modal gain calculations by considering total length of taper and waveguide and equivalent length. Total length values for waveguides are 1.1, 1.5, 2 and 3 mm and equivalent length for 0.4 m width waveguides are 0.85, 1.25, 1.75 and 2.75 mm. As expected, modal gain calculations give higher values for equivalent length than for total length calculations. Modal gain results for equivalent length are discussed in chapter 4. For total length calculations, we obtained modal gain only for waveguides with cladding 2 (Er:YSZ/PMMA) in both one-sided and double sided pumping for 1530 and 1535 nm. For Er emission at 1530 nm, modal gain was obtained for waveguides of 1.1, 1.5 and 2 mm length. While for Er emission at 1535 nm, we measured modal gain for 2 mm waveguide length.

21)

In which is called the enhancement coefficient (EH) thus,

(eq. A.22) (eq. A.23)

Adding EH to the net gain coefficient (G)

= (eq. A.24)

Finally, thee equivalent length is defined as, (eq. A.

 Waveguide one-side pumping

Modal gains for waveguides pumped on one-side for 1530 nm wavelength gave a value of 42±6, 30±4 and 8±2 dB/cm for equivalent lengths of 0.85, 1.25 and 1.75 mm, as observed in figure A.8 and chapter 4.

TABLE A .

 A 2 -Image of AFM tip interaction force with sample surface. In orange, repulsive force in contact mode, in green attractive force in tapping mode and in blue, non-contact mode.

J.R. Weber, J.J. Felten, B. Cho, and P.C. Nordine, "Glass fibres of pure and erbium-or neodymium-doped yttria-alumina compositions", Nature, vol.393, no.6687, pp.769-771, 1998. DOI: 10.1038/31662

J.D.B. Bradley, L. Agazzi, D. Geskus, F. Ay, K. Worhoff, and M. Pollnau, "2.0 dB/cm gain in an Al 2 O 3: Er 3+ waveguide on silicon", in 2009 Conference on Lasers and Electro-Optics and 2009 Conference on Quantum electronics and Laser Science Conference (pp. 1-2), 2009. IEEE. DOI: 10.1364/CLEO.2009.CThCC2

J.D. Bradley, and M. Pollnau, 2011. "Erbium-doped integrated waveguide amplifiers and lasers". Laser & Photonics Reviews, 5(3), pp.368-403. DOI: 10.1002/lpor.201000015

J. Rönn, J. Zhang, W. Zhang, Z. Tu, A. Matikainen, X. Leroux, E. Durán-Valdeiglesias, N. Vulliet, F. Boeuf, C. Alonso-Ramos, H. Lipsanen, L. Vivien, Z. Sun, and E. Cassan, "Erbiumdoped hybrid waveguide amplifiers with net optical gain on a fully industrial 300 mm silicon nitride photonic platform," Opt.Express 28, 27919-27926 (2020)

161 J. Mu, M. Dijkstra, J. Korterik, H. Offerhaus, and S. M. García-Blanco, "High-gain waveguide amplifiers in Si3N4 technology via double-layer monolithic integration," Photon. Res. 8, 1634-1641 (

2020) 162 J. Rönn, W. Zhang, A. Autere, X. Leroux, L. Pakarinen, et al.. Ultra-high on-chipoptical gain in erbium-based hybrid slot waveguides. Nature Communications, Nature PublishingGroup, 2019, 10 (1), 10.1038/s41467-019-08369-w. hal-02413614

Publications in peer-review jounals 2020

different cathodoluminescence microscopy techniques: CL scanning electron microscopy (CL-SEM) and CL transmission electron microscopy (CL-TEM). FIGURE 3.25 -Image of white spot corresponding to green upconversion emission of Er 3+ in a Er:YSZ multilayer thin film (sample 4) grown on a sapphire substrate when excited by a Ti:Sapphire laser at a wavelength of 800 nm.

Cathodoluminescence (CL-) SEM (Collaboration with Stéphane

Collin group ODIN at C2N) Cathodoluminescence scanning electron microscopy (CL-SEM) measurements were performed in collaboration with the group ODIN at C2N by Stéphane Collin. He used an Attolight Allalin 4027 Chronos quantitative CL-SEM with nanometer resolution at 3keV under high vacuum conditions, room temperature and exposure time of 0.1 s. SEM images are taken by focusing the electron beam on the surface of the sample. When beam electrons interact with the specimen surface, secondary electrons are emitted and detected by a secondary electron detector, creating a contrast image of the beam incident point. For luminescent materials, photons are emitted from the same excited point. To acquire CL data, a spectrometer and a CCD array detector is added to the SEM. After successfully depositing by PLD of RE-doped thin films on a sapphire substrate, structurally and optically characterize it and proving RE-doped YSZ can be photonic material, we integrate Er-doped YSZ thin film on silicon-based devices to achieve optical gain. First, we designed a SiNx waveguide including different lengths in which we will deposit a top cladding of Er 3+ -doped YSZ thin film. For this purpose, we simulated the mode propagation into the waveguide and we optimized the mode overlaps in the cladding, in order to excite the maximum number of Er 3+ ions in the doped YSZ layer. We aim a trade-off between confinement of pump to propagate on the waveguide and the highest optical power percentage in the cladding. In this regard, we simulated different Er-doped YSZ thicknesses and explored the addition of a second top cladding layer of poly(methyl) methacrylate (PMMA) with higher refractive index than air. On a second step, the two proposed cladding materials, Er-doped YSZ thin film and PMMA were deposited by PLD and spin coating, respectively. At a final step, signal enhancement measurements were performed for Er-doped YSZ cladding with and without PMMA (we will call them cladding 1 and 2, respectively for higher clarity in the text). This work is described in the part 4.1 of this chapter. Two input pumping configurations have been studied. The first approach consisted to pump in one side of the waveguide. The second approach was to pump from the two sides of the waveguide. This last two-sided pump configuration has been used in other signal enhancement studies [ 1 ] in which RE absorption of the pump hinders excitation of the whole waveguide. These measurements are presented in the part 4.2 of this chapter.

Waveguide design

Although silicon is the material for photonics platforms, this semiconductor has major limitations regarding light emission such as its indirect bandgap [ 2 ] and two photon absorption (TPA) limiting power propagation into Si waveguide. Consequently, other silicon-based materials such as SiO2 and SiNx have been used in silicon-based platforms to integrate light emission devices. For this thesis a silicon nitride platform was produced by depositing silicon dioxide and silicon nitride layers on silicon wafers by low-pressure or plasma-enhanced chemical vapor deposition. The vertical layer stack of the platform comprises silicon nitride (SiNx) layer of 600 nm in thickness deposited on 1.4 m thick silicon dioxide (SiO2). After deposition of SiNx layer, SiNx waveguides are fabricated in the silicon photonics platform at STMicroelectronics, formed by 248 nm deep-ultraviolet lithography followed by reactive ion etching. All etched strip waveguides present a trapezoid cross-section of 0.4, 0.6 and 0.8 m

(eq. A.2)

Refraction occurs in all media except for incident angles lower than the critic angle (c=sin -1 (nr/ni)) in which total internal reflection occurs.

The complex refractive index, n', (equation A.3) reflects in its real part the refractive index or phase velocity (n) while its complex coefficient is related to the Interplanar spacing (dhkl) and plane vector (ghkl) depend on the crystal system. Thus, lattice parameter (a,b,c) and diffraction plane orientation (h k l) can be found out from these variables as shown in table A.1. Depending on the structure of the Bravais lattice certain reflections are missing from the expected ones of a crystal system (figure A.4). This intensity extinction is linked to the electronic density (ρ) associated to the structure factor (F).

A.7. Table of samples' RE absorption and emission transitions.

In bold transitions and wavelengths with higher PL intensity peak.

Résume en français du manuscrit de thèse

Les oxydes fonctionnels constituent une classe de matériaux très intéressante en raison de leurs caractéristiques singulières comprenant la supraconductivité, le magnétisme, la ferroélectricité, l'activité catalytique, la commutation résistive ou les effets électro-optiques, entre autres [1]. L'ingénierie des matériaux est couramment utilisée pour régler et manipuler ces propriétés à volonté, étant ainsi des oxydes fonctionnels souvent utilisés pour construire des éléments reconfigurables actifs dans des systèmes complexes. À cet égard, la zircone stabilisée à l'yttria (YSZ) se présente comme un matériau intéressant car elle possède des propriétés thermiques et chimiques stables et offre une large plage de transparence allant du visible à la gamme de longueurs d'onde IR moyen. De plus, il a un indice de réfraction modéré de 2,1 qui offre un bon potentiel pour le développement de guides d'ondes à faible perte lorsqu'il est cultivé sur un substrat à faible contraste. Bien que ces propriétés optiques soient très intéressantes pour diverses applications, y compris les communications et la détection optiques sur puce, YSZ est restée presque inexplorée en photonique. À cet égard, nous avons récemment démontré des guides d'ondes YSZ avec des pertes de propagation aussi faibles que 2 dB / cm à une longueur d'onde de 1380 nm [2]. Sur la base des résultats préliminaires encourageants, nous avons récemment exploré la possibilité d'introduire des dopants actifs de terres rares dans les guides d'ondes YSZ pour démontrer les amplificateurs optiques sur puce basés sur YSZ.

Ce travail explore l'introduction des ions Er 3 + en utilisant une approche multicouche déposée par la technique de dépôt laser pulsé (PLD), offrant une luminescence exceptionnelle autour de λ = 1,55 µm, en correspondance avec la bande C des télécommunications. Ces couches actives ont été cultivées sur différentes plates-formes, y compris SiNx, saphir et Si. Les propriétés optiques des guides d'ondes YSZ dopés Er sous pompage résonnant et ses pertes de propagation seront discutées dans cet article. Ces résultats ouvrent la voie à la mise en oeuvre de nouveaux oxydes fonctionnels dopés aux terres rares dans des plates-formes photoniques hybrides de manière personnalisée et polyvalente, en ajoutant de nouvelles fonctionnalités d'amplification de la lumière.