Yves Rauch

Roland Salut

Jean-Marc Cote

Stefania Oliveri

Gautier Lecoutre

Raya El Beainou

Anurupa Shaw

Phong Trần

Etienne Coffy

Venan Cv

Xavier Romain

Tatiana Kovalevich

Bicky Marquez

Tintu Kuriakose

Franche-Comté (UBFC

List of symbols and abbreviations

General Introduction

Thin films are the workhorses of much today's technology. Entire technologies, from integrated circuits to spacecraft to candy wrappers are strongly dependent on thin films. In many cases, a scientifically observed property of thin films led to a creation of an entirely new field of technology. During the last decades, thin films and particularly nanostructured coatings, have been transformed from a laboratory curiosity to become a multi-billion euros industry worldwide.

Nanostructuring of transition metal oxide thin films has been considered as an attractive field, which differs from bulk materials since they have high surface-to-volume ratio. These nanostructured thin films can be categorized by their dimension: i) one dimensional nanostructures, such as nanowires, nanotubes, and nanobelts, ii) two dimensional nanostructures, which are usually used in lithographic techniques with a mask and then etched (hole type) [1] or growth (nanowire matrix) [2] to fabricate thin films, or deposit alternative multilayers nanostructures [3], or zigzag structures and iii) three dimensional nanostructures, which can be prepared by using templates (3 dimensionally ordered macro-porous systems) [4] or direct write lithography and refine deposited (square spiral arrays) [5]. Combined with the character of transition metal oxides, these thin films have exciting potential applications such as chemical gas sensors [6], photodegradation [7], catalysts [8], photonic crystals [9], resistant coatings [START_REF] Fenker | Precise control of multilayered structures of Nb-O-N thin films by the use of reactive gas pulsing process in DC magnetron sputtering[END_REF].

For the last years, numerous binary metal oxide thin films have been produced by reactive sputtering since this process is an attractive way to efficiently prepare semi-conducting and dielectric materials [11][START_REF] Safi | Recent aspects concerning DC reactive magnetron sputtering of thin films: A review[END_REF][START_REF] Musil | Reactive magnetron sputtering of thin films: Present status and trends[END_REF][START_REF] Pierson | Reactive magnetron sputtering of copper, silver, and gold[END_REF][START_REF] Sproul | Reactive sputter deposition of polycrystalline nitride and oxide superlattice coatings[END_REF]. There has been a continuous improvement towards fabricating tunable oxide coatings playing with the deposition parameters. Among the large panel of binary oxides, the growth and characterization of tungsten-based compounds have extensively been studied due to their remarkable optical, electrical and chemical properties [START_REF] Kulkarni | Synthesis, structural, compositional, morphological and optoelectronic properties of tungsten oxide thin films[END_REF][START_REF] Li | The deposition and microstructure of amorphous tungsten oxide films by sputtering[END_REF][START_REF] Niklasson | Electrochromics for smart windows: Thin films of tungsten oxide and nickel oxide, and devices based on these[END_REF]. Thus these W-O films have been used in a range of applications such as antireflective and electrochromic coatings [START_REF] Granqvist | Electrochromic tungsten oxide films: Review of progress 1993-1998[END_REF], gas sensors, catalysts [START_REF] Kim | Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film[END_REF] and so on. However, the majority of the reported investigations has been focused on stoichiometric WO3 or oxygen-deficient WO3-x thin films [START_REF] Ramana | Structural stability and phase transitions in WO3 thin films[END_REF][START_REF] Johansson | Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering[END_REF]. A few studies have been devoted to the behaviors of WOx thin films with a wide range of oxygen concentrations, i.e. 0 < x < 3.0 [START_REF] Patel | Growth, structural, electrical and optical properties of the thermally evaporated tungsten trioxide (WO3) thin films[END_REF]. This lack of knowledge is even more significant for W-O thin films prepared by reactive sputtering. Basically, when a metallic target like tungsten is sputtered in a reactive atmosphere (e.g. argon + oxygen gases), the process abruptly drops in a reactive mode as the oxygen mass flow is increasingly supplied [START_REF] Depla | Understanding the discharge voltage behavior during reactive sputtering of oxides[END_REF]. This typical nonlinear effect of the reactive sputtering process restrains the range of reachable compositions. Thus, it may become a challenge to accurately control the oxygen and tungsten elemental concentrations in the films without using feedback control systems or high pumping speeds [START_REF] Sproul | Advances in partial-pressure control applied to reactive sputtering[END_REF][START_REF] Hmiel | Partial pressure control of reactively sputtered titanium nitride[END_REF][START_REF] Kadlec | Hysteresis effect in reactive sputtering: A problem of system stability[END_REF]. Recently, the Reactive Gas Pulsing Process (RGPP) [START_REF] Martin | High rate and process control of reactive sputtering by gas pulsing: the Ti-O system[END_REF] was developed to deposit metallic oxide thin films with tunable compositions. It was successfully used to reach titanium, niobium, tantalum, iron, zirconium and W-O and oxynitride thin films with a wide range of metalloid contents [START_REF] Fenker | Precise control of multilayered structures of Nb-O-N thin films by the use of reactive gas pulsing process in DC magnetron sputtering[END_REF][START_REF] Martin | Correlation between processing and properties of TiOxNy thin films sputter deposited by the reactive gas pulsing technique[END_REF][START_REF] Banakh | Influence of process parameters on the properties of the tantalum oxynitride thin films deposited by pulsing reactive gas sputtering[END_REF][START_REF] Grafouté | Structural investigations of iron oxynitride multilayered films obtained by reactive gas pulsing process[END_REF][START_REF] Carvalho | ZrOxNy decorative thin films prepared by the reactive gas pulsing process[END_REF][START_REF] Parreira | Characterization of W-O coatings deposited by magnetron sputtering with reactive gas pulsing[END_REF][START_REF] Chappé | Titanium oxynitride thin films sputter deposited by the reactive gas pulsing process[END_REF]. That is the reason why this method has been chosen in this thesis as a chemical approach to deposit W-O thin films by reactive sputtering. The RGPP was implemented to produce adjustable oxygen and tungsten atomic concentrations in the films.

The strategy consisted in pulsing the oxygen gas during the sputter deposition, in order to alternate the process between metallic and oxidized sputtering modes and thus to get various WOx compounds from metallic W to over-stoichiometric WO3 compounds. The injection time of the oxygen gas was systematically changed, so as to reach a regular and well-controlled evolution of the growth rate, composition, optical behaviors and electronic transport properties. A gradual transition from metallic to insulating behaviors was finally demonstrated and connected to a single pulsing parameter, namely the duty cycle, and thus to the oxygen concentration in the films.

The design and fabrication of thin films have been the cornerstone to technological advances in many fields. The capability to further engineer film properties through physical structuring on the micro-and nano-scale and by an accurate control of architectures enable new opportunities for performance improvements. Among the structuring toolbox, vacuum processes involving the GLancing Angle Deposition (GLAD) technique recently appeared as an attractive approach because it easily creates a wide range of architectures (tilted columns, zigzags, spirals and so on) at the nanometric scale [START_REF] Robbie | Chiral sculptured thin films[END_REF]. The basis can be explained by initial numbers of nucleation on the substrate making self-shadowing incident angle particle flux. The nucleation sites keep growing and form voids between two columns. Therefore, during the GLAD process, there are two key parameters, which need to be mentioned: one is the incident angle α which is defined by the angle between the target normal and the substrate normal; the other one is the rotating speed ϕ, which is defined by a computer-controlled rotating speed of the substrate holder.

The incident angle α can be changed from 0 to 90 º, with a relevant column angle obtained from Tait's investigation [START_REF] Tait | Modelling and characterization of columnar growth in evaporated films[END_REF] explaining the empirical correlation between incident angle α and column angle β.

The rotating speed ϕ can produce different architectures. Zigzag structures can be prepared when the substrate holder rotates 180º after a certain period. Spiral structures can be prepared when the substrate holder rotates at a given speed, and the pitch height and coil diameter can be designed by the ratio between deposition rate and rotating speed.

Nanostructured thin films have found applications in many fields, exploiting the adjustable properties from the chemical approach (e.g. RGPP) or the physical structuring (e.g. GLAD). In this thesis, the focus will be on the use of W-O thin films as an active larger for gas sensors. Two gases have been chosen as pollutants: ozone and dodecane. Ozone is an oxidizing gas, which can cause the decreasing of oxygen vacancies concentration in the film, leading to an n-type sensor with a high resistivity. Ozone is the typical air pollutant, which is widely investigated in gas sensor field [START_REF] Boudiba | Sensing mechanism of hydrogen sensors based on palladium-loaded tungsten oxide (Pd-WO3)[END_REF][START_REF] Usta | Effects of annealing on morphological, structural and electrical properties of thermally evaporated WO3 thin films[END_REF][START_REF] Garcia-Sanchez | Thermal effects associated with the Raman spectroscopy of WO3 gas-sensor materials[END_REF][START_REF] Stoycheva | Micromachined gas sensors based on tungsten oxide nanoneedles directly integrated via aerosol assisted CVD[END_REF][START_REF] Guérin | A dynamic response model for the WO3-based ozone sensors[END_REF][START_REF] Belkacem | Cobalt nanograins effect on the ozone detection by WO3 sensors[END_REF][START_REF] Boulmani | Correlation between rfsputtering parameters and WO3 sensor response towards ozone[END_REF][START_REF] Labidi | Alternating current investigation and modeling of the temperature and ozone effects on the grains and the grain boundary contributions to the WO3 sensor responses[END_REF]. Jet fuels has large amount of compounds, but basic from the conception, researchers prepare to use small hydrocarbons (dodecane could represent petroleum-based jet fuel) instead of large hydrocarbon molecules to investigate such as reaction kinetic [START_REF] You | Detailed and simplified kinetic models of n-dodecane oxidation: The role of fuel cracking in aliphatic hydrocarbon combustion[END_REF] or gas leak monitor [START_REF] Yazdi | ZnO nano-tree active layer as heavy hydrocarbon sensor: From material synthesis to electrical and gas sensing properties[END_REF]. Dodecane is also typical of reduction air pollution, which could increase the depletion layer of oxygen vacancies concentration, giving rise to a resistivity decrease for an n-type gas sensor.

In this thesis, we report on the deposited W-O thin films prepared by RGPP, GLAD and co-sputtering.

The strategy consists in clarifying the role of oxygen gas during the sputter deposition in order to find out which composition of thin film exhibits the best gas sensing performances. Then, GLAD is implemented to get various architectures such as columnar, zigzag and spiral microstructures. We show that nanostructuring of W-O thin films allows sensing either oxidizing, or reducing chemical compound and how the films' architecture leads to a detection enhancement. To this aim, ozone and dodecane have been chosen since they exhibit oxidizing and reducing behaviors, respectively. In order to produce nanostructured films, two combined strategies have been involved to play with microstructure and composition of sputter-deposited W-O thin films. Finally, GLAD co-sputtering was used, to fabricate thin films with a low resistivity and an adequate electronic transport.

In chapter 1, W-O compounds are described as the starting materials with attractive tunable properties so as to apply as an active layer for gas sensors. Advantage of thin films compared to the conventional bulk materials is explained. Different methods to prepare W-O thin films are presented and, for each one, a few examples are given with more detailed information. Other works devoted to WOx films applied for gas sensors prepared by RGPP and GLAD techniques are mentioned and tested under different gases.

In chapter 2, all facilities and instruments used for the thesis are described, such as sputtering systems for the deposition of W-O thin films, morphologies and structured characterization by electronic microscopy, X-ray diffraction, resistivity and optical measurements. Experimental details are developed for the gas sensing tests.

In chapter 3, W-O thin films are prepared from one W target by RGPP using different duty cycles.

Electrical resistivity and optical transmittance are used to evaluate oxygen contents in WOx films as a function of duty cycle, the latter being announced as a key parameter influencing the characteristic of W-O thin films. Then, the GLAD technique is developed focusing on two more additional parameters: the incident angle α and the rotating speed ϕ. By controlling these two parameters, various architectures are fabricated. Optical and electrical properties are investigated as the function of the duty cycle dc and incident angle α.

In chapter 4, a two targets co-sputtering system is implemented. Firstly, two tungsten targets are cosputtered with a systematic change of the current applied on each target. The idea is to produce two components W/WO3 columnar microstructures. A ring close to one W target is used for the oxygen injection and its influence is discussed. Then, tungsten trioxide RF sputtered target is installed instead of one tungsten DC sputtered target. The current of the tungsten target IW and the WO3 self-bias potential UWO3 are separately modified to produce co-sputtered WOx thin films.

In chapter 5, some selected thin films prepared by RGPP, RGPP+GLAD or RGPP+GLAD co-sputtering are exploited as active layers for gas sensors. Ozone (oxidizing chemical compound) and dodecane (reducing chemical compound) are tested as pollutants. Influence of oxygen contents in thin film and microstructure are discussed.

Chapter 1: State-of-the-art

In this first chapter, the author intends to propose a short overview of W-O compounds and thin films.

Methods commonly developed for depositing WOx thin films are described and sputtering processes involving RGPP and GLAD approaches are discussed. Some recent investigations focused on WOx as an active layer for gas sensors are finally reported with an emphasis on W-Os produced by GLAD.

W-O compounds and thin films

Several W-O compounds can be reached as a function of the oxygen and tungsten concentrations (Figure 2.1) [START_REF] Wriedt | The OW (oxygen-tungsten) system[END_REF]. [START_REF] Wriedt | The OW (oxygen-tungsten) system[END_REF].

Figure 2.1 Phase diagram of the W-O system

Changing the oxygen content leads to typical stable phases such as W, WO2, WO3, or Magnéli phases such as W3O8, W5O14, W18O49, W17O47, W20O58, and W25O73, or other compounds with chemical formula WnO3n-1 or WnO3n-2. WO3 usually exhibits four phases vs. temperature: tetragonal structure, which is thermodynamically stable at temperatures above 740 ºC, orthorhombic structure stable for temperatures in-between 330 -740 ºC, monoclinic structure stable for temperatures in-between 17 -330 ºC and triclinic structure stable for temperatures below 17 ºC.

Among the various metal oxides, tungsten trioxide is a n-type semiconductor, which has a wide bandgap (2.4-2.8 eV) [START_REF] Zhao | Highly sensitive porous metal oxide films for early detection of electrical fire: Surface modification and high throughput screening[END_REF], and attracted the scientific community due to sensing properties. For n-type semiconductors, resistivity is increased when exposed to oxidizing gases (O3, NO2), or decreased with reducing gases (H2, CO, NH3). A sensing effect is based on electronic exchanges between gas and metal oxide surface. Tungsten oxide exhibits good sensing properties such as a low detection gas concentration, a low operation temperature, fast response and recovery times, and a high sensitivity. Tungsten oxide sensors also show excellent performances towards pollutants such as O3 [START_REF] Boulmani | Correlation between rfsputtering parameters and WO3 sensor response towards ozone[END_REF], NO2 [START_REF] Mane | NO2 sensing properties of nanostructured tungsten oxide thin films[END_REF], SO2, H2S [START_REF] Çiftyürek | Molybdenum and tungsten oxide based gas sensors for high temperature detection of environmentally hazardous sulfur species[END_REF], H2 [START_REF] Zhao | Ultrathin percolated WO3 cluster film and its resistive response to H2[END_REF], CO [START_REF] Tian | DFT study of CO sensing mechanism on hexagonal WO3 (001) surface: The role of oxygen vacancy[END_REF], NH3 [START_REF] Ho | Material and NH3-sensing properties of polypyrrole-coated tungsten oxide nanofibers[END_REF], hydrazine [START_REF] Shukla | Tungsten oxide (WO3) nanoparticles as scaffold for the fabrication of hydrazine chemical sensor[END_REF], acetone [START_REF] Chi | Tungsten trioxide nanotubes with high sensitive and selective properties to acetone[END_REF]. Its electronic properties are thus used during gas detection by recording the change of electrical resistance. It is well known that the mechanism of sensing performances is related to the surface-to-volume ratio (or surface area). Therefore surfaces exhibiting high roughness [START_REF] Usta | Effects of annealing on morphological, structural and electrical properties of thermally evaporated WO3 thin films[END_REF][START_REF] Stoycheva | Micromachined gas sensors based on tungsten oxide nanoneedles directly integrated via aerosol assisted CVD[END_REF][START_REF] Ho | Material and NH3-sensing properties of polypyrrole-coated tungsten oxide nanofibers[END_REF][START_REF] Zhao | Preparation and structure dependence of H2 sensing properties of palladium-coated tungsten oxide films[END_REF] and interfaces such as nano-necks [START_REF] Moon | Glancing angle deposited WO3 nanostructures for enhanced sensitivity and selectivity to NO2 in gas mixture[END_REF][START_REF] Hoa | Crystalline mesoporous tungsten oxide nanoplate monoliths synthesized by directed soft template method for highly sensitive NO2 gas sensor applications[END_REF] or junctions [START_REF] Ho | Material and NH3-sensing properties of polypyrrole-coated tungsten oxide nanofibers[END_REF][START_REF] Yu | A hydrogen/methane sensor based on niobium tungsten oxide nanorods synthesised by hydrothermal method[END_REF][START_REF] Vuong | Surface gas sensing kinetics of a WO3 nanowire sensor: Part 1oxidizing gases[END_REF][START_REF] Qin | Thermal-oxidative growth of aligned W18O49 nanowire arrays for high performance gas sensor[END_REF] play a key role in gas sensing performances.

Methods for producing WOx films

Various methods have ever been applied for preparing WOx films, such as sol-gel, hydrothermal, electrospinning, and PVD and CVD techniques. Several selected examples for each method are described below.

Sol-gel method

Sol-gel method can produce metal inorganic compounds, metal organic compounds or a mixture of these two compounds, through the hydrolysis polycondensation, gelation and annealing processes (preparation of oxides or other compounds). Maity et al. [START_REF] Maity | Understanding the anomalous conduction behavior in "n" type tungsten oxide thin film during hydrogen gas sensing: Kinetic analyses of conductance transients[END_REF] used tungsten precursor sol from tungsten (V) ethoxide in propan-2-ol and added HCl drop by drop during stirring. Then the sol was aged overnight before depositing with a high-speed spin coating. The coating was moved into a furnace at 350 ºC for 5 min to remove organic species. The procedure was repeated until the required films thickness. Mane et al. [START_REF] Mane | NO2 sensing properties of nanostructured tungsten oxide thin films[END_REF] used a sol-gel method to prepare WO3 nanoparticles. They mixed and stirred tungsten hexachloride and methanol at 60 ºC for 3 h to obtain the gel. Then the gel was heated to obtain a powder. It was washed, dried, and annealed at 700º C for 1 h. Finally, the powder was dissolved in m-cresol (solvent) and deposited on substrate by using drop-casting method.

Hydrothermal method

Hydrothermal method refers to a special sealed reactor, by using aqueous solution as a reaction medium.

The reaction system was heated to a critical temperature (or close to the critical temperature), to produce high-pressure environment for inorganic synthesis. It was also developed to prepare nanowires (or nanorods) arrays of WO3 thin films [START_REF] Qin | Nanowire (nanorod) arrays-constructed tungsten oxide hierarchical structure and its unique NO2-sensing performances[END_REF][START_REF] Zeng | Hydrothermal synthesis and gas sensing properties of WO3H2O with different morphologies[END_REF][START_REF] Susanti | Preparation of CO gas sensor from WO3 nanomaterial synthesized via sol-gel method followed by hydrothermal process[END_REF]. Zhang et al. [START_REF] Zhang | Temperature and acidity effects on WO3 nanostructures and gas-sensing properties of WO3 nanoplates[END_REF] used Na2WO4•2H2O solution as precursor, and HNO3 was added drop by drop during stirring. Then the substrate was mixed with the solution in the Teflon-lined stainless-steel autoclave at constant temperature for 12 h with hydrothermal method.

Finally, the substrate was washed by distilled water and ethanol for several times, and dried at 50 ºC.

Liu et al. [START_REF] Liu | Facile synthesis of tungsten oxide nanostructures for efficient photoelectrochemical water oxidation[END_REF] started with H2WO4 solution by adding H2O2 drop by drop with strong stirring, then mixed with NH4Cl solution. Different morphologies of WO3 nanostructures were controlled by different ratios of H2WO4 precursor, ethanol and deionized water. The solution was filled into Teflon-lined stainlesssteel autoclave with vertically oriented substrate, then sealed and kept at 180 ºC for 5 h. Finally, the substrate was cleaned by deionized water and ethanol and dried at 60 ºC for 12 h. Zhang et al. [START_REF] Zhang | A novel method in the gas identification by using WO3 gas sensor based on the temperature-programmed technique[END_REF][START_REF] Zhang | Temperature-programmed technique accompanied with high-throughput methodology for rapidly searching the optimal operating temperature of MOX gas sensors[END_REF] employed nonstoichiometric WO3 nanoparticles mixed with an organic solvent to prepare paste, which was applied with a screen printing technique, then drying at 80 ºC for 15 min, sintered at 350 ºC to remove organic species and calcinated at 600 ºC for 2 h to improve the mechanical strength.

Electrospinning method

Electrospinning method forces the surface of precursor to overcome the surface tension under electric field. It provides a simple and an efficient way for the fabrication of nanometer scale fibers. Chi et al. [START_REF] Chi | Tungsten trioxide nanotubes with high sensitive and selective properties to acetone[END_REF] developed single capillary electrospinning method. First, tungsten chloride as a precursor was mixed with N, N-dimethylformamide stirred for 30 min. Then, polyvinylpyrrolidone was added in the above solution and was stirred for 30 min. Afterwards, ethanol was added and stirred for 10 h. The precursor solution was filled into a syringe and a high-voltage power supply was connected to the pinhead. The fibers collected on the substrate were calcinated at 550 ºC for 3 h. Bai et al. [START_REF] Bai | Morphology, phase structure and acetone sensitive properties of copper-doped tungsten oxide sensors[END_REF] implemented an electrospinning method to prepare Cu-doped WO3 hollow fibers. H2WO4 was added into a mixed solvent (ethanol and H2O2) with ultrasonication for 40 min. Then citric acid and NH3•H2O aqueous solution were added drop by drop during stirring. Cu(NO3)2•3H2O as copper source was added into the sol. The latter was filled into a syringe pump and the pinhead high voltage was applied. Then the hollow fibers were collected and sintered in air at 500 ºC for 1 h.

Physical vapor deposition method

It is difficult to obtain large area homogeneous thin films by wet-chemical reaction. Then the physical vapor deposition methods were applied for preparation of W-O films. Zhao et al. [START_REF] Zhao | Ultrathin percolated WO3 cluster film and its resistive response to H2[END_REF] used Supersonic Cluster Beam Deposition (SCBD) technology to prepare tungsten films. An argon pulsing flow passes through a rotating tungsten rod. Then the time lag of electric pulses ignites the plasma of tungsten, which makes W atoms further to aggregate and produces W nanoclusters. The Ar flow carries the nanoclusters and passes through an expansion chamber, due to a pressure gradient. It leads to a supersonic expansion.

Then the nanoclusters flow passes an aerodynamic filter, to align the tungsten nanoclusters deposited on the substrate. Vuong et al. [START_REF] Vuong | Surface gas sensing kinetics of a WO3 nanowire sensor: Part 1oxidizing gases[END_REF] prepared WO3 nanowire thin films on single-walled carbon nanotubes template. First, they deposited platinum and titanium layers on the substrate as the electrodes. Then graphite rod as carbon source fabricated single-walled carbon nanotubes by an arc current in hydrogen gas. Last, DC magnetron sputter W was deposited on template and the thin films were put into an oven so as to oxidize the W to WO3 and burn the template. Stoycheva et al. [START_REF] Stoycheva | Micromachined gas sensors based on tungsten oxide nanoneedles directly integrated via aerosol assisted CVD[END_REF] developed aerosol assisted chemical vapor deposition (AACVD) to synthesize WO3 thin films. Dissolving hexaphenoxide in a mixture of acetone and toluene, a shadow mask was used to protect blank membrane. W-O films were grown on the membrane at 350, 450, and 500 ºC. After deposition, the films were annealed in air at 500 ºC for 2 h. Qin et al. [START_REF] Qin | Thermal-oxidative growth of aligned W18O49 nanowire arrays for high performance gas sensor[END_REF] used RF magnetron to sputter a metallic W target. Under 150 Pa at 650 ºC with Ar+ O2 gas mixture, aligned W18O49 nanowire arrays were prepared on the W film in a tube furnace.

Kumar et al. [START_REF] Kumar | Influence of annealing temperature on nanostructured thin films of tungsten trioxide[END_REF] prepared WO3 thin films by thermal evaporation. At first, the commercial grade WO3 powder was sintered into pellets, then using a high-energy electron beam (6 keV) to evaporate on the substrate. Afterwards, all the thin films were annealed at a certain temperature for 1 h. Parreira et al. [START_REF] Parreira | Synthesis, structural and mechanical characterization of sputtered tungsten oxide coatings[END_REF] deposited W-O by DC reactive magnetron sputtering. Various partial pressures of oxygen were used.

Oxygen content was systematically changed from 0 to 75 %. Deposition rate and X-ray diffraction were recorded vs. an increasing oxygen content. An optimized rate appears for the stoichiometric WO3 phase.

The deposition rate increased before this optimum and decreased after, which was explained by the poisoning effect of the tungsten target. Crystalline phases were produced α-W, β-W, amorphous phase, and quasi-amorphous structure. The adhesion of coating was also investigated. Parreira et al. [START_REF] Parreira | Thermal stability of reactive sputtered tungsten oxide coatings[END_REF] continued the research on DC reactive magnetron sputtering of the W-O system. Thermal stability of the W-O thin films was investigated by various annealing treatments and with or without neutral atmospheres. The results showed that sub-stoichiometric compounds start to oxidize at 600 ºC, and under neutral atmosphere, crystallization appears at 700 °C and follows the phase diagram. Li et al. [START_REF] Li | The deposition and microstructure of amorphous tungsten oxide films by sputtering[END_REF] prepared W-O films under various oxygen flow rates by DC reactive magnetron sputtering. As-deposited films were metal-rich WOx to amorphous WO3 compounds. The color changed from dark blue to transparent while oxygen supply increased during deposition. XPS indicated W 4+ , W 5+ , W 6+ ions in the films. The occurrence of W 5+ was attributed to the amorphous structure of the films. Optical bandgap widened for films deposited under high oxygen flow rate.

WOx films by RGPP

Parreira et al. [START_REF] Parreira | Characterization of W-O coatings deposited by magnetron sputtering with reactive gas pulsing[END_REF] developed the Reactive Gas Pulsing Process for the W-O system. They fixed the pulsing period of oxygen P = 10 s or 50 s and various TON times from 3 to 9 s or 25 to 45 s, respectively.

They also kept duty cycle dc = 0.9 with various periods P (from 10 to 100 s). All films exhibited a multilayered structure, which consisted of W-rich and O-rich periodic nanolayers. The main idea of RGPP is to alternate the reactive sputtering between metallic and compound sputtering modes. The deposition rate was decreased when the W target was fully poisoned. The authors suggested a hysteresis loop of W target potential and total pressure (Figure 2.2), which is rather a drift of the process. [START_REF] Parreira | Characterization of W-O coatings deposited by magnetron sputtering with reactive gas pulsing[END_REF].

Figure 2.2 Evolution of the target potential and total pressure vs. oxygen flow rate during reactive sputtering

By using RGPP, a significant increase of the deposition rate was measured and the deposition of compounds with controllable compositions. The structure was systematically changed from α-W or β-W to nanocrystalline WO3 phase only modifying the TON time.

Polcar et al. [START_REF] Polcar | Nanoscale colour control: W-O graded coatings deposited by magnetron sputtering[END_REF] used RGPP to obtain graded refractive index coatings, leading to films with different colors. Seven layers of a multilayered structure was built with the same pulsing period P but various duty cycles dc by adjusting the deposition time. Two samples with two series graded coatings were prepared. Reflectance vs. wavelength measurements were compared with simulations as shown in Figure 2.3. [START_REF] Polcar | Nanoscale colour control: W-O graded coatings deposited by magnetron sputtering[END_REF] Position and magnitude of the reflectance peak can be simulated. Controlling the sputtering conditions, such as gas flow rate or sputtering power, a desired thin film color can be achieved.

Figure 2.3 Reflectance spectra in the visible range of W-O thin films sputter deposited by RGPP. Comparison between simulation and experiment for coatings deposited with different periods P = 50 and 100 s, and TON time = 45 and 90 s.

WOx films by GLAD

Charles et al. [START_REF] Charles | Correlation between structural and optical properties of WO3 thin films sputter deposited by glancing angle deposition[END_REF] deposited WO3 thin film by using DC reactive magnetron sputtering from a W target in Ar+ O2 gas mixture. They first investigated the influence of incident angle α of the particle flux on optical properties, such as refractive index, extinction coefficient, optical band gap, Urbach energy and birefringence. They showed that the column angle β was changed from 0 to 54 º as the incident angle α was systematically increased from 0 to 80 º. When the incident angle α was higher than 50 º, a clearly columnar microstructure was observed. Refractive index at 550 nm dropped from 2.11 to 1.90 as a function of the incident angle. This optical property change was assigned to the increasing porous architecture coming from the shadowing effect during deposition.

More recently, Charles et al. also [START_REF] Charles | Optical properties of nanostructured WO3 thin films by GLancing Angle Deposition: Comparison between experiment and simulation[END_REF] studied the influence of different kinds of nanostructures on optical properties. By using the same deposition conditions, WO3 thin films were prepared with different incident angles and different zigzag periods. Refractive index and birefringence were investigated by experiments and simulations (MIT's MEEP code [START_REF] Farjadpour | Improving accuracy by subpixel smoothing in the finite-difference time domain[END_REF]). The maximum refractive index was obtained when the number of zigzag periods N = 4 (Figure 2.4), which could be considered as the less voided films produced when the period increases. A higher number of zigzag periods makes the columns trend to perpendicular columns to the substrate and thus a similar porous structure as tilted column. Deniz et al. [START_REF] Deniz | Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition[END_REF] fabricated tungsten and tungsten trioxide films by GLAD with an incident angle α = 80 º. W and WO3 films were grown in Ar or Ar + O2 gas mixtures, respectively. The films were annealed at 500 ºC in air for 5 hours. The annealing treatment did not change the morphology of the W nanorods, but it significantly transformed the WO3 nanorods into a nanoporous network. Simple cubic β-W phase and amorphous phase were obtained before annealing treatment, whereas films crystallized following the triclinic/monoclinic WO3 phase after the annealing treatment. Similarly, the XPS analysis of WO3 films showed that W4f 7/2 peaks shift from sub-stoichiometric to stoichiometric WO3 compound after annealing (Figure 2.5). [START_REF] Deniz | Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition[END_REF].

Figure 2.5 High resolution XPS W 4f peaks from as-deposited and post-deposition air annealed WO3 films

WOx films as an active layer for gas sensors

Horprathum et al. [START_REF] Horprathum | NO2-sensing properties of WO3 nanorods prepared by glancing angle DC magnetron sputtering[END_REF] made WO3 gas sensors by DC magnetron sputtering in Ar + O2 gas mixture. By using GLAD or without GLAD, incident angle was set at 85 º and 0 º, respectively. All the thin films were annealed in air for 3 hours at 400 and 500 ºC. Inclined WO3 nanorod thin films exhibited a better NO2 response compared to dense films, and such WO3 nanorods annealed at 500 ºC showed the most sensitive response using 0.1 ppm NO2 at an operating temperature of 250 ºC. In Figure 2.6, the response of the dense film is lower than that of the nanorods. Films annealed at 400 ºC exhibit a lower response than those annealed at 500 ºC. Such sensing performances were explained assuming that the nanorod architecture has a large surface area, and the highest annealing temperature (500 ºC) made these films as a highly porous cross-linked WO3 nanonetwork. Moon et al. [START_REF] Moon | Self-activated ultrahigh chemosensitivity of oxide thin film nanostructures for transparent sensors[END_REF] prepared their sensors by using RF sputtering with an incident angle of 85 º. Sensors were annealed in air at 500 ºC for 60 min. They compared with planar thin films fabricated with a normal incident angle (α = 0 º) with the same films thickness. NO2 gas was mainly tested, but other target gases like C2H5OH (ethanol), CH3COCH3 (acetone), NH3, H2, SO2 and CO were also investigated (Figure 2.7).

The lowest detected concentration was 1 ppm. They used self-heated sensors, which means that the resistance was measured under different bias voltages instead of operating temperatures. In Figure 2.7, the response vs. concentration is shown at the same bias voltage Vb = 5 V, which is a different standard compared to the operating temperature as commonly used by most researchers. More recently, the same group of Moon et al. [START_REF] Moon | Glancing angle deposited WO3 nanostructures for enhanced sensitivity and selectivity to NO2 in gas mixture[END_REF] used RF sputtering with an incident angle of 86 º to prepare gas sensor. The surface area was calculated to be 350 m 2 g -1 , which is 12 times higher than WO3 dense thin films. The sensor was sintered at 550 ºC for 60 min. Oxidizing gases made deeper additional depletion region of films involved in the sensing process, which means that a thicker thin film expresses a better response [START_REF] Ahmad | Investigation of RF sputtered tungsten trioxide nanorod thin film gas sensors prepared with a glancing angle deposition method toward reductive and oxidative analytes[END_REF]. The same group deposited Pd layer 2.5 nm thick as a catalyst on WO3 surface. The deposited films were annealed in air at 400 ºC for 3 h. Optical gas sensing absorbance was investigated under H2 or synthetic air. With an inclined structure, Pd catalyst layer exhibited a larger contact area to WO3 nanorods, which had also a much larger absorbance change than dense films (Figure 2.8). Wongchoosuk et al. [START_REF] Wongchoosuk | Carbon doped tungsten oxide nanorods NO2 sensor prepared by glancing angle RF sputtering[END_REF] used RF sputtering at an incident angle of 85 º. They injected acetylene (C2H2) as a carbon source into a gas mixture during WO3 deposition to prepare carbon-doped WO3 nanorods.

All the films were annealed at 400 ºC for 3 h. NO2 was selected as the target gas (Figure 2.9).

Figure 2.9 Gas responses of carbon-doped and undoped WO3 nanorods gas sensors to different concentrations of NO2

at the operating temperature of 150 ºC [START_REF] Wongchoosuk | Carbon doped tungsten oxide nanorods NO2 sensor prepared by glancing angle RF sputtering[END_REF].

They showed that carbon-doped WO3 films decreased the activation energy and increased the depletion layer thickness. Such a doping led to a better response, recovery time, operating temperature and selectivity.

Conclusion

The production of W-O compounds and thin films have been abundantly reported in the literature. It is nearly a challenging task to get an exhaustive review about physical and chemical deposition methods in liquid and/or vacuum environments leading to the growth of WOx thin films. Among the deposition methods, the reactive sputtering process is an attractive way to get tunable W-O thin films. The Reactive Gas Pulsing Process (RGPP) and the GLancing Angle Deposition (GLAD) have been unfrequently reported in the literature in spite of their potentiality to play with the nature and microstructure of the deposited metal oxides. Application of W-O as an active layer for gas sensing still remains an exciting scientific topic, especially when the films architecture as well as its composition can be adjusted by the deposition process.

Chapter 2: Materials and methods

In this chapter, the author reports on a brief description of the sputter deposition methods. The GLAD techniques with one and two targets are described, as well as pulsing parameters involved by RGPP.

Basic parameters of the characterization techniques are also given (rather than a detail description of principles, which can easily be found in the cited literature). Emphasis is put on the electrical resistivity and optical measurement setups. Sensor benches and operating conditions are finally explained for the detection of dodecane and ozone as pollutants.

Deposition setup and conditions

For W-O thin films, a homemade DC reactive magnetron sputtering system with a 40 L vacuum chamber was used. A 2-inch disk tungsten target (purity 99.9 at. %) was fixed 65 mm away from the center of the substrate holder as shown in Figure 3.1. Before depositing, an ultimate pressure about 10 -8 mbar was reached with a turbomolecular pump backed by a mechanical pump. Target current was fixed at IW = 100 mA (the corresponding current density was JW = 50 A m -2 ). A pre-sputtering time was applied for 5 min in order to remove the contamination layer on the target surface and stabilize the process. The argon flow rate was kept constant at qAr = 1.2 sccm.

A constant pumping speed S = 10 L s -1 was used leading to an argon partial pressure pAr = 2.810 -3 mbar.

Oxygen mass flow rate qO2 was pulsed during W-O deposition by means of the Reactive Gas Pulsing Process (RGPP). A constant pulsing period P = 16 s was used. The maximum oxygen flow rate was qO2Max = 2.4 sccm and the minimum was qO2min = 0 sccm (to see later about the choice of P, qO2Max and qO2min). For all depositions, the substrate holder was grounded, and no external heating was added. The angle of inclination of the substrate holder, namely α, could be changed from 0 to 90° compared to the substrate normal. The rotating speed ϕ was computer-controlled and could be adjusted from 0 to several rev h -1 (revolution per hour). A GLAD co-sputtering system was also used to deposit W-O thin films (Figure 3.2). A 2-inch disk tungsten trioxide (WO3) target (purity 99.9 at. %) was fixed 95 mm away from the substrate holder. A RF generator was used to sputter it. W and WO3 targets were focused on the center of the substrate holder and simultaneously sputtered using opposite and oblique angles αW = αWO3 = 80 ° from the substrate normal. The substrate holder was fixed for all GLAD co-sputtering (ϕ = 0 rev h -1 ).

The other deposition parameters were the same as those employed with a single W target DC sputtered.

It is important to note that the argon flow rate had to be increased since 1.2 sccm did not allow maintaining a stable ignition of the RF plasma on WO3 target (argon pressure was too low). As a result, the argon flow rate was set at qAr = 3.2 sccm leading to an argon partial pressure pAr = 3.0×10 -3 mbar and thus, stable co-sputtering conditions. The pulsing parameters of the oxygen gas were kept the same as those set for the single W target DC sputtered. Tungsten target current was systematically changed from IW = 40 to 100 mA whereas the WO3 self-bias potential was modified from UWO3 = 100 to 300 V in order to play with the films microstructure, as well as W and O concentrations.

When only one W target was DC sputtered, and in order to determine the oxygen pulsing parameter, argon was first injected at qAr = 1.2 sccm (the corresponding argon pressure was pAr = 2.810 -3 mbar).

W target was DC sputtered with IW = 100 mA and the oxygen gas was injected in the chamber. At first, oxygen flow rate was gradually injected from 0 to 2.4 sccm, and then reduced back to 0 sccm. The total sputtering pressure psp in the chamber and the W target voltage UW were recorded (Figure 3.3). Without oxygen injection, the W target voltage is more than 400 V and corresponds to a pure metallic sputtering mode. For the shortest oxygen flow rate (0.2 sccm), UW abruptly rises up to 470 W and then gradually reaches a maximum of 570 V for qO2 = 1.6 sccm. A further increase of the oxygen flow rate gives rise to a slight drop of the W target voltage. In-between 1.6-2.4 sccm, we can assume that the reactive sputtering process is mainly in reactive sputtering mode (i.e. W target surface is poisoned by the oxygen gas). Similarly, the total sputtering pressure slightly increases from 3.010 -3 up to 3.410 -3 mbar as qO2 changes from 0 to 1.6 sccm. Afterward, psp linearly increases vs. qO2, which correlates with a poisoning mode of the reactive sputtering process. Despite the low pumping speed implemented during the deposition stage (S = 10 L s -1 ), reducing the oxygen flow rate does not exhibit a hysteresis loop of the total sputtering pressure and target voltage as it can be commonly reported for other systems like Ti-O2 [START_REF] Martin | High rate and process control of reactive sputtering by gas pulsing: the Ti-O system[END_REF] or Zr-O2-N2 [START_REF] Carvalho | ZrOxNy decorative thin films prepared by the reactive gas pulsing process[END_REF]. Therefore, by setting an oxygen flow rate higher than 1.6 sccm, the sputtering process becomes mainly oxidized. To be sure to avalanche the process in reactive mode, qO2Max = 2.4 sccm has been chosen. In order to choose the pulsing period P of the oxygen mass flow rate, the duty cycle defined as dc = TON/P was set as 50 % of P keeping qO2Max = 2.4 sccm during the TON time and stopping the oxygen flow rate for the rest of the period (i.e. qO2min = 0 sccm during the TOFF time). The variation of W target voltage and total sputtering pressure between TON and TOFF, U and p, respectively, were both recorded as a function of an increasing pulsing period P from 4 to 100 s (Figure 3.4).

Changing the pulsing period from 4 to 10 s produces an abrupt increase of the total sputtering pressure variations, which is even more significant for the W target voltage. For pulsing periods higher than 10 s, the target voltage variation becomes stable and the sputtering pressure still rises but to some extent.

For these pulsing conditions (P > 10 s), the process completely alternates between the reactive and metallic sputtering modes. As a result, the pulsing period has to be long enough to be able to change the duty cycle keeping such an alternation between both modes. A pulsing period P = 16 s has been selected since it allows getting sputtering conditions to get fully metallic or oxidized sputtering mode during TOFF and TON times, respectively, and so tuning the oxygen concentration in the films.

Characterization techniques

The substrate material was glass and (100) silicon wafer. All substrates were cut to the following dimensions 25 mm × 12.5 mm. They were ultrasonically cleaned in acetone, ethanol and deionized water for 10 min and dried in an oven (60 ˚C). For testing the sensing performances, W-O thin films were sputter deposited on commercial MSP 769 Heraeus sensor platforms. The sensing procedure will be more described in the next paragraph.

Morphology of W-O films was observed with a Dual Beam SEM/FIB FEI Helios 600i microscope on the fractured cross-section and on the top view. Due to the limited number of samples that fit on the substrate holder since a strict placement of the substrates directly in front of the sputtered particle flux is crucial, a first series of samples was sputtered on silicon substrates. They were used for the deposition rate calculation (obtained from the film thickness measured by profilometry, and deposition time), and SEM cross-section and top imaging. Then, taking into account the different deposition rates, the sputtering time was adjusted in order to keep an overall thickness close to 400 nm for all films used in the subsequent characterization and sensing tests. X-ray diffraction (XRD) patterns of the films were collected by D8 focus diffractometer using a cobalt source (Co λKα =1.78897 Å) according to a Bragg-Brentano θ-2θ configuration. Scans were performed with a 2θ angle with a step of 0.02° per 0.2 s and a 2θ angle ranging from 20 to 80°.

Optical transmission spectra of the films deposited on glass substrate were recorded with a Lambda 900 UV-visible optical spectrometer. A mask (black piece) with a hole of 1 mm diameter was placed in the light path, to limit the observed area on the thin films. This mask allows probing a nearly constant thickness and avoid thickness gradient, which is especially substantial in GLAD films exhibiting an inclined columnar structure (intrinsic to the GLAD process). Optical transmittance spectra of thin films deposited on glass substrate were recorded in the visible range. The scanning wavelength ranged from 300 nm to 1100 nm, with 1 nm s -1 scanning speed.

The DC electrical resistivity of the films deposited on glass substrate was measured at room temperature or vs. temperature (from RT to 300 ºC) by the four-probe method. Measurements were carried out in air using a custom-made chamber, which was covered in order to achieve a dark environment. The measurement procedure is based on the van der Pauw's method [START_REF] Van Der Pauw | A method of measuring specific resistivity and Hall effect of discs of arbitrary shape[END_REF] (Figure 3.5). It requires four Ohmic contacts (golden tips) to be placed on the sample. They must be located at the boundaries of the sample (or as close to it as possible) and as small as possible compared to the distance between them. To make a measurement, a current is applied between 2 contacts (for instance, I12) and the voltage across the opposite edge (in this case, V34) is measured. From these two values, a resistance can be deduced using Ohm's law. Similar measurements are applied following the NIST procedure [START_REF]Hall Effect Measurements[END_REF] in order to get 8 resistances and finally, an average of two resistances, namely RA and RB, defined as: Knowing the film's thickness "d", the electrical resistivity "" is obtained following:

{ 𝑅 𝐴 = 𝑅 21,
ρ = 𝜋𝑑 ln 2 ( 𝑅 𝐴 + 𝑅 𝐵 2 ) 𝐹 ( m) (3.2)
where F is the van der Pauw's coefficient, which depends on the R = RA/RB ratio (or RB/RA since R must be > 1). The F coefficient is calculated using:

1 2 𝑒 ln 2 𝐹 = 𝑐𝑜𝑠ℎ ( 𝑅-1 𝑅+1 • ln 2 𝐹 ) (3.3)
Similarly, charge carrier mobility and concentration can be determined by Hall effect by means of the same procedure as electrical resistivity measurements, but applying a magnetic field B of 0.8 Tesla perpendicular to the sample surface. At first, the magnetic field +B is set, and a current is applied between 1 and 3 (I13). The resulting voltage between 2 and 4 (V24P) is measured. Similar measurements are performed with a current between contacts 3 and 1 (I31) and a voltage between 4 and 2 (V42P), and so on.

V24P, V42P, V13P, V31P associated to I13, I31, I24, I42 are obtained, respectively. Thus, the magnetic field is reversed, i.e. -B, and the same routine is used leading to V24N, V42N, V13N, V31N associated to I13, I31, I24, I42, respectively.

One can calculate VC, VD, VE and VF following:

VC = V24P -V24N; VD = V42P -V42N; VE = V13P -V13N; VF = V31P -V31N
The sample type is then determined from the polarity of the voltage sum

V = VC + VD + VE + VF. If V
is positive (negative), the film is p-type (n-type), I = average current (A), q = elementary charge (1.6×10 -19 A s). The carrier concentration is then given by:

[𝑝] = 8 × 10 -6 𝐼𝐵 𝑞𝑑Σ𝑉 (𝑐𝑚 -3 ) for p -type

[𝑛] = | 8 × 10 -6 𝐼𝐵 𝑞𝑑Σ𝑉 | (𝑐𝑚 -3 ) for n -type (3.4)
The carrier mobility is obtained from:

𝜇 𝑝 = 𝑑 𝑞[𝑝]𝜌 (𝑚 2 𝑉 -1 𝑠 -1
) for the hole mobility for p -type

𝜇 𝑛 = 𝑑 𝑞[𝑛]𝜌 (𝑚 2 𝑉 -1 𝑠 -1
) for the electron mobility for n -type (3.5) The ozone was generated by a commercial gas generator (O3 1000 ® ), by means of a UV radiation of air flow, to transform the oxygen into ozone, precisely and constantly. The dilution was controlled by a leak.

Sensor setups and conditions

A constant air flow rate (200 mL min -1 ) passed through the gas generator. Then a second route could be varied from 0 to 800 mL min -1 . The two routes were mixed before the controlled leak. Then the leak was modified to make sure that only 200 mL min -1 diluted ozone could enter the reactor.

Two dilution benches were used in order to expose the sensor layer under various concentrations of ozone (initial concentration equal to 220 ppb) or dodecane (initial concentration equal to 300 ppm) balanced in synthetic dry air flow (Figure 3.6) [START_REF] Gaddari | A novel way for the synthesis of tin dioxide sol-gel derived thin films: Application to O3 detection at ambient temperature[END_REF]. For each experiment, the sensor was exposed to a constant gas flow rate of about 200 mL min -1 . The electrical resistance change of the sensing film was measured as a function of time. In this study, the sensing properties of the films were studied at different working temperatures from room temperature up to 500 °C and under different concentrations of ozone or dodecane in synthetic dry air. These experiments allowed evaluating both the optimum operating temperature and then the limit of detection of various efficient W-O structures.

Typical WOx gas sensor response of a) ozone and b) dodecane, under concentration of [O3] at 500 ppb and [C12H26] at 300 ppm for 90 s (air exposure time is 360 s).

The gas response of the sensor is defined as S = |Rg-R0|/R0100 %, where Rg is the resistance () in the air mixed with ozone or dodecane and R0 is the resistance () under synthetic dry air. Since WOx film is an n-type sensor, the gas response exhibits a peak when the sensor is under ozone and a valley when it is exposed under dodecane. The response slope was calculated from the linear part of the response curve. The exposing time and air exposure time were fixed at 90 and 360 s, respectively.

Chapter 3: WO x films with one W target

This chapter aims at preparing W-O thin films with a suitable resistivity range and porosity microstructure. RGPP is an attractive method, which can gradually and precisely produce tunable compounds by adjusting deposition parameters, particularly duty cycle dc, so as to reach a regular and well-controlled evolution of the growth rate, composition, optical behaviors and electronic transport properties. To modify the films' microstructure, GLAD is applied for fabricating various architectures, such as columnar, zigzag and spiral W-O thin films. Electrical, optical properties, microstructure and morphology are finally investigated.

WOx films by RGPP

WOx films were prepared by RGPP, from pure metallic W to over-stoichiometric WO3 thin films. By modifying the duty cycle, thin films exhibit various electrical and optical properties. We focus on some optimized compositions to apply W-O thin film as gas sensors.

Structure, morphology and composition

SEM observations of the cross-section of the films show various morphologies as a function of the duty cycle (Figure 4.1). Without oxygen injection, a typical columnar microstructure is clearly viewed (Figure 4.1a). Taking into account qualitative structure zone models based-on Thornton's diagram [START_REF] Thornton | Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings[END_REF] or more recently improvements suggested by Anders [START_REF] Anders | A structure zone diagram including plasma-based deposition and ion etching[END_REF], densely packed fibrous grains are expected for DC sputtering of pure tungsten films. It may be suggested that W films tend to feature as a zone T microstructure rather than zone 1 since tungsten is a heavy atom, for which the reflection probability of Ar neutral is strongly favored [START_REF] Drüsedau | Energy transfer into the growing film during sputter deposition: An investigation by calorimetric measurements and Monte Carlo simulations[END_REF]. Thus, the contribution of reflected neutrals to the total deposited energy certainly increases the displacement and heating effects on the growing film.

Oxygen and tungsten atomic concentrations by EDS in W-O thin films as a function of the duty cycle dc. A gradual evolution is obtained by means of a systematic increase of dc. For duty cycles in-between 60 -80 % of P, substoichiometric WO3- to oxygen-rich WO3+ compounds are produced.

Most of the metallic targets sputtered in argon + oxygen atmosphere typically exhibits a sudden drop of the deposition rate as a function of the reactive gas flow rate [START_REF] Schiller | Reactive d.c. high-rate sputtering as production technology[END_REF]. Due to a poisoning effect of the target surface and since the sputtering yield of an oxide compound is much less than that of the metal, oxide thin films are normally deposited with a maximum rate of a few percent of the pure metal rate [START_REF] Billard | Attempted modelling of thickness and chemical heterogeneity in coatings prepared by d.c. reactive magnetron sputtering[END_REF]. With the Reactive Gas Pulsing Process, a gradual transition of the rate is rather measured when the duty cycle is incrementally increased [START_REF] Martin | Enhanced sputtering of titanium oxide, nitride and oxynitride thin films by the reactive gas pulsing technique[END_REF]. Again, the deposition rate of W-O films smoothly varies with the duty cycle, as shown in Firstly, deposition rate increases as duty cycle changes from 0 to 70 % of P, then reaches a maximum value higher than 2000 nm h -1 , which is twice the pure tungsten rate, for dc around 70 % of P. When the oxygen flow rate tends to be constantly injected (dc = 100 % of P), deposition rate drops down to 1400 nm h -1 , which is again significantly higher than the pure tungsten rate. This bell-shaped and enhancement of the deposition rate are mainly connected to the poisoning effect of the target surface, an increase of the oxygen concentration in the deposited films, and the density of deposited oxide compounds. A balance occurs between the sputtering yield of W-O compounds formed on the metallic target surface and its oxidation by the reactive gas. When oxygen is injected at the beginning of the TON time (Figure 4.4), the W target surface starts oxidizing and leads to an increase of the W target voltage, as shown in Our results agree with that of Parreira et al. [START_REF] Parreira | Characterization of W-O coatings deposited by magnetron sputtering with reactive gas pulsing[END_REF], who previously reported the growth of an amorphous phase in W-O films sputter deposited by the Reactive Gas Pulsing Process. Due to the periodic injection of the reactive gas, oxygen does not solely produce a compound formation on the target surface, but it also disturbs the growth of crystalline phases in the film. It is also worth noting that these amorphous films well correlate with the brittle microstructure previously evidenced from the cross-section viewed by SEM (Figure 4.1b and c).

Optical and electrical properties

The average optical transmittance at 633 nm (representative of the transparency of the material) of W-O films deposited on glass is also influenced by the duty cycle, particularly for dc > 70 % of P. Electrical conductivity measured at room temperature also depends on duty cycle (Figure 4.3). Without oxygen introduction, the conductivity of tungsten film is 25 ˚C= 4.8810 6 S m -1 , which is lower than the bulk value (25 ˚C of W bulk is higher than 10 7 S m -1 ) as expected in polycrystalline thin films. For the shortest duty cycle (10 % of P), conductivity reduces by more than an order of magnitude due to a significant oxygen incorporation in the film (28 at. % from Figure 4.2). As duty cycle rises from 10 to 60 % of P, the films become continuously and slightly more resistive, which is mainly assigned to their oxygen-enrichment. The transition zone corresponding to the absorbent-to-transparent evolution previously reported from optical transmittance well correlates with the abrupt drop of electrical conductivity. Semi-transparent W-O films produced with dc = 80 % of P reach the lowest conductivity with 25°C = 5.53 S m -1 . Films prepared with higher duty cycles are too much resistive to be measured with our four-probe system. Such a resistivity jump is again directly related to the oxygen concentration in the films since compositional analyses clearly demonstrated a regular evolution from O-deficient to O-rich WO3 compound in this range of duty cycles (Figure 4.2).

Electrical resistivity of the films vs. temperature was also measured from room temperature to 300 ºC. Without pulsing the oxygen gas, a typical metallic-like behavior is observed, i.e. a slight increase of the electrical conductivity as the temperature rises. The temperature coefficient of resistance calculated at room temperature (RT) is TCRRT = 3.710 -3 K -1 (Table 4.1). It is lower than that of the W bulk value (TCRRT of bulk W is 4.410 -3 K -1 [START_REF] Lide | CRC Handbook of Chemistry and Physics. Section 14, Geophysics, Astronomy, and Acoustics; Abundance of Elements in the Earth's Crust and in the Sea[END_REF]), which is commonly reported for thin films and mainly assigned to the scattering of electrons at grain boundaries [START_REF] Reiss | Grain-boundary resistance in polycrystalline metals[END_REF]. When oxygen is pulsed and up to dc = 30 % of P, conductivity remains in the order of 10 5 S m -1 and TCRRT reduces to 1.610 -3 K -1 . The metallic-like character of the films is kept in spite of an increasing oxygen concentration in the films (55 at. % from Figure 4.2), but the conductivity tends to exhibit an invariant temperature dependence. Increasing further the duty cycle leads to negative TCRs with -1.110 -2 K -1 for dc = 60 % of P (Table 4.1).

TCRRT (K -1 ) 3.7×10 -3 1.9×10 -3 1.5×10 -3 1.6×10 -3 -4×10 -3 -7×10 -3 -1.1×10 -2 - - - - Activation energy Ea (meV) - - - - - - 5 14 54 145
This shift from positive to negative TCR values has ever been reported for titanium oxide thin films similarly prepared by the Reactive Gas Pulsing Process [START_REF] Cacucci | The interdependence of structural and electrical properties in TiO2/TiO/Ti periodic multilayers[END_REF]. Taking into account that W-O films become amorphous for duty cycles higher than 40% of P, the change of sign of TCR cannot be assigned to the number of grain boundaries per electron mean free path as suggested for crystallized metallic materials When duty cycle varies from 60 to 65 % of P, there is a change from a low resistivity behavior (metalliclike) of W-O exhibiting a negative TCR to a semi-conducting-like behavior observed for oxygen deficient WO3- compound. A negative slope of Ln () = f(1000/T) is obtained for duty cycles higher than 65 % of P, which is connected to a thermally induced conductivity mechanism. The linear behavior in the Arrhenius plot leads to an activation energy Ea ranging from a few meV up to 145 meV (Table 4.1) for the highest resistive films measurable with our four-probe set-up (dc = 80 % of P). with an invariant temperature dependence. The carrier concentration is more than 10 21 cm -3 for films produced with dc = 65 % of P. This large increase of the carrier concentration is directly connected to the oxygen vacancies in WO3-x films produced for these deposition conditions. As the deviation from stoichiometric to strongly oxygen-deficient WO3 compound is favored by reducing the duty cycle in the 60-80 % of P range, the semiconductor-to-metal transition occurs. Deviation from stoichiometry creates donor levels due to oxygen vacancies [START_REF] Molenda | Electrical properties of nonstoichiometric WO3-y at temperatures 77 to 300 K[END_REF]. The increasing concentration of donor centers (as duty cycle reduces) leads to the formation of donor conduction band, which overlaps the conduction band and tends to exhibit metallic-like properties (i.e. n > 10 21 cm -3 ). It is interesting to note that for films prepared with dc = 65 % of P, the carrier concentration becomes temperature dependent. This reduction of the carrier concentration as the temperature rises may be related to the increasing structural defects in the films produced by oxygen vacancies. These defects act as trapping centers for the charge carriers (i.e. free electrons) in the films [START_REF] Machlin | Materials Science in microelectronics I: The relationships between thin film processing and structure[END_REF]. In addition, decreasing the oxygen concentration in sub-stoichiometric metallic oxide films enhances their reactivity and the films become more sensitive to oxidation with an increase of temperature.

Carrier mobility vs. temperature is less influenced by the duty cycle (Figure 4.9 b) and remains is in the order of 1 cm 2 V -1 s -1 for films prepared with dc = 75 and 80 % of P, with no significant variations as a function of the temperature. The lowest room temperature mobility is about 0.2 cm 2 V -1 s -1 for dc = 65 % of P and tends to 1 cm 2 V -1 s -1 as the temperature reaches 120 ºC. The temperature variation of the carrier mobility suggests that there are various carriers scattering mechanisms, which are temperature dependent.

At first, the carriers scattering can be related to the carriers mobility and intergranular potential according to a classical thermally activated process [START_REF] Patil | Synthesis and characterization of Co3O4 thin film[END_REF]. Nevertheless, the log  vs. 1/T plot of our films does not exhibit a clear linear evolution (Figure 4.9b). Therefore, the temperature dependence of carriers mobility has to be investigated taking into account lattice and impurity scattering mechanisms. It can be described by the empirical relationship   T w where w is a positive or negative constant for scattering due to impurities or lattice, respectively [START_REF] Peter | Fundamentals of semiconductors: physics and materials properties[END_REF]. Fitting from data reported in Figure 4.9b gives rise to a positive value with w = 0.67 for W-O films sputter deposited with dc = 70 % of P and reaches 2.78 for dc = 65 % of P. This temperature dependence is then associated to the scattering of carriers by defects (due to oxygen vacancies). Such defects act as neutral impurities since the w coefficient becomes higher than 3/2, this latter value being commonly assigned to ionized impurities scattering phenomenon [START_REF] Peter | Fundamentals of semiconductors: physics and materials properties[END_REF].

WOx films by GLAD + RGPP

As in the previous work ( § 4.1), WOx films were prepared combining GLAD and RGPP. GLAD technique was also introduced to produce various morphologies and structures. To this aim, a porous microstructure and appropriate W-O thin films were investigated so as to develop active layers for gas sensing applications. as shown in Figure 4.11. For incident angles α < 60 º, the columnar structure is not so clear as metallic films like Cr, Ti, W and so on [START_REF] Tait | Modelling and characterization of columnar growth in evaporated films[END_REF][START_REF] Sadeghi-Khosravieh | Morphology and crystal texture in tilted columnar microstructured titanium thin film coatings[END_REF][START_REF] Li | Periodic one-dimensional nanostructured arrays based on colloidal templates, applications, and devices[END_REF]. This kind of inclined columns exhibit a porous structure compare to conventional W-O films that we produced before without using GLAD ( Incident angle ()

Structure and morphology

Tait et al. [START_REF] Tait | Modelling and characterization of columnar growth in evaporated films[END_REF] dc = 60 % of P dc = 80 % of P dc = 100 % of P [START_REF] Tait | Modelling and characterization of columnar growth in evaporated films[END_REF]. Dashed region indicates that no columnar microstructure is observed in films prepared for α < 60º.

Figure 4.11 Column angle β vs. incident angle α of W-O films prepared with different duty cycles. β vs. α is also calculated from Tait et al. relationship

Based on the gas sensing performances of W-O thin films produced by RGPP (to see Figure 6.1 in chapter 5), and assuming that resistivity of porous architectures (inclined columns) is higher than that of conventional films (Figure 4.26), duty cycle of W-O films with a zigzag microstructure should be chosen at low duty cycle, in order to reach the suitable resistivity for zigzag gas sensors. Four turns are clearly viewed with voids and spaces between spirals. A porous microstructure is similar to those obtained in Figure 4.10 and Figure 4.12 but roughness and porosity seem to be less than that of inclined columns as shown in Figure 4.10.

In order to prepare W-O films as an active layer for gas sensors, inclined, zigzag and spiral microstructures were fabricated and post-annealed following their crystallinity by XRD. Inclined columns were prepared by the GLAD method with duty cycles dc = 20 to 50 % of P. Zigzag microstructures were also prepared using the same procedure. Duty cycle dc was changed from 30 to 50 % of P, zigzag number from 1 to 16, and spiral microstructures were grown with the same duty cycle range and spiral number fixed at 4. For all films, the annealing treatment was in air from 250 to 400 ºC for 12 hours. XRD patterns of films exhibiting all architectures, without annealing or with annealing lower than 300 ºC, are shown in Figure 4.14 a.

 For annealing temperatures lower than 300 ºC, no diffracted signals are recorded for any architecture.

The inclined thin films with an annealing higher than 300 ºC for 12 hours are shown in

Optical properties

At first, optical transmittance spectra in the visible region of W-O films exhibiting an inclined columnar microstructure were recorded (Figure 4.15). Films were prepared with a systematic change of duty cycle dc and incident angle α. For duty cycles lower than 65 % of P, films are completely absorbent. Semitransparent or transparent W-O films are produced for dc > 65 % of P and for any incident angle. For duty cycles higher than 90 % of P, typical interference fringes are clearly noticed, especially for the lowest incident angles. The highest amplitudes are recorded, which means that W-O films have a high refractive index. For incident angles α = 75 to 85º, amplitudes of fringes are significantly reduced. This is mainly connected to the reduction of refractive index. A porous microstructure is produced, as expected for the highest incident angles. It is also worth to note that the absorption edge shifts to higher wavelength, particularly for α = 80 and 85º. Such a shift has ever been reported by Charles et al. [START_REF] Charles | Correlation between structural and optical properties of WO3 thin films sputter deposited by glancing angle deposition[END_REF] and associated to the poorly crystallized or even amorphous structure of the films as previously shown from XRD results ( From optical transmittance spectra of transparent films deposited on glass substrate, the films porosity can be estimated. To this aim, we calculated the porosity π of the films according the Bruggeman effective medium approximation model [START_REF] Kasap | Handbook of Electronic and Photonic Materials[END_REF].

Figure 4.17 Illustration of Bruggeman effective medium approximation model in W-O thin films prepared with an incident angle α = 80º and a duty cycle dc = 80 % of P.

It uses to following equation:

χ 𝑎 ( 𝜀 𝑎 -𝜀 𝑒𝑓𝑓 𝜀 𝑎 + 2 × 𝜀 𝑒𝑓𝑓 ) + χ 𝑏 ( 𝜀 𝑏 -𝜀 𝑒𝑓𝑓 𝜀 𝑏 + 2 × 𝜀 𝑒𝑓𝑓 ) = 0 (4.1)
where εa and εb represent the dielectric permittivity of a and b components, and εeff is the effective permittivity of the film. χa and χb are the volume fraction of a and b components, respectively (χa + χb = 1), as shown in Figure 4.17. In our films, a is related to WO3 material and b represents the porosity.

The effective permittivity of the film is connected to the refractive index with:

𝜀 = 𝑛 2 (4.2)
The films porosity WO3 also takes into account the refractive index of bulk tungsten trioxide compound (nb = 2.5 at 633 nm [START_REF] Charles | Correlation between structural and optical properties of WO3 thin films sputter deposited by glancing angle deposition[END_REF]), and the refractive index neff of the film determined by the Swanepoel's method [START_REF] Swanepoel | Determination of the thickness and optical constants of amorphous silicon[END_REF]:

𝑛 𝑒𝑓𝑓 = √N + √𝑁 2 -𝑛 𝑔𝑙𝑎𝑠𝑠 2 (4.3)
Where

N = 2𝑛 𝑔𝑙𝑎𝑠𝑠 𝑇 𝑀 -𝑇 𝑚 𝑇 𝑀 𝑇 𝑚 + 𝑛 𝑔𝑙𝑎𝑠𝑠 2 + 1 2 (4.4)
nglass is the refractive index of optical glass, which is assumed to be 1.51 at 633 nm. TM and Tm are the optical transmittance from the envelope curves obtained from the peaks and valleys at 633 nm, respectively.

From equations 4.1 to 4.4, refractive index and porosity of the films were calculated. Figure 4.18 shows the evolution of refractive index n and porosity π as a function of the incident angle α and for duty cycles dc = 80, 90 and 100 % of P. It is worth noting that refractive index is significantly reduced when incident angle changes from 60 to 85º and for any duty cycle. The films porosity exhibits a reverse evolution vs.

incident angle. The most important variation of n and/or π is obtained for films prepared with dc = 80 % of P while the lowest porosity reaches 10 % for α = 60 º and the highest is 50 % for α = 85 º. The same trend is obtained for all duty cycles, which means that the incident angle is the key parameter leading to a more voided microstructure. As expected, by increasing the incident angle α, amplitude of the fringes becomes less important due to the increasing porosity for the highest angles (specially for α = 80 and 85º). It becomes even more significant when the zigzag number reduces. By decreasing the zigzag number, films tend to behave like inclined columnar microstructure.

Figure 4.19 Optical transmittance spectra in the visible region of W-O films with a zigzag microstructure vs. incident angle and different zigzag numbers. All films were 500 nm thick and prepared with a duty cycle dc = 80 % of P.

In addition, with a duty cycle dc = 60 % of P, films become more absorbent because composition is the predominant factor influencing the optical transmittance (rather than the incident angle). For duty cycle higher than 70 % of P, no clear effect of the zigzag number on the optical transmittance of the films is evidenced, except that films tend to behave as an inclined columnar microstructure when the zigzag number reduces down to 1.

Talking about the influence of zigzag number, the latter does not significantly affect the optical For both duty cycles, T633 vs. incident angle exhibit the same evolution. It shows again that the zigzag number does not influence the optical properties but rather the incident angle α, especially for dc = 70 % of P.

Electrical properties

In order to be measurable with our resistivity system, inclined columnar thin films were prepared varying duty cycle from 20 % to 70 % of P. Incident angle was systematically changed from 0 to 85º. Electrical resistivity of the films measured at room temperature is plotted vs. incident angle and for different duty cycles (Figure 4.21). All films systematically become more resistive as the incident angle increases. For the lowest duty cycle (dc = 20 % of P), ρRT = 2×10 -5 Ω m for α = 0º and it increases to one order of magnitude for α = 85º. Such resistivity increasing is more relevant for duty cycles dc = 30 to 70 % of P.

For such deposition conditions, resistivity slightly increases as the incident angle varies from 0 to 60º.

Afterwards, an abrupt jump is observed as the incident angle α becomes grazing (i.e. α tends to 85º). As an example, resistivity starts at 2×10 -5 Ω m for α = 0º and reaches near 2×10 -2 Ω m at α = 85º for dc = 60 % of P (i.e. 3 orders of magnitude). This sudden increase of resistivity from α ≥ 60º has ever been reported for GLAD metallic films [START_REF] Besnard | A theoretical model for the electrical properties of chromium thin films sputter deposited at oblique incidence[END_REF]. It is mainly attributed to the formation of voids in the inclined columnar microstructure. Films become more and more porous as α tends to 85º, which reduces the mean free path of electrons and thus favors a high electrical resistivity. One can also assume the effect of composition (oxygen pollution) especially at high incident angle. One side of the substrate far from the W target receives a lower W flux and thus more oxygen atoms. In our case, the resistivity of samples was systematically tested in the same area to exclude the impact of the distance (i.e. between the target and the substrate). For duty cycles higher than 60 % of P, the effect of oxygen becomes significant and for dc = 80% of P, the films resistivity is over 10 -1 Ω m and becomes too high to be measured with our system as the incident angle increases. Electrical resistivity of the films vs. temperature was also measured from room temperature to 300 ºC. angle. Films exhibit a metallic-like behavior, turns to semiconductor and finally insulator while duty cycle increases. When the duty cycle is higher than 80 % of P, the samples are too much resistive to be measured. It is important to mention that the incident angle also affects the hysteresis loop during the temperature cycle. Surprisingly and for duty cycles dc ≥ 30 % of P, resistivity can be reduced vs.

temperature cycle for films prepared at low incident angle (α ≤ 60 º), whereas the loop shows a reverse evolution at higher incident angles (α ≥ 80 º), i.e. an increase of resistivity after a temperature cycle. A thermal oxidation of the films mainly gives rise to such increase of resistivity. About the surprising resistivity reduction after the temperature cycle, one may suggest that some Magnéli phases exhibiting a metallic-like behavior are partially produced for our sputtering conditions [START_REF] Viswanathan | Crystal structure and charge carrier concentration of W18O49[END_REF].

Electrical resistivity vs. temperature of inclined columnar W-O thin films deposited on glass substrate for duty cycles lower than 80 % of P and with different incident angles.

Several sub-stoichiometric WOx compounds (where x changes from 2.6 to 2.9) have been reported by many authors [START_REF] Safi | Recent aspects concerning DC reactive magnetron sputtering of thin films: A review[END_REF][START_REF] Johansson | Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering[END_REF][START_REF] Parreira | Characterization of W-O coatings deposited by magnetron sputtering with reactive gas pulsing[END_REF]. Among these Magnéli phases, the charge carrier concentration becomes maximum (ne = 1.87×10 22 cm -3 ) when the W18O49 compound is formed (i.e. WO2.722). Such a compound exhibits a metallic-like behavior [START_REF] Migas | Tungsten oxides. II. The metallic nature of Magnéli phases[END_REF]. Increasing further the oxygen concentration leads to a reduction of the charge carrier concentration (ne = 2.90×10 21 cm -3 for W25O73; i.e. WO2.92) and finally the semiconducting nature is obtained reaching the WO3 stoichiometry.

For zigzag structured thin films, there are three main parameters, which need to be controlled: duty cycle, incident angle and the zigzag number. The electrical resistivity was measured at room temperature, with duty cycles varying from 20 to 70 % of P, incident angles changing from 60 to 85º, and zigzag numbers

Electrical resistivity vs. zigzag number for different duty cycles and incident angles. Zone 1 concerns all films for incident angle lower than 75 ˚, zone 2 for duty cycle ≤ 30 % of P and with an incident angle α = 80 ˚, zone 3 for duty cycle ≥ 40% of P and with an incident angle α = 80 ˚ and all films with an incident angle α = 85 ˚.

As previously noticed for inclined thin films, resistivity of zigzags shows the same behavior. Films prepared with low duty cycles and low incident angles exhibit the lowest resistivities (~10 -6 to 10 -5 Ωm).

For any duty cycle, increasing the zigzag number and/or incident angle, systematically leads to more resistive films. For incident angles lower than 75º, duty cycle mainly influences the films resistivity, especially for dc higher than 40 % of P. For grazing incident angles (i.e. α ≥ 80º), the zigzag number clearly influences the films resistivity for the highest duty cycles. Increasing the zigzag number to values

Three types of electrical resistivity evolution vs. temperature cycle of W-O thin films for a) an incident angle α = 85 º with any duty cycles and an incident angle α = 80 º with duty cycles higher than 40 % of P, b) incident angle α = 80 º with duty cycles lower than 30% of P, c) incident angles α lower than 75 º with any duty cycle.

When the resistivity vs. temperature is investigated, there are three different types of evolution for films exhibiting a zigzag microstructure (Figure 4.24). The resistivity monotonically increases after one thermal cycle. This evolution occurs for zigzag films prepared with the highest incident angle of 85º and for duty cycles higher than 40 % of P ( For W-O films prepared with a spiral microstructure, the resistivity vs. temperature was measured for a few samples, i.e. those prepared with an incident angle α = 80º, 4 pitches and duty cycle changing from 30 to 60 % of P (Figure 4.25). For duty cycles higher than 40 % of P, films exhibit an increasing resistivity with temperature cycle. Spiral films deposited with duty cycles lower than 30 % of P similarly show the resistivity reducing previously discussed for some inclined columnar and zigzag films (not shown here). The other films show a hysteresis loop evolution of resistivity vs. temperature typical of a thermal oxidation process, i.e. an increase of resistivity after the temperature cycle.

Conclusion

Electrical resistivity measured at room temperature as a function of the duty cycle for W-O films exhibiting inclined, zigzag, spiral and normal columnar architectures, = resistivity range for gas sensing.

Films prepared with normal incidence (α = 0 º) exhibit the lowest resistivity due to a typical dense structure. Increasing the duty cycle gives rise to W-O films with a growing amount of oxygen, as previously reported and discussed in this chapter. For inclined columns, zigzag and spiral microstructures, resistivity systematically increases with duty cycle mainly due to an increasing oxygen concentration in the films. Such resistivity variation is more relevant for inclined columns, than zigzag and finally spiral microstructures. This is mainly ascribed to the porosity in the GLAD films. Such a porosity is more important in inclined columns than in zigzags and finally in spirals. One can expect the best sensing performances according to the same order (to see chapter 5).

Chapter 4: WO x films by GLAD co-sputtering

Inclined columnar structures are produced implementing the GLAD co-sputtering technique by means of two opposite and inclined targets. Structure, morphology and electrical resistivity of the films are investigated. The strategy is first to use two W targets. By changing the second target current intensity and duty cycle dc of RGPP, one can expect an original control of the reactive co-sputtering process. In a second approach, oxygen is pulsed with a ring close to the second W target. Then two opposite W and WO3 targets are co-sputtered in a reactive atmosphere using RGPP. The tungsten target current intensity IW and WO3 self-bias potential UWO3 are separately modified, in order to tune the final properties of the films.

From two W targets

The two targets process was first implemented using two DC co-sputtered W targets. Due to different distances between each target and the substrate, two different fluxes can be expected, especially with RGPP. A ring was added near one target to further improve oxygen concentration of the flux from this target. This approach aims at producing a two components columnar microstructure in co-deposited thin films.

Structure and morphology by classical O2 injection

Inclined columnar thin films prepared with duty cycle dc = 40, 60, 80 % of P were first observed by SEM. The current intensity of the second tungsten target IW was changed from 20 to 100 mA, whereas that of the first target was kept constant at 100 mA. It must be noted that the second tungsten target was sputtered without ring. An inclined columnar structure is clearly observed from the cross-section views (Figure 5.1).

Figure 5.1 cross-section SEM views of W-O films co-sputtered by GLAD with duty cycle dc = 60 % of P, the current intensity of the second tungsten target IW changed from 20 to 100 mA, keeping that of the first tungsten target at 100 mA.

As expected, the column angle β decreases from 41 to 15º as the second IW rises as shown in Figure 5.1.

The column angle is not completely perpendicular to the substrate surface, while the two targets current are the same and equal to 100 mA. This can be explained by the distances between the targets and the substrate center, which are not the same (65 mm and 95 mm from Figure 3.2). As a result, the column angle is 15º due to the highest W atoms flux coming from the W target the closest to the substrate (i.e. at 65 mm). It is worth to note that the column angle is always higher for W-O films co-sputtered with the highest duty cycle (80 % of P in Figure 5.2). For dc = 40 and 60 % of P, the co-sputtering process alternates between metal and oxidized sputtering modes. Thus, columnar growth is disturbed for such sputtering conditions, whereas the process becomes mainly in oxidized mode for dc = 80 % of P.

Optical transmittance spectra in the visible region of W-O films with different duty cycles and current intensities on the second tungsten target.

Films becomes transparent and typical interference fringes are produced in the visible region spectrum for duty cycles higher than 80 % of P and second tungsten target current IW = 20 mA, as shown in Figure 5.3. While the second tungsten target current IW is increased, the transmittance decreases for any duty cycle due to a larger amount of W in thin films. They tend to exhibit a metallic-like behavior.

O2 injection close to one W target

In order to poison a W target surface but keeping the other W target in the metallic sputtering mode, the UW1 and UW2 both follow a periodic variation vs. time, which directly correlates with the oxygen pulsing signal (TON and TOFF times of 9.6 and 6.4 s, respectively). With and without ring, both targets become poisoned after a few seconds of TON time (upper UW values) and are restored to the metallic mode at the end of the TOFF time. ΔUW of the second target is nearly the same with or without ring, which means that the oxygen pollution is nearly equivalent. On the other hand, ΔUW of the first target reaches more than 10 V lower value at the end of the TOFF time (Figure 5.4 a). This would suggest a less poisoned state with the ring. However, the high target potential (higher than 550 V) rather corresponds to an oxidize sputtering mode. As a result, the use of such a ring does not allow poisoning of one W target while keeping the other one in the metallic state. In addition, the voltage differences without ring and for both targets are always higher than with a ring (Figure 5.5), which indicates that the ring does not change the poisoning state during TON and TOFF times. Thus, the ring does not influence the poisoning of both targets and cannot change one target sputtering mode while the other one is sputtered in another mode.

From W target and WO3 target

Based-on results from § 5.1, the two W targets co-sputtering process did not allow getting a metallic state for one target and an oxidized one for the other one. As a result, a RF WO3 target was sputtered instead of a DC W target.

Varying the DC sputtering of W target

Co-sputtering GLAD process was developed using two kinds of targets during deposition. Since many parameters may influence the film properties, univariate analysis was considered. We only changed one parameter at a time. Current intensity of DC sputtering of W target was firstly changed.

a) Structure and morphology

W-O films were prepared by reactive co-sputtering of tungsten and tungsten trioxide targets. Both targets were glanced at 80° to the substrate normal. Deposition rate and column angle were determined by using SEM cross-section views (Figure 5.6).

Figure 5.6 SEM views (cross-section and top) of W-O films co-sputtered by GLAD with self-bias potential of WO3 target UWO3 = 300 V and different current intensities on W target without RGPP.

The self-bias potential of WO3 target was kept constant at UWO3 = 300 V whereas the current intensity of W target IW was changed from 40 to 100 mA (20 mA per step). As expected, deposition rate rapidly increases vs. IW from 230 to 400 nm h -1 (Figure 5



Figure 5.8 X-ray diffraction patterns of W-O thin films deposited on glass with various current intensities on W target and a constant self-bias potential UWO3 = 300 V on the WO3 target. = W (pdf # 04-0806); ♦ = β-W (pdf # 65-6453).

In addition, peak at 2θ = 51.53° vanishes and it tends to give rise to a preferential orientation along (200) plans. These results show that a significant amount of oxygen still remains in the films in spite of a high W target current.

b) Electrical properties

An annealing treatment in air was applied to W-O thin films prepared by GLAD co-sputtering. They were all heated for 12 hours in air at 300 ºC. The goal of such annealing was to investigate and understand the evolution of the films electrical properties, especially for a further application as gas sensors.

Electrical resistivity was measured vs. time during such a treatment (Figure 5.9). For the lowest W target current intensity (IW = 40 mA), resistivity remains quite stable as a function of the annealing time (ρ ~ 5 -6×10 -5 Ω m after 12 hours). This relative stability can be connected to the dense microstructure previously observed from SEM pictures (Figure 5.6). For IW = 60, 80 and 100 mA, the same ρ vs. time evolution is recorded. An increasing IW favors the metallic-like behavior of the films (ρ drops from 6.5×10 -5 to 3.2 ×10 -5 Ω m before annealing) mainly due to an increase of their tungsten content.

Electrical resistivity vs. time of W-O films prepared on glass substrates and annealed in air at 300 ºC for 12 hours with various tungsten target current intensities and a constant WO3 self-bias potential UWO3 = 300 V.

Films become more porous and tungsten-rich as IW increases. Despite the more voided microstructure, the film oxidation during annealing becomes less and less significant when IW increases from 60 to 100 mA, since the amount of W rises in these co-sputtered inclined columnar films.

Varying the RF sputtering of WO3 target

A constant current intensity of the W target is implemented whereas the self-bias potential of tungsten trioxide target is systematically changed from UWO3 = 100 V (minimum potential to ignite the RF plasma) up to 300 V. Tuning UWO3 in this range allows a stable co-sputtering process with composition changing from W to WO3-rich films. This means that sputtering flux of the W target prevails over that of WO3 target.

It is also interesting to note that two kinds of columnar microstructures can be distinguished from the SEM top views whatever the WO3 self-bias potential. The first one consists of narrow columns (width of a few tens nanometers) and more or less circular. The second one exhibits columns with an elliptical shape and a strong anisotropic microstructure, i.e. a big columnar width (a few hundreds of nanometers) in the direction normal to the W particle flux. This type of anisotropic microstructure is typical of tungsten thin films prepared by GLAD [START_REF] Moon | Glancing angle deposited WO3 nanostructures for enhanced sensitivity and selectivity to NO2 in gas mixture[END_REF]. As a result, it supports that for these deposition conditions (IW = 100 mA and UWO3 = 100 to 300 V), particle flux coming from the W target mainly influences the final microstructure of W-O films produced by GLAD co-sputtering from W and WO3 targets.

Figure 5.10 SEM views (cross-section and top) of W-O films co-sputtered by GLAD with a constant W target intensity IW = 100 mA and various WO3 target self-bias potentials.

The evolution of deposition rate and column angle as a function of the WO3 target self-bias potential corroborates the predominance of the particle flux coming from the W target (Figure 5.11). Deposition rate slightly increases from 364 to 397 nm h -1 as UWO3 rises from 100 to 300 V. In addition, the column angle is a little influenced by UWO3 since it is reduced from 37 to 34 º. Similarly, an increase of UWO3 favors the WO3 target flux, which leads to a low rising of the total deposition rate. The crystallographic structure of these films prepared with various UWO3 is significantly influenced as shown from X-ray diffraction patterns reported in Figure 5.12. For the lowest self-bias potential (UWO3 = 100 V), peaks are mainly assigned to the pure W phase (pdf # 04-0806) with a ( 110

Figure 5.12 X-ray diffraction patterns of W-O thin films deposited on glass with various WO3 target self-bias potentials and a constant W target current intensity. IW = 100 mA;  = W (pdf # 04-0806); ♦ =β-W (pdf # 65-6453); □ = WO3-x (pdf # 53-0434). Inset is a zoom of (200) peak at 2θ = 41.58 º. Full width at half-maximum (FWHM) is indicated.

It is also worth to notice that a shoulder appears close to the peak related to the (110) planes of the Wphase. Such a shoulder is linked to the occurrence of the β-W phase, i.e. (210) planes at 2θ = 47.11°. For the films prepared with UWO3 =150 V and IW = 100 mA, a diffracted signal is recorded at 2θ = 33.96 ° and is assigned to the WO3-x phase (pdf # 53-0434). The occurrence of this WO3-x phase still remains an open question.

b) Electrical properties

The same annealing treatment was applied to the films as before, i.e. annealing in air at 300 ºC for 12 hours. Figure 5.13 shows the evolution of electrical resistivity vs. time during this annealing. As UWO3 increases, the samples become more resistive since more oxygen is introduced in the films. For all films, resistivity slightly increases after 12 hours annealing and remains in the order of 10 -5 Ω m. However, films deposited with UWO3 = 100 and 150 V keep their metallic-like characteristic after the annealing treatment, whereas those obtained with UWO3 = 250 and 300 V rather exhibit a semi conducting-like behavior after 12 hours in air at 300 ºC (from resistivity vs. time during the cooling stage, i.e. from 760 to 820 min). For the highest UWO3, films become oxygen-rich, which agrees with the β-W phase occurrence noticed from XRD results. We have to note that these films are too much conductive to be used as an active layer for gas sensing.

Conclusion

W-O thin films were reactively deposited by GLAD co-sputtering using two targets with a tilt angle of 80º to the substrate normal. At first, two W targets were DC sputtered. Despite a pulsing introduction of oxygen by RGPP using a ring for oxygen introduction close to one target, it was not possible to get the co-sputtering process maintaining one target in a metallic mode, whereas the other one was in reactive mode in order to achieve tunable and original columnar architectures. Thus, WO3 material was RF sputtered as the second target. By changing W target current intensity and WO3 self-bias potential, inclined columnar microstructures were prepared with tunable columnar angles. Dense or porous architectures were produced with adjustable microstructures leading to nearly the same range of electrical resistivities (i.e. ~10 -5 Ω m). Such resistivities are not suitable for gas sensing tests. An annealing post-deposition stage is thus necessary in order to reach the required range.

WOx films with one W target

At first, the influence of the oxygen supply (duty cycle in RGPP) is considered on sensing performances of W-O thin films produced without GLAD. Afterwards, the nanostructuring by GLAD of W-O thin films are studied and applied as an active layer for gas sensor.

Role of oxygen

W-O films produced with duty cycles lower than 60 % of P did not produce any sensing behavior when exposed to dodecane vapor. It is mainly due to their metallic-like properties as previously shown in WOx thin films sputter deposited with other duty cycles (dc = 60, 80 and 100 % of P) were tested by using the same sensing procedure, i.e. 10 to 300 ppm of dodecane injections for 90 s followed by 360 s of air exposure time. An annealing treatment in air (12 hours at 300 ºC) was applied to W-O thin films before the sensing tests.

For dc = 60 % of P, WOx are poorly sensitive to the dodecane injections since resistance changes are a few m. In addition, the drift of the baseline as well as saturation at about 150 ppm are noticeable. These poor sensing performances agree with the metallic-like behavior previously noticed from optical (Figure 4.7) and electrical (Figure 4.8) characteristics of the films. On the other hand, WOx films prepared with a continuous oxygen supply (dc = 100 % of P) exhibit an improved sensitivity to dodecane. The resistance change is negligible at 10 ppm but becomes significant at 50 and 100 ppm. However, the recovery time is longer than the sensor produced with dc = 60 % of P. The resistance increases when dodecane injection is stopped but never recovers its starting value. The baseline tends to be stabilized with time, but a saturation of the detection is once again close to 150 ppm.

The best sensing performances are finally achieved for films obtained with a duty cycle of 80 % of P. A very low drift of the baseline signal can be noticed after several dodecane injections. From resistance vs.

time measurements, response and recovery times are improved compared to WOx films fabricated with other duty cycles. Sensitivity of the sensor also seems to be enhanced and saturation is not reached (higher than 300 ppm). It is interesting to note that these deposition conditions (dc = 80 % of P) corresponds to the transition from sub-to over-stoichiometric WO3 compounds (Figure 4.2). Oxygen vacancies as well as oxygen inserted in the films are both involved in the sensing mechanism.

As a result, the adsorption-desorption phenomena on WOx thin films surface remain an open question since for sub-and over-stoichiometric WO3 thin films, dodecane is detected. However, one can suggest that for over-stoichiometric WO3+' compounds, some surface oxygen vacancies are created by the reaction of products from the decomposition of C12H26 with oxygen adsorbates and the resulting desorption of these reaction products (certainly CO2 and H2O). These surface oxygen vacancies created by the alkane decomposition can lead to the generation of W 5+ centers. The additional electron associated with the resulting W 5+ center favors the release of a free electron and gives rise to an increase of conductivity [START_REF] Frühberger | Surface chemistry of H2S-sensitive tungsten oxide films[END_REF].

For sub-stoichiometric WO3- compounds, a mechanism involving interaction of charged surface oxygen species and the hydrocarbon molecules can be suggested taking into account Rout et al. investigations [START_REF] Rout | High-sensitivity hydrocarbon sensors based on tungsten oxide nanowires[END_REF]. Assuming that hydrocarbon sensing by an oxygen-deficient WO3 compound involves negative oxygen species O -and O 2-, which are adsorbed on the film surface, these latter react with the hydrocarbon molecule to release an electron (CO2 and H2O gases are also produced) and finally decrease the resistivity.

The following reactions have been suggested by these authors [START_REF] Rout | High-sensitivity hydrocarbon sensors based on tungsten oxide nanowires[END_REF]:

1 2 𝑂 2 + 𝛿𝑒 -→ 𝑂 𝛿-(𝑎), 𝛿 = 1,2 (6
These reactions release electrons leading to a decrease of the film's resistivity.

Exploiting the GLAD approach

It is well known that operating temperature of metal oxide material plays an important role on the sensing performances of resistive gas sensors because of the catalytic reactions involved during the detection process [START_REF] Mcaleer | Tin oxide based gas sensors[END_REF][START_REF] Kohl | Surface processes in the detection of reducing gases with SnO2-based devices[END_REF]. The influence of sensing material temperature has been studied measuring electrical responses of each selected chemical gas sensor under 300 ppm and 1 ppm of dodecane and ozone, respectively at different temperatures ranging from 50 to 500 °C. W-O thin films exhibiting dense, inclined, zigzag and spiral columnar microstructures, and sputter-deposited with various duty cycles were systematically tested as gas sensors. At first, it is worth noting that inclined and zigzag columnar structures gave rise to the best sensing performances towards dodecane and ozone environments. For these two kinds of structures, Figure 6.2a and b show the evolution of the gas sensor response vs. thin film temperature under dodecane and ozone, respectively and for duty cycles dc = 30, 40 and 50 % of P.

These measurements clearly show that the optimal temperature changes as a function of the architecture and the nature of the chemical compound to detect. For a given temperature, the inclined structures systematically exhibit the best relative response. Zigzag structures tested with a temperature higher than 350°C did not produce a significant improvement of the sensing response. Such a temperature correlates with the crystallization of the films as previously reported from XRD analyses (Figure 4.14).

Figure 6.2 Responses of the W-O-based sensors to a) 300 ppm of dodecane and b) 1 ppm of ozone as a function of the temperature for inclined and zigzag architectures prepared with various duty cycles (only the optimized duty cycles and architectures are presented). The best sensing response is obtained at 250°C with dc = 40% of P and dc = 50% of P for dodecane and ozone, respectively (green dashed lines).

It is also interesting to note that heating zigzags and inclined structures at temperatures higher than 350°C led to a less defined architecture. Inclined columns and particularly zigzags become less distinct (film's densification), which is certainly connected to the lowest sensing performances at temperatures higher than 400°C. In addition, for both gaseous species tested with the inclined structures, the optimized sensing temperature ranges from 250 °C to 400 °C. As a result, the best responses are obtained for inclined structures at 250 °C with duty cycle dc = 40 % of P and 50 % of P for dodecane and ozone, respectively (green dashed lines in Figure 6.2a and b). The same trend is measured for inclined and zigzag structures tested with dodecane and ozone, i.e. a higher resistance and an improvement of the response as the duty cycle increases. This duty cycle effect is mainly connected to the oxygen vacancies in the films. Oxygen-deficient WO3 compound is favored by reducing the duty cycle in the 60-80 % of P range [START_REF] Xu | Structure, composition and electronic transport properties of tungsten oxide thin film sputter-deposited by the reactive gas pulsing process[END_REF]. Deviation from stoichiometry creates donor levels due to oxygen vacancies [START_REF] Molenda | Electrical properties of nonstoichiometric WO3-y at temperatures 77 to 300 K[END_REF]. The increasing concentration of donor centers (as duty cycle reduces) leads to the formation of donor conduction band, which overlaps the conduction band and tends to exhibit metallic-like properties (i.e. free carrier concentration > 10 21 cm -3 ). In addition, decreasing the oxygen concentration in sub-stoichiometric metallic oxide films enhances their reactivity and the film becomes more sensitive to an oxidized environment.

It is also interesting to note that the sensor responses improvement is in the following order: dense (normal sputtering), zigzags and inclined columns, which agrees with previous results reported in Figure 6.2. Clearly, sensors with a high resistance under synthetic air exhibit the best responses under dodecane or ozone. More specially, inclined columns prepared with duty cycles dc = 40 % and 50 % of P produce the highest response under dodecane and ozone, respectively in comparison with zigzags and normal structures. This is mainly assigned to the porous architecture (Figure 4.10 and Figure 4.12), which can be particularly emphasized in GLAD films with an inclined columnar structure [START_REF] Krause | Surface area characterization of obliquely deposited metal oxide nanostructured thin films[END_REF]. In fact, inclined columns exhibit the most porous structure in comparison with the other ones [START_REF] Charles | Optical properties of WO3 thin films modeled by finitedifference time-domain and fabricated by glancing angle deposition[END_REF], leading to a high quantity of chemisorbed gaseous species (reactive surface species) and a high permeability for the fast adsorption and gas diffusion. From these results, it can be concluded that the inclined columnar structures with duty cycles dc = 40 % and 50 % of P appear as the most efficient architectures for the detection of dodecane and ozone, respectively.

Repeatable test of W-O thin films gas sensor towards different concentrations of dodecane were also performed at an operating temperature of 250 ºC (Figure 6.4). Films exhibiting an inclined columnar microstructure and prepared with dc = 40 % of P (i.e. the best dodecane response from results shown in Figure 6.3) were tested. Various concentrations (from 10 to 300 ppm) were applied 3 times for 90 s followed by 360 s of air exposure time. For the lowest concentrations (i.e. 10, 50 and 100 ppm), the resistance systematically drifts. The response signal becomes more stable for dodecane concentrations higher than 150 ppm. It is also worth to remark that after 360 s of air exposure time, resistance never comes back to its original value, even for the lowest dodecane concentration. It leads to a significant drift of the baseline as clearly seen after 10 000 s of test. As a result, adsorption and desorption mechanisms of dodecane on tungsten oxide surfaces appear as complex process. Implementation of porous thin films produced by GLAD sputtering makes even more complicated understanding of sensing mechanisms and a whole area of work remains to be done in this respect. For inclined columnar architectures, we systematically studied the influence of pollutant concentration on the gas sensor electrical responses at the optimized thin film temperature (250 °C). Figure 6.5a and b depict the real-time gas sensor response curves vs. time for various dodecane and ozone concentrations.

Since WO3 is an n-type semiconductor, the electrical resistance of the sensors decreases when exposed to dodecane (reducing compound) and increases when exposed to ozone (oxidizing compound).

Figure 6.5 Normalized sensor responses (with a base line correction) at 250°C towards various gas concentrations of a) dodecane with duty cycle dc = 40 % of P and b) ozone with duty cycle dc = 50 % of P for W-O films exhibiting an inclined columnar architecture. Dodecane or ozone is injected for 90 s (injection) and stopped for 360 s (air exposure).

As expected, amplitudes of sensor electrical responses gradually decrease with decreasing dodecane or ozone concentration in synthetic air. It is also worth noticing that both sensors exhibited a poor repeatability over the time since the amplitude of the response for the same concentration is quite different. In addition, and especially for dodecane (Figure 6.5a), the response signal is still varying (back to the baseline is incomplete) when the next dodecane injection begins. At first, this phenomenon can be explained by the operating temperature. Testing at 250 °C is too low to rapidly and completely desorb pollutant from the sensing surface. On the other hand, a relative short air exposure time (pollutant off = 360 s) is implemented between each pollutant exposition, which certainly leads to a non-total regeneration of activated sites. From Rout et al. [START_REF] Rout | High-sensitivity hydrocarbon sensors based on tungsten oxide nanowires[END_REF] and Carotta et al. [START_REF] Carotta | Sensing of volatile alkanes by metal-oxide semiconductors[END_REF] investigations, one may suggest that the initial step of metal oxide sensor vs. alkane interactions consists of C-H bonds breaking and so, the formation of reactive alkyl radicals. Such radicals adsorb on the W-O surface with an oxidation of carbon (CO, CO2) and generation of cracking sub-products, leading to poisoning of active sites.

For ozone, the sensor response becomes more stabilized at the end of the air exposure time (before the next ozone injection) since lower pollutant concentrations are involved (maximum of 1000 ppb or less).

In addition, mechanisms of ozone sensor response would suggest a more efficient desorption phenomenon for ozone than dodecane for W-O active sites [START_REF] Guérin | Thermal modelling of a WO3 ozone sensor response[END_REF]. Similarly, a faster adsorption mechanism of ozone may also explain the highest response signals systematically measured when the pollutant is incrementally injected (from 3000 to 6000 s in Figure 6.5b) compared with those recorded as the ozone concentration is reduced (from 0 to 3000 s in Figure 6.5b). One may suggest the ozone adsorption mechanism on metal oxide surfaces by Ferrer and Somorjai [START_REF] Ferrer | Ultraviolet photoelectron spectroscopy study of the adsorption of oxygen on reduced SrTiO3 surfaces[END_REF] and continued by Labidi et al. [START_REF] Labidi | Impedance spectroscopy on WO3 gas sensor[END_REF]. The negatively charged ions adsorbed at the gain boundaries create a depletion layer in the n-type sample. This increases the bulk resistance and the gain boundary resistance by filling in the surface vacancies. The oxygen due to ozone adsorption also decreases the concentration of oxygen vacancies. Therefore, a decrease in the electron concentration is induced and thus an increase of the film's resistance.

Looking at the linear variation of the sensor responses during the first seconds of pollutant exposition (from Figure 6.5a and b), calculations of the slope value give information about the reactivity of each gas sensor under dodecane and ozone (as explained in Figure 3.7). The slopes of the W-O sensors were then measured and plotted as a function of the dodecane and ozone concentrations (Figure 6.6). The slope gradually increases as the pollutant concentration rises until a concentration close to 10 ppm and 300 ppb for dodecane and ozone, respectively. Above these concentrations, each gas sensor exhibits a smooth plateau, which is probably due to the rapid saturation of the W-O sensitive layer when increasing pollutant concentrations in the surrounding atmosphere of the gas sensor.

Figure 6.6 Absolute value of the slope as a function of the pollutant concentration calculated from the normalized sensor responses vs. time measurements (calculated as described in Figures 3.7 a and b) and data from Figure 6.5 when a) dodecane or b) ozone is injected. Duty cycle dc (% of pulsing period P) used for depositing W-O films is also indicated.

For dodecane, the log |slope| vs. concentration can be separated into 2 parts: below 20 ppm, and over 100 ppm. Below 20 ppm, the sensor sensitivity is the highest. A saturation state is reached for dodecane concentration higher than 100 ppm. Such a behavior has even been reported for the detection of hydrocarbon by metal oxide thin films [START_REF] Yazdi | ZnO nano-tree active layer as heavy hydrocarbon sensor: From material synthesis to electrical and gas sensing properties[END_REF][START_REF] Kanda | Development of a WO3 thick-film-based sensor for the detection of VOC[END_REF]. This abrupt increase of sensitivity followed by a more or less defined saturation strongly depends on the number of carbon atoms in the targeted molecule [START_REF] Malyshev | Gas sensitivity of SnO2 and ZnO thin-film resistive sensors to hydrocarbons, carbon monoxide and hydrogen[END_REF].

Since the gas sensing mechanism is closely linked to the complex surface reactions of adsorbed dodecane and the sensing materials (among other parameters), no clear explanations can be proposed about this strong increase of slope for the lowest dodecane concentrations. Similarly, the ozone saturation becomes significant close to 400 ppb. Interestingly, the slopes (in log scale) for concentrations from 100 to 300 ppb and from 400 to 1000 ppb could be well fitted into two linear curves with different slopes, respectively. The larger slope curve in the low concentration range (< 300 ppb) demonstrates a relatively high increase of the sensitivity (high-sensitive mode), whereas the sensitivity increases more slowly (low-sensitive mode) in the high concentration range (> 400 ppb). As the |slope| depends on resistance and thus conductivity of the film, which is the product of carrier concentration and carrier mobility, we may suggest that the switch from the high-to low-sensitive mode could be assigned to the reduced carrier mobility related to the scattering effects of adsorbed O3 molecules [START_REF] Cui | Ultrahigh sensitivity and layer-dependent sensing performance of phosphorene-based gas sensors[END_REF]. Therefore, for low ozone concentrations (< 300 ppb), the reduction of carrier mobility could be negligible due to sparse O3 molecular adsorption (high-resistive mode). When ozone concentration is high enough (low-resistive mode), carrier mobility decreases significantly due to abundant O3 adsorption. From top and cross-section views, voids can be clearly seen between the columns. When the annealing temperature increases to 300 ºC, columns become more aggregated and coalescent to form a more connected network grain growth (Figure 6.7b). Voids and spaces between columns remain but are less defined. From the cross-section view, columns begin to be destroyed while the annealing temperature is higher than 400 ºC (Figure 6.7c). It becomes even worst while the annealing temperature reaches 450 ºC (Figure 6.7d). At 500 ºC, columns are still distinguishable but are rather formed by small crystalline grains, and the column angle can be still observed. The column boundaries appear from the top view and voids are limited between the columns. It is worth noting that despite the annealing temperature higher than 450 ºC, voids still exist. Columns are sintered and connected to each other. They form nano-necks, which may enhance the electron transport properties between the column boundaries. X-ray diffraction patterns of co-sputtered W-O thin films show that the crystalline structure is also influenced by the annealing temperature (Figure 6.8). Two series of samples were co-sputter deposited. Three types of co-sputtered W-O thin films were annealed and followed by resistivity vs. measurements as reported in Figure 5.9 and Figure 5.13. These reactive sputtering conditions were performed for films exhibiting various and inclined columnar microstructures as shown in Figure 5.6 and for the two IW extrema (i.e. the densest and the more voided). For the first series, the tungsten target current intensity was IW = 40 mA and WO3 self-bias potential was UWO3 = 300 V. Using these deposition conditions, the column angle was nearly perpendicular to the substrate surface and a dense microstructure was observed (Figure 5.6). About the second series, the co-sputtering conditions were as follows: IW = 100 mA and UWO3 = 100 V (highest IW and lowest UWO3), which produced inclined and narrow columns with a porous microstructure (Figure 5.10). For the third series, IW = 100 mA and UWO3 = 300 V (highest IW and highest UWO3). The column angle was the highest (β = 35 º) and the SEM top view showed an anisotropic microstructure (i.e. large columns in the direction perpendicular to the W particle flux as shown in Figure 5.10). However, the electrical resistivity of those 3 series are not suitable to be used as gas sensor (ρ ~ 10 -5 Ωm, which is less than one order of magnitude of the required range). The RGPP was used to inject oxygen flow rate during the co-sputtering process. The duty cycle was chosen in order to reach a resistivity range suitable for the gas sensing tests (i.e. ρ ~ 10 -4 to 10 -2 Ωm). Afterward, all samples were annealed in air at 300 ºC for 12 hours and the resistivity evolution vs. time was recorded as shown in Figure 6.9 (gas sensor aging process, as similarly applied for all sensors). As expected and for the same IW and UWO3 conditions, W-O films prepared with the highest duty cycle are the most resistive (dashed line in Figure 6.9). Such films are oxygen-rich compared to those deposited with the lowest duty cycle. As a result, they are very sensitive to the annealing time and become too much resistive to be measured with our system (ρ > 10 -1 Ωm). It is also worth to note that for the films prepared with dc = 10 % of P, IW = 40 mA and UWO3 = 300 V (the most oxygen-rich sample), the resistivity is higher than 10 -1 Ωm only after 120 min, whereas those produced with dc = 30 % of P, IW = 100 mA and UWO3 = 300 V (tungsten-rich) exhibit a resistivity ρ = 2×10 -1 Ωm after more than 700 min, despite its porous microstructure.

WOx films with W and WO3 co-sputtered targets

columnar structure. As a result, crystallization, microstructure and oxygen vacancies in the films certainly change with the post-deposition thermal treatment. In addition, we focused on dodecane vapor detection for which a few investigations can be found in the literature [START_REF] Yazdi | ZnO nano-tree active layer as heavy hydrocarbon sensor: From material synthesis to electrical and gas sensing properties[END_REF][START_REF] Xu | Tungsten Oxide Thin Films Sputter Deposited by the Reactive Gas Pulsing Process for the Dodecane Detection[END_REF] and detection mechanisms certainly differ from common gases such as NO2, CO, O3, among others. When the temperature is higher than 400 º C, the relevant kinetic parameters of adsorption or desorption phenomena between gas atoms and surface reaction sites increase, leading to a better response. The resistance in air (R0) also decreases due to the high operating temperature, which consequently increases the response ΔR/R0. Dynamic responses of sensors at 500 ºC when exposed to 325 ppm dodecane vapor for 90 s followed by 360 s air exposure time were measured for the same three groups of samples at 500 ºC (Figure 6.11).

Here again, the best performances are clearly obtained for films deposited with dc = 30 % of P, tungsten target current intensity IW = 100 mA and WO3 self-bias potential UWO3 = 300 V. Response is higher than 90 % and slightly drifts after a few cycles. The response time is also the best one for these films and saturation of R/R0 clearly occurs at the end of all dodecane injections (i.e. after a few ten seconds of injection, R/R0 is constant). Despite the incomplete dodecane desorption (steep and negative slope of R/R0 vs. time before the next dodecane injection), sensor remains sensitive to dodecane vapor. For the other films, response is in-between 20 -80 %.

In addition, R/R0 still drifts and increases after each dodecane injection, particularly for films sputtered with dc = 30 % of P, IW = 100 mA and UWO3 = 100 V. It worth to notice that the influence of duty cycle on the sensors response is stronger than that of WO3 self-bias potential. Films prepared with the lower duty cycles in series II (i.e. dc = 30 % of P, IW = 100 mA and UWO3 = 300 V) exhibit the lowest sensing performances. With a slight increase of duty cycle (i.e. dc = 40 % of P for these films), the sensing performance is among the highest with a response of about 80 %. One can notice that the sensor prepared with dc = 10 % of P, IW = 40 mA and UWO3 = 300 V, is too much resistivity to be tested, as shown in Figure 6.9. The most thermally stable sample (i.e. dc = 3 % of P, IW = 40 mA and UWO3 = 300 V) does not exhibit the best performances, which can be explained by its quite dense microstructure as shown in Figure 5.6.We also have to highlight that the best dodecane sensing performances are connected to the high porous microstructure of W-O films sputter-deposited with dc = 30 % of P (series III) combined with a relatively stable electrical resistivity after the annealing treatment (Figure 6.9).

Conclusion

W-O films prepared by RGPP with one W target and without GLAD, exhibited some sensing performances to dodecane vapor at 300 ºC, mainly in terms of response and recovery time, for duty cycles in-between 60 -100 % of P. The best performances were obtained for dc = 80 % of P, which is closely connected to the transition from sub-to overstoichiometric WO3 compound.

For sensors obtained with films sputter-deposited using one W target and combining RGPP and GLAD processes, inclined columns, zigzag and spiral architectures were tested. Oxidizing and reducing chemical compounds were involved, namely dodecane and ozone. For both pollutants, the best sensing responses were obtained at 250 ºC for W-O films prepared with an optimized duty cycle and showing tilted columns. These enhanced responses were mainly assigned to the high porous architecture, which is especially produced in GLAD films exhibiting an inclined columnar structure.

For GLAD co-sputtering thin films obtained from tilted W and WO3 targets, some selected W-O films were used as sensitive layers for the detection of dodecane vapor. The best sensing responses were measured for films exhibiting the highest porous structure and quite stable electrical resistivity. Metal oxide thin films fabricated by the GLAD co-sputtering technique associated with the Reactive Gas Pulsing Process appears as a highly promising strategy and a large open field for gas sensing applications.

Conclusions and perspectives

The At first, W-O thin films were successfully deposited by conventional reactive magnetron sputtering. W target was the starting material and oxygen gas was pulsed periodically by RGPP using a constant and short rectangular signal (P = 16 s), whereas the duty cycle dc was systematically changed from 0 to 100 % of P. Beyond the alternation of the process between the metallic and oxidized sputtering modes, a gradual evolution of the film properties was clearly achieved. The smooth transition from metal to semiconductor and finally insulator was possible by adjusting the duty cycle parameter by RGPP. The most interesting range was obtained for dc in-between 60-100 % of P corresponding to the transition from sub-to overstoichiometric WO3 materials. Similarly, electrical and optical behaviors of thin films were largely tuned, and they produced the most interesting sensing performances for dc = 80 % of P when exposed to dodecane vapor as pollutant.

Combining GLAD and RGPP techniques, a large panel of compositions and architectures were produced in W-O thin films. Inclined columns with tilted angles up to 55 º, zigzags with a periods number varying from 1 to 16, and spirals with a few pitches, were successfully grown. An accurate control of some geometrical characteristics such as column angle, zigzag number, etc. with an incremental change of duty cycle allowed a progressive evolution of optical properties: from absorbent, to semi-transparent and finally interferential compounds. A wide range of electrical resistivities (more than 5 orders of magnitude) especially for films exhibiting an inclined columnar microstructure was also reached. This extended range of electrical properties was made possible mostly with the two key parameters: the incident angle α and the duty cycle dc. The first one gave rise to an enhanced thin films porosity (about 50 % of voids) and the second one led to play with the oxygen amount in the films. A suitable range of electrical resistivities was consequently defined in order to test these nanostructured W-O films as an active layer for gas sensors. Such a range strongly depends on the GLAD and RGPP key parameters, and the post-deposition annealing treatment as well.

The more complex GLAD co-sputtering process always associated with RGPP was finally developed.

WO3 target material was RF sputtered with a W target in DC mode. Both targets were inclined with a tilt angle of 80 º compared to the substrate normal: the goal being to favor a growth competition between WO3 and W fluxes in order to get tunable and original architectures. Playing with WO3 self-bias potential and W target current intensity, columnar architectures were systematically prepared. The column angle could be modified from 11 to 37º as well as the columns width and thus, the resulting voided microstructure. A few films showing different morphologies were tested as gas sensors when exposed to dodecane. Some sensing performances were obtained for active layers exhibiting the most porous microstructure. Last but not least, the most interesting sensing responses were reached for W-O films sputter-deposited with one W target and associating GLAD and RGPP. Favoring the targeted range of films resistivity (between 10 -4 to 10 -2 Ωm) by means of a precise adjustment of the deposition parameters and the required annealing procedure, these nanostructured W-O films were applied as an active layer for reducing and oxidizing pollutants, namely dodecane and ozone. For both pollutants, the best sensing performances were obtained for films prepared with an optimized duty cycle (dc = 40 % of P for dodecane and dc = 50 % of P for ozone) and showing tilted columns. These improved responses were assigned to the high porous architecture, which is particularly produced in GLAD films with an inclined columnar microstructure. A drawback in the performance of these sensors was however brought to the fore by their relatively poor repeatability.

One of the first future works would be a focus on creating even more porous microstructures taking advantage of the GLAD co-sputtering process. By co-deposition of more or less miscible metals (e.g.

W-Cu, Mo-Cu or even Sn-Cu), some original bicomponent columnar films could be achieved (e.g.

Janus-like columns). Removing one material by chemical etching to further improve the surface-tovolume ratio of the films and with a post-thermal oxidation treatment to get the sensitive metal oxide compounds (e.g. WOx, SnOy, etc.), the sensing behaviors should surely be improved with deeper investigations focused on all sensing performances (sensitivity, response and recovery time and so on).

Another attractive prospect would be the doping of binary metal oxide films (e.g. Me: WO3, Me: SnO2, etc.) or the possibility to get ternary oxides exhibiting inclined columns and thus, a porous microstructure.

The choice of metals and their thermal stability to normal environment would obviously remain a challenging task to manage and understand, in correlation with their electronic transport properties.

Finally, the open and porous microstructure associated with GLAD films make them very exciting candidates not only for sensing applications, but also for other micro and/or nano systems. This strong advantage makes them a strategic tool for integrating into a wide range of sensing and analytical devices.

  sccm: standard cubic centimeter per minute JW: target current density (A m -2 ) P: period of rectangular signal (s) TON: time of injection of the oxygen gas (s) TOFF: time without any oxygen injection (s) dc: duty cycle TON/P : incident angle of substrate (˚) W: angle between the W target normal and the substrate normal (˚) WO3: angle between the WO3 target normal and the substrate normal (˚) ϕ: computer-controlled rotating speed of the substrate holder (rev h -1 ) n: refractive index π: porosity ε: relative dielectric constant psp: total sputtering pressure (mbar) pAr: argon partial pressure (mbar) qAr: argon mass flow rate (sccm) pO2: oxygen partial pressure (mbar) qO2: oxygen mass flow rate (sccm) S: pumping speed (L s -1 ) IW: W target current (mA) UW: W target DC potential (V) UWO3: self-bias potential of the WO3 target (V)

Figure 2 . 4

 24 Figure 2.4 Comparison between experimental and simulated refractive indices n at λ = 550 nm for zigzag structures with N = 0.5, 1, 2, 4 and 8 zigzags [76].

Figure 2 . 6

 26 Figure 2.6 Response vs. NO2 concentration with dense and nanorod thin films, annealed at 400 and 500 ºC[START_REF] Horprathum | NO2-sensing properties of WO3 nanorods prepared by glancing angle DC magnetron sputtering[END_REF].

Figure 2 . 7

 27 Figure 2.7 Response of the dense-planar and nanocolumnar WO3 thin film sensors as a function of gas concentrationfor various target gases at bias voltage Vb= 5 V[START_REF] Moon | Self-activated ultrahigh chemosensitivity of oxide thin film nanostructures for transparent sensors[END_REF].

Figure 2 . 8

 28 Figure 2.8 Sensor response of Pd/WO3 dense films and Pd/WO3 nanorods vs. various H2 concentrations at 100 ºC[START_REF] Wisitsoorat | Optical H2 sensing properties of vertically aligned Pd/WO3 nanorods thin films deposited via glancing angle rf magnetron sputtering[END_REF].

Figure 3 . 1 :

 31 Figure 3.1: Illustration of the deposition system. All the following parameters can be adjusted:  = the angle of inclination of the W target compared to the normal of the substrate, ϕ = the rotating speed of the substrate holder, P = the pulsing period of oxygen gas (rectangular signal), TON = the time of injection of the oxygen gas.

Figure 3 . 2 :

 32 Figure 3.2: Illustration of the GLAD co-sputtering system. W target was DC sputtered, whereas WO3 target was RF sputtered. Oxygen gas was pulsed, and argon was constantly supplied. W = the angle of inclination of W target compared to the normal of the substrate = 80 ° and WO3 = the angle of inclination of WO3 target compared to the normal of the substrate = 80 °.

Figure 3 . 3 :

 33 Figure 3.3: Evolution of the total sputtering pressure (■) and W target potential () vs. oxygen flow rate. The pumping speed S = 10 L s -1 , argon pressure pAr = 2.8×10 -3 mbar and W target current IW = 100 mA.

Figure 3 . 4 :

 34 Figure 3.4: Variation of the total sputtering pressure (■) and W target potential () as a function of the pulsing period of the oxygen gas. Duty cycle dc = 50 % of P, qO2Max = 2.4 sccm, qO2min = 0 sccm, Pumping speed S = 10 L s -1 , argon partial pressure PAr = 2.8×10 -3 mbar and W target current IW = 100 mA.

Figure 3 . 5 :

 35 Figure 3.5: Illustration of the van der Pauw's procedure[START_REF] Van Der Pauw | A method of measuring specific resistivity and Hall effect of discs of arbitrary shape[END_REF].

W

  -O thin films were sputter deposited on commercial MSP 769 Heraeus sensor platforms(Figure 3.6). Such a device is made of interdigitated combs and allows a simultaneous heating, temperature and thin film electrical resistance measurements vs. time during the gas testing. The MSP 769 module combines a heater layer of Pt 25 (25.0 ± 2.5  at 0°C with a characteristic curve according to DIN EN 60 751), and a temperature sensor thin film of Pt 1000 (1000  at 0°C, measuring values and characteristic curve according to DIN EN 60 751) on a ceramic substrate covered with an insulating glass layer. Interdigitated gold electrodes are deposited on the insulating glass layer, onto which a sensing layer can be deposited by thin film technology. Before testing, the sensor was aged under synthetic dry air flow (200 mL min -1 , 5% RH) at 300°C for 12 hours to test the stabilization of sensors resistance. Dodecane was thermalized (vaporized) by means of a water bath at 35 °C and synthetic dry air carrying dodecane through the tubes, where they were warped by electric resistance heating tape, which kept the tubes at 40 °C, in order to avoid the dodecane condensation in the tubes. After passing through the reactor, the exhaust went through the humidity sensor.

Figure 3 . 6

 36 Figure 3.6 Schematic view of systems used to test W-O films exposed to a) dodecane vapor and b) ozone gas.

Figure 3 .Figure 3 .

 33 Figure 3.7 Typical WOx gas sensor response of a) ozone and b) dodecane, under concentration of [O3] at 500 ppb and [C12H26] at 300 ppm for 90 s (air exposure time is 360 s).

Figure 4 . 1 Figure 4 .

 414 Figure 4.1 SEM cross-sections of tungsten and W-O thin films deposited on silicon substrate with various duty cycles: (a) dc = 0 %, (b) dc = 20 %, (c) dc = 40 %, (d) dc = 100 % of P.

  Figure 4.3.

1 )Figure 4 . 3

 143 Figure 4.3 Deposition rate (■), DC electrical conductivity at room temperature  25ºC () and optical transmittance T633 at 633 nm (▲) of W-O thin films as a function of the duty cycle dc.

Figure 4 . 5 Figure 4 . 4

 4544 Figure 4.4b, curve 1-2. Afterwards, the process avalanches in the oxidized sputtering mode (Figure 4.4

Figure 4 . 5 W

 45 Figure 4.5 W target voltage vs. time for various duty cycles.

Figure 4 . 6 X

 46 Figure 4.6 X-ray diffraction patterns of tungsten and W-O thin films deposited on silicon substrate with various duty cycles: dc = 0, 20, 40 and 100 % of P. S = silicon substrate;  = W (pdf # 04-0806); ♦ = W18O49 (pdf # 05-0392); □ = WO2.83 (pdf #36-0103); ◊ = WO3 (pdf #05-0388).

  Films are completely absorbent for the lowest duty cycles and a transition zone is achieved in the range 70 < dc < 85 % of P, as previously shown in Figure 4.3. Films become transparent and typical interference fringes are produced in the visible region spectrum for duty cycles higher than 85 % of P, as shown in Figure 4.7. These results correlate with the evolution of W and O concentrations vs. duty cycle (Figure 4.2). Semi-transparent W-O films are obtained in this transition zone and correspond to the change from subto over-stoichiometric WO3 compounds. dc = 100 % of P

Figure 4 . 7

 47 Figure 4.7 Optical transmittance spectra in the visible region of W-O thin films deposited on glass substrate for dutycycles higher than 65 % of P.

Figure 4 .Figure 4 . 8

 448 Figure 4.8 shows that the electrical resistivity vs. temperature is largely affected by the duty cycle. Films

[ 97 ]

 97 . In our W-O films, this change from positive to negative values is directly connected to the growing amount of oxygen in the films. From Goldfard et al. investigations [99], conductivity and TCR of W-O films are related to the O 2p-band relative intensity in the valence band spectral region. The O 2p-band grows with oxygen concentration, at the expense of the W d-band. As a result, the decrease of conductivity and TCR vs. duty cycle is due to the reverse evolution of the O 2p and d-bands contribution to the intensity of the valence band region. A more oxygen concentration causes depletion of the d-band charge carriers in favor of the O 2p valence band (carrier concentration decreases).

Figure 4 . 9 (

 49 Figure 4.9 (a) Carrier concentration and (b) Hall mobility vs. reciprocal temperature and temperature of W-O thin films for duty cycles dc in-between 65-80 % of P. Black line is a fit obtained following a power-law temperature dependence of the mobility.

  Cross-section views of inclined thin films prepared with a duty cycle dc = 100 % of P are observed by SEM for an incident angle α = 70, 75, 80 and 85 ° (Figure4.10). An inclined columnar structure is clearly obtained, whatever the incident angle.

Figure 4 .

 4 Figure 4.10 Inclined columnar structures of W-O thin films prepared with a duty cycle dc = 100 % of P, and various incident angles a) α = 70 °, b) α = 75 °, c) α = 80 °, and d) α = 85 °. Column angle β gradually increases from 45 ° to 55 ° as the incident angle changes from α = 70 to 85 º,

Figure 4

 4

Figure 4 .

 4 12 exhibits zigzag structures prepared with an incident angle α = 80 º, a duty cycle dc = 30 % of P. Zigzag numbers are 1/2, 1, 2, 4, 8 and 16. Whatever the zigzag number, the average column angle is about 48 ° (taken from both directions). This value can be compared to those obtained in Figure4.11, and correlates with Tait et al. investigations[START_REF] Tait | Modelling and characterization of columnar growth in evaporated films[END_REF]. With a zigzag structure, one can also expect a porous thin film, as shown from the cross-section observations and compounds produced in conventional films (Figure4.1d).

Figure 4 .

 4 Figure 4.12 Zigzag structures of W-O thin films prepared with dc = 30 % of P, an incident angle α = 80 º and various zigzag numbers a) 1/2, b) 1, c) 2, d) 4, e) 8, and f) 16.

Figure 4 .

 4 Figure 4.13 Spiral microstructures of W-O thin films with different duty cycles a) dc = 50 % and b) dc = 60 % of P, an incident angle α = 80 º and a number of pitches = 4.

  spiral at T = 350 C (c) zigzag at T = 350 C (b) inclined T  350 C (a) all films T  300 C

Figure 4 .

 4 Figure 4.14 X-ray diffraction patterns of inclined, zigzag and spiral architectures of W-O thin films annealed in air for 12 h. a) An amorphous structure remains for all films annealed at temperature lower than 300 ºC. b) Diffracted signals clearly appear for films with inclined columns annealed at temperature higher than 300 ºC. c) Small peaks are recordedfor zigzag structures annealed at 350 ºC, and d) spiral structures annealed at 350 ºC. = WO3 (pdf # 20-1324); ♦ = W18O49 (pdf # 05-0392).

Figure 4 .

 4 14b. Spiral and zigzag architectures annealed at 350 ºC exhibit a nearly amorphous structure since only one weak peak is recorded at 2θ = 28 °. This peak corresponds to the (001) plans of WO3 phase (pdf # 20-1324). Inclined columns annealed at temperature ≥ 350 ºC exhibit clear diffracted signals. Peaks are mainly assigned to WO3 (pdf # 20-1324) and W18O49 (pdf # 05-0392) phase mixture. For the pattern shown in Figure 4.14b, a strong peak related to (001) planes of WO3 phase (pdf # 20-1324) is recorded at 2θ = 26.97 °. Other peaks can be noticed at 2θ = 27.53 °, 28.33 °, which correspond to (020), (200) of WO3 phase (pdf # 20-1324), and (010), (1 ̅ 11) of W18O49 phase (pdf # 05-0392), respectively. Another intense peak (021) of the WO3 phase (pdf # 20-1324) is measured at 2θ = 38.88 ° and another one (512) due to the W18O49 phase (pdf # 05-0392) appears at 2θ = 39.93 °. The rest are small peaks corresponding to these two phases.

Figure 4 .Figure 4 . 16

 4416 Figure 4.16 Average optical transmittance at 633 nm vs. incident angle α and for different duty cycles.

Figure 4 .

 4 Figure 4.18 Refractive index n at 633 nm and porosity π vs. incident angle α for different duty cycles, a) dc = 80 %, b) dc = 90 %, and c) dc = 100 % of P.

  transmittance. Plotting the average transmittance at 633 nm as a function of the incident angle α and for duty cycles dc = 70 and 80 % of P (Figure 4.20), one can compare with the value from inclined microstructure (Figure 4.16).

Figure 4 . 20

 420 Figure 4.20 Average optical transmittance at 633 nm vs. incident angle α of W-O films with 4 zigzags and prepared withduty cycles dc = 70 and 80 % of P.

Figure 4 . 21

 421 Figure 4.21 Electrical resistivity at room temperature vs. incident angle of inclined columnar W-O films for different duty cycles.

Figure 4 .

 4 Figure 4.22 supports again that the electrical resistivity strongly depends on the duty cycle and incident

Figure 4 .

 4 Figure 4.22 Electrical resistivity vs. temperature of inclined columnar W-O thin films deposited on glass substrate for duty cycles lower than 80 % of P and with different incident angles.

from 1 to 16 (Figure 4 .

 164 Figure 4.23 Electrical resistivity vs. zigzag number for different duty cycles and incident angles. Zone 1 concerns all films for incident angle lower than 75 ˚, zone 2 for duty cycle ≤ 30 % of P and with an incident angle α = 80 ˚, zone 3 for duty cycle ≥ 40% of P and with an incident angle α = 80 ˚ and all films with an incident angle α = 85 ˚.

higher than 8

 8 gives rise to columns, which are normal to the surface. The zigzag architecture still remains as shown from SEM cross-section observation in Figure4.12f. As a result, this wide range of electrical resistivities obtained for α = 85º and dc = 60 % of P, and by increasing the zigzag number, is mainly due to a reduction of the column length (a few 10 nm for 16 zigzags) and thus a small electron mean free path.

Figure 4 .

 4 Figure 4.24 Three types of electrical resistivity evolution vs. temperature cycle of W-O thin films for a) an incident angle α = 85 º with any duty cycles and an incident angle α = 80 º with duty cycles higher than 40 % of P, b) incident angle α = 80 º with duty cycles lower than 30% of P, c) incident angles α lower than 75 º with any duty cycle.

Figure 4 .

 4 24a or Figure 4.23, Zone 1). This resistivity increase is mainly due to the thermal oxidation of the zigzag structure. Zigzag films deposited with duty cycles lower than 30 % of P and high incident angles α ≥ 80 º show again a surprising resistivity vs. temperature evolution (cross-like behavior as shown in Figure 4.24b or Figure 4.23, Zone 2). For zigzag films prepared with incident angles lower than 75 º (and any duty cycle or zigzag number), resistivity reduces after a temperature cycle as shown in Figure 4.24c or Figure 4.23, Zone 3. (as previously reported for inclined columns in Figure 4.22). Again, this resistivity decreasing may be assigned to the partial formation of the W18O49 Magnéli phase exhibiting metallic-like properties[START_REF] Viswanathan | Crystal structure and charge carrier concentration of W18O49[END_REF][START_REF] Migas | Tungsten oxides. II. The metallic nature of Magnéli phases[END_REF].

Figure 4 . 25

 425 Figure 4.25 Electrical resistivity vs. temperature cycle for spiral microstructures of W-O films with different duty cycles, an incident angle α = 80 º and number of pitches = 4.

W

  -O thin films were deposited by DC reactive magnetron sputtering from a W target in argon + oxygen atmosphere. The Reactive Gas Pulsing Process (RGPP) was firstly involved to accurately change the chemical composition in the films from pure tungsten to over-stoichiometric WO3 compound. These tunable W-O films were produced using a rectangular pulsing signal with a constant period P = 16 s and by means of a systematic change of the duty cycle dc from 0 to 100 % of P. These different and adjustable compositions have a clear influence on optical and electrical behaviors of the films since a gradual transition from metallic-to-semiconducting and finally insulating-like behaviors is evidenced. Various columnar architectures were produced by means of the GLancing Angle Deposition (GLAD) technique. Such a technique allowed the growth of different architectures: normal columns, inclined columns, zigzags and spirals. A wide range of electrical conductivities, from metallic to semiconducting-like behaviors, were reached playing with the duty cycle of RGPP and the two-key parameters of the GLAD technique, which are the incident angle () and the rotating speed of the substrate holder (ϕ). As a result, a large panel of compositions and architectures can be produced in W-O thin films combining GLAD + RGPP processes. In order to test these nanostructured films as an active layer for gas sensing, a suitable range (experience range further discussed in chapter 5) of the films' resistivity is required (Figure4.26).

Figure

  Figure 4.26 Electrical resistivity measured at room temperature as a function of the duty cycle for W-O films exhibiting inclined, zigzag, spiral and normal columnar architectures, = resistivity range for gas sensing.

Figure 5 . 2 Figure 5 .

 525 Figure 5.2 Column angle vs. current intensity of the second W target for 3 different duty cycles.

Figure 5 . 4 W

 54 Figure 5.4 W targets potential of the first (UW1) and second target (Uw2) vs. time when oxygen is supplied with and without a ring close to the target 2. Duty cycle dc = 60 % of P, IW1 = 100 mA, and IW2 = 60 mA.

Figure 5 . 5

 55 Figure 5.5 Voltage difference ΔUW for the first W target and second W target vs. second W target current IW2 and fordifferent duty cycles with or without ring for the oxygen supply, IW1 = 100 mA.

  .7). For IW = 40 mA (minimum IW required to ignite the plasma on W target) and UWO3 = 300 V, the column angle β = 11 º, which is nearly perpendicular to the substrate surface. From SEM top views, columns become bigger and more inclined as IW increases. The column angle increases from 11 to 34º as IW rises from 40 to 100 mA. Column angle β also becomes higher while the W target current intensity increases, because the deposition rate (due to the second W target) increases linearly vs. IW. The resulting total deposition rate due to both targets increases as well, and the column angle is mainly influenced by the W target (β increases from 11 to 34 º). A dense film with a low column angle (β = 11 º) is clearly produced for the lowest IW = 40 mA, whereas larger columns are observed from IW = 60 mA with an increasing column angle. This microstructural evolution supports again that W target flux prevails over that of WO3.

Figure 5 . 7 5 . 8 .

 5758 Figure 5.7 Deposition rate and column angle vs. tungsten target current.

Figure 5 .

 5 Figure 5.9 Electrical resistivity vs. time of W-O films prepared on glass substrates and annealed in air at 300 ºC for 12 hours with various tungsten target current intensities and a constant WO3 self-bias potential UWO3= 300 V.

a)Figure 5 . 10 .

 510 Figure 5.10. An inclined columnar microstructure is clearly observed for any WO3 self-bias potential.

Figure 5 . 11

 511 Figure 5.11 Deposition rate and column angle vs. WO3 target self-bias potential. W target was DC sputtered with IW = 100 mA. Deposition rate and column angle axes have the same scale as in Figure 5.7.

  ) preferential orientation. Increasing UWO3 gives rise to intense diffracted signals at 2θ = 41.58° and 47.20° corresponding to the (200) and (211) planes of the β-W phase (pdf # 65-6453), respectively. The (200)peak becomes more and more intense as UWO3 increases, which correlates with an oxygen-enrichment of the films (β-phase formation is favored). Taking in to account the diffracted intensity of (200) phase of the β-W phase (pdf # 65-6453), the full width at half maximum (FHWM) does not show a clear evolution as a function of the WO3 self-bias potential (inset in Figure5.12). As a result, the crystal size is not significantly influenced by the WO3 flux.

Figure 5 .

 5 Figure 5.13 Electrical resistivity vs. time of W-O films prepared on glass substrates with various WO3 target self-bias potentials and a constant W target current intensity IW = 100 mA. Annealing was performed in air at 300 ºC for 12 hours.

Figure 4 . 8 .

 48 Figure 4.8. Oxygen content in the films is too low and duty cycles higher than 60 % of P are required to

Figure 6 . 1

 61 Figure 6.1 Resistance vs. time of WOx sensors prepared with 3 different duty cycles (dc = 60, 80 and 100 % of P). A growing dodecane concentration (10, 50 and up to 300 ppm, with a 50 ppm increment) was systematically used. Dodecane is injected for 90 s (injection time) and stopped for 360 s (air exposure time) with an operating temperature of 300 ºC.

. 1 )

 1 O δ-(a) is a charged oxygen species adsorbed on the oxide surface. Hydrocarbons (HC) is first adsorbed on the oxide surface to give the HC(a) species, which react with O δ-(a) following: 𝐻𝐶(𝑎) + 𝑂 𝛿-(𝑎) → 𝐻𝐶: 𝑂(𝑎) + 𝛿𝑒 - (6.2) 𝐻𝐶: 𝑂(𝑎) + 𝑂 𝛿-(𝑎) → 𝐶𝑂 2 + 𝐻 2 𝑂 + 𝛿𝑒 -

Figure 6 .

 6 Figure 6.3 depicts the relationship established for each sensor prepared at their optimized duty cycle (dc

Figure 6 . 3

 63 Figure 6.3 Dodecane and ozone responses vs. electrical resistance of W-O coated platform in air at the optimized temperature for films exhibiting normal, inclined and zigzag architectures. For each type of architecture, the duty cycle dc is indicated in % of the oxygen pulsing period P as well as the optimized temperature (in ºC) in parentheses. The best response is systematically obtained for inclined columns as indicated by the dashed line.

Figure 6 . 4

 64 Figure 6.4 Resistance vs. time with different dodecane concentrations at 250°C of W-O thin films with an inclined columnar microstructure (α = 80 º) prepared with duty cycle dc = 40 % of P. Dodecane is injected for 90 s and stoppedfor 360 s. Each concentration is repeated 3 times.

6. 2 . 1

 21 Annealing treatment before gas sensing An annealing treatment in air was applied to W-O thin films prepared by GLAD co-sputtering. They were all heated for 12 hours in air at 300, 400, 450 and 500 ºC. The sample were prepared with duty cycle dc = 40 % of P, tungsten target current intensity IW = 100 mA and WO3 self-bias potential UWO3 = 100 V. SEM observations of the cross-section and top view of the films show various morphologies as a function of the annealing temperature (Figure 6.7). Without annealing treatment, a typical columnar microstructure is clearly viewed (Figure 6.7a).

Figure 6 . 7

 67 Figure 6.7 SEM views (cross-section and top) of W-O as-deposited films prepared with IW = 100 mA, UWO3 = 100 V, dc = 40 % of P and after annealing in air for 12 hours at: a) room temperature, b) 300 ºC, c) 400 ºC, d) 450 ºC and e) 500 ºC.

  The first one corresponds to films prepared with duty cycle dc = 40 % of P, tungsten target current intensity IW = 100 mA and WO3 self-bias potential UWO3 = 100 V (Figure6.8a). Such conditions produced inclined and narrow columns (Figure5.10). The second one is related to films exhibiting a porous and anisotropic microstructure (i.e. large columns in the direction perpendicular to the W particle flux as shown in Figure5.10). They were deposited with duty cycle dc = 30 % of P, tungsten target current intensity IW = 100 mA and WO3 self-bias potential UWO3 = 300 V. All films were annealed in the temperature range changing from 300 to 500 ºC. Up to 300 ºC, all samples exhibit a nearly amorphous structure. As the annealing temperature reaches 400 ºC, clear diffracted signals are recorded for both series. Peaks are mainly assigned to WO3 (pdf # 20-1324) and W3O8 (pdf # 65-1175) phase mixture.With a higher annealing temperature, a (001) strong preferential orientation of the WO3 phase (pdf # 20-1324) is clearly recorded at 2 = 26.87 °, as well as another peak at 2 = 27.37 ° corresponding to the (001) planes of the W3O8 phase (pdf # 65-1175). In addition, weak but significant diffracted signals can also be noticed at 2 = 33.51 and 37.80 °, which are assigned to (320) and (330) planes of the substoichiometric W3O8 phase (pdf # 65-1175), respectively. The peak is related to the SiO2 phase coming from the glass substrate at high temperature annealing treatment.

Figure 6 . 8 X

 68 Figure 6.8 X-ray diffraction patterns of W-O co-sputtered thin films with different annealing temperatures. a) IW = 100 mA, UWO3 = 100 V and dc = 40 % of P, b) IW = 100 mA, UWO3 = 300 V and dc = 30 % of P;  = WO3 (pdf # 20-1324); ♦ = W3O8 (pdf # 65-1175); □ = SiO2 (pdf # 43-0745).

Figure 6 . 9

 69 Figure 6.9 Electrical resistivity vs. time of some selected W-O films prepared on glass substrates and annealed in air at300 ºC for 12 hours.

Figure 6 .

 6 Figure 6.10 Sensor response vs. temperature for three series of W-O thin films GLAD co-sputtered, annealed 12 h in airat 300 ºC and exposed to 325 ppm of dodecane for 90 s.

Figure 6 .

 6 Figure 6.11 Sensor response vs. time of the 3 series of W-O thin films exposed to 325 ppm of dodecane for 90 s followed by 360 s of air exposure time at their own optimized temperature determined from Figure 6.10.

  present work was based on the reactive sputter deposition of W-O thin films. Although W-O compounds have been abundantly investigated for these last decades, clear relationships between deposition conditions and fundamental material properties are often rather straight forward and partly well established in the literature. To tune the nature and the microstructure of WOx films by means of new reactive sputtering processes still remains an exciting scientific motivation and the development of original strategies is always a pending task. To this aim, some attempts to modify W-O thin films properties have been performed following two approaches: the modification of the composition implementing the Reactive Gas Pulsing Process (RGPP) and the sculpturing of the films architecture at the micro-and/or nanoscale by means of the GLancing Angle Deposition (GLAD) technique.

Table 4 .1 Oxygen-to-tungsten atomic concentrations ratio [O]/[W], electrical conductivity at room temperature σ25 ˚C, temperature coefficient of resistance at room temperature TCRRT and activation energy Ea of W-O thin films prepared with various duty cycles.

 4

	Duty cycle dc (% of P)	0	10	20	30	40	50	60	65	70	75	80
	[O]/[W] ratio	0.06	0.38	0.84	1.24	1.63	2.00	2.53	-	3.08	-	3.40
	σ25˚C (S•m -1 )	4.9×10 6	4.5×10 5	2.9×10 5	2.2×10 5	1.6×10 5 9.3×10 4 5.3×10 4	2.4×10 4 7.0×10 3 1.1×10 3 5.5

Acknowledgements

This work of thesis was fulfilled in team of MIcro NAno MAterials & Surfaces (MINAMAS), This work was performed in cooperation with the LABEX ACTION program (contract ANR-11-LABX-01-01) and the RENATECH network. I got financial support from the China Scholarship Council (CSC China).

homogeneous as those measured for films with dc = 90 and 100 % of P. Films are semi-transparent and an absorption band close to 600-700 nm becomes significant. This range of duty cycles correspond to WO3-x compounds, with a large amount of oxygen deficiencies. Transmittance is strongly affected by the incident angle due to the voids in the microstructure, which scatter the light. For higher incident angles, films become more and more porous and the average transmittance is enhanced.

Chapter 5: Gas sensing performances

In the previous chapter, we presented how to develop an original way to get adjustable chemical compositions of WOx thin films by means of the Reactive Gas Pulsing Process, namely RGPP. It was shown that we can gradually and precisely produce an accurate and progressive transition of WOx film properties -from metal to semiconductor and finally to insulatorby using suitable deposition parameters of the RGPP technique. Such tunable WOx films are now applied for the detection of dodecane vapor and the sensing performances are optimized in terms of response and recovery times, by simply taking into account the duty cycle parameter.

In order to produce nanostructured films, two combined strategies were implemented to play with microstructure and composition of sputter-deposited W-Os. The GLAD technique was developed so as to produce various architectures (inclined columns, zigzags, spirals) and similarly the Reactive Gas Pulsing Process, namely RGPP, was used for adjusting the oxygen concentration in the films and so, their electrical properties. The gas sensing performances of these W-O nanostructured films are now tested when separately exposed to dodecane vapor and ozone gas. We try to establish and discuss relationships between deposition parameters, architecture of W-O thin films and gas sensing performances.

We finally study GLAD co-sputtered W-O thin films as dodecane sensors. Inclined columnar structures are produced by tuning the co-sputtering conditions by means of two opposite and inclined W and WO3 targets. Structure, morphology and electrical resistivity of the films are investigated, and an optimized series is prepared pulsing the oxygen gas during the growth to adjust the films composition and to tune the electrical properties. These W-O films are tested as dodecane sensors at operating temperatures ranging from 100 to 500 ºC.

For a given series, decreasing the duty cycle reduces the resistivity drift vs. annealing time. It is particularly significant for films prepared with dc = 20 % of P, IW = 100 mA and UWO3 = 300 V and even more marked when dc = 3 % of P, IW = 40 mA and UWO3 = 300 V. For this latter, resistivity is nearly temperature invariant (slight reduction of resistivity after 720 min of annealing treatment). This quite stable resistivity evolution of the films co-sputtered with the highest UWO3 and lowest IW is directly linked to the densest microstructure. As previously discussed in § 4.2.3, this reduction of resistivity may be connected to the formation of metallic Magnéli phases in WOx compounds. Last but not least, all these films are certainly incompletely oxidized after 12 hours in air at 300 ºC, which probably contributes to the resistivity drift. However, and after this thermal treatment, the films resistivity remains in the suitable range for testing as gas sensors.

Sensing performances

After the process annealing in air for 12 hours at 300 ºC, all WOx thin films sputter-deposited according to the 3 series (i.e. different dc, IW and UWO3) were tested as active layers for the dodecane detection. All the sensors were tested following the same procedure: a constant dodecane concentration (325 ppm) and measurements of the response vs. temperature every 50 ºC from 100 to 500 ºC (Figure 6.10). For temperatures lower than 350 ºC, all sensors exhibit no response. The dodecane detection becomes significant for temperatures higher than 350 ºC, except for films prepared with IW = 100 mA, UWO3 = 300

V and dc = 30 % of P (biggest and widest columns from Figure 5.6 ) for which a response is clearly recorded from 200 ºC.

For the other films, response becomes relevant when temperature reaches 450 ºC and the best one is obtained for films prepared with dc = 30 % of P. This operating temperature is higher than those recently reported by others [START_REF] Ahmad | Investigation of RF sputtered tungsten trioxide nanorod thin film gas sensors prepared with a glancing angle deposition method toward reductive and oxidative analytes[END_REF][START_REF] Wongchoosuk | Carbon doped tungsten oxide nanorods NO2 sensor prepared by glancing angle RF sputtering[END_REF][START_REF] Horprathum | Ultrasensitive hydrogen sensor based on Pt-decorated WO3 nanorods prepared by glancingangle dc magnetron sputtering[END_REF] on the sensing properties of WO3 nanorods produced by the GLAD method. However, it is important to remark that before testing our sensors, our annealing treatment is different, especially the involved temperature. Films were annealed in air for 12 hours at 300 ºC, which is lower than the 400 to 500 ºC typically reported [START_REF] Horprathum | NO2-sensing properties of WO3 nanorods prepared by glancing angle DC magnetron sputtering[END_REF] but this annealing treatment allows preserving the

Summary

This thesis is focused on the reactive sputter deposition of W-O thin films. In order to play with their composition, the Reactive Gas Pulsing Process (RGPP) is implemented and allows tunable oxygen and tungsten concentrations. Similarly, the GLancing Angle Deposition (GLAD) technique is developed to produce various architectures, namely inclined columns, zigzags and spirals, and increases the surfaceto-volume ratio of the films. The GLAD co-sputtering approach is also investigated by means of two inclined and separated W and WO3 targets. Relationships between microstructure, composition, electronic and optical properties of W-O films are systematically studied. They are finally applied as active layers for resistive sensors in order to improve detection of dodecane vapor and ozone gas. The high porous microstructure of inclined columns produced by GLAD combined to the suitable composition adjusted by RGPP leads to define a range of W-O films attractive for sensing performances.

Keywords

W-Os, thin films, RGPP, GLAD, co-sputtering, gas sensor, dodecane, ozone.

Résumé

Mots-clés

Oxydes de tungstène, films minces, RGPP, GLAD, co-pulvérisation, capteurs de gaz, dodécane, ozone.