2017 -09/2021) was to assess the potentialities of the ZnSnN2 material line as a potential candidate for an inorganic thin film

First of all, I would like to thank my supervisor Dr. Jean-Pierre VILCOT who has accepted me into his project, he also made my European dream come true. I not only learned from him knowledge about research and science but also many other things, such as optimism, communication, future plan and how to relieve stress. He gave me encouragement, enthusiastic help, the effective orientation to perform my thesis. At times, he looks busy and tired but the door of his office is always open and greets everyone with a welcoming

State of the art

Along with the rapid development of science and technology, the basic human needs for live also enhance day by day. More and more energy is consumed to make life better and more comfortable. Besides the earth is being cooked and fossil energy resources are gradually running out. Today, renewable energy sources such as hydropower, bio-fuels, solar and wind are researched and developed. They play an important role as sustainable solutions for electricity supply as well as reducing greenhouse gas emissions that are a reason of climate change. In the Global Energy Review 2020 of the International Energy Agency (Figure I. 1), solar energy is represented as furnishing around 3% of global electricity generation. Depending on own feeling, this can be judged as a quite big leap compares to data in 2013 (around 1%) or a very modest step but, in any case, it is still a small part of overall electricity supply.

Figure I. 1: Worldwide electricity generation shares between fossil (coal), nuclear and lowcarbon sources [1]

The solar resource is enormous and electricity issued from solar energy exhibits low greenhouse gas emissions (no emission during use, only during the fabrication of solar panels). There are two technologies to convert directly solar energy to electricity: photovoltaic (PV) and concentrated solar power (CSP). PV can be used either for individual or large-scale (solar farms) photovoltaic generation units and CPV is mainly used for largescale units.

Focusing on cell research, it is incessantly targeting cells with higher power conversion efficiencies, lower material usage and reduced manufacturing complexity and cost. [START_REF]Best Research-Cell Efficiency Chart | Photovoltaic Research[END_REF] Highest power conversion efficiencies are obtained by the use of III-V material compounds. Especially, 29.1% for single-junction gallium arsenide (GaAs) device announced by Alta Devices manufacturer [START_REF]Alta Devices Sets 29.1% Solar Efficiency Record; NASA Selects Alta Devices for International Space Station Test[END_REF] and using six-junction III-V solar cell, the efficiency grows up to 39.2% under 1 sun (PV) and 47.1% under 143 suns concentration (CPV) [START_REF] Geisz | Six-junction III-V solar cells with 47.1% conversion efficiency under 143 Suns concentration[END_REF]. Nevertheless, the increase of efficiency by a factor, let us say, around 1.5, does not bear comparison with the increase in fabrication costs that are at least a decade higher making such a technology unrealistic for large scale terrestrial application.

The crystalline silicon solar cell, named as first-generation cell, has been studied and put into practical use long time ago: first genuine photovoltaic cell exhibiting roughly 6% efficiency was demonstrated in 1954 by researchers of Bell Laboratories. Since that time, its technology has continuously evolved, more specially this last decade, so that current industrial panels can achieve around 25% efficiency together with a large decrease in fabrication cost (roughly a decade). Moreover, its base raw material is the second more abundant element on Earth. Then, it is not surprising that it dominates the current market share. According to the International Technology Roadmap for PhotoVoltaic (ITRPV) published in April 2020 [START_REF]Photovoltaic Roadmap (ITRPV): Eleventh Edition Online[END_REF], the market share of mono-crystalline silicon (c-Si) cells is close to 75 percent and multi-crystalline silicon (mc-Si) cells is around 20 percent. Those two last years, great efforts have been made in silicon purification processes that lead, today, to almost the vanishing of cell production using mc-Si. So, silicon based solar cells are decreasing in production costs and their conversion efficiency asymptotically tends towards the highest achievable value that has been theoretically evaluated to 29.43%: current [START_REF] Richter | Reassessment of the limiting efficiency for crystalline silicon solar cells[END_REF] record values are 27.6% [START_REF] Slade | 27.6% Efficient Silicon Concentrator Solar Cells for Mass Production[END_REF] and 26.7% [START_REF] Yamamoto | High-efficiency heterojunction crystalline Si solar cell and optical splitting structure fabricated by applying thin-film Si technology[END_REF] for homojunction cells (CPV configuration) and heterojunction cells (PV configuration), respectively.

Owing to the light absorption properties of thin -ranging from hundreds of nanometers to few microns-PV materials, thin-film PV cells, known as second generation cells, were seen as a solution decreasing raw material consumption and so the weight-per-unit-power figure of merit and ultimately the overall cost of solar cells. Amorphous Si was firstly used; it effectively led to a low-cost technology but also a low efficiency one, the highest efficiency value is 14.04% for triple-junction solar cell [START_REF] Sai | Stabilized 14.0%-efficient triple-junction thinfilm silicon solar cell[END_REF]. CdTe, and more recently copper indium gallium di-selenide (CIGS), materials were then used. The efficiency of cells based on such materials is now comparable to the one of crystalline Si cells. More specifically, Solar Frontier published their results on CIS with an efficiency of 23.35% [START_REF]Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 23.35%[END_REF] and First Solar reported 22.1% (cell efficiency) and 18.2% (module efficiency) on thin-film CdTe [START_REF]Leading the World's Sustainable Energy Future[END_REF]. Nevertheless, due to the sharp concurrence of silicon technology as well as some process or material toxicities concerns (e.g. Cd for CdTe and Se for CIGS cells) and raw material possible shortage (i.e. In and Ga for CIGS), those technologies do not succeed to affect first generation market share supremacy. Their current fabrication cost is still slightly higher than that of Si cells and they really get inevitable in flexible cells where firstgeneration cells are definitely not usable.

The huge challenge for PV is how to maximize the conversion efficiency while minimizing the fabrication costs. The efficiency of single junction cells are limited by the Shockley-Queisser limit that was initially (in 1961) assessed to 33%. Since that time the model has been revised and the limit is now evaluated around 29.5% as mentioned earlier. Even if silicon has not the optimum bandgap maximizing the efficiency -1.4 eV-, it allows approaching this limit very close. The way that is now unanimously recognized to outperform the Shockley-Queisser limit is to use tandem solar cells, i.e. two-junction cells The studies that have been made on multi-junction cells have shown the limits of the approach, particularly in the materials and the fabrication cost issues. Although other materials could have been used, best way to achieve a sustainable tandem cell is to use a crystalline Si (c-Si) bottom cell since Si cell technology is already well handled, leads to high efficiency cells and Si is a sustainable material (as mentioned before, it is the second earth more abundant material after oxygen). According to Thomas P. White [START_REF] White | Tandem solar cells based on highefficiency c-Si bottom cells: Top cell requirements for >30% efficiency[END_REF], tandem solar cells based on c-Si bottom cell and a thin-film top cell present a practical route toward low-cost cells with efficiencies above 30%. From his tandem cell model, the characteristics of the top cell can be derived. Following his results (Figure I.4), the higher the material bandgap is, the lower the requirement in efficiency is needed achieving a tandem cell with a given efficiency (η = 25%, 27.5% or 30%). The choice of top cell materials that have suitable bandgap is then more flexible. There is currently no technology achieving a sustainable and cost-efficient top cell.

Therefore, the first step before designing such a tandem cell is to get a top cell material with adequate bandgap and low environmental footprint which technology can match the requirements of its integration, as a thin film, onto a c-Si cell. By experiment, tandem solar cells based on III-V semiconductors reach high efficiency; specially achieve 33.3% efficiency for triple-junction GaInP (Eg = 1.90 eV)/ GaAs (Eg = 1.43 eV)/ Si ((Eg = 1.12 eV) cell [START_REF] Cariou | III-V-on-silicon solar cells reaching 33% photoconversion efficiency in two-terminal configuration[END_REF]. Besides, the experimental results of GaAs (Eg = 1.42 eV) single-junction solar under one-sun illumination record the conversion efficiency of 27.6% [START_REF] Kayes | 27.6% Conversion efficiency, a new record for single-junction solar cells under 1 sun illumination[END_REF] and 29.3% [START_REF] Green | Solar cell efficiency tables (version 54)[END_REF]. In the case of perovskite, the conversion efficiency reaches 13.37% when the bandgap is 1.38 eV [START_REF] Hu | Sub-1.4eV bandgap inorganic perovskite solar cells with long-term stability[END_REF]. According B. Ehrler et al. [START_REF] Ehrler | Photovoltaics reaching for the shockley-queisser limit[END_REF], dual-junction tandem cells were made from combinations of III-V semiconductors, combinations of perovskite, perovskite/Si or perovskite/CIGS have the efficiency higher than 23.4%, the record efficiency and bandgap of the sub-cells are summarized in Table I. 1. We can conclude that semiconductors with bandgap between 1.4 and 2.0 eV can be used as top-cell of tandem cell. Today, the energy conversion potentialities of a material are not the only focus, its sustainability, fabrication environmental footprint and lifetime properties added: that was not always the case in previous approaches. We will focus in this work on new inorganic materials using earth-abundant and non-toxic elements. Among them ternary zinc nitride semiconductors have been pointed out as potential candidates. [START_REF] Hinuma | Discovery of earth-abundant nitride semiconductors by computational screening and high-pressure synthesis[END_REF]. According to their research, the combination of zinc and IA (Li, Na, K), IIA (Be, Mg, Ca, Sr, Ba) elements and nitrogen produces materials with bandgap in the range from 1.5 to 2.0 eV approximately. They are so promising as to be used as top cell absorber material of tandem cell as an alternative to III-V and perovskite. However, in ambient condition, the IA and IIA react quickly with water and oxygen to create hydroxide and oxide layers, this leads to difficult ternary material fabrication, as well as durability and stability. We focus hereby on the ZnSnN2 material line, the material bandgap ranges from 1.0 to 2.0 eV based on reported computational and experimental results. It is so a potential candidate satisfying the criteria of the availability (earth-abundant) and low cost of used raw materials. Nevertheless, even if computational results are promising, few experimental data are available, it is then a fully innovative research way.

Zn-IV-N2 semiconductors

The first-principle study of ternary Zn-IV-N2 semiconductors, where IV = Si, Ge or Sn, was done in 2008 by Tula R. Paudel and Walter R. L. Lambrecht to show the ground-state and phonon related properties [START_REF] Paudel | First-principles study of phonons and related ground-state properties and spectra in Zn-IV-N2 compounds[END_REF]. It indicates that the crystal structure of these compounds is wurtzite-derived-type orthorhombic as well as their ground-state structural properties and phonon related quantities look close to those of III-N compounds. Other properties have continued to be calculated, such as the elastic constants and compliances, the piezoelectric tensor and spontaneous polarization, the non-linear optical and linear electro-optic coefficients that are determined in the local density approximation (LDA) [START_REF] Paudel | First-principles calculations of elasticity, polarization-related properties, and nonlinear optical coefficients in Zn-IV-N2 compounds[END_REF], [START_REF] Punya | Electronic and lattice dynamical properties of II-IV-N2 semiconductors[END_REF].

Figure I. 6:

The band gaps versus "a" lattice constant for III-nitrides and Zn-IV-nitrides [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF] Furthermore, the crystal structure parameters of Zn-IV-N2 semiconductors were calculated by using the full-potential linearized muffin-tin orbital (FP-LMTO) using both the local density approximation (LDA) and the generalized gradient approximation (GGA) [START_REF] Punya | Electronic and lattice dynamical properties of II-IV-N2 semiconductors[END_REF], [START_REF] Punya | Quasiparticle band structure of Zn-IV-N2 compounds[END_REF]. Figure I. [START_REF] Richter | Reassessment of the limiting efficiency for crystalline silicon solar cells[END_REF] shows the relationship between bandgap and "a" lattice constants of Zn-IV-Nitrides compared to III-Nitrides; the same behaviour [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF] can be observed.

The results of band structure elucidate ZnSiN2 as an indirect band gap material, whereas ZnSnN2 and ZnGeN2 are determined as direct bandgap materials; the band gap energies are calculated by A.Punya et al. [START_REF] Paudel | First-principles calculations of elasticity, polarization-related properties, and nonlinear optical coefficients in Zn-IV-N2 compounds[END_REF] by using LDA, GGA and quasiparticle self-consistent GW methods (QSGW), summarized in Table I. 2. ZnSnN2: From theory to experiment

I.3.1 Theory review

ZnSnN2 is a member of ternary Zn-IV-N2 semiconductors, the most stable structure of bulk ZnSnN2 was given to be the orthorhombic Pna21 phase by calculating the total energy per unit cell, band structure and electronic density of states [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF] (Figure I. 7). Paul C. Quayle et al [START_REF] Quayle | Charge-neutral disorder and polytypes in heterovalent wurtzitebased ternary semiconductors: The importance of the octet rule[END_REF] illuminated a wurtzite-like diffraction spectrum by described octet-rule-preserving arrangements of the atoms in hetero-valent ternary nitride structures.

In addition, they used a pseudopotential plane wave method, LDA, GGA and FP-LMTO to calculate the lattice parameters, total energy, formation energies; and QSGW to calculate band structure of ZnSnN2 and ZnGeN2. Their results reveal the similarities between Pna21 and Pmc21 in band gap and lattice parameters of ZnSnN2; and the difference in the formation energies between Pna21 and Pmc21 is only 16 meV/formula unit in LDA calculation. The formation energy of ZnSnN2, which is defined by the enthalpy change of the following reaction, Zn (hcp) + Sn (diamond) + N2 (gas) → ZnSnN2

where zinc is in hexagonal close-packed structure, tin is in diamond structure and nitrogen is at gas state. Based on FP-LMTO method [START_REF] Punya | Electronic and lattice dynamical properties of II-IV-N2 semiconductors[END_REF], the calculated formation energies of ZnSnN2 are -1.67 eV/formula unit (in LDA) and -2.32 eV/formula unit (in GGA). On the other hand, the results obtained by S. Chen et al [START_REF] Chen | Phase stability and defect physics of a ternary ZnSnN2 semiconductor: First principles insights[END_REF] gave very small values, -0.17 eV/formula unit (α = 0.31), -0.13 eV/formula unit (α = 0.25) by using the hybrid functional calculation. Besides that, when they used the developed scheme proposed by V. Stevanovic et al [START_REF] Stevanovic | Correcting density functional theory for accurate predictions of compound enthalpies of formation : Fitted elemental-phase reference energies Fitted elemental-phase reference energies[END_REF], the energy of formation is equal to -0.14 eV/formula unit. In addition, six possible defects of ZnSnN2 are given and explained. The challenge is achieving single-phase synthesis due to the very narrow window of equilibrium phase stability shown in Figure I.

8 [START_REF] Chen | Phase stability and defect physics of a ternary ZnSnN2 semiconductor: First principles insights[END_REF]. obtained by calculated chemical potentials of the component elements (µZn, µSn, µGe, µN) [START_REF] Chen | Phase stability and defect physics of a ternary ZnSnN2 semiconductor: First principles insights[END_REF].

The cation disorder degree plays an important role in crystallography, the predicted structures of ZnSnN2 are ordered orthorhombic Pna21 and disordered wurtzite-like lattice structure [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF], [START_REF] Quayle | Charge-neutral disorder and polytypes in heterovalent wurtzitebased ternary semiconductors: The importance of the octet rule[END_REF], [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF]- [START_REF] Arca | Band edge positions and their impact on the simulated device performance of ZnSnN2-based solar cells[END_REF]. The X-ray powder diffraction spectra calculation of L. Lahourcade et al. [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF] are described in Figure I. 9(a). Observably, the characteristic peak positions of Pmc21, Pna21 and average wurtzite structures are relatively similar, except Pna21 has 2 characteristic peaks at position 2θ = 20 -21°. Furthermore, the fundamental band gap of ZnSnN2 in the range 1 -2 eV depend on degree of cation disorder and Burstein -Moss effect [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF], [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF], shown in electron density of ZnSnN2. [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF] Effective mass of ZnSnN2 (𝑚 𝑒 * ) is also a well-researched parameter, most of results give small 𝑚 𝑒 * values, approx. 0.1m0 [START_REF] Punya | Quasiparticle band structure of Zn-IV-N2 compounds[END_REF], [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF], [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF], [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF] (where m0 denotes the free electron mass). The relationship between effective mass and electron concentration is represented by the following equations (1), [START_REF]Best Research-Cell Efficiency Chart | Photovoltaic Research[END_REF].

𝑛 𝑒 = 𝑁 𝐶 𝑒𝑥𝑝 (-𝐸 𝐶 -𝐸 𝐹 𝑘𝑇 )

(1)

𝑁 𝐶 = 2 ( 2𝜋𝑚 𝑒 * 𝑘𝑇 ℎ 2 ) 3 2 (2)
where 𝐸 𝐹 is the Fermi level, 𝐸 𝐶 is the minimum energy of the conduction band, and 𝑁 𝐶 is a concentration coefficient that depends on temperature, 𝑘 is Boltzmann's constant and ℎ is Planck's constant.

The electron mobility is defined by the equation ( 3), [START_REF] Geisz | Six-junction III-V solar cells with 47.1% conversion efficiency under 143 Suns concentration[END_REF]:

𝜐 ⃗ = 𝜇Ε ⃗⃗ (3) 𝜇 = 𝑒𝜏 ̅ 𝑚 𝑒 * ( 4
)
where 𝑒 is the electron charge, 𝜏̅ the average scattering time, and Ε ⃗⃗ the electric field.

According to these equals, small 𝑚 𝑒 * values lead to the high electron mobility (μ) and the low carrier density (n).

The work function and valence-band onset were measured by using ultraviolet photoelectron spectroscopy (UPS) or X-ray photoelectron spectroscopy (XPS), approximately 4.0 eV and 1.2 eV below the vacuum level, respectively [START_REF] Qin | Effects of surface condition on the work function and valence-band position of ZnSnN2[END_REF]. Additionally, the work function is described as proportional to the amount of oxygen contamination; the more oxygen content, the higher the work function is.

Up to now, ZnSnN2 is still a new material and was only introduced in research in recent years, its first synthesis was published by Coronel et al [START_REF] Coronel | Earth-abundant ZnSnxGe1-xN2 alloys as potential photovoltaic absorber materials[END_REF] and Feldberg et al [START_REF] Feldberg | ZnSnN2: A new earth-abundant element semiconductor for solar cells[END_REF] in 2012.

Currently, this material is being studied more extensively, the calculation results of the crystal structure, the electronic structure will help to evaluate and interpret the experimental results.

I.3.2 Preparation of ZnSnN2

I.3.2.1 Plasma-assisted vapor-liquid-solid method Quayle et al. synthesized polycrystalline ZnSnN2 by using a plasma-assisted vapor-liquidsolid method [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF], this experiment was carried out at high temperatures, 390°C and 525°C, and the Zn composition in the layers was 8 and 45%, respectively. Their X-ray diffraction (XRD) results show the diffraction peaks that are characteristic of Sn and Zn phases beside the ZnSnN2 phase. In addition, the lattice parameters of the orthorhombic structure were a=6.75 Å, b=5.84 Å and c=5.42 Å. The optical bandgap was obtained by photoluminescence excitation (PLE) spectroscopy and was around 1.7eV.

I.3.2.2 Plasma-assisted molecular beam epitaxy (PAMBE)

ZnSnN2 thin films were fabricated on yttrium-doped zirconia substrates by using PAMBE system [START_REF] Feldberg | Growth, disorder, and physical properties of ZnSnN2[END_REF], [START_REF] Feldberg | Growth of ZnSnN2 by molecular beam epitaxy[END_REF] in active nitrogen (10 -5 torr) and 150 W of RF power. The effects of substrate temperature (300°C to 700°C) and Zn:Sn flux ratios (1:1 to 20:1) were analysed.

The ZnSnN2 thin films obtained by this method are high-purity and monocrystalline as well as highly textured and polycrystalline, all the films show a monoclinic structure of ZnSnN2.

However, the subsequent publications by T. Veal et al. [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF] and R. Makin et al. [START_REF] Makin | Growth of ordered and disordered ZnSnN2[END_REF] demonstrate the existence of both wurtzite structure (cation disordered) and orthorhombic structure (cation order).

I.3.2.3

High-pressure metathesis reaction F. Kawamura et al. show that a high-pressure metathesis reaction can be used to synthesize

ZnSnN2 crystals [START_REF] Kawamura | Synthesis of ZnSnN2 crystals via a high-pressure metathesis reaction[END_REF], ZnSnN2 is synthesized by the following equation:

ZnF2 + SnF4 + 2Li3N + 3LiF → ZnSnN2 + 9LiF
The temperature and pressure applied in this study ranged from room temperature to 1000ºC and from 2.5 to 7.7 GPa, respectively. Single-phase ZnSnN2 crystals were synthesized by this method, and the bandgap was measured at 1.4 eV by using the diffuse reflection method.

I.3.2.4 Magnetron sputtering deposition:

The sputtering PVD-technique is one of the most commonly used methods for producing, tailoring and engineering materials to improve thin-film device performance. The primary advantages of using sputtering are high deposition rates, applicability to a large variety of materials, high-purity and adhesive films, ability to coat heat-sensitive substrates (glass substrates, plastic substrates) and automation capabilities [START_REF] Swann | Magnetron sputtering[END_REF].

In addition, the fabrication of ZnSnN2 thin films by vapor-liquid-solid method, molecular beam epitaxy or high-pressure metathesis reaction are all done at high temperatures, up to 1000°C that, in fine, are not compatible with the targeted application. Therefore, the sputtering method is used predominantly in the synthesis of ZnSnN2, this makes sense in reducing production costs based on reduced power usage. Table I. 2 summarizes the theoretical and experimental reported values of the electrical properties of ZnSnN2: the band gap varies in range 0.58 to 2.62 eV, the highest carrier mobility equals to 24 cm/(V.s) [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF] and the lowest carrier concentration equals to 4x10 16 cm -3 [START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF]. Table I. [START_REF]Alta Devices Sets 29.1% Solar Efficiency Record; NASA Selects Alta Devices for International Space Station Test[END_REF] reports the same kind of information for the lattice parameters of ZnSnN2: a rather good agreement is obtained between calculation and experiment results for Orthorhombic Pna21 phase structure. Plasma-assisted MBE [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF] 1.33 -2.38 1.4x10 19 to 1.1x10 21 High-pressure metathesis reaction [START_REF] Kawamura | Synthesis of ZnSnN2 crystals via a high-pressure metathesis reaction[END_REF] 1.4 RF Magnetron Sputtering [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF] 1.9 -2.2 5.0x10 19 to 1.0x10 21 10 RF Magnetron Sputtering [START_REF] Cao | Conductionband effective mass and bandgap of ZnSnN2 earth-abundant solar absorber[END_REF] 1.25 -1.51 2.8x10 20 to 5.1x10 20 Co-sputtering in RF mode [START_REF] Fioretti | Combinatorial insights into doping control and transport properties of zinc tin nitride[END_REF] 1.0 -1.4 1.8x10 18 to 4.0x10 20 0.5 -11

Co-sputtering in RF mode [START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF] 4.0x10 16 to 1.96x10 18 0.28 -8.76

Co-sputtering in RF mode [START_REF] Qin | Effects of surface condition on the work function and valence-band position of ZnSnN2[END_REF] 1.9 3.6x10 20

2.4

Co-sputtering in RF mode [START_REF] Kumar Chinnakutti | Tailoring optoelectronic properties of earth abundant ZnSnN2 by combinatorial RF magnetron sputtering[END_REF] 1.96 -1.77 4.99x10 18 11.1 -15.5

Co-sputtering in RF mode [START_REF] Chinnakutti | Investigation on structural and optoelectronic properties of in-situ post growth annealed ZnSnN2 thin films[END_REF] 3.97x10 18 to 2.42x10 18 14.5 -11.1 DC Magnetron Sputtering [START_REF] Deng | Determination of the basic optical parameters of ZnSnN2[END_REF] 1.9 2.3x10 17

Manuscript overview

ZnSnN2 has only been studied for about a decade; while the first calculation was done in 2008 [START_REF] Paudel | First-principles study of phonons and related ground-state properties and spectra in Zn-IV-N2 compounds[END_REF], the synthesis of ZnSnN2 started in 2013 [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF], [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF], [START_REF] Feldberg | Growth, disorder, and physical properties of ZnSnN2[END_REF]. The structure and basic properties of ZnSnN2 are still being studied by varying parameters during fabrication.

However, the relationship between the previous results and the applicability of ZnSnN2 in the photovoltaics field is still unknown.

Based on the literature review of ZnSnN2 material, my thesis will focus deeply on the study of the effect of the different parameters that support the sputtering process to elaborate ZnSnN2 thin films. The effects of sputtering atmosphere and target biasing will be described in Chapter II. Other sputtering conditions such as sputtering pressure, substrate temperature and post deposition annealing (RTA) will be described in Chapter III. The difference of co-sputtering with two elemental targets and sputtering with compound target will be explained in Chapter IV. The fabrication of aluminum doped zinc oxide and p-type tin oxide that will be used to fabricate photodetector devices will be shown in Chapter V.

An initial assessment of the photoelectric properties will be presented in Chapter VI.

SPUTTERING ATMOSPHERE AND TARGET BIASING

After a brief introduction to the sputtering process and the equipment that has been used all along this work, this chapter will focus on the effects of the sputtering atmosphere as well as of the target biasing on the properties of the ZnSnN2 thin films.

Whatever the sputtering atmosphere is, two metallic targets of Zn and Sn are used in a reactive co-sputtering scheme. Target biasing can be made in a RF or DC mode. Sputtering atmosphere is then either a pure nitrogen, a nitrogen/argon mix, or a nitrogen/hydrogen/argon mix.

Physical properties of the deposited films are investigated by SEM, EDX and XRD.

Their electrical and optical properties are determined from resistivity and Hall measurements and UV-Vis spectroscopy, respectively.

Reactive sputtering

Reactive sputter deposition is a process in which the reaction takes place between the atoms removed from the target and the gaseous species, such as oxygen or nitrogen (called "reactive gas"). Reactive gases can be present in either molecular or "activated" species, activation process can occur according to the following equations to form new fragments that are more chemically reactive and easier to absorb [START_REF] Mattox | Physical Sputtering and Sputter Deposition (Sputtering)[END_REF]:

-N2 + e -→ 2No and NH3 + e -→ No + 3Ho -O2 + e -→ 2Oo then Oo + O2 → O3 However, nitrogen and oxygen are light ions, with atomic masses 14 u (or Da) * and 16 u, respectively; they are unattractive in sputtering . This is explained by the relationship between sputtering yield and energy transfer mass factor [START_REF] Simon | Sputter Processing[END_REF], described in following equations: Y = (6.4 × 10 -3 )M t γ 5/3 E i 0.25 (1 -

E th E i ) 3.5 (2.1)
where Y is the sputtering yield, M t is the target particle mass (in atomic mass unit), E i is the initial energy of the incident atom (in eV), E th is the threshold energy for sputtering (in eV), γ is the energy transfer mass factor defined by:

γ = 4M t M i (M t +M i ) 2 (2.2)
with M i the incident particle mass (in atomic mass unit).

In addition, the energy loss of the reflected particle is dependent on the atomic masses M t , M i and the angle of impact,θ , as depicted in conservation of energy and the conservation of momentum [START_REF] Mattox | Substrate ('Real') Surfaces and Surface Modification[END_REF], the energy that is transferred by the physical collision between hard spheres, E t , is given by Equation 2.3:

E t E i = 4M t M i (M t +M i ) 2 cos 2 θ
(2.3)

Figure II. 1: Collision of incident particle (Mi) and target particle (Mt); after collision Mt moves

along path connecting the center line of the sphere µt [START_REF] Mattox | Substrate ('Real') Surfaces and Surface Modification[END_REF].

As it can be deduced from the equation (2.3), the transferred energy is maximum when cosθ = 1 ( = zero degree) and M t = M i , that means the atomic mass of the bombarding ion is important to the sputtering yield. For sputtering heavy elements, krypton (84 u), xenon (131 u), and mercury (201 u) are then favorably used, and light ions such as nitrogen (14 u) are unattractive and not effective in sputtering. Thus, to aid reactive sputtering, a heavy inert gas is used in conjunction with the reaction gas. Based on its cost and nontoxicity, argon (40 u) is widely used.

Thus, a sputtering environment formed by a mixture of argon and nitrogen has been used in most studies related to the fabrication of ZnSnN2 thin films by the sputtering method [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF], [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF], [START_REF] Qin | Effects of surface condition on the work function and valence-band position of ZnSnN2[END_REF], [START_REF] Cao | Conductionband effective mass and bandgap of ZnSnN2 earth-abundant solar absorber[END_REF]- [START_REF] Chinnakutti | Investigation on structural and optoelectronic properties of in-situ post growth annealed ZnSnN2 thin films[END_REF], [51]. Concerning the free carrier density that is the macroscopic property that is often considered as characteristic of the material potentialities, the lowest value (4x10 16 cm -3 ) was obtained by using RF co-sputtering and post-annealing in 5%H2 in N2 atmosphere [START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF], while the highest (22.7 cm.V -1 s -1 ) was obtained using ZnRFSnDC † cosputtering in mixed N2/Ar atmosphere [51]. However, literature results report on fixed sputtering gas composition, its parametric study has not been reported, yet.

Sputtering system and characterization methods

Figure II. 2: Schematic of the magnetron-sputtering chamber of the Alliance Concept CT200

equipment.

The deposition system that was used all along my work is an Alliance Concept CT200 equipment. This equipment has 3 separate deposition chambers, a transfer chamber and a loadlock. It is composed of three 2" target holders (cathodes) that are configured in a confocal arrangement, the distance between the targets and substrate holder is approximately 5 cm and the sputtering gun is inclined at 20°. All cathodes are magnetron equipped. The substrate is located on a rotating holder that improves the film uniformity.

Different gas inlets are available; three of them have been used for Ar, N2 and H2/N2 (5% H2) gas inputs. A schematic of the sputtering chamber is shown in Figure II.2. Each target can be fed using either a DC or a RF generator. Metals are deposited using either the co- † All along this manuscript, the following writing rule will be adopted concerning the way the films have been deposited: ZnXXSnYY where XX and YY are either DC or RF and correspond to the power supply that has been used on each target during deposition.

sputtering of two separate Zn (purity 99.99%) and Sn (purity 99.99%) targets or the sputtering of a single Zn42.2Sn57.8 target (purity 99.95%). A preliminary assessment shows that the plasma cannot be ignited and maintained when the gas flow is less than 10 sccm and the deposition pressure less than 0.01 mbar. In this work, the rotation speed of the substrate holder was set to 20 rpm and, except otherwise noted, the argon flow was fixed to 10 sccm and film deposition was made without intentional heating. The operating pressure in the chamber is 2.10 -2 mbar (base pressure of 10 -8 mbar). The effect of operating pressure will be discussed in Chapter III.

After the film deposition, the film thickness is measured using surface profilometry after a physical mark was made on the thin film (DektakXT stylus surface profiler).

The surface morphology of the ZnSnN2 thin films is observed by using scanning electron microscopy SEM (field emission gun (FEG) equipment: Zeiss Ultra 55). The film composition is investigated by energy-dispersive X-ray spectroscopy (EDS Quanta 200 / Flash 4010 from Bruker inserted in the SEM). The composition in light elements (such as O and N) cannot be retrieved, only the Zn and Sn elements are measured after sensor calibration. ZnSnN2 is considered stoichiometric when Zn/(Zn+Sn) is equal to 50%. When this ratio is higher than 50%, material is Zn-rich, otherwise it is Sn-rich.

Electrical characteristics, resistivity, carrier concentration and mobility, are determined from Van der Pauw resistivity measurement method and Hall voltage measurements made with a 4-probe system (Nanometrics HL 5500 PC).

The optical properties are determined from UV-Vis spectrometry (Perkin Elmer).

The crystal structure is analyzed by using X-ray diffraction equipment (Rigaku SMARTLAB) in Bragg-Brentano mode with Cu-Kα radiation.

Except otherwise noted, all samples have been deposited on Eagle XG substrates of 1"square dimensions.

Investigation of sputtering rate in argon environment

II.3.1 Each metallic element separately

The purpose of this survey is the determination of the sputtering rate of each metallic target (Zn and Sn) under pure argon atmosphere. The deposition rate was evaluated by measuring the thickness of thin films obtained from a 10 minutes-long process. This experiment was performed on new targets to ensure the surface area of targets and so the power density.

Effectively, as processes are going on, the magnetron sputtering process makes erosion of the target. Its effective surface area is then modified time running, as shown in Figure II.

3, and so might affect the power density (new targets are 6 mm thick). The sputtering rate of Zn and Sn is different, consequently, the erosion groove of each target is also different after the same period of use. It implies that the calibration of the deposition process shall be done regularly in order to amend the deposition rates. The deposition rate of Sn is almost 2 times higher when using DC mode than using RF mode. Even if deposition conditions were different, such a ratio range in deposition rate values was observed for other metals as aluminum [START_REF] Nyaiesh | The dependence of deposition rate on power input for dc and rf magnetron sputtering[END_REF](1.86) and copper [START_REF] Samuel | No Title[END_REF](1.89). In contrast, the sputtering rate of Zn is higher using RF mode than when using DC mode. This is rather usual for poorly conductive materials but less common for metals. Nevertheless, the DC sputtering rate of Sn is almost double than that of Zn that is a rather common figure [START_REF] Semicore | Material sputtering yield table[END_REF]. Based on the theoretical values of density and molecular weight for each metal and assuming that both targets have the same porosity, the predicted thickness ratio of Zn to Sn is given by Equation 2.5:

d Zn d Sn = ρ Sn M Zn ρ Zn M Sn = 0.588 (2.5)
where d is the thickness of corresponding films (nm), ρ Sn and ρ Zn are the density of Sn -ZnRFSnDC configuration: 25 W RF applied to Zn; 29 W DC applied to Sn.

-ZnDCSnDC configuration: 25 W DC applied to Zn; 22 W DC applied to Sn.

-ZnRFSnRF configuration: 25 W RF applied to Zn; 51 W RF applied to Sn

II.3.2 Compound ZnxSn1-x metallic thin films

Based on the results presented in Section II.3, first experiments to deposit stoichiometric metallic ZnSn layers used the ZnDCSnRF configuration with parameters mentioned in section II. 2, i.e. the Ar flow rate was 10 sccm and deposition pressure 2.10 -2 mbar, 25W-DC ‡ and 40W-RF power supplies applied to Zn and Sn target, respectively. After the deposition, the composition determined from EDS measurements was 47.8% Zn and 52.1%

Sn so not exactly 1:1 but not so far. The RF power applied to Sn target was then decreased to 35 W and the zinc/tin ratio so obtained was practically 1:1, 50.09% Zn and 49.91% Sn from EDS measurements.

Sputtering in mixed nitrogen and argon environment

II.4.1 First tests of ZnxSn1-xN2 thin films

Then, nitrogen was introduced into the deposition chamber together with the argon. The There is a significant change in the composition of the thin film with the nitrogen flux value. The film becomes zinc-rich as soon as nitrogen (5 sccm) is added in the sputtering atmosphere; the atomic percent of zinc increases then considerably from 50% to 65%. Then it increases slightly, to approximately 68%, for 10 sccm (N2/Ar = 1) of nitrogen and exhibits less pronounced changes, remaining at values around approximately 70% for higher ‡ All along this manuscript, the following writing rule will be adopted concerning the way the power is applied on the different targets: XXW-YY where XX is the power applied and YY the sputtering mode, either DC or RF.

nitrogen fluxes. Besides that, the thickness of the layer exhibits a fast decrease, from 530 nm (pure argon atmosphere) down to 280 nm (for 30 sccm flow rate of nitrogen, (N2/Ar = 3). It can be concluded that nitrogen decisively modifies the sputtering process, especially having a strong effect in the Sn composition of the films because they become Zn-rich as soon as nitrogen is added. Several origins may be put forward:

(i) The addition of nitrogen increases the gas density in the reaction chamber, reducing the probability of a collision between the gas and the target surface.

(ii) The change in mass of the gas mixture and the different effect of each gas (argon and nitrogen) affecting the sputtering efficiency of both metals.

Since the atomic mass of Sn (118.71 u) is high, it is more efficiently sputtered by argon ions (65.37 u) than nitrogen ions (14 u). The combined effect of above two reasons might explain that Zn-rich compounds are mainly linked to a lower Sn incorporation than a higher Zn one.

(iii) The targets can react with nitrogen to form compounds on their surface, like SnN2 When the nitrogen content is equal to or less than the amount of argon used (10 sccm, N2/Ar <= 1), it is difficult to identify grain boundaries, the surface is similar to granular background, the average grain size after deposition is small and the structure tightly arranged. As N2/Ar ratio is in the range 1.5 to 2.5, the difference of surface morphology is difficult to assess and much more cauliflower-like, interstitial voids can be perceived. At higher N2/Ar, the grain size is "visually" smaller and the voids become ambiguous.

XRD patterns are presented on Figure II. 7. As N2/Ar ratio is lower than 1.0, the spectra do not exhibit particular peak and the films are rather of amorphous nature. This is consistent with the surface views of Figure II. 121), ( 122), (320), ( 123) and (322) directions [59] § . § In the following of the manuscript, these seven particular peaks will be referred as the seven characteristic peaks of ZnSnN2.

the two characteristic peaks, corresponding to 110 and 011 reflections, at respectively 2θ = 20° and 22° cannot be pointed out on the XRD results (perhaps an occurrence for N2/Ar = 3). N2/Ar ratios, compared to experiment result [START_REF] Ye | Preparation, Characterization, and Device Applications of Zinc Tin Nitride and Zinc Tin Oxynitride Materials[END_REF] and simulation result [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF].

The explanation may be that the intensity of these peaks is too weak and the baseline noise overlaps. Furthermore, another, even smaller, peak that is predicted around 2θ = 38.5° can be observed on the N2/Ar = 2 deposited sample. Another hypothesis is that ZnSnN2 has a wurtzite-like crystal structure [START_REF] Quayle | Synthesis, lattice structure, and band gap of ZnSnN2[END_REF], [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF], [START_REF] Quayle | Charge-neutral disorder and polytypes in heterovalent wurtzitebased ternary semiconductors: The importance of the octet rule[END_REF], based on the random distribution of the Zn and Sn atoms. Even if we tried to get the same sample thickness for all experiments, it should be noted that it is not practically the case; care shall be taken comparing the peak intensities between all samples. Based on the presence and the shape (narrow and sharp) of the peak corresponding to [START_REF] Tynell | Atomic layer deposition of Al-doped ZnO thin films[END_REF] plane (2θ = 34.3°) for gas ratios greater than 1.0, it has been used as a reference. There is no particular explanation for the change of peak intensity when increasing N2/Ar ratio that is seen in ( 002), ( 122) and ( 123) planes: their intensity increases as increasing N2/Ar ratio, then decreases at high N2/Ar ratio (≥ 3.0).

The average crystallite size (D) has been evaluated from the (002) peak width using the Scherrer formula [START_REF] Agostmo | Determination of thin metal film thickness by x-ray diffractometry using the Scherrer equation , atomic absorption analysis and transmission / reflection visible spectroscopy[END_REF] (2.6),

D = Kλ βcosθ (2.6)
where K is the shape factor (~ 0.93) [START_REF] Agostmo | Determination of thin metal film thickness by x-ray diffractometry using the Scherrer equation , atomic absorption analysis and transmission / reflection visible spectroscopy[END_REF], λ the wavelength of CuK radiation (0.15406 nm) and β the full half width maximum (FHWM).

It shows that when the N2/Ar ratio increases from 1.5 to 4.0, the crystallite size decreases from 19.0 to 6.2 nm. These values are higher than A. Shing et al. [START_REF] Shing | Semiconducting ZnSnxGe1-xN2 alloys prepared by reactive radio-frequency sputtering[END_REF] and R. Quin et al [START_REF] Qin | Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions[END_REF] reports, but smaller than K. Chinnakutti et al. [START_REF] Kumar Chinnakutti | Tailoring optoelectronic properties of earth abundant ZnSnN2 by combinatorial RF magnetron sputtering[END_REF]. The carrier mobility illustrates low values from 0.05 to 0.53 cm 2 V -1 s -1 . All the samples exhibit n-type semiconductor polarity. The lower nitrogen content is, the lower carrier density is. It increases by 2 decades, from 1x10 18 to 1x10 20 cm -3 , when the N2/Ar ratio rises from 1 to 2 and stays almost constant afterwards.

The UV-Vis spectra represented in Figure II.10a show a shift of the transmittance cut-off towards lower wavelengths (blue shift) as the N2/Ar ratio increases.

The optical bandgap was evaluated using using Tauc's relation (2.7):

αhv = β(hv -E g opt ) n (2.7)
where hν represents the photon energy, β is a fixed constant, E g opt is the optical bandgap energy, and n is the allowed direct band with a value of 1/2.

The absorption coefficient (α) is determined by the following equation:

α = 1 d ln (1-R 1 )(1-R 2 )(1-R 3 ) T (2.8)
where T represents the film transmittance, R is the reflection and d its thickness.

In this study, the reflection of ZnSnN2 has not measure yet, the Fresnel equation can be used to calculate the reflection: As assuming no reflection, we obtain α < α * , α * will be used to calculate the optical bandgap in next measurements. ), it is perceptible that higher the concentration is, larger the gap is. This relationship is explained by the Burstein-Moss effect, which has been mentioned in previous studies [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF], [START_REF] Veal | Band Gap Dependence on Cation Disorder in ZnSnN 2 Solar Absorber[END_REF], [START_REF] Coronel | Earth-Abundant Zinc-IV-Nitride Semiconductors[END_REF]. The relationship of carrier concentration (ne) and optical bandgap (𝐸 𝑔 𝑜𝑝𝑡 ) is expressed by the following equation [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF], [START_REF] Cao | Conductionband effective mass and bandgap of ZnSnN2 earth-abundant solar absorber[END_REF]:

R
𝐸 𝑔 𝑜𝑝𝑡 = 𝐸 𝑔 + ∆𝐸 𝑔 𝐵𝑀 -(∆𝐸 𝑒𝑒 + ∆𝐸 𝑒𝑖 ) (2.9)
where 𝐸 𝑔 is the intrinsic bandgap energy; ∆𝐸 𝑒𝑒 and ∆𝐸 𝑒𝑖 are the bandgap narrowing based on electron-electron and electron-impurity interactions, respectively; ∆𝐸 𝑔 𝐵𝑀 is the band gap widening induced by the Burstein-Moss effect, described as:

∆𝐸 𝑔 𝐵𝑀 = ℏ 2 2𝑚 𝑉𝐶 * (3𝜋 2 𝑛 𝑒 ) 2 3
(2.10)

where ℏ is the Plank constant and mvc* is the reduced effective mass given by:

𝑚 𝑉𝐶 * = ( 1 𝑚 𝑣 * + 1 𝑚 𝑐 * ) -1 (2.11)
where mc*, mv* denote the conduction-band and valence-band effective mass, respectively. In this series and contrary to the previous series, all sputtering parameters were fixed, except for nitrogen flux (i.e. no tunings were made either on target applied power or sputtering time). 30W-RF and 50W-DC powers were applied to Zn and Sn targets, respectively. The operating pressure in the chamber was fixed at 2.10 -2 mbar, the argon nitrogen on thin film composition and sputtering rate are described in Figure II. 11.

II.4.2.2 ZnSnN2 thin film -ZnRFSnDC series

A rather similar behavior that observed on the ZnDCSnRF series has to be noticed (see Table II. 1), i.e. the effect of nitrogen flux on film composition is no more really perceptible after a certain N2/Ar ratio; here the switchover is around a ratio N2/Ar between 1 and 1.5 (it was 2.5 for the ZnDCSnRF series). The sputtering rate shows also the same trend, i.e. decreases rapidly, from 40.7 to 25.7 nm/min, at low N2/Ar ratios (below 1), then reduces continuously as N2/Ar ratio grows up to 4.0.

The SEM images that are shown in Figure II.12 illustrate the change in surface morphology of the ZnRFSnDC series of ZnSnN2 thin films. For N2/Ar ratio of 0.5, the interstitial voids are clearly perceived and they look like cracks in the surface; the shape of grains is random. At higher N2/Ar ratios, the difference in surface morphologies is difficult to assess, the grain size is visually smaller as N2/Ar ratio increases. As N2/Ar equals 0.5, the ZnSnN2 thin film is of amorphous nature. At N2/Ar ratio of 1.0, there are only 2 peaks corresponding to (200) and (320) planes. They are located at of 30.1° and 53.6° respectively, both of them are sharp and narrow. When N2/Ar ratio is equal or higher than 1.5, the peak of ( 121) plane appears at 2θ equal to 34.2°. At higher N2/Ar ratios, other peaks appear at 32.4°, 45.0°, 59.2° and 65.3°, corresponding to (002), ( 122), [START_REF] Kuroyanagi | Crystallographic characteristics and electrical properties of Aldoped ZnO thin films prepared by ionized deposition[END_REF] and (322) directions [START_REF] Ye | Preparation, Characterization, and Device Applications of Zinc Tin Nitride and Zinc Tin Oxynitride Materials[END_REF], respectively. The seven characteristics peaks of ZnSnN2 phase are present as N2/Ar ratio is equal or higher than In particular, the resistivity of ZnSnN2 remains around 0.1 Ω.cm. The carrier mobility also illustrates low variation, ranging from 0.15 to 0.42 cm 2 V -1 s -1 as well as the carrier density that ranges from 4.0x10 20 to 1.1x10 20 cm -3 . All the samples exhibit n-type semiconductor characteristics.

Sputtering in 5%H2/N2 mixed Ar environment:

Hydrogen plays an important role in the optical and electrical properties of semiconductors.

According to S.J. Pearton [START_REF] Pearton | Hydrogen in Crystalline Semiconductors[END_REF], [START_REF] Pearton | GaN: Processing, defects, and devices[END_REF], the nitride semiconductors with nitrogen vacancy, VN, defects are not stable and may decompose and oxidize during storage in the ambient air.

Furthermore, hydrogen can be used to passivate the electrical activity, by combining with dangling and defective bonds, as it used, for example, in amorphous silicon. Hydrogen has also be pointed out as decreasing the carrier density in ZnSnN2 material [START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF].

In this section, 5%H2/N2 (0.05HN) gas was used instead of pure nitrogen. The gas flux was varied from 5 to 35 sccm with a 5 sccm step, the argon flux was fixed at 10 sccm. A 25W-DC feeding was used for Zn and a 40W-RF for Sn (it is then a ZnDCSnRF series like in section II.4.2.1). It can be noticed that a lower Sn power was used than that used in section II.4.2.1 (it was 60 to 70 W for achieving a stoichiometric composition (Table II. 1)) so a Zn rich material is awaited. The operating pressure in the chamber is unchanged and fixed at 2.10 -2 mbar. The sputtering rate and Zn composition obtained for different 0.05HN/Ar ratios is shown in Figure II. 16. When the 0.05HN/Ar ratio is higher than 1.0, it does not affect, or slightly, the deposition rate and the Zn/(Zn+Sn) ratio (nearly 52.5%). This behavior is similar to the one obtained for deposition without hydrogen (see Table II. 1) except in that later case, the switchover was around a 0.05HN/Ar ratio of 2-2.5. As awaited, material is Zn rich.

The surface morphology is illustrated in Figure II.17. When 0.05HN/Ar ratio is equal to 0.5, the shape is much more cauliflower-like. However, the morphology changes to long granular grains for ratios between 1.0 and 2.0; then it seems returning to a cauliflower-like structure. This behavior is different from the one obtained without hydrogen (see Figure appear as a singular point, the highest mobility is 6.0 cm 2 V -1 s -1 that is a decade higher than when sputtering in N2/Ar atmosphere (see Figure II. 9). This value corresponds to that (5.8 cm 2 V -1 s -1) reported by A. Fioretti et al. [START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF] when incorporating hydrogen in the sputtering atmosphere. Contrary to previous cases, the material polarity when 0.05HN/Ar ratio is higher than 1.5 is ambiguous since both polarities can be returned. Nevertheless, up to a two-decades decrease in carrier concentration can be noted over the full range .Such an amphoteric behavior has been reported by Coronel et al. for ZnSnxGe1-xN2 [START_REF] Coronel | Earth-Abundant Zinc-IV-Nitride Semiconductors[END_REF], the reason for this phenomenon is still unclear. Besides that, the change in electrical properties of ZnSnN2 are preferably explained by the defects inside the crystal [START_REF] Chen | Phase stability and defect physics of a ternary ZnSnN2 semiconductor: First principles insights[END_REF], [START_REF] Tsunoda | Electrically Benign Defect Behavior in Zinc Tin Nitride Revealed from First Principles[END_REF]. In n-doped ZnSnN2, the carrier density lowering is caused by the complexation between excess Zn

Sputtering in H2 mixed N2 environment:

Following the results presented in section II.5 that point out a decrease of the carrier concentration when the hydrogen is added to the sputtering atmosphere, another sample series was fabricated in a H2/N2 sputtering environment. case. The Zn/(Zn+Sn) ratio varies from 52 to 53% so also rather the same values that using a gas mix with Ar (cf. Figure II.16). The sputtering rate is between 13 to 15 nm/min so roughly 25% faster than when using a gas mix with Ar. So, no particular effect of hydrogen concentration can be observed either on sputtering rate or material composition.

The surface morphology is shown in Figure II.21. In all cases, the aspect is much more cauliflower-like. For the lower (0.5%) and higher (≥ 4%) H2/N2 ratios, the morphology is rather similar with apparent bigger grain size.

Comparison of sputtering in different environments

In order easier comparing the effects of each environment, three samples were fabricated in the same period and under the same sputtering conditions (ZnDCSnRF series), except for varying the deposition environment (Table II. 4). Compared to previous experiments, the pure nitrogen sputtering atmosphere was a new case and the mix H2/N2/Ar environment (Section II. 5) is not represented. The power supplies were 25W-DC and 70 W-RF for Zn and Sn, respectively. The operating pressure in the chamber was fixed at 2.10 -2 mbar and the overall gas flow rate is 40 sccm.

Table II. 4 : Sputtering gas composition (gas flow rates in sccm) used for the fabrication of the different types (N, NA and NH) of samples. corresponding to (002), ( 121), ( 122), ( 123) and (322). Only NA samples exhibited all characteristic peaks of ZnSnN2, located at 30.1°, 32.4°, 34.3°, 45.0°, 53.6°, 59.2° and 65.3° corresponding to ( 200), ( 002), ( 121), ( 122), ( 320), ( 123) and (322). Furthermore, all samples were recorded the same thickness, approximately 1 µm. The peak of ( 200) and

Gas

(320) planes of HN sample and those of ( 121) and (322) planes of N sample are narrower and sharper than those of NA samples. The deposition of ZnSnN2 was made using two metallic (Zn and Sn) targets in a confocal arrangement. Starting from the deposition rate of the metallic targets that were obtained in a pure argon atmosphere, the power supply of each target was determined in order achieving a stoichiometric compound. Nevertheless, as soon as nitrogen is inserted in the sputtering atmosphere, the deposited films shift to a Zn-rich composition. So, the calibration of the film composition can get a guess by the deposition of each elementary metal but the real calibration needs to be done using a nitrogen atmosphere. This Zn-rich characteristic has been attributed to a lower incorporation of Sn than to a higher one of Zn due to the respective mass of metallic and sputtering gas species (see section II. Table II. 5 attempts to summarize the main effects of those actions, modifying either the sputtering atmosphere or the biasing mode of the metallic targets.

Action

Film composition Deposition rate Crystallography

Changing biasing mode of targets.

Can be adjusted by power tuning of each power supply.

Higher in ZnRFSnDC mode.

No significant change.

Concerning the electrical characteristics of the materials, without hydrogen into the sputtering atmosphere (either within a N2 or a N2/Ar atmosphere), films are almost degenerate with very high electron density, around some 10 20 cm -3 (see Figures II. [START_REF] Sai | Stabilized 14.0%-efficient triple-junction thinfilm silicon solar cell[END_REF] and II. 14). Adding hydrogen (either within a N2 or a N2/Ar atmosphere) to a ratio H2/N2 of 2-3% leads to decrease the carrier concentration by at least two decades, some 10 17 cm -3 can then be achieved (see Figures II. [START_REF] Hinuma | Discovery of earth-abundant nitride semiconductors by computational screening and high-pressure synthesis[END_REF] and II. 28). This effect is jointly obtained with an amphoteric behavior since n-type or p-type material is retrieved from Hall measurements (see Figures II. 9, II. 14, II. 18, II. [START_REF] Punya | Quasiparticle band structure of Zn-IV-N2 compounds[END_REF] and II. 28). The carrier mobility is very low of some cm 2 .V -1 .s -1 or even less (see Figures II. [START_REF] Hinuma | Discovery of earth-abundant nitride semiconductors by computational screening and high-pressure synthesis[END_REF] and II. 28). Hence, the resistivity is low, in the order of 0.1-1 .cm for materials deposited without hydrogen in the sputtering atmosphere.

One or even two orders of magnitude can be gained adding hydrogen, reaching then around 10 .cm.

The optical bandgap deduced from UV-Vis spectroscopy measurements show a very huge large range, from 1.6 eV up to 2.2 eV depending on the different sputtering conditions. In all cases, the transmittance spectrum does not exhibit an abrupt transition. Even if the Burstein-Moss effect has been suspected for the first case that was made, i. Right now, no clear conclusion can be made on the link between carrier concentration and optical bandgap.

PRESSURE, TEMPERATURE AND RAPID THERMAL ANNEALING

In this chapter, we will firstly mention the effect of sputtering pressure on properties of ZnSnN2 thin films.

Secondly, we will report on the effect of temperature on the film properties. Temperature has been applied either during or after deposition. The in-situ heating is achieved owing to the in-built substrate heater within the deposition chamber. The post-annealing is made using rapid Thermal Annealing (RTA) in a chamber that is under N2 atmosphere.

The in-situ heating extends greatly the processing time since several hours are needed for temperature to be reached and the same time for system to cool down before sample extraction.

Effect of sputtering pressure:

According to Wasa [START_REF] Wasa | Handbook of Sputtering Technology[END_REF], the deposition process under low vacuum pressure can influence the morphology of the deposited thin films, especially their porosity, leading to poor adhesion. The adhesion between the material and the substrate is dependent on the stored energy at unit area (us) of the deposited film, expressed by equation (3.1) [START_REF] Wasa | Handbook of Sputtering Technology[END_REF]:

u s = u v d = σ 2 d 2Y ( J/m 2 ) (3.1)
where uv indicates the elastic energy, d is the thickness of thin the film, σ is the internal stress, and Y is the Young's modulus.

The thin film is removed from the substrate when the thickness exceeds the critical thickness or the stored energy at unit area is larger than the adhesive energy. It is possible to minimize the us by minimizing the stress σ. The thermal expansion coefficient and the lattice structure of the substrate should be suitable with deposited material. The internal stress, σ, is given by equation (3.2)

σ = εY 2v (3.2)
where ε indicates the strain and ν indicates the Poisson's ratio of the films.

Additionally, the pressure in the sputtering chamber is also an important parameter in preventing the undesirable contamination of oxygen [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF]. Although oxygen is not introduced into the sputtering chamber, unintentionally oxygen doped ZnSnN2 films are mentioned in previous literature reports [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF], [START_REF] Cao | Conductionband effective mass and bandgap of ZnSnN2 earth-abundant solar absorber[END_REF], [START_REF] Fioretti | Combinatorial insights into doping control and transport properties of zinc tin nitride[END_REF], [START_REF] Alnjiman | Chemical environment and functional properties of highly crystalline ZnSnN2 thin films deposited by reactive sputtering at room temperature[END_REF], [51], [START_REF] Cao | Electron-transport properties of degenerate ZnSnN2 doped with oxygen[END_REF]. Its origin may issue from two options that are not necessarily exclusive: from oxygen impurities in Ar and N2 gas (≤ 5ppm) during deposition, and adsorbed oxygen during air exposure after deposition.

The base pressure was 10 -8 mbar, the sputtering pressure was varied from 0.01 to 0.05 mbar, and other parameters were fixed. A 30W-RF and 50W-DC powers were supplied to to perform, the obtained image was not clear, the grain boundaries are tough to assess even if large voids seem present. At higher pressure, it is difficult to identify the difference in surface morphology that is much more cauliflower-like; interstitial voids can be observed in all cases and their size seems to expand with increasing pressure. Large voids may be the cause of the increase of the amount of oxygen adsorbed on the surface and reducing the adhesion between thin film and substrate. The SEM cross-section image shows the microstructure is columnar. This result is consistent with the explanation of John A.

Thornton [START_REF] Thornton | Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings[END_REF], [START_REF] Thornton | Internal stresses in titanium, nickel, molybdenum, and tantalum films deposited by cylindrical magnetron sputtering[END_REF]. The grain size becomes "visually" smaller by increasing sputtering pressure. As in previous chapter, assuming that the crystal structure of ZnSnN2 is orthorhombic, the lattice parameters determined from XRD data are reported in The average crystal size (D), evaluated from the (200) peak width using the Scherrer formula (2.6), shows that the crystal size decreases from 22 to 12 nm when the pressure increases from 0.01 to 0.05 mbar; that is consistent with SEM top view images of Figure

III. 2.

The strain of thin film can be measured determined from XRD data by using Williamson-Hall plot method [START_REF] Williamson | X-ray line broadening from filed aluminium and wolfram[END_REF]:

β tot = β D + β ε (3.3)
where 𝛽 𝑡𝑜𝑡 is the total broadening, 𝛽 𝐷 is the broadening (determined at full width at half maximum (FWHM)) due to crystalline size and 𝛽 𝜀 is the broadening due to strain.

β D = Kλ D cosθ (3.4)
β ε = 4ε tanθ (3.5)
where K is the shape factor (~ 0.93), λ the wavelength (0.15406 nm), 𝜃 is the peak position in radians, D is the average crystal size and 𝜀 the strain.

Combining equations (3.3), (3.4) and (3.5), we obtain:

β tot cosθ = Kλ D + 4ε sinθ (3.6)
Using a graphical representation, 𝜀 is the slope of the β tot cosθ = f(4sin𝜃) linear curve.

increases from 0.01 to 0.05 mbar, the strain value increases by one order of magnitude, from 2.8x10 -3 to 2.3x10 -2 . From equation (3.2) it can be predicted that the internal stress of the film increases with sputtering pressure. Since the carrier concentration is decreasing when increasing the sputtering pressure whereas the optical bandgap value increases, a relationship between those two involving Burstein-Moss effect cannot be clearly established.

Effect of substrate temperature

In Chapter II, the ZnSnN2 thin films were deposited at room temperature, or more precisely without any intentional heating (the substrate holder is cooled during deposition). Small crystal sizes were obtained. Specially, the XRD data of ZnRFSnDC (Section II.4.2.2) do not show all the characteristic peaks of ZnSnN2 when deposition was made at low nitrogen flux. In addition, the increase in nitrogen flux reduced the crystal size. To improve the microstructure of thin film, kinetics of growth is a factor that should be concerned. At high substrate temperature, the kinetic energy of incident species increases leading to an increase in the grain size [START_REF] Wasa | Handbook of Sputtering Technology[END_REF]. The change in microstructure, optical and electrical properties based on substrate temperature were reported at for ITO [START_REF] Meng | Properties of indium tin oxide films prepared by rf reactive magnetron sputtering at different substrate temperature[END_REF], ZnO [START_REF] Chaabouni | Effect of the substrate temperature on the properties of ZnO films grown by RF magnetron sputtering[END_REF], and NiO [START_REF] Ai | Influence of substrate temperature on electrical and optical properties of p-type semitransparent conductive nickel oxide thin films deposited by radio frequency sputtering[END_REF]. So far, most of the published results on ZnSnN2 fabrication refer to deposition at substrate temperatures between 100°C and 450°C [START_REF] Cai | Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN 2[END_REF], [START_REF] Qin | Effects of surface condition on the work function and valence-band position of ZnSnN2[END_REF], [START_REF] Cao | Conductionband effective mass and bandgap of ZnSnN2 earth-abundant solar absorber[END_REF]- [START_REF] Chinnakutti | Investigation on structural and optoelectronic properties of in-situ post growth annealed ZnSnN2 thin films[END_REF], [51], [START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN2[END_REF]. Furthermore, here, ZnSnN2 is deposited on glass substrates that are stable in this temperature range in order to limit the diffusion of oxygen into the material during the sputtering process [51].

In this section, the substrate temperature was varied from room temperature (i.e. non intentionally heated, approximately 25°C) to 400°C. The limit in temperature is governed by technical aspects linked to the rotating substrate holder and its related vacuum joint.

30W-RF and 50W-DC power supplies were applied to Zn and Sn targets, respectively. difficult to identify the difference in surface morphology that is much more cauliflowerlike for all samples. The SEM cross-section image shows the microstructure is columnar.

The grain size and column become "visually" bigger by increasing substrate temperature. The corresponding XRD data is presented in Figure III. 9. While the substrate temperature is lower than 400°C, the seven characteristic peaks [START_REF] Ye | Preparation, Characterization, and Device Applications of Zinc Tin Nitride and Zinc Tin Oxynitride Materials[END_REF] The substrate temperature does not significantly affect the electrical properties of the material (Figure III. 10). The resistivity is in the range 0.08 -0.25 Ω.cm, the mobility in the 0.17 to 0.30 cm 2 V -1 s -1 range and the carrier density in the 1.4x10 20 to 3.8x10 20 cm -3 . All the samples exhibit n-type polarity. effect of temperature is seen on the optical properties of ZnSnN2. This is quite in good agreement with the stability of carrier concentration that has been observed.

As we saw in the previous paragraph, the substrate temperature during deposition does not affect significantly the electrical and optical properties of ZnSnN2. Besides, the heating and cooling processes take long time. The second way applying temperature to the samples is using post-deposition annealing. Rapid thermal annealing (RTA) is a practical way doing so, control of heating and cooling gradients, easy temperature control and even some possibilities to choose the annealing atmosphere. It also makes sense in reducing production costs. Furthermore, the RTA step is an easy upscalable process that is used widely in the preparation of thin films in the semiconductor industry. The annealing treatment may relief strain due to lattice mismatch and also modify the surface morphology, increasing the grain size and crystalline quality [START_REF] Wasa | Handbook of Sputtering Technology[END_REF]. [80] cannot be clearly pointed out. On the contrary, ZnO pattern (JCPDS #79-0206) [START_REF] Wang | Zn/ZnO Heterostructure for the Application of MO Degradation and NO Removal[END_REF] especially via the shallow peaks on both sides of 35° and around 47° could be discerned. The lattice parameters are described in Table III. 3; a very slight decrease of a, b and c values could be observed with temperature increase, whether this is really significant since it shall be within the experimental determination error..

III.3.1 RTA in

III.3.2 RTA on ZnDCSnRF mode and under 2%H2/N2 (ZnDCSnRF -HN)

As reported by Atwater et al. [START_REF] Atwater | US 2013/0240026 A1 Photovoltaic semiconductive materials[END_REF], the TEC of ZnSnN2 can explain to the appearance of cracks in the films. In their experiments and as in previous experiments (Section III.3.1), ZnSnN2 thin films were deposited in N2 mixed Ar environment [START_REF] Atwater | US 2013/0240026 A1 Photovoltaic semiconductive materials[END_REF]. As it has been seen in section II.7, the use of a H2 mixed N2 atmosphere during deposition can drastically reduce the free carrier concentration. In this section, the effect of using such an atmosphere on the morphology and crystal structure of ZnSnN2 thin film after annealing treatment is studied:

will the TEC value be modified by the sputtering conditions? III.3.2.1 Deposition of ZnDCSnRF-HN thin film on Eagle XG substrate:

The power feeding of targets is the same as in the previous experiments, i.e., 25W-DC and 70W-RF supplies were applied to Zn and Sn targets, respectively. The operating pressure in the chamber was still fixed at 2.10 -2 mbar, the nitrogen flux was 24 sccm, the 5%H2/N2 flux was 16 sccm (overall H2/N2 is then 2%), and the sputtering process took time was 25 minutes. RTA on ZnRFSnDC series under N2/Ar atmosphere (ZnRFSnDC-NA)

In this section, all sputtering conditions are kept similar as in section III.3.1.1 except the target power supplies. This will allow to compare with the results obtained on ZnDCSnRF series.

After their deposition, the films are annealed in pure N2 (300 sccm) and in a temperature The values of lattice parameters are described in Table III. [START_REF] Slade | 27.6% Efficient Silicon Concentrator Solar Cells for Mass Production[END_REF] and do not show any significant change under the temperature of the annealing treatment.

Figure III

. [START_REF] Kawamura | Synthesis of ZnSnN2 crystals via a high-pressure metathesis reaction[END_REF] shows the strain of ZnRFSnDC-NA thin films on Eagle XG substrate calculated from XRD data and equation (3.6). The rise in the annealing process temperature from 100°C to 300°C leads to increase the strain from 1.5x10 -2 to 2.5x10 -2 . At 400°C, the strain decreases; this can be linked to the appearance of cracks on the surface (cf. Figure

III. 29).

The strain value is three times higher than for ZnDCSnRF-NA case (cf. 27, and III. 30), this cannot be seen as a rule. When using a hydrogenated argon-free deposition atmosphere, cracks appear on both substrates, glass and silicon but at higher temperatures (300°C) on Si than on glass (150 °C). As already observed in Chapter II, when using an argon-free deposition, only two main peaks are observed on the XRD patterns of films.

Table III. 8 attempts to summarize the main effects of pressure or temperature on the physical properties of films. No significant effect on electrical and optical properties of ZnSnN2 thin film were recorded under the different deposition or annealing conditions that have been used.

ZnxSn1-xN2: Co-sputtering with two elemental targets

IV.1.1 ZnDCSnRF series

ZnDCSnRF series was deposited on Eagle XG substrate at room temperature and in a mixed N2/Ar atmosphere (so, same configuration as in section II.4.2.1). The N2/Ar ratio was fixed at 2.0 and the sputtering chamber pressure at 0.02 mbar. The different samples of this series

were not fabricated at the same period; Zn-rich to stoichiometric samples were done 6 months sooner than Sn-Rich samples. As mentioned in Chapter II, due to target erosion, same power density will request more power over time. So, no relationship between applied power and film composition could have been drawn on this series. The thickness of the different films is in the 400 to 500 nm range. The power applied to Zn and Sn targets were varied to get the zinc concentration, x, in the 0.27 -0.74 range.

Up to x = 0.5, i.e. Sn-rich films, the surface aspect is more cauliflower-like (Figure IV.1).

For 0.54 ≤ x ≤ 0.60, i.e. Zn-rich films, surface aspect changes to elongated grains with huge interstitial voids. For higher x values, the surface aspect changes back to a cauliflower-like structure. The grain size "visually" increases as increasing the Zn composition. When Zn composition increases, the carrier mobility gains roughly one decade, from 0.20 to 2.56 cm 2 V -1 s -1 and the carrier concentration reduces dramatically, the lowest value is equal to 2.5 x 10 18 cm -3 for Zn0.74Sn0.26N2. All the samples demonstrated n-type polarity.

For Sn-rich up to stoichiometric compositions, there is no major difference in the carrier density that ranges in the 10 20 cm -3 and more precisely between 1.2 x 10 20 and 3.5 x 10 20 cm -3 . The decrease of carrier concentration versus the increasing in Zn composition matches previous reports [START_REF] Chen | Phase stability and defect physics of a ternary ZnSnN2 semiconductor: First principles insights[END_REF], [START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF], [START_REF] Fioretti | Combinatorial insights into doping control and transport properties of zinc tin nitride[END_REF] that mention the ZnSn defects as acceptor defects, counterbalancing the inherent high n-type behaviour of the material. observations of previous chapters, the decrease of optical bandgap value with the decrease of carrier density when the Zn composition increases could be related to a Burstein-Moss effect.

IV.1.2 ZnRFSnDC series

ZnRFSnDC series was fabricated on the same time slot than the previous series. The deposition parameters were kept identical, i.e. deposition on Eagle XG substrate, at room temperature and in a mixed N2/Ar of 2.0 with a sputtering chamber pressure at 0.02 mbar (so, same configuration as in section II.4.2.2). Power feeding was 30W-RF and 30W to 59W-DC for Zn and Sn targets, respectively. to 0.60. A Burstein-Moss effect could also be suspected in this series. Nevertheless, the correlation between carrier concentration and bandgap does not match since, in this case, a lower bandgap variation is obtained for rather the same variation of carrier concentration.

IV.1.3 ZnDCSnDC series

All along this thesis, the possibilities of the sputtering equipment evolve and particularly, the possibility feeding both targets in DC mode was set lately. Always keeping the same sputtering atmosphere, 20W-DC and 17W to 37W-DC was used for Zn target and Sn targets, respectively. sputtering rate shifts linearly versus power. The lattice parameters (Figure IV. 21a) do not change by the apply power. However, the crystal size evaluated from ( 200), ( 121) and (320) plane increase as increasing the power.

IV.2.2 Comparision of ZnSnRF mode and ZnSnDC mode

In this section RF supply is used instead of DC for the compound target. All other parameters have been kept identical as in section IV. Optical and electrical properties also differ; the optical bandgaps obtained for the same compound composition are lower than in case using two elementary targets, 1.61 eV with RF and 1.77 eV with DC (Figure IV. 26). Besides that, Hall measurements show a very high carrier mobility of 23.7 cm 2 V -1 s -1 and a low carrier density of 1.60 x1 0 18 cm -3 in RF mode. In DC mode, these values change to 3.02 cm 2 V -1 s -1 and 8.30 x 10 18 cm -3 , respectively.

Discussions and partial conclusions

In this chapter, we compared the properties of ZnxSn1-xN2 films versus their composition (x parameter). As reported in previous chapters, owing to the co-sputtering technique, several configurations exist that are linked to the DC or RF power that is applied to each and IV. 17).

Using a compound target (Zn42.2Sn57.8), experiments have been limited since problems due to reflected power in RF mode have been encountered. In DC mode, the obtained films are always Sn-rich. Below a switchover point around 3W/cm 2 (60W on a 2" target), composition is rather stable and reflects that of target. Above this point, Sn concentration decreases and films tend to become stoichiometric. Nevertheless, they are always more or less Sn-rich and the carrier concentration is around some 10 20 cm -3 as observed in the previous series.

Aluminum doped zinc oxide (AZO)

V.1.1 State of the art

V.1.1.1 Structural
ZnO is a member of binary II-VI semiconductors, its crystal structure is either cubic zincblende or hexagonal wurtzite. Its characteristics peaks exhibit at (100), ( 002) and ( 101)

planes [START_REF] Khuili | Comparative first principles study of ZnO doped with group III elements[END_REF]. By calculating the total energy per unit cell [START_REF] Jaffe | LDA and GGA calculations for high-pressure phase transitions in ZnO and MgO[END_REF], [START_REF] Özgür | A comprehensive review of ZnO materials and devices[END_REF], the most energetically stable ZnO structure is hexagonal wurtzite; hence, it is obtained in almost all reports on AZO, with a dominant orientation along the (002) direction, having c-axis perpendicular to the substrate [START_REF] Ruth | Aluminum doped zinc oxide thin films deposited by ion beam sputtering[END_REF], [START_REF] Park | The physical properties of Al-doped zinc oxide films prepared by RF magnetron sputtering[END_REF], [START_REF] Zhou | Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties[END_REF]- [START_REF] Jellal | Experimental and ab-initio investigation of the microstructure and optoelectronic properties of FCM-CVD-prepared Al-doped ZnO thin films[END_REF], [START_REF] Kim | Structural, electrical and optical properties of zirconium-doped zinc oxide films prepared by radio frequency magnetron sputtering[END_REF]- [START_REF] Yoo | High transmittance and low resistive ZnO:Al films for thin film solar cells[END_REF].

Besides, group III atoms such as Al, Ga, In acting as dopants were found not modifying the crystal structure, as most dopants. Due to difference in the ionic radius of doping materials and Zn 2+ (74 pm), a shift of diffraction peaks was observed if they substitute Zn +2 in the lattice. In Al doped ZnO, the ionic radius of Al +3 is 53 pm, smaller than Zn 2+ , the diffraction peak of (002) plane shifts toward higher angle [START_REF] Jeong | Deposition of aluminum-doped zincoxide films by RF magnetron sputtering and study of their structural, electrical and optical properties[END_REF], [START_REF] Tyona | Dye-sensitized solar cells based on Al-doped ZnO photoelectrodes sensitized with rhodamine[END_REF], [START_REF] Murugesan | Properties of Al-doped zinc oxide and In-doped zinc oxide bilayer transparent conducting oxides for solar cell applications[END_REF]- [START_REF] Dasgupta | Atomic Layer Deposition of Al-doped ZnO Films : Effect of Grain Orientation on Conductivity[END_REF].

In this study, we design a structure of ZnSnN2/AZO because of their lattice mismatch is smaller than 5% [START_REF] Fang | Band gap engineering of ZnSnN2/ZnO (001) short-period superlattices via built-in electric field[END_REF]- [START_REF] Punya | Band offsets between ZnGeN2, GaN, ZnO, and x ZnSnN2 and their potential impact for solar cells[END_REF]. It is possible to restricting the strain in interface and the degradation of device.

V.1.1.2 Electrical and optical properties of AZO

In most studies on AZO, the resistivity has been reported as low value, ranges in the order of 10 -4 Ω.cm to the order of 10 -2 Ω.cm. A post-annealing treatment contributes to the reduction of resistivity [START_REF] Ruth | Aluminum doped zinc oxide thin films deposited by ion beam sputtering[END_REF], [START_REF] Oh | Post-annealing of Al-doped ZnO films in hydrogen atmosphere[END_REF], but no effect of substrate temperature was mentioned [START_REF] Yoo | High transmittance and low resistive ZnO:Al films for thin film solar cells[END_REF], [START_REF] Mass | Effect of high substrate temperature on Al-doped ZnO thin films grown by pulsed laser deposition[END_REF]. Additionally, in sputtering method, the higher working pressure is, the higher resistivity is obtained [START_REF] Song | Optimisation of ZnO:Al films by change of sputter gas pressure for solar cell application[END_REF].

It is largely agreed that AZO is a n-type semiconductor with high electron density, ranging between 10 18 cm-3 and 10 21 cm -3 , depending on deposition techniques and parameters [START_REF] Ruth | Aluminum doped zinc oxide thin films deposited by ion beam sputtering[END_REF], [START_REF] Park | The physical properties of Al-doped zinc oxide films prepared by RF magnetron sputtering[END_REF], [START_REF] Jin | Optical and electronic properties of transparent conducting ZnO and ZnO:Al films prepared by evaporating method[END_REF]- [START_REF] Asemi | Preparation of highly conducting Aldoped ZnO target by vacuum heat-treatment for thin film solar cell applications[END_REF], [START_REF] Kim | Structural, electrical and optical properties of zirconium-doped zinc oxide films prepared by radio frequency magnetron sputtering[END_REF], [START_REF] Yang | Transparent conducting ZnO:Al films deposited on organic substrates deposited by r.f. magnetronviii sputtering[END_REF], [START_REF] Oh | Post-annealing of Al-doped ZnO films in hydrogen atmosphere[END_REF], [START_REF] Lu | Carrier concentration dependence of band gap shift in n-type ZnO:Al films[END_REF]- [START_REF] Ghosh | A novel blanket annealing process to achieve highly transparent and conducting Al doped ZnO thin films: Its mechanism and application in perovskite solar cells[END_REF], [START_REF] Kluth | Texture etched ZnO:Al coated glass substrates for silicon based thin film solar cells[END_REF]. When the doping level of Al increases, the carrier density increases [START_REF] Kim | Structural, electrical and optical properties of zirconium-doped zinc oxide films prepared by radio frequency magnetron sputtering[END_REF], due to AlZn donor defect (Al substitutes Zn).

The carrier mobility varies from 0.6 cm 2 V -1 s -1 [START_REF] Ghosh | A novel blanket annealing process to achieve highly transparent and conducting Al doped ZnO thin films: Its mechanism and application in perovskite solar cells[END_REF] to 60 cm 2 V -1 s -1 [START_REF] Neubert | Improved conversion ef fi ciency of a-Si : H / m c-Si : H thinfi lm solar cells by using annealed Al-doped zinc oxide as front electrode material[END_REF]. The highest mobility is obtained when using DC magnetron sputtering.

In the visible region (400 to 800 nm), the optical transmittance is very high (≥ 75%). It is associated with an optical bandgap value between 3.15 eV [119] and 3.9 eV [START_REF] Jin | Optical properties of sputter-deposited ZnO:Al thin films[END_REF].

V.1.1.3 Deposition techniques of AZO thin film

There are several deposition techniques to fabricate AZO thin films, such as atomic layer deposition (ALD) [START_REF] Dasgupta | Atomic Layer Deposition of Al-doped ZnO Films : Effect of Grain Orientation on Conductivity[END_REF], [START_REF] Tynell | Atomic layer deposition of Al-doped ZnO thin films[END_REF], [START_REF] Yang | Energy Level-Graded Al-Doped ZnO Protection Layers for Copper Nanowire-Based Window Electrodes for Efficient Flexible Perovskite Solar Cells[END_REF], chemical bath deposition (CBD) [START_REF] Tyona | Dye-sensitized solar cells based on Al-doped ZnO photoelectrodes sensitized with rhodamine[END_REF],

electrochemical deposition (ECD) [START_REF] Henni | Studies on the structural , morphological , optical and electrical properties of Al-doped ZnO nanorods prepared by electrochemical deposition[END_REF], [START_REF] Pauporté | Al-Doped ZnO Nanowires by Electrochemical Deposition for Selective VOC Nanosensor and Nanophotodetector[END_REF], electron beam evaporation [START_REF] Kuroyanagi | Crystallographic characteristics and electrical properties of Aldoped ZnO thin films prepared by ionized deposition[END_REF], electrospraying method [START_REF] Mahmood | Controlling the surface nanostructure of ZnO and Al-doped ZnO thin films using electrostatic spraying for their application in 12% efficient perovskite solar cells[END_REF], mist chemical vapor deposition [START_REF] Lu | Carrier concentration dependence of band gap shift in n-type ZnO:Al films[END_REF], [START_REF] Jellal | Experimental and ab-initio investigation of the microstructure and optoelectronic properties of FCM-CVD-prepared Al-doped ZnO thin films[END_REF], plasma-enhanced metalorganic chemical vapor deposition [START_REF] Volintiru | Evolution of the electrical and structural properties during the growth of Al doped ZnO films by remote plasma-enhanced metalorganic chemical vapor deposition[END_REF], pulsed laser deposition [START_REF] Mass | Effect of high substrate temperature on Al-doped ZnO thin films grown by pulsed laser deposition[END_REF], [START_REF] Kim | Epitaxial growth of Al-doped ZnO thin films grown by pulsed laser deposition[END_REF], [START_REF] Kaur | Pulsed laser deposited Al-doped ZnO thin films for optical applications[END_REF],

sol-gel coating [START_REF] Zhou | Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties[END_REF], [START_REF] Tonny | Electrical, optical and structural properties of transparent conducting Al doped ZnO (AZO) deposited by sol-gel spin coating[END_REF], [START_REF] Sharmin | Depositions and characterization of sol -gel processed Al -doped ZnO ( AZO ) as transparent conducting oxide ( TCO ) for solar cell application[END_REF], [START_REF] Murugesan | Properties of Al-doped zinc oxide and In-doped zinc oxide bilayer transparent conducting oxides for solar cell applications[END_REF], [START_REF] Jin | Optical and electronic properties of transparent conducting ZnO and ZnO:Al films prepared by evaporating method[END_REF], [START_REF] Srinatha | Spin-coated Al-doped ZnO thin films for optical applications: Structural, micro-structural, optical and luminescence studies[END_REF], spray pyrolysis [START_REF] Crossay | Solar Energy Materials & Solar Cells Spray-deposited Al-doped ZnO transparent contacts for CdTe solar cells[END_REF], [START_REF] Islam | XPS and X-ray diffraction studies of aluminum-doped zinc oxide transparent conducting films[END_REF], [START_REF] Gençyılmaz | Deposition and Ellipsometric Characterization of Transparent Conductive Al-doped ZnO for Solar Cell Application[END_REF],

thermal evaporation [START_REF] Jin | Optical and electronic properties of transparent conducting ZnO and ZnO:Al films prepared by evaporating method[END_REF], and finally sputtering (ion beam, DC or RF mode) [START_REF] Chen | Surface characterization of transparent conductive oxide Al-doped ZnO films[END_REF]- [START_REF] Yoo | High transmittance and low resistive ZnO:Al films for thin film solar cells[END_REF], [START_REF] Pei | Optical and electrical properties of direct-current magnetron sputtered ZnO:Al films[END_REF]. Sputtering is obviously the most popular method, owing to its advantages mentioned in Section I.3.2.4. In addition, the deposition of AZO thin films by sputtering will be more convenient in a full process making a device.

V.1.1.4 Application of AZO thin films

According to its electrical and optical properties, AZO thin films are used as transparent conductive oxide (TCO) for solar cells, mainly of 2 nd generation. The experimental conversion efficiencies of CdTe, perovskite and Si solar cells using AZO films are summarized in Table V.1

V.1.2 Fabrication of AZO by DC-sputtering

The deposition system is the same than for the ZnSnN2, i.e. the Alliance Concept CT200

equipment. Here a 4-inch diameter 3%Al doped ZnO (purity 99.99) is used in a planar configuration. The distance between the target and substrate holder is approximately 5 cm.

A DC generator was used and 80W were applied to the target. The rotation speed of substrate holder was 20rpm and the operating pressure in the chamber was 7.10 -3 mbar (base pressure of 10 -8 mbar), the Ar gas flow was fixed at 10 sccm. During the deposition, the substrate is not heated, or least not intentionally heated. After deposition on Eagle XG substrate in 9 min, the film thickness was measured using surface profilometry and has been recorded approximately to 270nm. corresponding to ( 100), ( 002), ( 101), ( 102), ( 103) and ( 004) directions [START_REF] Jeong | Deposition of aluminum-doped zincoxide films by RF magnetron sputtering and study of their structural, electrical and optical properties[END_REF]. The orientation is more preferentially along the (002) direction. The average crystal size (D) is equal to 27 nm, evaluated from the (200) peak width using the Scherrer formula (2.6). The full width at half maximum is equal to 0.30316, the a and c parameters are equal to 3.26 Å and 5.20

Å respectively, in good agreement with other reports [START_REF] Jellal | Experimental and ab-initio investigation of the microstructure and optoelectronic properties of FCM-CVD-prepared Al-doped ZnO thin films[END_REF], [START_REF] Murugesan | Properties of Al-doped zinc oxide and In-doped zinc oxide bilayer transparent conducting oxides for solar cell applications[END_REF], [START_REF] Kuroyanagi | Crystallographic characteristics and electrical properties of Aldoped ZnO thin films prepared by ionized deposition[END_REF], [START_REF] Islam | XPS and X-ray diffraction studies of aluminum-doped zinc oxide transparent conducting films[END_REF]. eV. This result is similar to the optical band gap of intrinsic ZnO [START_REF] Kim | Structural, electrical and optical properties of zirconium-doped zinc oxide films prepared by radio frequency magnetron sputtering[END_REF], [START_REF] Jellal | Experimental and ab-initio investigation of the microstructure and optoelectronic properties of FCM-CVD-prepared Al-doped ZnO thin films[END_REF], and close to the result of AZO deposited by EDC [START_REF] Henni | Studies on the structural , morphological , optical and electrical properties of Al-doped ZnO nanorods prepared by electrochemical deposition[END_REF] and sol gel spin coating [START_REF] Sharmin | Depositions and characterization of sol -gel processed Al -doped ZnO ( AZO ) as transparent conducting oxide ( TCO ) for solar cell application[END_REF]. Optical bandgap of AZO deposited by sputtering in previous study gave the value higher 3.4 eV [START_REF] Song | Optimisation of ZnO:Al films by change of sputter gas pressure for solar cell application[END_REF], [START_REF] Jeong | Deposition of aluminum-doped zincoxide films by RF magnetron sputtering and study of their structural, electrical and optical properties[END_REF],

[101], [START_REF] Jiang | Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices[END_REF], [START_REF] Drewelow | Work function investigations of Al-doped ZnO for band-alignment in electronic and optoelectronic applications[END_REF].

The electrical properties of AZO were measured by using Hall system. The resistivity is equal to 2.82x10 -3 Ωcm, the carrier density is equal to 1.84x10 20 cm -3 and the mobility is equal to 12 cm 2 V -1 s -1 , in excellent agreement with the result of Grilli et al. [START_REF] Grilli | Transparent and conductive Al-doped ZnO films for solar cells applications[END_REF]. Our electrical results of AZO are approx. the value AZO thin film that was fabricated by A.

Crossay et al. [START_REF] Crossay | Solar Energy Materials & Solar Cells Spray-deposited Al-doped ZnO transparent contacts for CdTe solar cells[END_REF] and applied for TCO of CdTe solar cell.

contribute to electric conductivity and Sni defect shows both positive-and negative-charge states near the valence band maximum (VBM) and conduction band minimum (CBM), respectively. The acceptor defect Vsn is the origin of p-type conductivity of SnO.

Besides, the electrical properties of SnO are greatly influenced by the fabrication conditions and the post-anneal treatment, as mentioned in Section V.2.1.1. When the transformation of SnO into SnO2 occurs, it leads to change the type of charge carrier from p-type to n-type [START_REF] Leja | Electrical properties of nonstoichiometric tin oxide films obtained by the d.c. reactive sputtering method[END_REF]- [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF], [START_REF] Hayashi | Selective fabrication of n-and p-type SnO films without doping[END_REF].

For sputtered films, their optical bandgap increases as the working pressure [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF] and oxygen partial pressure [START_REF] Garzon-Fontecha | SnOx thin films with tunable conductivity for fabrication of p-n homo-junction[END_REF], [START_REF] Luo | Structural, chemical, optical, and electrical evolution of SnO x films deposited by reactive rf magnetron sputtering[END_REF] increase. The post-anneal treatment also leads to expand the optical bandgap [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF], [START_REF] Barros | Role of structure and composition on the performances of P-type tin oxide thin-film transistors processed at low-temperatures[END_REF]. The optical band gap is between 2 eV and 3.5 eV [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF], smaller than optical bandgap of SnO2 [START_REF] Luo | Structural, chemical, optical, and electrical evolution of SnO x films deposited by reactive rf magnetron sputtering[END_REF].

V.2.1.3 Deposition techniques of SnO thin films

The p-type SnO thin films can be deposited by atomic layer deposition [START_REF] Kim | Fabrication of high-performance p-type thin film transistors using atomic-layer-deposited SnO films[END_REF], electron beam evaporation [START_REF] Guo | Microstructure, optical, and electrical properties of p -type SnO thin films[END_REF], [START_REF] Pan | Tin oxide thin films grown on the (1012) sapphire substrate[END_REF], pulsed laser deposition [START_REF] Hayashi | Selective fabrication of n-and p-type SnO films without doping[END_REF], [START_REF] Ogo | P -channel thin-film transistor using p -type oxide semiconductor, SnO[END_REF], [START_REF] Nomura | Ambipolar oxide thin-film transistor[END_REF] and sputtering [START_REF] Leja | Electrical properties of nonstoichiometric tin oxide films obtained by the d.c. reactive sputtering method[END_REF],

[155]- [START_REF] Chiu | Gate-bias stress stability of P-type SnO thin-film transistors fabricated by RF-sputtering[END_REF].Here, sputtering is used in order to get compatibility between the different deposition technique and, in fine, to be able to grow deposit all the layers without exposing the samples to air.

V.2.2 Fabrication of SnO by magnetron sputtering

The deposition system arrangement is the same than used for AZO (see section V.1.2) except the Ar gas flow that was here fixed at 50 sccm and the process pressure to 2.10 -2 mbar. A SnO (purity 99.99%) target is used. Both DC and RF mode were tested.

V.2.2.1 SnO deposition in RF mode

The power density was close to 1.2 W/cm 2 , the sputtering rate obtained was close to 28 nm/min. After deposition, the SnO thin film was characterized by XRD and Hall effect measurements. The XRD data showed an amorphous nature and electrical properties could not be recorded. So, SnO thin films were fabricated again and post-annealed in nitrogen atmosphere at different temperatures, from 100°C to 400°C with a 50°C step. The annealing time was 15 minutes and the heating-up/cooling-down rate was 100°C/min. It has to be noted that all the deposited samples were made in a single batch; their thickness is close to 250 nm. confirm the pure SnO phase, no diffraction peaks characteristic of secondary phases, neither β-Sn nor SnxOy (SnO2, Sn3O4…). There are two possible hypotheses: 1) the concentration of secondary phases is very small compared to that of SnO, the intensity of their characteristics peaks is too low to be observed, 2) the secondary phases do not exist in a crystalline form when the annealing temperature is below 400°C, and are so not clearly distinguishable by XRD. 101), ( 110) and ( 002) plane of SnO fabricated on RF mode after annealing from 250°C to 400°C. ). The strain increases slightly from 6.4x10 -3 to 6.9x10 -3 when the temperature is ranging between 250°C and 300°C. At higher temperatures, the strain reduces around 5x10 -3 that can be explained by the increase of the density of cracks causing then stressrelief. The crystallite size (D), evaluated from the ( 101), ( 110), (002) peak width using the Scherrer formula (2.6), is reported in Figure V. 6b. The result obtained using (002) peak (the orientation plane used to calculate was not mentioned) [START_REF] Caraveo-Frescas | Record mobility in transparent p-type tin monoxide films and devices by phase engineering[END_REF]. and after annealing from 100°C to 400°C. Please, remind that all the samples have the same thickness (approx. 250 nm), the transmittance increase as increasing the annealing temperature. The highest one obtained in sample that annealing at 300°C, the maximum transmittance reaches 71% in visible light region (400 -800 nm), and approaches 96% at wavelength equals to 820 nm. A progressive shift of the cut-off of the transmittance value towards lower wavelengths (blue shift) is observed when the annealing temperature increases; the corresponding optical bandgap varies from 2.26 eV to 2.90 eV. Those values are lower than that reported by S. Lin et al [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF] for SnO thin films that were deposited by RF magnetron sputtering at working pressures from 5.10 -3 to 3.10 -2 mbar (here, we are at 2.10 -2 mbar) that were all determined as above 2.9 eV. 16 Ω.cm. The mobility decreases from 0.39 to 0.14 cm 2 V -1 s -1 as annealing temperature varies from 250°C to 350°C, then grows slightly up to 0.20 cm 2 V -1 s -1 at 400°C.

When SnO thin films were annealed at temperature below 400°C, p-polarity is observed, hole concentration is in the range 1.5x10 19 -2.5x10 19 cm -3 . At 400°C, the polarity is uncertain with approximately the same concentration (5x10 18 cm -3 ). This change in type of polarity under temperature has been explained in previous reports [START_REF] Leja | Electrical properties of nonstoichiometric tin oxide films obtained by the d.c. reactive sputtering method[END_REF], [START_REF] Lee | Composition, Microstructure, and Electrical Performance of Sputtered SnO Thin Films for p-Type Oxide Semiconductor[END_REF], [START_REF] Barros | Role of structure and composition on the performances of P-type tin oxide thin-film transistors processed at low-temperatures[END_REF], [START_REF] Yabuta | Sputtering formation of p-type SnO thin-film transistors on glass toward oxide complimentary circuits[END_REF], [START_REF] Huo | Preparation and characterization of SnO films via reactive sputtering for ambipolar thin-film transistor applications[END_REF]. At low annealing temperatures, the main defects that are present are Oi and VSn that act as acceptors. Because SnO is less thermodynamically stable than SnO2, at higher temperature (400°C here), SnO is easily transformed to SnO2. However, XRD data in Figure V. 5 does not record the presence of crystalline SnO2 phase, with the characteristic ( 110) peak located at 26.0°, according to JCPDS # 77-0452 [START_REF] Garzon-Fontecha | SnOx thin films with tunable conductivity for fabrication of p-n homo-junction[END_REF]. At 400°C, SnO2 cannot be completely converted or it does not exist in a crystalline form. According to XPS result of J. Huo et al [START_REF] Huo | Preparation and characterization of SnO films via reactive sputtering for ambipolar thin-film transistor applications[END_REF], Sn +4 was present on the surface of the thin film after annealing at 400°C, the intensity of O-Sn +4 and O-Sn +2 are almost equal, their result is in good agreement with this study.

V.2.2.2 SnO deposition in DC mode

The DC power density was close to 0.6 W/cm 2 and the achieved sputtering rate was approximately 28 nm/min. Comparing to RF mode (see Section V.2.2.1), the sputtering rate in DC mode is twice than in RF mode. As-deposited SnO has an amorphous nature and electrical properties could not be recorded, recording so identically to the RF mode. SnO thin films was fabricated again and post-annealed at different temperatures from 100°C to 400°C with a 50°C step in nitrogen atmosphere. The annealing time was 15min and the heating-up/cooling-down rate was 100°C/min. It has to be noted that all the samples were made in a single batch, and their thickness is approximately 250 nm. Ultimately, the same fabrication process has been used for both modes! phenomenon. In this study, c parameter of Zn0.47Sn0.53N2 is approx. 5.50 Å (see Figure IV.

3), it is close to c parameter of AZO (5.20 Å), according to the result in Section V.1.2. The evaluated lattice mismatch ∆c/c equals to 5.4%. It has to be noted that the AZO layer was deposited immediately after the Zn0.47Sn0.53N2 film, in the same deposition chamber without any exposure to ambient atmosphere. The current under 1 Sun illumination as the photocurrent values obtained following Eq. 6.1 versus cell area are illustrated in Figure VI. 5. When the cell area increases, the photocurrent density at 500 mV reduces from 218 to 2.9 mA/cm 2 , approx. two orders of magnitude. The qualitative behavior is so the same that for the dark current density but the quantitative behavior is strongly different (roughly one order of magnitude higher). Beside that the JPh/JD ratio decrease from 0.58 to 0.036. According to these results, 0.05 cm 2 surface area allows achieving the highest photocurrent values, so we will use this size for the next fabrication run (Structure1.2).

VI.1.2 Second fabrication run

VI.1.2.1 Effect of x value

So, for this second fabrication run, only 0.05 cm 2 surface area samples have been fabricated, the overall fabrication process is the same than for Structure1.1 devices (see section VI.1.1), except the power supplied to the target Sn that has been varied in order to achieve the different compositions (see Table VI. 1). We are so still in a ZnDCSnRF series sweeping from Zn rich (x> 0.5) to Sn rich (x< 0.5) samples. Each sample consists of 9 elementary cells of same composition, 5 samples have been so made in order achieving the different compositions. Almost all devices reported in this chapter have a completely different structure since by using the above-mentioned technological processes, the carrier transport is now in the transverse direction (direction of material deposition) instead of a longitudinal one:

-1) it corresponds to the configuration that would be used in a potential PV application, -2) it is done, along the film thickness, i.e., 300 nm and no more between two surface contacts that were distant by at least 1 cm in previous chapters.

To check the effect of carrier transport direction, Structure1. Right now, it is difficult to discern the influence of the distance and direction of carrier transport on the modification of the electric and photoelectric properties of the films. So, specific samples have been fabricated and sent to one partner of the OPERA project, the Laboratoire de génie électrique de Paris (GeePs), for characterization. This will be discussed in the next section.

VI.1.5 Conductivity measurements in transverse and longitudinal electrode configurations

All samples have been fabricated on Eagle XG substrate (1 inch x 1 inch). The 300 nm of ZnSnN2 was deposited in ZnRFSnDC mode at room temperature, the working pressure was 0.02 mbar, the N2/Ar ratio was fixed at 2.0, 30W-RF and 50W-DC were used for Zn and ) is almost constant with temperature and is between 2 and 3 x10 -6 S/cm. The conductivity obtained on the return way to ambient temperature is slightly higher indicating that material conductivity has been slightly modified by the temperature increase (even for the one that was deposited at 300°C). The main difference is observed by comparison to the measurements carried out on the lateral transport samples (Structure1.M2) that exhibit a conductivity value 10 6 order of magnitude higher. The large difference in conductivity value that is measured on transverse and lateral configurations shows that the electrical properties of ZnSnN2 strongly depends how the carrier transport is.

M2). (data obtained by GeePs researchers)

From all results obtained on Structure1 samples, it appears that:

-lateral and transverse conduction properties are completely different. Up to now, the presence of a thin film of conductive oxide, linked to oxygen incorporation during deposition might be an explanation for the high conductivity values reported in longitudinal transport devices, -only transverse transport samples exhibited photodetection capabilities.

It appears then that material characterization shall also be conducted on transverse conduction devices that compels technology to be done to get the right electrode configuration. So, a longer study would be required comparing to just depositing a thin film. The dark J-V curve (Figure VI. 19a) does not relate to a diode functionality and is similar to a conductive behavior. In addition, when the probe was located on TiAu1, the obtained dark current density is higher almost one order of magnitude than when located on SnO indicating that direct probe contact on SnO is highly resistive. Whereas there is no significant difference when placed on TiAu2 or AZO.

Under 1-sun illumination, a small photocurrent is obtained for all configurations. As can be awaited from dark measurements, the JPh/JD ratio as probe is located on TiAu1 is higher than on SnO. The best result is obtained when the two probes are connected to TiAu1 and TiAu2 contacts, then the JPh/JD ratio at 5V equals to 0.32. On the contrary, the worst case is obtained when the two probes are connected directly to SnO and AZO layer, the JPh/JD ratio at 5V equals to 0.08, mainly due to the direct contact on SnO.

VI.2.2 p-Si/ZnSnN2 heterojunction

In this section, ZnSnN2 was deposited on p-type boron-doped silicon wafer at room temperature, the working pressure was 0.02 mbar, the N2/Are ratio was fixed at 2.0, and the applied power scheme is described in This composition was then fixed for the remaining studies.

A 300°C post-anneal under N2 atmosphere during 15 minutes has also been seen as beneficial for the electrical properties. Such a behavior was not observed during the parametric study of chapters II to IV in longitudinal carrier transport conditions.

Structure2 topologies aimed achieving a junction device. Whatever the materials used, p-SnO or p-Si, all fabricated samples react as photoconductors and a photodiode behavior cannot be observed. Target biasing conditions (DC or RF feeding of Zn and Sn targets)

have also been reiterated and results do not exhibit significant modifications of electrical properties as it was observed in the parametric study in longitudinal conduction conditions. The electrical characterization of materials deposited in H2-free atmosphere shows that they are almost degenerated with a very high electron density, around some 10 20 cm -3 . The carrier mobility is very low ranging from 0.15 to 0.42 cm 2 V -1 s -1 and the resistivity is then also very low, in the order of 0.1 -1 .cm. Insertion of hydrogen in the deposition atmosphere leads to decrease the carrier concentration by at least two decades, some 10 17 cm -3 can even be achieved, the carrier mobility and resistivity are also reported at slightly higher values than without hydrogen.

The optical bandgap deduced from UV-Vis spectroscopy measurements show a very huge large range, from 1.6 eV up to 2.2 eV depending on the different sputtering conditions.

-Deposition pressure:

The increase in sputtering pressure leads to increase the Sn content, the optical band gap, and the carrier mobility as to decrease the carrier density. However, at high pressure, the crystallinity is reduced, voids exist on the surface and the strain of thin films increases.

Those two can affect material stability.

-Temperature o during deposition, by heating the substrate:

Heating the substrate during deposition does not really impact the sputtering rate as well as the film composition. The crystallite size is also not really modified. However, the number and size of voids increase with the increase of temperature.

o after deposition using Rapid thermal annealing treatment:

Using a post-anneal using an RTA under N2 has no significant effect on ZnSnN2 crystallography and its electrical properties as long as temperature stays below 400°C. At higher temperatures, the films disaggregate and Sn balls can be observed at their surface.

Cracks in the films appear approximately above 300°C for material deposited on glass since none can be observed when on silicon. Using a hydrogenated argon-free deposition atmosphere, cracks appear on both those substrates, but at higher temperatures (300°C) on Si than on glass (150°C).

Whatever the way temperature was applied, during or after deposition, no significant effect on electrical and optical properties of ZnSnN2 thin films were recorded.

-Material composition and target biasing:

The characteristics XRD peaks of ZnSnN2 are rather always observed for near stoichiometric or moderately Zn rich material (0.45 < x < 0.6). Highly Zn rich films seem amorphous and moderately to highly Sn rich films do not show all diffraction peaks. For 0.45 < x < 0.6, the optical bandgap ranges from roughly 2 eV down to 1.85 eV. This has been observed in the two main deposition modes that have been tested by swapping DC and RF bias conditions on Zn and Sn targets. The electrical parameters exhibit also almost the same behavior versus the Zn composition whatever the deposition mode is. It can be noticed that electron concentration decreases by two orders of magnitude, from 10 20 down to 10 18 cm -3 for highly Zn rich films.

Optoelectronic properties

Finally, the optoelectronic characteristics of ZnSnN2 have been examined by fabricating two device structures that were originally intended to constitute n-n and p-n device structures.

-Photoconductor (Structure 1): no junction exists in this structure and the device using a slightly Sn rich material, Zn0.47Sn0.53N2, exhibited the highest photocurrent density and JPh/JD ratio. When Ti/Au is used as bottom electrode, the obtained dark current density is very high. It is mainly attributed to gold diffusion inside the material. In addition, since technology was used on those samples, several electrode spacings were fabricated of which the electrical characterization revealed that a spacing-dependent conductivity value.

o Photodiode (Structure2):

-p-SnO/n-ZnSnN2/AZO structure: the J-V characteristics did not relate a rectifying behavior and is rather similar to that of a resistance. The JPh/JD ratio equals to 0.36 (at 5V bias) for samples with a 600nm-thick SnO layer.

-p-Si/n-ZnSnN2/AZO structure: on this structure that involves transverse carrier transport, some difference was observed depending the target bias that was used. The behavior is almost ohmic in ZnDCSnRF mode while it looks like exhibiting some rectifying behavior in ZnRFSnDC mode. Nevertheless, curves always pass through the 0,0 point and no particular photovoltaic effect can be pointed out.

OPENING

The rather exhaustive parametric study of ZnSnN2 electrical properties versus the deposition conditions did not show obvious drastic behavior modifications except on carrier density that can sometimes be significantly reduced.

The measurement of the photoelectric behavior cannot, right now, be correlated to the electrical parameters obtained during this parametric study. Such measurements involved the development of technological steps in order to build photodetector devices. Contrary to the parametric study that was carried out on as deposited films and so using longitudinal carrier transport conditions, those structures implied transverse carrier transport designs.

The results obtained on those latter, by my own characterizations but also by a deeper study at one OPERA project partner (GeePs), showed a completely different electrical behavior:

transverse carrier conduction exhibits several orders of magnitude higher in resistivity values than longitudinal one. Other material studies conducted in another OPERA project partner (IJL) [XX] showed that oxygen atoms are present in the film. They can be included during deposition or absorbed in the film after deposition. The low resistivity values that are measured can then express the existence of a parallel conductive oxide film that shortcuts the ZnSnN2 material in longitudinal transport devices. IJL partner has the possibility implementing the sputtering deposition technique using a substrate bias and showed that using such a technique reduces significantly the porosity of the film avoiding post deposition oxygen absorption. Next step will be then to coupe such a deposition technique with the technology developed her to build transverse carrier transport structures.

The measurement of the photoelectric properties of ZnSnN2 is at the very beginning. In this work, we cannot succeed obtaining a diode functionality, although some photocurrent was recorded.

Realizing a junction is still a challenge using this material. The high carrier density levels that have been observed seem to kill any hope achieving an homojunction, p-type material being difficult to fabricate (some amphoteric behavior has been recorded in longitudinal transport devices but this has not been clarified right now). If any junction could potentially be made, it might most probably be a heterojunction. The preliminary results we obtained using p-Si could be a route. The use of hydrogen in deposition atmosphere that seems hindering the formation of donor defects, such as nitrogen vacancies VN and unintentional ON impurities has not been tested in transverse carrier transport devices yet. It can also constitute a route achieving higher photoelectric performance and, may be, allow some promise to reverse doping polarity. Post-deposition implantation of species, particularly of group III elements (such as B, Al, Ga), has also shown some potentiality to decrease the free carrier density.

In the OPERA project, and in collaboration with another partner of the project (CEA-INES), owing to its high carrier concentration and wide bandgap, ZnSnN2 has been investigated as selective electrode for silicon heterojunction (SHJ) solar cells. First preliminary experiments have been made that way depositing ZnSnN2 on top of a Si cell.

Right now, sputtering damage on the Si cell passivation layer is prohibitive but such experiments are in their infancy.

List of Abbreviations

AZO

ABSTRACT

By literature reports, ZnSnN2 has been pointed out as a potential high bandgap semiconductor material. We particularly investigated on it as an earth abundant and low environmental footprint material for photovoltaic application, and more specifically as a thin film inorganic top absorber for tandem cells based on silicon bottom cell. In this work, ZnSnN2 has been deposited using magnetron sputtering and a parametric study of the influence of its deposition parameters on the material properties has been carried out. The effect of sputtering atmosphere, pressure, temperature and composition were so investigated. The crystal structure of the films has been characterized by X-ray diffraction (XRD) and their composition by energy-dispersive X-ray spectroscopy (EDS). Electrical characterization has been made using Hall Effect measurements and optical properties determined from UV-Vis spectroscopy. Wide variations of carrier density and optical bandgap have been experimentally observed, from 10 17 to of 10 20 cm -3 and from 1.6 to 2.3 eV, respectively. The photoelectric characteristics of ZnSnN2 have been assessed by fabricating photoconductor and photodiode structures. Although some photocurrent could have been retrieved under 1 sun solar illumination, no photodiode behavior has been obtained. Key words: ZnSnN2, earth abundant II-IV-N2 material, magnetron sputtering, parametric study, photovoltaics

RESUME

Les résultats théoriques publiés sur le matériau ZnSnN2 le font apparaitre comme un matériau semiconducteur à grand gap. Dans ce travail, ses qualités de matériau abondant et à faible empreinte écologique nous ont particulièrement intéressés, en particulier, pour son utilisation en tant que matériau inorganique en film mince pour la réalisation de l'absorbeur de la cellule supérieure d'une cellule tandem dont la cellule inférieure serait en silicium. Nous avons réalisé une étude paramétrique de l'influence des conditions de dépôt, atmosphère, pression, température par pulvérisation cathodique et de la composition ainsi obtenue sur les propriétés cristallines, électriques et optiques des films. La cristallographie a été analysée par diffraction de rayons X (DRX) et la composition par spectroscopie de rayons X à dispersion d'énergie (EDX). Des mesures par Effet Hall ont permis d'obtenir les caractéristiques électriques et la spectroscopie UV-Visible l'évaluation du gap optique. En fonctions des paramètres de dépôt, de grandes variations à la fois sur la densité de porteurs, de 10 17 to of 10 20 cm -3 , et sur la valeur du gap optique, de 1.6 to 2.3 eV ont été obtenues. Des structures de type photoconducteur et photodiode ont été fabriquées pour évaluer les propriétés photoélectriques. Bien qu'un photocourant ait pu être mesuré, aucune fonctionnalité de type "photodiode" n'a pour l'instant pu être mise en évidence.

Mots clefs: ZnSnN2, matériau abondant II-IV-N2, pulvérisation cathodique, étude paramétrique, photovoltaïque

  smile. I would like to thank Mathieu HALBWAX, Cathy SION, Boubakeur AYACHI for being my tutorials during my first days in IEMN. They had supported me with knowledge from theory to practice. Thanks to Guillaume COCHEZ, Christophe BOYAVAL, Florent BLANCHARD, Marc DEWITTE, Isabelle ROCH-JEUNE, Pascal TILMANT, Francois VAURETTE, David GUERIN, Vanessa AVRAMOVIC and Etienne OKADA for their technical help. I am very grateful to the collaborators in the OPERA project: Jean Lamour Institute (JLI), The Group of electrical engineering of Paris (GeePs) and The National Institute for Solar Energy (INES). I would like to thank Nora BENBAHLOULI and Andy LEDENT who are always available and have helped me in all the administrative steps and procedures. I would like to thank the committee members, Mireille RICHARD and Philippe DANIEL who reviewed my work as Wilfried FAVRE and Christophe LETHIEN who examinated it.INTRODUCTION 4

Figure I. 2

 2 shows the well-known panorama of research photovoltaic cell efficiency record versus time made by the National Renewable Energy Laboratory (NREL).

Figure I. 2 :

 2 Figure I. 2: Record Lab-Cell Power Conversion Efficiencies (from NREL)[START_REF]Best Research-Cell Efficiency Chart | Photovoltaic Research[END_REF]

(

  Figure I.3).

Figure I. 3 :

 3 Figure I. 3: Schematic of the tandem cell including two elementary cells using materials with different bandgap. Light with higher energies (lower wavelengths) is absorbed in the higher bandgap top cell. Light with lower energies goes through and is absorbed in the bottom cell (lower bandgap material).

Figure I. 4 :

 4 Figure I. 4: Top cell conversion efficiency required for a c-Si bottom-cell based tandem cell to reach a conversion efficiency of η = 25% (blue solid curve), 27.5% (green dashed curve), and 30% (red solid curve). The shaded area indicates areas where η < 25%. Dots indicate experimental cell efficiencies reported for several high bandgap materials. Inset: output generated power of the bottom cell when the top cell absorbs all photons with energy above the bandgap [12].

Figure I. 5 :Figure I. 5

 55 Figure I. 5:Theoretical and experimental bandgap of ternary zinc nitride[START_REF] Hinuma | Discovery of earth-abundant nitride semiconductors by computational screening and high-pressure synthesis[END_REF]

Figure I. 7 :

 7 Figure I. 7: (a) Schematic of the predicted wurtzite-derived structure of ZnSnN2 including two possible orthorhombic structures, Pmc21 or Pna21, (b) Band structure and (c) electronic density of states of orthorhombic Pna21 ZnSnN2[START_REF] Lahourcade | Structural and optoelectronic characterization of RF sputtered ZnSnN 2[END_REF]

Figure I. 8 :

 8 Figure I. 8: The yellow regions provide stabilized single-phase ZnSnN2 (a) and ZnGeN2 (b),

Figure I. 9 (

 9 b).

Figure I. 9 :

 9 Figure I. 9: (a) Simulated powder XRD patterns of ZnSnN2 [24]; (b) Bandgap versus free

Figure II. 1 .

 1 According to the laws of * 1 u = 1 Da (Dalton) = 1.66053906660 x 10 -27 kg.

Figure II. 3 : 4 .

 34 Figure II. 3: Due to the magnetron sputtering process, a circular pattern of erosion is created on targets as they are used (example of Sn target).

Figure II. 4 :

 4 Figure II. 4: Thickness and corresponding sputtering rate in DC and RF modes for: a) Sn and b) Zn (Ar flow rate was 10 sccm and deposition pressure 2.10 -2 mbar).

( 7 . 4 ,

 74 625 g/cm3 ) and Zn (7.14 g/cm3 ), and M Sn and M Zn the atomic mass of Sn(118.71 u) and Zn (65.37 u), respectively. The ZnxSn1-xN2 thin film is considered as stoichiometric when the atomic percentage of Zn and Sn are equal (x=0.5). Combined with the sputtering yield rates derived from Figure II. the stoichiometry for ZnSn could be achieved, for example, by those configurations: -ZnDCSnRF configuration: 25 W DC applied to Zn; 40 W RF applied to Sn.

  power supplies were then 25 W-DC and 35 W-RF, for Zn and Sn targets, respectively. The nitrogen gas flow rate was varied from 0 to 30 sccm with a 5 sccm step, the argon flux is fixed at 10 sccm (N2/Ar ratio is then in the range 0 to 3) and the deposition pressure is kept as 2.10 -2 mbar. Deposition process was carried out for 25 minutes to ensure sufficient thickness of the films for EDS measurements. The change in film composition and thickness are illustrated in Figure II. 5. The data at nitrogen flux =0 correspond to the metallic film of section II.4.1.

Figure II. 5 :

 5 Figure II. 5: Zn/(Zn+Sn) ratio and thickness of ZnSnN2 (ZnDCSnRF series) thin films versus nitrogen flow rate (Ar flow rate:10 sccm, deposition pressure: 2.10 -2 mbar, 25W-DC applied to Zn and 35W-RF applied to Sn targets).

  electron microscopy (SEM) was employed to investigate the surface morphology of the ZnSnN2 films, views are shown in Figure II. 6.

Figure II. 6 :

 6 Figure II. 6: Top surface views of ZnDCSnRF series under different N2/Ar ratio deposition atmospheres.

Figure II. 7 :

 7 Figure II. 7: XRD diffractograms of the ZnSnN2 films (ZnDCSnRF series) deposited under different

  Carrier concentration and mobility were characterized by Van der Pauw resistivity measurement method and Hall voltage measurements. In order carrying out these characterizations, samples are cut into 1cm×1cm pieces and indium balls are used as electrodes. They are deposited at the four corners of those pieces and annealed at 200°C in 5%H2/N2 atmosphere during 4 minutes (see Figure II. 8 right).

Figure II. 8 :

 8 Figure II. 8: I-V curves obtained from Van Der Pauw resistivity measurements of sample N2/Ar = 2.5 (insert shows the way contacts are taken).

Figure II. 9 :

 9 Figure II. 9: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnDCSnRF series (determined from Hall effect measurements) versus N2/Ar ratio.

Figure II. 8

 8 Figure II.8 shows typical results of I-V measurements where the ohmicity of contacts can be noticed. The results are shown in Figure II. 9. The resistivity of ZnSnN2 decreases by two decades as the nitrogen content increases (ranging from 27.47 down to 0.16 Ω.cm).

Figure II. 10 :

 10 Figure II. 10: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF samples deposited under varied N2/Ar ratios, b) Corresponding optical bandgap values calculated using Tauc's relation.

Figure II. 11 :

 11 Figure II. 11: Zn/(Zn+Sn) ratio and sputtering rate of ZnSnN2 (ZnRFSnDC series) thin films versus nitrogen flow rate (Ar flow rate:10 sccm, deposition pressure: 2.10 -2 mbar, 35W-RF applied to Zn and 50W-DC applied to Sn targets).

Figure II. 12 :

 12 Figure II. 12: Top view of ZnRFSnDC series deposited at different N2/Ar ratios.

Figure II. 14 :

 14 Figure II. 14: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnRFSnDC series (determined from Hall effect measurements) versus N2/Ar ratio.

Figure II.15a shows

  Figure II.15a shows a shift of the cut-off in transmittance towards lower wavelengths (blue shift) as the nitrogen flux increases, the optical bandgap is calculated using Tauc's relation (2.7) (Figure II.15b). It increases from 1.59 eV to 1.95 eV when N2/Ar ratio increases from 1.0 to 4.0. So even with a slight variation (decrease) of carrier density, a rather large increase of optical bandgap is observed that is moreover shifting the other way it should have done if the Burstein-Moss effect was its origin.

Figure II. 15 :

 15 Figure II. 15: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC samples deposited under varied N2/Ar ratios , b) Corresponding optical bandgap values calculated using Tauc's equation.

Figure II. 16 :

 16 Figure II. 16: Sputtering rate and Zn/(Zn+Sn) ratio versus 0.05HN/Ar ratio.

II. 6 )

 6 by the granular structure obtained for middle range of N2/Ar ratios. Moreover, for those ratios, the density of voids appears much less than on the pure nitrogen case.

Figure II. 17 :

 17 Figure II. 17: Top surface SEM images of ZnDCSnRF series deposited in different mixed 0.05HN/Ar environments.

(

  Figure II. 16 shows all samples are Zn-rich) and hydrogen, activating the ZnSn acceptor.This situation hinders the formation of donor defects, such as nitrogen vacancies VN and unintentional ON impurities. Such a behavior is consistent with the analysis of Fioretti et al.[START_REF] Fioretti | Effects of Hydrogen on Acceptor Activation in Ternary Nitride Semiconductors[END_REF].

Figure II. 18 :

 18 Figure II. 18: Resistivity (R), carrier mobility (µ) and carrier density of ZnSnN2 films (determined from Hall effect measurements) versus 0.05HN/Ar ratio.

Figure II. 19 :

 19 Figure II. 19: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF samples deposited under varied 0.05HN/Ar ratios, b) Corresponding optical bandgap values calculated using Tauc's equation.

Figure II. 20 :

 20 Figure II. 20: Sputtering rate and Zn/(Zn+Sn) ratio versus H2/N2 ratio.

Figure II. 21 :

 21 Figure II. 21: Top surface SEM images of ZnDCSnRF series deposited using different H2/N2 ratios.

Figure II. 22 :

 22 Figure II. 22: XRD patterns of the ZnSnN2 films deposited in different percentage of hydrogen

Figure II. 23 :

 23 Figure II. 23: Resistivity (R), carrier mobility (µ) and carrier density of ZnDCSnRF series (determined from Hall effect measurements) versus H2/N2 ratio.

Figure II. 23

 23 Figure II.[START_REF] Punya | Quasiparticle band structure of Zn-IV-N2 compounds[END_REF] shows the electrical properties. The resistivity increases with the hydrogen concentration and a higher value is also observed at 3% concentration as observed for the previous case, i.e. with a mix hydrogen, nitrogen, argon atmosphere. This similarity could indicate that it could be rather a trend than a singular experimental point. The mobility varies from 0.1 to 1.3 cm 2 V -1 s -1 , the highest value obtained at 2% H2/N2. The electron density decreases 2 orders of magnitude as the hydrogen concentration increases as observed in the previous case. At 0.5% H2/N2, electron density is not significantly different from that of samples deposited in pure nitrogen. But at hydrogen ratio of 1%, the carrier

Figure II. 24 :

 24 Figure II. 24: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF samples deposited under varied H2/N2 ratios sputtering atmospheres, b) Corresponding optical bandgap values calculated using Tauc's equation.

  Figure II. 27 shows the change in crystallinity states of the different series. For NH samples, XRD data show only four peaks located at 2θ equal to 30.1°, 34.0°, 53.4° and 62.6°, corresponding to (200), (121), (320) and (322) planes, respectively. Meanwhile, N samples have five peaks located at 2θ equal to 32.4°, 34.3°, 53.6°, 59.2° and 65.3°

Figure II. 27 :

 27 Figure II. 27: Comparison of XRD diffractograms of the ZnSnN2 films deposited in different sputtering environments.

Figure II. 28 :

 28 Figure II. 28: Resistivity (R), carrier mobility (µ) and carrier density of ZnSnN2 films (determined from Hall effect measurements) depending on their sputtering environment.

Figure II. 29

 29 Figure II. 29 shows the UV-Vis spectra of the different samples and the deduced optical bandgap using Tauc's relation (2.7). This bandgap is 1.94, 2.04 and 2.19 eV corresponding to N, NA and HN series, respectively. Without hydrogen, bandgap values are rather similar.

Figure II. 29 :

 29 Figure II. 29: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF series deposited under varied sputtering atmospheres, b) Corresponding optical bandgap values calculated using Tauc's equation.

  4.2).For all sputtering atmospheres containing argon (see Figures II. 5, II. 11 and II.[START_REF] Hu | Sub-1.4eV bandgap inorganic perovskite solar cells with long-term stability[END_REF]), the deposition rate decreases as the nitrogen concentration (from 0 up to some percents) increases. This could be linked to the formation of SnN2 or ZnN2 compounds at target surface that are heavier species to sputter than pure metallic ions. When no argon is involved (H2/N2 atmosphere, Figure II. 20), the deposition rate is almost constant whatever the H2/N2 ratio is (in the 0.5-5% range that is been studied). Following the same argumentation than above, i.e. taking into account the respective mass of the species that are present inside the sputtering atmosphere, the lighter mass of hydrogen (1 u) does not modify the sputtering process that is govern by the nitrogen ions.Using a mixed N2/Ar sputtering atmosphere, whatever the power biasing of targets is, Zn concentration of films increases as nitrogen concentration increases (see Figures II.[START_REF]Photovoltaic Roadmap (ITRPV): Eleventh Edition Online[END_REF] and II. 11). The N2 flux shall at least be the same that the Ar flux (N2/Ar >1) in order the film not to ne amorphous (see Figures II.[START_REF] Slade | 27.6% Efficient Silicon Concentrator Solar Cells for Mass Production[END_REF] and II. 13). However, after a switchover point in the composition of the mixed N2/Ar atmosphere that depends on power biasing conditions, the Zn concentration of films tends to stabilize.From the XRD measurements, it is rather impossible to discern the crystal structure of the material, orthorhombic or wurtzite, since both of them exhibit an almost identical spectrum (see Figure II. 7). Since the orthorhombic structure is reported as the most stable one, we, as many authors, assume that it was the case here. All samples fabricated in an argon containing atmosphere show the characteristics peaks of ZnSnN2 (see Figures II.[START_REF] Slade | 27.6% Efficient Silicon Concentrator Solar Cells for Mass Production[END_REF] and II. 13). The lattice parameters (see Tables II. 1 and II. 2) are not really impacted by the N2/Ar ratio or the target bias arrangement. Without argon, only 2 main diffraction peaks are observed as it has been reported also for materials fabricated under pure nitrogen atmosphere. Top views of films show a cauliflower-like structure for almost all cases (see Figure II. 6, II. 12, II. 17, II. 21). Only sputtering in pure N2 atmosphere exhibits a completely different structure (see Figure II. 26).

  e the ZnDCSnRF one (see Figures II.[START_REF] Sai | Stabilized 14.0%-efficient triple-junction thinfilm silicon solar cell[END_REF] and II. 10), all other subsequent experiments do not support this. Same range of variation of bandgap has been obtained for the ZnRFSnDC (see Figures II.[START_REF] Kayes | 27.6% Conversion efficiency, a new record for single-junction solar cells under 1 sun illumination[END_REF] and II. 15) where the carrier concentration is slightly changing but the reverse way of the bandgap variation if the Burstein-Moss effect was applying. Nevertheless, except the points corresponding to the first investigated N2/Ar ratios of the ZnDCSnRF configuration and the measurement accuracy, a huge bangap variation (from 1.6 to 2 eV) should have been linked to a small carrier concentration modification (from 1 to 5 10 20 cm -3 ).Thinks get shaky when hydrogen is added since with a decrease of carrier concentration by two decades (see Figures II.[START_REF] Hinuma | Discovery of earth-abundant nitride semiconductors by computational screening and high-pressure synthesis[END_REF] and II. 19 or Figures II. 23 and II. 24), the optical bandgap increases or remains almost stable. Same trend is also observed on the small comparative series depicted in section II. 7 where the optical bandgap value varies inversely to the carrier concentration (see Figures II. 28 and II. 29).

  Zn and Sn targets, respectively. Argon flux was 10 sccm, nitrogen flux was 20 sccm (N2/Ar ratio = 2), sputtering time was 11.5 minutes. All the samples were deposited on Eagle XG substrate. The effects of pressure on thin film composition and sputtering rate are shown in Figure III.1. At lowest pressure (0.01 mbar), the material is Zn-rich (54% of Zn). When the pressure increases, the films become Sn-rich, Zn/(Zn+Sn) is in range 48% to 49%. Whereas the composition does not change significantly while pressure increases from 0.02 to 0.05 mbar, the sputtering rate decreases by nearly 2 times.

Figure III. 1 :

 1 Figure III. 1: Zn/(Zn+Sn) ratio and sputtering rate via sputtering pressure (ZnRFSnDC series).

Figure III. 2 :

 2 Figure III. 2: SEM images, top-views (on the left hand side) and cross-sections (on the right hand side) of ZnSnN2 thin films (ZnRFSnDC series) deposited at pressure of 0.01 to 0.05 mbar.

Figure III. 3 :

 3 Figure III. 3: XRD diffractograms of ZnSnN2 films (ZnRFSnDC series) deposited at different sputtering pressure.

Figure III. 4 :

 4 Figure III. 4: Calculated strain,, of ZnSnN2 films (ZnRFSnDC series) deposited on Eagle XG substrate via versus the sputtering pressure (N2/Ar =2).

Figure III. 5 :

 5 Figure III. 5: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC samples deposited under varied deposition pressures (N2/Ar = 2), b) Corresponding optical bandgap values calculated using Tauc's equation.

Figure III. 6 :

 6 Figure III. 6: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnRFSnDC series (determined from Hall measurements) versus deposition pressure.

  Argon flux and nitrogen flux were fixed at 10 sccm and 20 sccm, respectively. The sputtering pressure was 0.02 mbar, and rotation speed was 20 rpm. All samples films were deposited on Eagle XG substrates and the process time was 11.5 minutes.The variations of thin film composition and sputtering rate versus the sputtering temperature are shown in Figure III.7. At room temperature, the obtained ZnSnN2 is almost stoichiometric (50% of Zn). When the substrate temperature is increased, the films become Zn-rich, Zn/(Zn+Sn) is in range 52% to 53%. This result shows the same behavior as reported by Fioretti et al[START_REF] Fioretti | Combinatorial insights into doping control and transport properties of zinc tin nitride[END_REF]. Moreover, the deposition rate curve exhibits a rather Ushape behavior with a minimum at 200°C. Its variation is somewhat limited with a sputtering rate value of 27.8  0.8 nm/min over the full temperature range.

Figure III. 7 :

 7 Figure III. 7: Zn/(Zn+Sn) ratio and sputtering rate via sputtering pressure.

Figure III. 8 :

 8 Figure III. 8: SEM images, top-views (on the left hand side) and cross-sections (on the right hand side) of ZnSnN2 thin films deposited at different temperatures, from room temperature up to 400°C.

Figure III. 9 :

 9 Figure III. 9: XRD diffractograms of the ZnSnN2 films deposited at different temperatures, from 25°C to 400°C (Note: the 25°C curve corresponds to the N2/Ar curve of Figure II.13).

Figure III. 10 :

 10 Figure III. 10: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnRFSnDC series (determined from Hall measurements) versus substrate temperature.

Figure III. 11 :

 11 Figure III. 11: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC samples deposited at different substrate temperature, b) Corresponding optical bandgap values calculated using Tauc's equation.

  ZnDCSnRF mode and under N2/Ar atmosphere (ZnDCSnRF -NA) III.3.1.1 Deposition of ZnDCSnRF-NA thin films on Eagle XG SubstratesIn this section, 25W-DC and 70W-RF supplies were applied to Zn and Sn targets, respectively. The operating pressure in the chamber was fixed at to 2.10 -2 mbar, the argon flux to 10 sccm, and the nitrogen flux to 20 sccm. This is the same set of process parameters that has been used in section II.4.2.1 with N2/Ar value of 2. The process time was 25 minutes. The as-deposited ZnSnN2 thin film obtained nearly stoichiometry (approximately 49.8% Zn/(Zn+Sn) ratio) and its thickness was 440 nm. For the post-deposition annealing step, the temperature has been varied from 100 o C to 550 o C with a 50 o C step in pure N2 atmosphere (300 sccm). The annealing time was 15 minutes and the heating-up/coolingdown rate was 100 o C/min. The change in the composition of the films was recorded using EDX and its thickness was measured (Figure III. 12). Below 500 o C, the Zn/(Zn+Sn) ratio decreases monotonously in the range 49.8% down to 48.1% but increases drastically up to 64.7% at 550 o C. Such a trend has not been recorded on the thickness that is rather stable, taking into account the measurement uncertainties, in the 420 to 440 nm range.

Figure III. 12 :Figure III. 13 :

 1213 Figure III. 12: Zn/(Zn+Sn) ratio and thickness of ZnDCSnRF-NA samples deposited on Eagle XG substrate versus post-deposition annealing temperature.

Figure III. 14 :

 14 Figure III. 14: SEM view (left hand side) and EDX mapping (right hand side) of ZnDCSnRF-NA thin film on glass post-annealed at 550°C.

  Bottom right picture presented in Figure III. 13 could effectively suggest the formation of ZnO particles.

Figure III. 15 :

 15 Figure III. 15: XRD diffractograms of ZnDCSnRF-NA series thin films on glass for different postannealing temperatures: a) from 100°C to 500°C, b) 550°C sample data spectrum compared to XRD data of Sn on red peaks and ZnO on blue peaks

Figure III. 16

 16 Figure III.[START_REF] Hu | Sub-1.4eV bandgap inorganic perovskite solar cells with long-term stability[END_REF] shows the strain of ZnDCSnRF-NA thin films deposited on glass calculated from XRD data and equation(3.6). The strain of as-deposited sample is 5.3x10 -3 , it increases to 7.0x10 -3 as annealing temperature rises 100°C. At higher, the strain that should intuitively have been increased due to the difference in TEC value of ZnSnN2 and glass does not exhibit a clear trend. The apparition of cracks in the films that can be observed on SEM pictures of Figure III. 13 shall so have a stress-relief effect. The average crystal size (D), evaluated from the (200) peak width using the Scherrer formula (2.6), is in range 14.5 to 21.3 nm, so in the same order of values than on previous experiments (cf. section III. 1).

Figure III. 17 :

 17 Figure III. 17: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF-NA series deposited on glass at room temperature and post-annealed at different temperature, b) Corresponding optical bandgap values calculated using Tauc's equation.

Figure III. 18 :

 18 Figure III. 18: Resistivity (R), carrier mobility (µ) and carrier density of ZnSnN2 films (determined from Hall effect measurements) depending on the post-annealing temperatures.

Figure III. 19 :

 19 Figure III. 19: SEM images of ZnDCSnRF-NA thin films on Si (100) after RTA treatment (the scale of 450° and 500°C sample images is 4 times greater compared to others).

Figure III. 20 : 4 .

 204 Figure III. 20: XRD diffractograms of ZnDCSnRF-NA thin films on Si (100) for different postannealing temperatures: a) from 100°C to 400°C, b) 450°C and 500° data compared to XRD data of Sn on red peaks.

Figure III. 21

 21 Figure III.[START_REF] Punya | Electronic and lattice dynamical properties of II-IV-N2 semiconductors[END_REF] shows the strain of ZnDCSnRF-NA thin films on Si (100) calculated from XRD data and equation(3.6). The strain value and particularly the average crystallite size, evaluated from the (200) peak width using the Scherrer formula (2.6), exhibit less variation than on glass substrate.

Figure III. 21 :

 21 Figure III. 21: Strain and crystal size of ZnDCSnRF-NA series thin films deposited on Si(100) and post-annealed from 100°C to 500°C.

Figure III. 22 :

 22 Figure III. 22: SEM images of DCZnRFSn-HN thin films on Eagle XG after RTA treatment (note: magnification of 500°C sample is 4-time smaller compared to others)

Figure III. 23 :

 23 Figure III. 23: XRD diffractograms of ZnDCSnRF-HN thin films on Eagle XG substrate for different annealing temperatures: a) from 100°C to 400°C, b) 500°C data compared to XRD data of Sn on red peaks.

Figure III. 24 :

 24 Figure III. 24: Crystal size ZnDCSnRF-HN thin films on Eagle XG Substrate after annealing from 100°C to 450°C

Figure III. 25 :

 25 Figure III. 25: SEM images of ZnDCSnRF-HN thin films on Si (100) after RTA treatment from 100°C to 500°C

  The average crystal size (D), evaluated from the (200) peak width using the Scherrer formula (2.6), is in the range 15.4 to 22.8 nm and decreases with the increase of the annealing temperature (Figure III. 27).

Figure III. 27 :

 27 Figure III. 27 : Crystal size of (200) plane of DCZnRFSn-HN thin films on Eagle XG Substrate after annealing from 100°C to 450°C

range from 100

 100 Figure III. 28. This figure has to be compared with Figure III. 12. The Zn/(Zn+Sn) ratio is rather stable, in the range 48.1% to 49.8%. The deposition rate shows a small trend increasing with the increase in temperature. So, no significant variation is observed on those two parameters when changing the post-anneal temperature.

Figure III. 28 : 29 .

 2829 Figure III. 28: Zn/(Zn+Sn) ratio and thickness of ZnRFSnDC-NA samples deposited on Eagle XG glass versus post-deposition annealing temperature (N2 atmosphere).

Figure III. 31 :

 31 Figure III. 31 : Strain and crystal size of (200) plane of ZnRFSnDCRFZnDCSn-NA thin films onEagle XG substrate after annealing from 100°C to 500°C.

Figure III. 16 )

 16 by order of magnitude. The average crystal size (D), evaluated from the (200) peak width using the Scherrer formula (2.6), is in the range 17.6 to 18.9 nm, so rather the same value than in the ZnDCSnRF-NA case (cf. Figure III. 16). The UV-Vis spectra and optical bandgap of ZnRFSnDC-NA thin films on glass after annealing from 100°C to 550°C are reported in Figure III. 32. As the temperature is in the range in 100°C to 300°C, the anneal does not affect at all the optical properties of the material. The optical bandgap is equal to 1.70 eV. At 400°C, this value increases to 1.83 eV.

Figure III. 32 :

 32 Figure III. 32: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC-NA series deposited on glass at room temperature and post-annealed at different temperature, b) Corresponding optical bandgap values calculated using Tauc's equation.

Figure III. 33 :

 33 Figure III. 33: Resistivity (R), carrier mobility (µ) and carrier density of ZnRFSnDC-NA series (determined from Hall effect measurements) depending on the post-annealing temperature.

  sputtering pressure leads to increase the optical bandgap (see Figure III. 5), the carrier mobility and decreasing the carrier density; the latter two by two orders of magnitude (see Figure III. 6). As observed in Chapter II, this is contrary to a Burstein-Moss effect.

  The corresponding XRD patterns are reported in Figure IV. 2. As 0.27 ≤ x ≤ 0.34, there are only 3 peaks corresponding to (200), (121) and (320) planes and located at 30.1°, 34.3° and 53.6°, respectively. At x = 0.42, small peak locates at position 32.2°, indicating the (002) plane is recorded. For 0.46 ≤ x ≤ 0.50, XRD data match all the seven characteristic peaks of ZnSnN2: this corresponds to the case recorder in section II.4.2.1 were stoichiometry was rather sought for. The change in grain shape observed on SEM images (see Figure IV. 1) for 0.54 ≤ x ≤ 0.60 connects to the disappearing of (200) and (320) peaks in XRD patterns. When x value increases up to 0.67, XRD data illustrate the seven characteristic peaks of ZnSnN2, but the crystalline quality degrades with an enlarging of peaks. At x = 0.74, no particular order appears and the ZnSnN2 thin film reports as of amorphous nature.

Figure IV. 1 :

 1 Figure IV. 1: SEM surface images of the ZnxSn1-xN2 (ZnDCSnRF series) thin films with 0.27 ≤ x ≤ 0.74.

Figure IV. 2 :Figure IV. 3 )

 23 Figure IV. 2: XRD diffractograms of ZnxSn1-xN2 (ZnDCSnRF series) thin film with 0.27 ≤ x ≤ 0.74.

Figure IV. 3 :

 3 Figure IV. 3: a) Lattice parameters and b) crystal size of (121) plane calculated from XRD data of ZnxSn1-xN2 thin film with 0.27 ≤ x ≤ 0.74 (ZnDCSnRF series).

Figure IV. 4 :

 4 Figure IV. 4: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 (ZnDCSnRF series) thin film with 0.27 ≤ x ≤ 0.74.

Figure

  Figure IV. 5a shows a shift of the cut-off in transmittance towards higher wavelengths (red shift) as the Zn composition increases. Correspondingly, the optical bandgap (Figure IV. 5b) is primarily reducing, from 2.06 eV to 1.77 eV.

Figure IV. 5 :

 5 Figure IV. 5: a) Transmittance vs. wavelength and b) Optical bandgap of ZnxSn1-xN2 thin film with 0.27 ≤ x ≤ 0.74, fabricated in ZnDCSnRF mode.

Figure IV. 6

 6 shows the Zn composition and the sputtering rate versus the DC power applied to the Sn target. The sputtering time was optimized to get the thickness of films approximately to 300 nm, whatever the bias conditions were.

Figure IV. 6 :

 6 Figure IV. 6: Composition and sputtering rate via DC power bias of Sn of ZnxSn1-xN2 (ZnRFSnDC series) thin film (attention: x axis is reversed).

Figure IV. 7 :

 7 Figure IV. 7: Top-view images of the ZnxSn1-xN2 thin films with 0.45 ≤ x ≤ 0.60 (ZnRFSnDC series).

Figure IV. 8 :

 8 Figure IV. 8: XRD diffractograms of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.60, fabricated in ZnRFSnDC mode.

Figure IV. 9 :

 9 Figure IV. 9: a) Lattice parameter and b) crystal size of (121) plane calculated by XRD data of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.65, fabricated in ZnDCSnRF mode.

Figure IV. 10 :

 10 Figure IV. 10: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.60, fabricated in ZnRFSnDC mode.

Figure IV. 11 :

 11 Figure IV. 11: a) Transmittance vs. wavelength and b) Optical gap of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.60, fabricated in ZnRFSnDC mode

Figure IV. 12

 12 shows the Zn composition and the sputtering rate versus the DC power applied to the Sn target. Here too, the sputtering time was optimized to get a film thickness of approximately 300 nm.

Figure IV. 12 :

 12 Figure IV. 12: Composition and sputtering rate via Sn power of ZnxSn1-xN2 (ZnDCSnDC series) thin films.

Figure IV. 13 14 )

 1314 Figure IV. 13 illustrates the surface morphology of ZnxSn1-xN2 thin films, fabricated in ZnDCSnDC mode. For x ≤ 0.53, the difference of surface morphologies is difficult to assess and presents a cauliflower-like surface. For x = 0.60, the surface morphology changes to elongated grains with voids as observed in the previous cases for such x values. For x = 0.55, surface morphology appears as a kind of transition between cauliflower-like structure and elongated grains. For Sn-rich samples (x< 0,5), the XRD diffractograms (Figure IV. 14) shows three dominant peaks located at 30.2°, 34.2° and 53.6°, related to (200),[START_REF] Tynell | Atomic layer deposition of Al-doped ZnO thin films[END_REF] and (320) planes, as rather observed on previous deposition modes. For stoichiometric samples, the seven characteristic peaks of ZnSnN2[START_REF] Ye | Preparation, Characterization, and Device Applications of Zinc Tin Nitride and Zinc Tin Oxynitride Materials[END_REF] are recorded. For Zn-rich samples, the disappearing of (200) and (320) planes rather follow previous observations and can be clearly correlated to the change in surface morphology that are observed by SEM (see Figure IV. 13).

Figure IV. 13 :

 13 Figure IV. 13: Top-view SEM images of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.60.

Figure IV. 14 :

 14 Figure IV. 14: XRD diffractograms of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.60.

Figure IV. 15 :

 15 Figure IV. 15: a) Lattice parameter and b) crystallite size calculated from XRD data ((121) plane for crystallite size) of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.65.

Figure IV. 16 :

 16 Figure IV. 16: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.60.

Figure IV. 18 :

 18 Figure IV. 18: Composition and sputtering rate via DC power of ZnxSn1-xN2 thin films, (ZnSnDC series).

Figure IV. 19 :

 19 Figure IV. 19: Top-view SEM images of the ZnxSn1-xN2 (ZnSnDC series).

Figure IV. 20 :

 20 Figure IV. 20: XRD diffractograms of ZnxSn1-xN2 thin films (ZnSnDC series)via DC power.

Figure IV. 21 :

 21 Figure IV. 21: a) Lattice parameter and b) crystallite size calculated from XRD data ((121) plane for crystallite size) of ZnxSn1-xN2 thin films (ZnSnDC series).

Figure IV. 22 :

 22 Figure IV. 22: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 thin films (ZnSnDC series) via DC power

Figure IV. 24 : 123 Figure IV. 25 :

 2412325 Figure IV. 24: SEM image of ZnSnN2 film deposited in (a) 100W-DC mode (ZnSnDC sample) and (b) 100W-RF mode (ZnSnRF sample) from a compound target.

Figure IV. 26 :

 26 Figure IV. 26: Optical bandgap of the ZnSnN2 films deposited in DC mode (ZnSnDC sample) and RF mode (ZnSnRF sample) from a compound target.

  target. All configurations have been tested except a dual RF mode since only one RF generator is available. A compound ZnSn target has also been used in both deposition modes. Focusing on the co-sputtering experiments, the characteristics XRD peaks of ZnSnN2 are rather always observed (see Figures IV. 2, IV. 8 and IV. 14) for near stoichiometric or moderately Zn rich material (0.45 < x < 0.6). Highly Zn rich films seem amorphous (e.g. x = 0.74 in Figure IV. 2) and moderately to highly Sn rich films do not show all peaks (e.g. x < 0.45 in Figure IV. 2). The lattice parameters and the crystallite size versus composition show almost the same behavior whatever the target bias configuration is. a and b parameters are unchanged whatever the x value is, c parameter decreases by roughly 10% as film starts becoming Zn rich. The crystallite size mainly increases with Zn composition with a mean value around 10 nm (see Figures IV. 3, IV. 9 and IV. 15). The electrical parameters exhibit also almost the same behavior versus the Zn composition whatever the deposition mode is (see Figures IV. 4, IV. 10 and IV. 16). Electron concentration decreases by two order of magnitude, from 10 20 down to 10 18 cm -3 when Zn concentration increases. Particularly, Sn rich films always exhibit high carrier concentration around 10 20 cm -3 and a switchover point seems to exist at stoichiometric ratio: the two-decade decrease is so mainly observed for x values between 0.5 and 0.6. For 0.45 < x < 0.6, the optical bandgap is decreasing from roughly 2eV down to 1.85 eV, whatever the deposition mode is (see Figures IV. 5, IV. 11

Figure V. 1

 1 shows the top and cross-sectional views of AZO films. The shape of grain is random and the size is smaller than 100 nm. The cross-section image shows the microstructure is columnar.

Figure V. 1 :

 1 Figure V. 1 : SEM top (on the left) and cross-sectional (on the right) views of AZO thin films.

Figure V. 2 :

 2 Figure V. 2 : XRD pattern of AZO thin films on Eagle XG substrate.

Figure V. 3 :

 3 Figure V. 3 : UV-Vis result of AZO thin film: a) Transmittance vs. wavelength spectra, b) Optical band gap calculated using Tauc's equation (2.7).

Figure V. 4 :

 4 Figure V. 4 : SEM images of SnO thin films deposited in RF mode after post-annealing treatment at different temperatures.

Figure V. 5 : 2 .

 52 Figure V. 5: XRD diffractograms of SnO thin films deposited in RF mode versus post-annealing temperature.

Figure V. 6 :

 6 Figure V. 6 : a) Calculated strain (ε) from XRD data and b) crystal size of (101), (110) and (002)

Figure V. 6a shows

  Figure V. 6a shows the strain of SnO thin films on glass calculated by XRD data and equation (3.6). The strain increases slightly from 6.4x10 -3 to 6.9x10 -3 when the temperature

Figure V. 7 :

 7 Figure V. 7 : a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for SnO films fabricated using RF mode, as-deposited and after annealing from 100°C to 400°C:, b) Corresponding optical bandgap calculated using Tauc's equation.

Figure V. 7a shows

  Figure V. 7a shows the optical transmittance data of SnO films fabricated using RF mode

Figure V. 8 :

 8 Figure V. 8 : Resistivity (R), carrier mobility (µ) and carrier density of SnO films fabricated under RF mode versus annealing temperature.

Figure V. 8

 8 Figure V. 8 shows the electrical properties of SnO thin films. The resistivity increases from 0.97 Ω.cm to 7.16 Ω.cm. The mobility decreases from 0.39 to 0.14 cm 2 V -1 s -1 as annealing

Figure V. 9 :

 9 Figure V. 9 : SEM images of SnO thin films deposited in DC mode after post-annealing treatment at different temperatures.

Figure V. 9

 9 Figure V. 9 illustrates the effect of the annealing process on surface morphology of the SnO thin films fabricated in DC mode. There is no significant difference between samples deposited in RF and DC mode.

Figure V. 10 :

 10 Figure V. 10 : XRD diffractograms of SnO thin films deposited in DC mode versus postannealing temperature.

Figure V. 11 :

 11 Figure V. 11 : a) Calculated strain (ε) from XRD data and b) crystal size of (101), (110) and (002) plane of SnO fabricated on RF mode after annealing from 250°C to 400°C.

Figure V. 12 :

 12 Figure V. 12 : UV-Vis result of of SnO fabricated on DC mode and after annealing from 100°C to 400°C: a) Transmittance vs. wavelength spectra, b) Optical gap calculated using Tauc's equation.

Figure V. 12

 12 Figure V. 12 shows transmittance results and corresponding optical bandgap of SnO thin films deposited in DC mode. The change in properties due to the amorphous-crystalline

Figure V. 13 :

 13 Figure V. 13 : Resistivity (R), carrier mobility (µ) and carrier density of SnO films fabricated under DC mode versus annealing temperature.

Figure VI. 3 :

 3 Figure VI. 3: Structure1.1 (first fabrication run): a) Schematic cross-section b) Top view of fabricated devices with surface areas from 0.05 cm 2 to 0.25cm 2 and c) Cross-sectional SEM view

Figure VI. 4 :

 4 Figure VI. 4: Dark J-V curves of Structure1.1 devices versus their active surface area, from 0.05 cm 2 to 0.25 cm 2

Figure VI. 5 :

 5 Figure VI. 5: Structure1.1 samples a) J-V curve under 1-sunAM1.5 solar spectra illumination (100 mW/cm 2 ) at room temperature, b) JPh/JD average and photocurrent obtained at 500mV versus cell area

Figure VI. 6

 6 Figure VI. 6 shows a schematic cross-section of Structure1.2 samples as the result of their fabrication on Eagle XG substrate, the active area of each cell is 0.05 cm 2 .

Figure VI. 6 : 4 ,

 64 Figure VI. 6: a) Cross-section and b) Final view of Structure1.2 devices (0.05cm 2 cells).

Figure VI. 7 :Figure VI. 8 :

 78 Figure VI. 7: Structure1.2 samples a) dark J-V curves, b) J-V curves under 1-sun illumination and c) photocurrent density and JPh/JD values (averaged values other the 9 devices of same x composition @ 500mV bias)

Figure VI. 9

 9 Figure VI. 9 shows a schematic cross-sectional view of Structure1.3 and final view of fabricated samples on Eagle XG substrate. The active area of elementary cells is 0.05 cm 2 .Therefore, except the bottom electrode deposition process, all other fabrication and deposition steps were conducted in the same conditions that in Section VI.1.2. Samples were post-annealed at 300°C in N2 atmosphere during 15 minutes.

Figure VI. 9 :

 9 Figure VI. 9: Structure1.3 a) Schematic cross-section and b) Final view of the 0.05cm 2 surface area samples.

4 11 Figure VI. 11 :

 41111 Figure VI. 11: Structure1.4: a) Schematic cross-section and b) final view of 0.05cm 2 surface area samples.

Figure VI. 12 :

 12 Figure VI. 12: a) Dark J-V characterization of Structure1.4 samples, b) JPh -V curve under 1sun illumination for as-deposited and post-annealed-samples.

  Sn target power feeding. The thin film composition was measured by EDS and exhibited an x value of 0.48. AZO and Ti/Au were fabricated in the same parameters as in Section VI.1.1, the main steps of fabrication process are presented in Figure VI. 13.

Figure VI. 13 :

 13 Figure VI. 13: Schematic process flow of samples a) Structure1.M1 and b) Structure1.M2

Figure VI. 14 :

 14 Figure VI. 14: Top (left) and cross-sectional (right) views of a) Structure1.M1 and b) Structure1.M2 samples.

Figure VI. 15 :

 15 Figure VI. 15: Conductivity versus temperatures varying between 25°C up to 245°C of a) transverse conduction samples (Structure1.M1) and b) longitudinal conduction (Structure1.M2) samples (2mm electrode spacing). (data obtained by GeePs researchers)

Figure VI. 16 :

 16 Figure VI. 16: Conductivity versus electrode spacing for lateral transport samples (distance of Structure1.M2). (data obtained by GeePs researchers)

  /ZnSnN2 hetero junction VI.2.1.1 Fabrication of ZnSnN2 in ZnDCSnRF mode:In this section, SnO thin film was deposited on Eagle XG first following the fabrication process described in Section V.2.2.2. Deposition time was 18 minutes and the sample was post-annealed at 250°C in N2 atmosphere during 30 minutes. Then ZnSnN2 thin film was deposited at room temperature during 12.5 minutes, the working pressure was 0.02 mbar, the N2/Ar ratio was fixed at 2.0, 24W-DC and 80W-RF were used for Zn and Sn target, respectively. The EDS measurement of only the ZnSnN2 layer deposited on glass displayed slightly a Sn-rich material with a x value of 0.48. Next step consisted in the sputtering deposition of 300 nm of AZO which deposition process was presented in Section V.1.2. Finally, 20 nm of Ti and 300 nm of Au were evaporated as electrode contacts. The main steps of fabrication process are presented in Figure VI.17.

Figure VI. 17 :

 17 Figure VI. 17: Schematic process flow of p-SnO/ZnSnN2 heterojunction structure.

Figure VI. 18 :

 18 Figure VI. 18: Structure2.1 sample with Zn0.48Sn0.52N2 fabricated in ZnDCSnRF mode: a) Top design b) Cross-section design and c) Cross-sectional SEM image of p-SnO/Zn0.48Sn0.52N2/AZO

Table VI. 2 :

 2 Probe positioning for the different measurements shown in Figure IV.

Figure VI. 19 :

 19 Figure VI. 19: a) J-V characterization of Structure2.1 samples with Zn0.48Sn0.52N2 fabricated in ZnDCSnRF mode in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm 2 ) at room temperature

Figure VI. 21

 21 Figure VI. 21 illustrates the J-V characterization in dark and the JPh -V characterization under 1-sun AM1.5 solar spectra illumination. The overall behavior, ether in dark or under

Figure VI. 20 :

 20 Figure VI. 20: Cross-section SEM image of Structure2.2 (identical to Structure2.1 except an increased SnO bottom contact layer thickness).

Figure VI. 21 :

 21 Figure VI. 21: a) J-V characterization of Structure2.2 sample in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm 2 ) at room temperature

Figure VI. 22 :

 22 Figure VI. 22: Cross-section SEM image of Structure2.3: p-SnO/Zn0.48Sn0.52N2/AZO with ZnSnN2 layer deposited in ZnRFSnDC mode

Figure VI. 23 :

 23 Figure VI. 23: a) J-V characterization of Structure2.3 in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm 2 ) at room temperature

Figure VI. 24 :

 24 Figure VI. 24: Cross-section SEM image of p-SnO/Zn0.48Sn0.52N2/AZO with sputtering time of SnO increased to 27 min, and ZnSnN2 was deposited in ZnDCSnRF mode (Structure2.4).

Figure VI. 25

 25 Figure VI. 25 reports the J-V measurements. By comparison with identical structure in ZnDCSnRF mode (Structure2.2), this Structure2.4 (ZnRFSnDC mode) does not display

Figure VI. 25 :

 25 Figure VI. 25: a) J-V characterization of Structure2.4 sample in dark and b) JPh -V under 1-sun AM1.5 solar spectra illumination (100 maw/cm 2 ) at room temperature

Figure VI. 26 :

 26 Figure VI. 26: Main steps of the process flow of p-Si/ZnSnN2 heterojunction sample (Structure2.5).

Figure VI. 27 :

 27 Figure VI. 27: Structure2.5 sample with of p-Si/Zn0.48Sn0.52N2/AZO: a) top design, b) crosssection design structure, c) cross-section SEM image with ZnSnN2 fabricated in ZnDCSnRF mode and d) in ZnRFSnDC

5 Figure I. 2 : 6 Figure I. 3 : 8 Figure I. 4 : 9 Figure I. 5 : 10 Figure I. 6 : 11 Figure I. 7 : 13 Figure I. 8 : 14 Figure I. 9 : 15 Figure 24 Figure II. 2 : 25 xviiiFigure II. 3 :Figure II. 4 :

 52638495106117138149152422534 Figure I. 1: Worldwide electricity generation shares between fossil (coal), nuclear and low-carbon sources [1]......................................................................................................... 5 Figure I. 2: Record Lab-Cell Power Conversion Efficiencies (from NREL) [2] ................ 6 Figure I. 3: Schematic of the tandem cell including two elementary cells using materials with different bandgap. Light with higher energies (lower wavelengths) is absorbed in the higher bandgap top cell. Light with lower energies goes through and is absorbed in the bottom cell (lower bandgap material). ................................................................................. 8 Figure I. 4: Top cell conversion efficiency required for a c-Si bottom-cell based tandem cell to reach a conversion efficiency of η = 25% (blue solid curve), 27.5% (green dashed curve), and 30% (red solid curve). The shaded area indicates areas where η < 25%. Dots indicate experimental cell efficiencies reported for several high bandgap materials. Inset: output generated power of the bottom cell when the top cell absorbs all photons with energy above the bandgap [12]. ........................................................................................... 9 Figure I. 5: Theoretical and experimental bandgap of ternary zinc nitride [18] ................ 10 Figure I. 6: The band gaps versus "a" lattice constant for III-nitrides and Zn-IV-nitrides [22] ..................................................................................................................................... 11 Figure I. 7: (a) Schematic of the predicted wurtzite-derived structure of ZnSnN2 including two possible orthorhombic structures, Pmc21 or Pna21, (b) Band structure and (c) electronic density of states of orthorhombic Pna21 ZnSnN2 [24] ............................... 13 Figure I. 8: The yellow regions provide stabilized single-phase ZnSnN2 (a) and ZnGeN2 (b), obtained by calculated chemical potentials of the component elements (µZn, µSn, µGe, µN) [26]. ................................................................................................................... 14 Figure I. 9: (a) Simulated powder XRD patterns of ZnSnN2 [24]; (b) Bandgap versus free electron density of ZnSnN2.[28] ....................................................................................... 15

Figure III. 9 : 73 Figure III. 10 : 75 Figure 75 Figure III. 12 : 77 Figure III. 13 : 79 Figure III. 14 : 80 Figure III. 15 : 81 xxiFigure III. 16 : 83 Figure

 97310757512771379148015811683 Figure III. 9: XRD diffractograms of the ZnSnN2 films deposited at different temperatures, from 25°C to 400°C (Note: the 25°C curve corresponds to the N2/Ar curve of Figure II.13). .................................................................................................................. 73 Figure III. 10: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnRFSnDC series (determined from Hall measurements) versus substrate temperature. ..................... 75 Figure III. 11: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC samples deposited at different substrate temperature, b) Corresponding optical bandgap values calculated using Tauc's equation. ................................................ 75 Figure III. 12: Zn/(Zn+Sn) ratio and thickness of ZnDCSnRF-NA samples deposited on Eagle XG substrate versus post-deposition annealing temperature. .................................. 77 Figure III. 13: SEM images, top-views (on the left hand side. To be noted that the zoom on the 550°C sample is 20 times greater compare to others) and cross-sections (on the right hand side) of ZnDCSnRF-NA series thin films deposited at room temperature with different post-deposition rapid thermal anneals from 100°C up to 550°C. ....................... 79 Figure III. 14: SEM view (left hand side) and EDX mapping (right hand side) of ZnDCSnRF-NA thin film on glass post-annealed at 550°C. ............................................. 80 Figure III. 15: XRD diffractograms of ZnDCSnRF-NA series thin films on glass for different post-annealing temperatures: a) from 100°C to 500°C, b) 550°C sample data spectrum compared to XRD data of Sn on red peaks and ZnO on blue peaks .................. 81

Figure IV. 1 : 106 Figure IV. 2 : 107 Figure 108 Figure IV. 4 : 108 Figure 109 Figure IV. 6 : 110 Figure IV. 7 : 60 ( 111 Figure IV. 8 :

 11062107108410810961107601118 Figure IV. 1: SEM surface images of the ZnxSn1-xN2 (ZnDCSnRF series) thin films with 0.27 ≤ x ≤ 0.74. ........................................................................................................ 106 Figure IV. 2: XRD diffractograms of ZnxSn1-xN2 (ZnDCSnRF series) thin film with 0.27 ≤ x ≤ 0.74. ................................................................................................................ 107 Figure IV. 3: a) Lattice parameters and b) crystal size of (121) plane calculated from XRD data of ZnxSn1-xN2 thin film with 0.27 ≤ x ≤ 0.74 (ZnDCSnRF series). ............. 108 Figure IV. 4: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 (ZnDCSnRF series) thin film with 0.27 ≤ x ≤ 0.74. ........................................................ 108 Figure IV. 5: a) Transmittance vs. wavelength and b) Optical bandgap of ZnxSn1-xN2 thin film with 0.27 ≤ x ≤ 0.74, fabricated in ZnDCSnRF mode. ..................................... 109 Figure IV. 6: Composition and sputtering rate via DC power bias of Sn of ZnxSn1-xN2 (ZnRFSnDC series) thin film (attention: x axis is reversed). .......................................... 110 Figure IV. 7: Top-view images of the ZnxSn1-xN2 thin films with 0.45 ≤ x ≤ 0.60 (ZnRFSnDC series).......................................................................................................... 111 Figure IV. 8: XRD diffractograms of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.60, fabricated in ZnRFSnDC mode. ...................................................................................... 111 Figure IV. 9: a) Lattice parameter and b) crystal size of (121) plane calculated by XRD data of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.65, fabricated in ZnDCSnRF mode. ... 112

Figure IV. 11

 11 Figure IV. 11: a) Transmittance vs. wavelength and b) Optical gap of ZnxSn1-xN2 thin film with 0.45 ≤ x ≤ 0.60, fabricated in ZnRFSnDC mode .............................................

  Figure IV. 17: a) Transmittance vs. wavelength and b) Optical bandgap of ZnxSn1-xN2 thin films (ZnDCSnDC series)with 0.45 ≤ x ≤ 0.60. .......................................................

Figure IV. 18 :

 18 Figure IV. 18: Composition and sputtering rate via DC power of ZnxSn1-xN2 thin films, (ZnSnDC series). .............................................................................................................

Figure IV. 19 :

 19 Figure IV. 19: Top-view SEM images of the ZnxSn1-xN2 (ZnSnDC series). ...............

Figure IV. 20 :

 20 Figure IV. 20: XRD diffractograms of ZnxSn1-xN2 thin films (ZnSnDC series)via DC power. ..............................................................................................................................

Figure IV. 21

 21 Figure IV. 21: a) Lattice parameter and b) crystallite size calculated from XRD data ((121) plane for crystallite size) of ZnxSn1-xN2 thin films (ZnSnDC series). ...............

Figure IV. 22 :

 22 Figure IV. 22: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 thin films (ZnSnDC series) via DC power.......................................................................

Figure IV. 23

 23 Figure IV. 23: a) Transmittance vs. wavelength and b) Optical bandgap of ZnxSn1-xN2 thin films (ZnSnDC series) via DC power.......................................................................

Figure IV. 24 :Figure IV. 26 : 131 Figure V. 2 : 131 Figure V. 3 : 132 Figure V. 4 : 135 Figure V. 5 : 138 Figure V. 8 : 139 Figure V. 9 : 140 Figure V. 10 : 141 Figure V. 12 : 142 Figure V. 13 : 146 xxvFigure VI. 2 : 148 Figure VI. 3 : 149 Figure VI. 4 : 150 Figure VI. 5 : 151 Figure 153 Figure VI. 8 : 155 Figure VI. 9 : 156 Figure 158 Figure 159 Figure VI. 13 : 160 Figure VI. 14 : 161 Figure VI. 15 : 162 xxviFigure VI. 16 :

 242613121313132413551388139914010141121421314621483149415051511538155915615815913160141611516216 Figure IV. 24: SEM image of ZnSnN2 film deposited in (a) 100W-DC mode (ZnSnDC sample) and (b) 100W-RF mode (ZnSnRF sample) from a compound target. ...............Figure IV. 25: XRD diffractograms of ZnSnN2 deposited in DC mode (ZnSnDC sample) and RF mode (ZnSnRF sample) from a compound target. ..............................................

Figure VI. 17 :

 17 Figure VI. 17: Schematic process flow of p-SnO/ZnSnN2 heterojunction structure. .....

Figure VI. 18 :

 18 Figure VI. 18: Structure2.1 sample with Zn0.48Sn0.52N2 fabricated in ZnDCSnRF mode: a) Top design b) Cross-section design and c) Cross-sectional SEM image of p-SnO/Zn0.48Sn0.52N2/AZO ............................................................................................

Figure VI. 19

 19 Figure VI. 19: a) J-V characterization of Structure2.1 samples with Zn0.48Sn0.52N2 fabricated in ZnDCSnRF mode in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm2) at room temperature...............................................

Figure VI. 20 :

 20 Figure VI. 20: Cross-section SEM image of Structure2.2 (identical to Structure2.1 except an increased SnO bottom contact layer thickness). .........................................................

Figure VI. 21

 21 Figure VI. 21: a) J-V characterization of Structure2.2 sample in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm2) at room temperature .........................................................................................................................................

Figure VI. 22 :

 22 Figure VI. 22: Cross-section SEM image of Structure2.3: p-SnO/Zn0.48Sn0.52N2/AZO with ZnSnN2 layer deposited in ZnRFSnDC mode ........................................................

Figure VI. 23

 23 Figure VI. 23: a) J-V characterization of Structure2.3 in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm2) at room temperature ...............

Figure VI. 24 :

 24 Figure VI. 24: Cross-section SEM image of p-SnO/Zn0.48Sn0.52N2/AZO with sputtering time of SnO increased to 27 min, and ZnSnN2 was deposited in ZnDCSnRF mode (Structure2.4). ........................................................................................................

Figure VI. 25

 25 Figure VI. 25: a) J-V characterization of Structure2.4 sample in dark and b) JPh -V under 1-sun AM1.5 solar spectra illumination (100 mW/cm2) at room temperature .....

Figure VI. 26 :

 26 Figure VI. 26: Main steps of the process flow of p-Si/ZnSnN2 heterojunction sample (Structure2.5). ..................................................................................................................

Figure VI. 27 :

 27 Figure VI. 27: Structure2.5 sample with of p-Si/Zn0.48Sn0.52N2/AZO: a) top design, b) cross-section design structure, c) cross-section SEM image with ZnSnN2 fabricated in ZnDCSnRF mode and d) in ZnRFSnDC .........................................................................

Figure VI. 28

 28 Figure VI. 28: a) J-V characterization of Structure2.5 sample (p-Si/Zn0.48Sn0.52N2/AZO) in dark and b) JPh -V curve under 1-sun AM1.5 solar spectra illumination (100 mW/cm2) at room temperature, the probes of solar simulator are located on Ti/Au and AZO ..............................................................................................

4 :

 4 Main results of n-ZnSnN2/n-AZO, p-SnO/n-ZnSnN2 and p-Si/n-ZnSnN2 heterojunction .................................................................................................................. xxx Publications and communications -T. Bui, M.Halbwax and J-P. Vilcot, Effect of Sputtering Atmosphere on ZnSnN2 Thin Films Electrical and Optoelectronic Properties, ECS Transactions, 97 (4) 47-55 (2020) -J-P. Vilcot., T. Bui, B. Ayachi, P. Miska, J-F. Pierson, C. Longeaud, S. Le Gall, W. Favre, V.S. Nguyen, ZnSnN2: material investigations for photovoltaic applications, 37 th European Materials Research Society Spring Meeting, E-MRS Spring 2019, Nice, France, may 27-31, 2019, Symposium E -Earth-abundant next generation materials for solar energy -III, invited paper E 9.1 -T. Bui, M. Halbwax and J-P. Vilcot, Effect of sputtering atmosphere on ZnSnN2 thin films electrical and optoelectronic properties, accepted for oral presentation in 237 th Meeting of the Electrochemical Society, Montréal, Canada, may 10-14, 2020, but conference cancelled by organizers (sanitary crisis) -T. Bui, M. Halbwax, J-P. Vilcot, Effects of sputtering environment on the optical, electrical and optoelectronic properties of ZnSnN2, oral presentation in JNPV (December 2019, Dourdan) -B. Ayachi, T. Bui, C. Sion, J-P Vilcot, Reactively co-sputtered ZnSnN2 thin films: a promising material/process for low cost/high efficiency sustainable tandem cells, poster presentation in JNPV (December 2017, Dourdan) -S. Le Gall, B. Ayachi, N. Beddelem, T. Bui, C. Longeaud, A. Goldani, J. Alvarez, A. Jaffres, J-P. Kleider, J-P. Vilcot, P. Miska, Etude des propriétés de photoconductivité de couches minces de ZnSnN2 par implantation de dopants et selon différentes conditions de croissance, poster presentation in JNPV (December 2018, Dourdan) -V.S. Nguyen, T. Bui, J-P. Vilcot, A. Valla, A. Chabli, W. Favre, Influence of growing conditions on optical and electrical properties of ZnSnN2 thin films for PV applications, poster presentation in JNPV (December 2018, Dourdan) -L. Kopprio, S. Le Gall, C. Longeaud, T. Bui, B. Ayachi, J-P Vilcot, A. Virfeu, J-F. Pierson, Evaluation of the optoelectronic properties of ZnSnN2 thin films under different growth conditions and doping implantation of As and P at different concentrations, poster presentation in JNPV (December 2019, Dourdan) -T. Bui, M. Halbwax, J-P. Vilcot, Comparison of DC and RF Sputtering: Effect on Structural and Optical Properties of ZnSnN2, poster presentation in JNPV (December 2020, Dourdan)

Table I . 1 :

 I1 The efficiency of dual junction tandem cells and bandgap of sub-cells were recorded by B. Ehrler et al.[START_REF] Ehrler | Photovoltaics reaching for the shockley-queisser limit[END_REF]

	Materials	Record efficiency (%)	Bandgap (eV)
	GaInP/ GaAs	32.8	1.95/ 1.42
	GaAsP/ Si	23.4	1.73/ 1.12
	Perovskite/ Si	29.2	1.70/ 1.12
	Perovskite/ Perovskite	24.2	1.82/ 1.27
	Perovskite/ CIGS	24.2	1.70/ 1.13

Table I . 2 :

 I2 Bandgap (eV) of Zn-IV-N2 semiconductors calculated by A.Punya et al.[START_REF] Punya | Quasiparticle band structure of Zn-IV-N2 compounds[END_REF]

	Compound		LDA	GGA	QSGW
	ZnSiN2	Indirect	3.60	3.23	6.01
		Direct	3.84	3.45	6.26
	ZnGeN2	Direct	1.66	1.57	3.99
	ZnSnN2	Direct	0.55	0.35	2.64

Table I . 3 :

 I3 Reported optical and electrical properties of ZnSnN2 determined by computation and

	experiment
	Methods	Bangap	n	µ
		(eV)	(cm -3 )	(cm.V -1 s -1 )
	Computation
	LDA [21]	0.58
	QSGW [21]	1.94
	0.8∑ approximation[21]	1.65
	HSE06 density functional [41]	1.40
	QSGW [41]	2.02
	PBE0 funcrional [42] -monolayer	2.39
	PBE0 funcrional [42] -bilayer	2.62
	Experiment
	Co-sputtering in RF mode [41]	2.10
	Vapor-liquid-solid method[22]	1.70
	Plasma-assisted MBE [36]	2.10
	Plasma-assisted MBE [37]	2.21 -2.38	3.0x10 20 to 1.1x10

  21

Those results are consistent with the visual evaluation from SEM images (Figure II. 6).

	Assuming that the crystal structure of ZnSnN2 is orthorhombic, the lattice parameters
	determined from our measurements are compared to predicted and experimental reported
	values in Table II.2. The lattice parameters of ZnSnN2 are not significantly changed by the
	N2/Ar ratio used during deposition.

Table II . 2: Lattice

 II parameters and crystallite size for ZnDCSnRF series deposited at different N2/Ar

	ratios, determined from XRD measurements. Values are compared to literature results for
	orthorhombic phase structure of ZnSnN2.
	N2/Ar ratio or literature	a (Å)	b (Å)	c (Å)	D (nm)
	LDA [23]	5.70	6.59	5.41
	GGA [23]	5.80	6.70	5.53
	Hydrid HSE06 [24]	5.84	6.72	5.46
	Experimental by VLS [22]	5.84	6.75	5.462
	Experimental by sputtering	5.79	6.74	5.59
	[46]
	N2/Ar = 1.5	5.92	6.83	5.52	19.0
	N2/Ar = 2.0	5.93	6.84	5.52	17.3
	N2/Ar = 2.5	5.93	6.84	5.52	17.9
	N2/Ar = 3.0	5.90	6.82	5.50	10.9
	N2/Ar = 4.0	5.92	6.85	5.51	6.2

Figure II. 12). The lattice parameters are described in Table II. 3, the

	3.0. Although those results are similar to
	the results of ZnDCSnRF series (Figure II. 7), peaks are here less clearly defined. The
	average crystal size (D), evaluated from the (200) peak width using the Scherrer formula
	(2.6), decreases from 22.5 to 13.1 nm when the N2/Ar ratio increases. These results are
	consistent with visual evaluation from SEM images above (

c value declines slightly by the increase of N2/Ar. The dispersion in values is lower than that of ZnDCSnRF series (Table

II. 2) Figure II. 13: XRD

	Table II. 3: Lattice parameters and crystallite size for ZnRFSnDC series for different N2/Ar ratios,
	determined from XRD.
	N2/Ar ratio	a (Å) b (Å) c (Å) D (nm)
	1.5	5.92	6.80	5.63	22.5
	2.0	5.93	6.82	5.61	18.8
	2.5	5.92	6.82	5.58	13.1
	3.0	5.92	6.84	5.53	13.7
	3.5	5.92	6.83	5.55	13.1
	4.0	5.93	6.84	5.54	13.1

diffractograms of ZnRFSnDC series versus N2/Ar ratio.

Table II .

 II

5: Main effects of the tuning of sputtering parameters on the film properties.

Table III . 1 :

 III1 Lattice parameters calculated from XRD data for ZnSnN2 thin films (ZnRFSnDC series) deposited at different pressure.

	Pressure (mbar)	a (Å)	b (Å)	c (Å)	D (nm)
	0.01	5.90	6.79	5.57	21.7
	0.02	5.93	6.82	5.60	18.1
	0.03	5.93	6.82	5.65	15.3
	0.04	5.93	6.84	5.61	12.4
	0.05	5.93	6.86	5.59	12.4

  of ZnSnN2 can be distinguished even if some are weak. Only (200), (121) and (320) planes are dominant with high intensity, narrow and sharp peaks. At 400°C, the sputtering rate is almost equal to sample deposited on 25°C substrate, but the intensity of (200) peak increases dramatically by 3 times while

	some others completely disappear and even newer one (around 63°) appears. The average
	crystal size (D), evaluated from the (200) peak width using the Scherrer formula (2.6),
	increases from 17 to 21 nm when the substrate temperature increases. These results are
	consistent with visual evaluation from SEM images above (Figure III. 8). The lattice
	parameters are described in Table III.2, the c value increases slightly with the increase of
	substrate temperature.

Table III . 2 :

 III2 Lattice parameters and crystallite size determined from XRD data for ZnSnN2 thin films deposited at different substrate temperature.

	Temperature (°C)	a (Å)	b (Å) c (Å)	D (nm)
	25	5.93	6.82	5.60	17.0
	100	5.92	6.81	5.59	16.6
	200	5.92	6.81	5.58	17.7
	300	5.92	6.81	5.60	19.3
	400	5.93	6.85	5.76	21.0

Table III . 3 :

 III3 Lattice parameters calculated from XRD data of ZnDCSnRF-NA thin films deposited on glass and post-annealed at 100°C to 500°C in N2 atmosphere during 15 min.

	Temp. (°C)	a (Å)	b (Å)	c (Å)
	As-dep	5.93	6.83	5.52
	100	5.92	6.83	5.51
	150	5.91	6.82	5.51
	200	5.90	6.81	5.50
	250	5.91	6.81	5.50
	300	5.90	6.81	5.50
	350	5.90	6.81	5.50
	400	5.89	6.81	5.50
	450	5.89	6.81	5.51
	500	5.88	6.80	5.49

Table III . 4 :

 III4 Lattice parameters calculated from XRD data of ZnDCSnRF-NA thin films deposited on Si (100) and post-annealed at 100°C to 400°C in N2 atmosphere during 15 min.

	Temp. (°C)	a (Å)	b (Å)	c (Å)
	As-dep	5.90	6.82	5.50
	100	5.90	6.83	5.49
	150	5.90	6.83	5.49
	200	5.89	6.82	5.48
	250	5.89	6.82	5.48
	300	5.91	6.83	5.49
	350	5.88	6.81	5.47
	400	5.87	6.79	5.47

Table III . 5 :

 III5 Lattice parameters calculated from XRD data of ZnDCSnRF-HN thin films on Eagle XG

	substrate, annealed at 100°C to 450°C during N2 in 15 min.
	RTA Temp. (°C)	a (Å)	b (Å)	c (Å)
	As-dep	5.93	6.81	5.71
	100°C	5.92	6.80	5.69
	150°C	5.90	6.79	5.67
	200°C	5.89	6.78	5.66
	250°C	5.93	6.81	5.68
	300°C	5.87	6.77	5.63
	350°C	5.87	6.76	5.60
	400°C	5.86	6.76	5.60
	450°C	5.85	6.75	5.57

Table III . 6 :

 III6 Lattice parameters calculated from XRD data of ZnDCSnRF-HN thin films on Si[START_REF] Henni | Studies on the structural , morphological , optical and electrical properties of Al-doped ZnO nanorods prepared by electrochemical deposition[END_REF], annealed at 100°C to 450°C in N2 during 15 minutes.

	Temp. (°C) a (Å) b (Å) c (Å)
	As-dep	5.91	6.81	5.64
	100°C	5.88	6.79	5.62
	150°C	5.87	6.77	5.60
	200°C	5.86	6.77	5.57
	250°C	5.91	6.81	5.66
	300°C	5.85	6.76	5.56
	350°C	5.83	6.74	5.55
	400°C	5.82	6.72	5.48

Table III . 7 :

 III7 Lattice parameters calculated from XRD data of ZnRFSnDC-NA thin films on Eagle XG substrate, annealed from 100°C to 400°C in N2 during 15 minutes.

	Temp. (°C)	a (Å) b (Å) c (Å)
	As-dep	5.93	6.82	5.60
	100	5.90	6.78	5.60
	200	5.92	6.82	5.58
	300	5.90	6.78	5.60
	400	5.92	6.82	5.61

16, III. 21, III. 24, III.

15, III. 20, III. 23, III. 26 and III. 29). Above 400°C, the films disaggregate and Sn balls can be observed at surface of the films (see Figure III. 14). When deposited on glass, cracks in the films appear approximately at 300°C whatever the power configuration of targets (see Figures III. 13 and III. 29). Those cracks do not appear when deposited on silicon substrate (see Figure III. 19). Even if some slight decrease (maximum around 10%) of crystallite size can be observed with the increase in temperature, (see Figures III.

Table III .

 III

8: Main effects of process pressure and temperature, either during or post deposition, s on the film properties.

  2.1. For low applied powers of 30 W, the sputtering rate is very slow, approximately 1nm/min. Reversely, sputtering at high RF powers cannot be done due to high level of reflected power that cannot be decreased. As a

result, a RF power of 100 W is selected, the x value equals to 0.44. The sputtering rate is then of 32 nm/min that is smaller than 61 nm/min obtained in DC mode but a predictable behavior. The material surface (Figure IV. 24) is, a cauliflower-like and no particular difference is observed with the result obtained in DC mode.

Table V . 1 :

 V1 Comparison of the application of AZO as conductive electrode in of CdTe, perovskite

				24.7	75.9	15.6	J. Perrenoud et al [134]
		Spray pyrolysis	845	20.2	70.8	12.1	A. Crossay et al [117]
	perovskite	Electro-Spraying	1045	15.1	76	12	K. Mahmood et al [124]
	solar cells	RF sputtering	990	17.9	53.4	9.5	S. Ghosh et al [101]
		ALD	1080	18.6	70.8	14.2	H. Yang et al [122]
	Si Solar cell	RF sputtering DC sputtering	411 1413	30 11.4	66.7 70.5	8.2 11.4	D. Song et al [93] S. Neubert et al [118]
	and Si solar cell
		AZO fabrication	Voc (mV)	Jsc (mA/cm 2 )	FF (%)	Efficiency (%)	Ref
	CdTe Solar cell	RF sputtering RF sputtering	763 814	20.3 23.6	55.7 73.2	8.6 14	A. N. Tiwari et al [132] A. Gupta et al [133]

Table V . 2 :

 V2 Comparison of lattice parameters issued from literature results and from here fabricated SnO thin films versus annealing temperature (determined from XRD measurements, (101) plane).

		a (Å)	c (Å)
	F. Izumi [160]	3.80	4.84
	A. Togo et al [148]	3.88	4.98
	S. Lee et al [141]	3.80	4.84
	A. Garzon-Fontecha et al	3.80	4.84
	[139]
	SnO-RF-RTA250°C	3.75	4.78
	SnO-RF-RTA300°C	3.74	4.79
	SnO-RF-RTA350°C	3.74	4.79
	SnO-RF-RTA400°C	3.75	4.79

Table VI . 1 :

 VI1 Change in x value by adjusting the plasma power of Sn target (x value is obtained from EDS measurements)

	Sn Power (W)	Time (min)	Thickness (nm)	x
	25	40	290	0.58
	35	33	305	0.50
	40	30	310	0.47
	55	23	301	0.34
	70	19	298	0.26

Table VI . 3 :

 VI3 Bias applied power of ZnSnN2 layer to fabricate Structure2.5 samples

		Applied Power	Sputtering time
		Zn target	Sn target	(min)
	pSi-ZnDCSnRF	DC -24 W	RF -80W	12.5
	pSi-ZnRFSnDC	RF -30 W	DC -50 W	11.5

Table VI. 3. It was replicated from previous

experiments and chosen in order to obtain an x value close to 0.48. AZO and electrode deposition steps are the same that used in Section V.1.2. The main steps of fabrication process are presented in Figure VI. 26.

Table VI . 4 :

 VI4 Main results of n-ZnSnN2/n-AZO, p-SnO/n-ZnSnN2 and p-Si/n-ZnSnN2 heterojunction

		Action	Voltage	Jd	Jph	Jph/Jd
			(V)	(mA/cm 2 )	(mA/cm 2 )
	Structure 1.1	Surface area (cm 2 )
	TiAu/ZnSnN2/AZO	0.05	0.5	442.8	256.8	0.58
		0.10	0.5	310.1	68.2	0.22
		0.15	0.5	265.6	45.2	0.17
		0.20	0.5	152.6	24.4	0.16
		0.25	0.5	103.4	3.7	0.036
	Structure 1.2	Zn/(Zn+Sn)
	TiAu/ZnSnN2/AZO	x = 0.58	0.5	0.26	0.0016	0.0062
	Area = 0.05 cm 2	x = 0.5	0.5	13.1	0.46	0.035
		x = 0.47	0.5	206.0	98.9	0.48
		x = 0.34	0.5	0.48	0.13	0.27
		x = 0.26	0.5	0.09	0.029	0.31
		RTA 300°C/15min/N2
		x = 0.47	0.5	3975	240	0.060
		x = 0.34	0.5	The electrical contact cannot be
		x = 0.26	0.5		accessed
	Structure 1.3

AZO/Zn0.47Sn0.53N2/AZO Area = 0.05 cm 2

	results; nevertheless, the crystal structure of the material cannot be clearly concluded
	between orthorhombic and wurtzite. Without argon in deposition atmosphere, there are
	only 2 main diffraction peaks obtained for materials deposited in pure N2, and 4 for those
	fabricated in H2/N2 atmosphere.
	Top views of films show a cauliflower-like structure for almost all cases. Only sputtering
	in pure N2 atmosphere exhibits a completely different structure with large
	triangular/pyramidal shape.
		As-dep	0.5	36.2	18.1	0.50
		RTA 300°C/15min/N2	0.5	16.2	10.4	0.64
	Structure 1.4	As-dep	0.5	330	39.2	0.12
	Zn0.47Sn0.53N2/AZO Area = 0.05 cm 2	RTA 300°C/15min/N2	0.5	24.5	4.36	0.15
	Structure 2.1&2.2	SnO thickness (nm)
	p-SnO/ZnDCSnRFN2/AZO	610	5	0.30	0.097	0.32
	x = 0.58	870	5	1.92	0.19	0.1
	Structure 2.3&2.4	SnO thickness (nm)
	p-SnO/ZnRFSnDCN2/AZO	600	5	0.43	0.16	0.36
	x = 0.58	790	5	2.07	0.22	0.1
	Structure 2.5	ZnDCSnRF	-1	-0.62	-0.23	0.37
	p-Si/ZnSnN2		1	0.78	-0.24	-0.30
		ZnRFSnDC	-1	-0.22	-0.54	2.42
			1	0.60	-0.36	-0.60

  . 5: Zn/(Zn+Sn) ratio and thickness of ZnSnN2 (ZnDCSnRF series) thin films versus nitrogen flow rate (Ar flow rate:10 sccm, deposition pressure: 2.10-2 mbar, 25W-DC applied to Zn and 35W-RF applied to Sn targets). ......................................................Figure II. 6: Top surface views of ZnDCSnRF series under different N2/Ar ratio deposition atmospheres. .....................................................................................................Figure II. 7: XRD diffractograms of the ZnSnN2 films (ZnDCSnRF series) deposited under different N2/Ar ratios, compared to experiment result [59] and simulation result [22]. ....................................................................................................................................Figure II. 13: XRD diffractograms of ZnRFSnDC series versus N2/Ar ratio. .................. Ar ratio. .......................... Tauc's equation. ............................................................Figure II. 16: Sputtering rate and Zn/(Zn+Sn) ratio versus 0.05HN/Ar ratio. ................... Resistivity (R), carrier mobility (µ) and carrier density of ZnSnN2 films (determined from Hall effect measurements) versus 0.05HN/Ar ratio. ............................Figure II. 19: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF samples deposited under varied 0.05HN/Ar ratios, b) Corresponding optical bandgap values calculated using Tauc's equation. ............................................................Figure II. 20: Sputtering rate and Zn/(Zn+Sn) ratio versus H2/N2 ratio. ..........................Figure II. 21: Top surface SEM images of ZnDCSnRF series deposited using different H2/N2 ratios. ......................................................................................................................Figure II. 22: XRD patterns of the ZnSnN2 films deposited in different percentage of hydrogen ............................................................................................................................Figure II. 23: Resistivity (R), carrier mobility (µ) and carrier density of ZnDCSnRF series (determined from Hall effect measurements) versus H2/N2 ratio. .................................... ..........................................................................................................................................Figure II. 29: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF series deposited under varied sputtering atmospheres, b) Corresponding optical bandgap values calculated using Tauc's equation. ................................................Figure III. 1: Zn/(Zn+Sn) ratio and sputtering rate via sputtering pressure (ZnRFSnDC series). ................................................................................................................................ XRD diffractograms of ZnSnN2 films (ZnRFSnDC series) deposited at different sputtering pressure. ............................................................................................. 66 Figure III. 4: Calculated strain,, of ZnSnN2 films (ZnRFSnDC series) deposited on Eagle XG substrate via versus the sputtering pressure (N2/Ar =2). .................................. 68 Corresponding optical bandgap values calculated using Tauc's equation. ....................... 69 Figure III. 6: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnRFSnDC series (determined from Hall measurements) versus deposition pressure. ........................ 70 Figure III. 7: Zn/(Zn+Sn) ratio and sputtering rate via sputtering pressure. ..................... 71 Figure III. 8: SEM images, top-views (on the left hand side) and cross-sections (on the right hand side) of ZnSnN2 thin films deposited at different temperatures, from room temperature up to 400°C. ................................................................................................... 73

	Figure II. 14: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnRFSnDC series (determined from Hall effect measurements) versus N2/Figure II. 15: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC samples deposited under varied N2/Ar ratios , b) Corresponding optical bandgap values calculated using Figure III. 5: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC samples deposited under varied deposition pressures (N2/Ar = 2), b)

Figure II. 8: I-V curves obtained from Van Der Pauw resistivity measurements of sample N2/Ar = 2.5 (insert shows the way contacts are taken). ....................................................

Figure II. 9: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnDCSnRF series (determined from Hall effect measurements) versus N2/Ar ratio. ..........................

Figure II. 10: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF samples deposited under varied N2/Ar ratios, b) Corresponding optical bandgap values calculated using Tauc's relation. ..............................................................

Figure II. 11: Zn/(Zn+Sn) ratio and sputtering rate of ZnSnN2 (ZnRFSnDC series) thin films versus nitrogen flow rate (Ar flow rate:10 sccm, deposition pressure: 2.10-2 mbar, 35W-RF applied to Zn and 50W-DC applied to Sn targets)..............................................

Figure II. 12: Top view of ZnRFSnDC series deposited at different N2/Ar ratios. ..........

Figure II. 17: Top surface SEM images of ZnDCSnRF series deposited in different mixed 0.05HN/Ar environments................................................................................................... xix Figure II. 18: Figure II. 24: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnDCSnRF samples deposited under varied H2/N2 ratios sputtering atmospheres, b) Corresponding optical bandgap values calculated using Tauc's equation. .......................

Figure II. 25: Comparison of sputtering rate and Zn/(Zn+Sn) ratio of ZnSnN2 samples deposited in different environments. .................................................................................

Figure II. 26: SEM (top) and AFM (bottom) images of ZnSnN2 films for NA, NH and N samples...............................................................................................................................

Figure II. 27: Comparison of XRD diffractograms of the ZnSnN2 films deposited in different sputtering environments. .....................................................................................

Figure II. 28: Resistivity (R), carrier mobility (µ) and carrier density of ZnSnN2 films (determined from Hall effect measurements) depending on their sputtering environment. .Figure III. 2: SEM images, top-views (on the left hand side) and cross-sections (on the right hand side) of ZnSnN2 thin films (ZnRFSnDC series) deposited at pressure of 0.01 to 0.05 mbar. ...................................................................................................................... xx Figure III. 3:

  .......................................................................................................................... 84 Figure III. 18: Resistivity (R), carrier mobility (µ) and carrier density of ZnSnN2 films (determined from Hall effect measurements) depending on the post-annealing temperatures. ...................................................................................................................... 84 Figure III. 19: SEM images of ZnDCSnRF-NA thin films on Si (100) after RTA treatment (the scale of 450° and 500°C sample images is 4 times greater compared to others). ............................................................................................................................... 85 Figure III. 20: XRD diffractograms of ZnDCSnRF-NA thin films on Si (100) for different Crystal size ZnDCSnRF-HN thin films on Eagle XG Substrate after annealing from 100°C to 450°C ........................................................................................ 91 Figure III. 25: SEM images of ZnDCSnRF-HN thin films on Si (100) after RTA treatment from 100°C to 500°C ......................................................................................... 92 Sn on red peak). ............................................................................................ 93 SEM images, top-views (on the left hand side) and cross-sections SEM (on the right hand side) of ZnRFSnDC-NA series deposited on Eagle XG glass after RTA treatment from 100°C to 500°C. ........................................................................................ 97 Figure III. 30: XRD diffractograms of ZnRFSnDC-NA thin films on Eagle XG substrate for different annealing temperatures, from 100°C to 500°C. ............................................ 97 Figure III. 31 : Strain and crystal size of (200) plane of ZnRFSnDCRFZnDCSn-NA thin films on Eagle XG substrate after annealing from 100°C to 500°C. ................................. 98 Figure III. 32: a) Transmittance vs. wavelength spectra (UV-Vis spectroscopy) for ZnRFSnDC-NA series deposited on glass at room temperature and post-annealed at different temperature, b) Corresponding optical bandgap values calculated using Tauc's equation. ............................................................................................................................. 99

	Figure III. 26 : XRD diffractograms of ZnDCSnRF-HN thin films on Si (100) substrate
	for different annealing temperatures: a) from 100°C to 400°C, b) 500°C data compared to
	XRD data of Figure III. 27 : Crystal size of (200) plane of DCZnRFSn-HN thin films on Eagle XG

post-annealing temperatures: a) from 100°C to 400°C, b) 450°C and 500° data compared to XRD data of Sn on red peaks. ....................................................................................... 86 Figure III. 21: Strain and crystal size of ZnDCSnRF-NA series thin films deposited on Si(100) and post-annealed from 100°C to 500°C. ............................................................. 88 Figure III. 22: SEM images of DCZnRFSn-HN thin films on Eagle XG after RTA treatment (note: magnification of 500°C sample is 4-time smaller compared to others) .. 89 Figure III. 23: XRD diffractograms of ZnDCSnRF-HN thin films on Eagle XG substrate for different annealing temperatures: a) from 100°C to 400°C, b) 500°C data compared to XRD data of Sn on red peaks............................................................................................. 90 Figure III. 24: Substrate after annealing from 100°C to 450°C ................................................................ 94 Figure III. 28: Zn/(Zn+Sn) ratio and thickness of ZnRFSnDC-NA samples deposited on Eagle XG glass versus post-deposition annealing temperature (N2 atmosphere). ............ 95 xxii Figure III. 29: Figure III. 33: Resistivity (R), carrier mobility (µ) and carrier density of ZnRFSnDC-NA series (determined from Hall effect measurements) depending on the post-annealing temperature. ..................................................................................................................... 100

  . 12: Composition and sputtering rate via Sn power of ZnxSn1-xN2 (ZnDCSnDC series) thin films. .......................................................................................Figure IV. 13: Top-view SEM images of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.60. ........................................................................................................Figure IV. 14: XRD diffractograms of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.60. ................................................................................................................ .65. ........................................................................................................................

	Figure IV. 15: a) Lattice parameter and b) crystallite size calculated from XRD data
	((121) plane for crystallite size) of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45
	≤ x ≤ 0

Figure IV. 16: Resistivity (R), carrier mobility (µ) and carrier density (n) of ZnxSn1-xN2 thin films (ZnDCSnDC series) with 0.45 ≤ x ≤ 0.60. ......................................................

Figure IV. 17

Table I .

 I 3: Reported optical and electrical properties of ZnSnN2 determined by computation and experiment .............................................................................................. 18 Table I. 4: Comparison of calculated and experimental lattice parameters of ZnSnN2..... 19 Table II. 1: Process parameters for different N2/Ar flux ratios of sputtering atmosphere. (Power of generators and processing time have been adjusted to get 50/50 Zn/Sn composition and 400nm thick films. (Common parameters are 10 sccm argon flux and 2x10-2 mbar process pressure) .......................................................................................... 31 Table II. 2: Lattice parameters for ZnDCSnRF series deposited at different N2/Ar ratios, determined from XRD measurements. Values are compared to literature results for orthorhombic phase structure of ZnSnN2. ......................................................................... 35 Table II. 3: Lattice parameters for ZnRFSnDC series for different N2/Ar ratios, determined from XRD. ...................................................................................................... 42 Table II. 4 : Sputtering gas composition (gas flow rates in sccm) used for the fabrication of the different types (N, NA and NH) of samples. ........................................................... 52 Table III. 1: Lattice parameters calculated from XRD data for ZnSnN2 thin films (ZnRFSnDC series) deposited at different pressure. ......................................................... 67 Table III. 2: Lattice parameters determined from XRD data for ZnSnN2 thin films deposited at different substrate temperature. ..................................................................... 74 Table III. 3: Lattice parameters calculated from XRD data of ZnDCSnRF-NA thin films deposited on glass and post-annealed at 100°C to 500°C in N2 atmosphere during 15 min. ........................................................................................................................................... 82 Table III. 4: Lattice parameters calculated from XRD data of ZnDCSnRF-NA thin films deposited on Si (100) and post-annealed at 100°C to 400°C in N2 atmosphere during 15 min. .................................................................................................................................... 87 xxix Table III. 5: Lattice parameters calculated from XRD data of ZnDCSnRF-HN thin films on Eagle XG substrate, annealed at 100°C to 450°C during N2 in 15 min. ...................... Table III. 6: Lattice parameters calculated from XRD data of ZnDCSnRF-HN thin films on Si (100), annealed at 100°C to 450°C in N2 during 15 minutes................................... Table III. 7: Lattice parameters calculated from XRD data of ZnRFSnDC-NA thin films on Eagle XG substrate, annealed from 100°C to 400°C in N2 during 15 minutes. ........... Table V. 1: Comparison of the application of AZO as conductive electrode in of CdTe, perovskite and Si solar cell .............................................................................................. Table V. 2: Comparison of lattice parameters issued from literature results and from here fabricated SnO thin films versus annealing temperature (determined from XRD measurements, (101) plane). ............................................................................................ Table VI. 1: Change in x value by adjusting the plasma power of Sn target (x value is obtained from EDS measurements) ................................................................................. Table VI. 2: Probe positioning for the different measurements shown in Figure IV. 19 Table VI. 3: Bias applied power of ZnSnN2 layer to fabricate Structure2.5 samples .....

Table VI .

 VI

Acknowledgments

After 3 years of research, my thesis has been done at the Institut d'Electronique de Microélectronique et de Nanotechnologie -IEMN, it would not be complete without the help of many people. Now, this is the right time to send gratitude to everyone.

for the fabrication of tandem cells. The project consortium comprised four partners: -Institut Jean Lamour (IJL) in Nancy for material deposition and deep crystallography analyses, -Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN) in Lille for material deposition, photodetector fabrication and photoelectric properties assessment,

and morphology looks like pyramidal shape, corresponding roughness is also larger than for the other samples. The UV-Vis spectra and optical band gap of ZnDCSnRF-NA thin films on glass after annealing from 100°C to 550°C are shown in Figure III. 17. The optical gap increases slowly from 1.77 to 1.99 eV as the annealed temperature grows up to 500°C. Such an increase in optical bandgap by after annealing treatment has also been reported by some authors [START_REF] Kumar Chinnakutti | Tailoring optoelectronic properties of earth abundant ZnSnN2 by combinatorial RF magnetron sputtering[END_REF]- [START_REF] Deng | Determination of the basic optical parameters of ZnSnN2[END_REF], [START_REF] Javaid | Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction[END_REF]. The large increase to 2.26 eV observed at 550°C is obviously due to the film de-structuring as shown in [START_REF] Tynell | Atomic layer deposition of Al-doped ZnO thin films[END_REF] and (320) planes are dominant with high intensity, narrow and sharp peaks. At 500°C, XRD

TUNING THE COMPOSITION

In this chapter, we will investigate how the change in ZnSnN2 thin film composition affects the microstructure, the electrical and the optical properties of the material. To do so, the power of the generators feeding the targets will be tuned. In the preceding chapters, the film composition was kept stoichiometric, or we tried to get it as close as possible.

Besides that, the effect of generators used to apply power for the target is also clarified.

All samples exhibit n-type polarity (Figure IV. 16). The carrier density and carrier mobility

values behave mainly as observed on previous cases, too As the Zn composition increases, the resistivity increases up to approximately 10 Ω.cm that can be considered as the highest resistivity compared to that of other series. However, the mobility and carrier density do not exhibit huge variation compared to other series and display the same behavior. In a predictable manner according to previous results, a progressive shift of the cut-off wavelength in transmittance towards higher values (red shift) is observed as the Zn composition increases (Figure IV. 17) with so the same comments.

ZnSnN2: Sputtering with compound-target Zn42.2Sn57.8

IV.2.1 ZnSnDC mode:

A compound target (Zn42.2Sn57.8) has been sputtered in pure DC on an Eagle XG glass, the applied power was varied from 30 W to 100 W and the atmosphere was N2/Ar. Other parameters are the same (i.e. deposition at room temperature, N2/Ar ratio fixed to 2.0, and sputtering chamber pressure to 0.02 mbar). The change in film composition and sputtering rate are illustrated in Figure IV. 18, the composition does not change significantly as supply power is smaller than 60 W, x value is in range 0.40 to 0.41 and rather corresponds to the target composition. When the power increases above this switchover point, the x Ω.cm, the carrier varies from 0.09 to 0.17 cm 2 V -1 s -1 . the carrier concentration is always close to 4 x 10 20 cm -3 .

FABRICATION OF ALUMINUM DOPED ZINC OXIDE AND TIN OXIDE

Since the beginning of the studies on ZnSnN2 semiconductor, a decade ago, the scientific reports focused mainly on the structure and electrical properties of the material. Although, the computational and experimental results (discussed in Chapter I) are promising for this material to be used in photovoltaics, few data on photoelectric properties have been recorded, yet. Even if simulation results of p-CuCrO2/n-ZnSnN2 device display more than 20% of power conversion efficiency [START_REF] Arca | Band edge positions and their impact on the simulated device performance of ZnSnN2-based solar cells[END_REF], [START_REF] Laidouci | Numerical study of solar cells based on ZnSnN2 structure[END_REF], experimental results are in sharp contrast since only 0.37 % [START_REF] Javaid | Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction[END_REF] and 1.54% [START_REF] Javaid | Band Offset Engineering in ZnSnN2-Based Heterojunction for Low-Cost Solar Cells[END_REF] of conversion efficiency were obtained on p-SnO/n-ZnSnN2 structures. Besides, the solar structure based on p-Si and n-ZnSnN2 [START_REF] Qin | Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions[END_REF] was investigated, and no data of conversion efficiency was reported.

In this chapter, I will focus on the fabrication (always by using PVD, and especially, the sputtering technique) of complementary materials aiming building a cell structure. First, I will report on a rather usual n-type transparent conductive oxide that is aluminum-doped zinc oxide (AZO). AZO has been preferred to ITO since it is in the same spirit of sustainable development that ZnSnN2 material although ITO use is controversial due to its indium content (exhibiting nevertheless slightly better electrical properties). As p-type material that can also acts as a transparent window, I will report on tin oxide (SnO). Both will be used trying making a cell in next chapter.

p-SnO thin film

V.2.1 State of the art:

According to E. Leja et al [START_REF] Leja | Phase composition of SnOx thin films obtained by reactive d.c. sputtering[END_REF], [START_REF] Leja | Electrical properties of nonstoichiometric tin oxide films obtained by the d.c. reactive sputtering method[END_REF], α-SnO and SnO2 are two stable compounds obtained from the combination of tin and oxygen, and both of them exhibit a tetragonal structure. In addition, α-SnO is a thermodynamically unstable phase; as temperature is higher than 300°C, it easily decomposes and recombines into SnO2 by the following reaction (5.1).

The formation of a stable α-SnO phase is limited to a narrow window. Specially in reactive sputtering technique, the oxygen partial pressure should be lower than 4.5% (working pressure  7x10 -3 mbar) [START_REF] Kim | Transparent SnOx thin films fabricated by radio frequency reactive sputtering with a SnO/Sn composite target[END_REF] or lower than 9.5% (working pressure  2.4x10 -3 mbar) [START_REF] Garzon-Fontecha | SnOx thin films with tunable conductivity for fabrication of p-n homo-junction[END_REF].

The post deposition annealing treatment will help to improve the crystalline quality, but it should not be higher than 200°C to avoid the appearance of a second phase SnO2 [START_REF] Leja | Electrical properties of nonstoichiometric tin oxide films obtained by the d.c. reactive sputtering method[END_REF], [START_REF] Kim | Transparent SnOx thin films fabricated by radio frequency reactive sputtering with a SnO/Sn composite target[END_REF], [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF].

Although most studies agree that SnO exists in the form of tetragonal crystals [START_REF] Garzon-Fontecha | SnOx thin films with tunable conductivity for fabrication of p-n homo-junction[END_REF]- [START_REF] Fortunato | Transparent p-type SnOx thin film transistors produced by reactive rf magnetron sputtering followed by low temperature annealing[END_REF] with P4/nmm space group [START_REF] Moreno | Cation nonstoichiometry in tinmonoxide-phase with tweed microstructure[END_REF], [START_REF] Hsu | Sputtering deposition of P-type SnO films with SnO2 target in xiii hydrogen-containing atmosphere[END_REF]- [START_REF] Pan | Tin oxide thin films grown on the (1012) sapphire substrate[END_REF], a metastable orthorhombic form was mentioned by S. Lin et al [START_REF] Lin | Structural and physical properties of tin oxide thin films for optoelectronic applications[END_REF], the type of space group was not discussed.

V.2.1.2 Electrical and optical properties of SnO

A. Togo et al. [START_REF] Togo | First-principles calculations of native defects in tin monoxide[END_REF] have calculated and suggested native defects in SnO: they are the O interstitial (Oi), the O vacancy (VO), the Sn interstitial (Sni), the Sn vacancy (VSn). The formation energy of Vsn, VO and Oi defects were recorded the similar order of magnitude.

When the SnO obtained Sn-rich nature, the VO defect is dominating, whereas Oi and VSn defects dominate. Depending on these defects are acceptor like or donor like, the charge state obtained is negative or positive, respectively. Additionally, the Oi defect does not

Discussions and partial conclusions:

-AZO thin film

The deposition process of AZO was previously set in IEMN, so we did not go into details the factors affecting the properties of AZO thin film in this study. The AZO thin film fabricated by DC magnetron sputtering shows a very high optical transmittance (approx.

97%) and good crystallinity. In addition, the electrical properties are promising ones for the potential use of AZO thin film as a n-type transparent conductive electrode for a solar cell. We will use in the next chapter, AZO films made in DC mode (80W applied power)

without any post-annealing treatment and exhibiting a bandgap value of 3.3 eV.

-SnO thin film

The SnO thin films were fabricated in RF and DC mode, the as-deposition samples and those annealed at low temperature (< 250°C) show amorphous nature. At high annealing temperatures, cracks appear and the surface of the thin film is no longer homogenous (see There are very few reports on the selection of p-type layer to create p-n junction with ZnSnN2. n-ZnSnN2/p-SnO heterojunction has been published by K. Javiad et al [START_REF] Javaid | Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction[END_REF], [START_REF] Javaid | Band Offset Engineering in ZnSnN2-Based Heterojunction for Low-Cost Solar Cells[END_REF] but no detailed information was given on the used SnO material. Based on above results we selected a process in DC mode (50W applied power) with a post-annealing treatment at 250°C and exhibiting a bandgap value of 2.9 eV.

PHOTOELECTRIC PROPERTIES

In this chapter, we will examine the photoelectric properties of ZnSnN2 by constructing 2 photodetection structures such as reported by Dharmadasa et al [START_REF] Dharmadasa | New ways of developing glass/conducting glass/CdS/CdTe/metal thin-film solar cells based on a new model[END_REF] (Figure V. 1). In the hereafter denominated Structure1, aluminum doped zinc oxide (AZO) was used as wide bandgap n-type layer and transparent electrode and Ti/Au as backside metallic electrode.

Such a configuration amounts to a photoconductor design. In Structure2 , a p-n junction structure is made using a p-type tin oxide (p-SnO) or p-doped silicon (p-Si), as done by Javaid et al [START_REF] Javaid | Thin Film Solar Cell Based on ZnSnN2/SnO Heterojunction[END_REF], [START_REF] Javaid | Band Offset Engineering in ZnSnN2-Based Heterojunction for Low-Cost Solar Cells[END_REF] and Quin et al [START_REF] Qin | Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions[END_REF], respectively. This structure corresponds to a photodiode design. Structure2) [START_REF] Dharmadasa | New ways of developing glass/conducting glass/CdS/CdTe/metal thin-film solar cells based on a new model[END_REF].

The optoelectronic properties were determined using a 2 probes Oriel Sol3A Solar Simulator under AM1.5 illumination conditions.

Structure 1 -Photoconductor

VI.1.1 First attempt: overview of the fabrication process

All samples have been fabricated on Eagle XG substrate (1 inch x 1 inch). The 20nm-Ti/300nm-Au back contact was deposited by evaporation on a cleaned substrate.

Following, the 300 nm-thick ZnSnN2 film was deposited at room temperature, working pressure of 0.02 mbar, N2/Ar ratio of 2.0, power feeding of 25W-DC and 40W-RF for Zn 18, the resistivity of the film is one order of magnitude lower after a 300°C anneal that corresponds to the observed increase of the dark current density. On the contrary, this order of magnitude is lost on the photoelectric properties since the JPh/JD is equal to 0.06 for the annealed sample (it was 0.6 for the as-deposited sample). From those results, we can conclude that the annealing process does not affect the AZO/ZTN/AZO structure and even helps to improve the photoelectrical properties of the device. The opposite phenomenon that was observed after annealing in section VI.1.2.2 may originate from the Ti/Au electrode. According A. Hiraki [START_REF] Hiraki | Room-temperature interfacial reaction in Au-semiconductor systems[END_REF], [START_REF] Hiraki | Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?[END_REF], at low temperature or even at room temperature, gold can diffuse into semiconductors that have a bandgap smaller than 2.5 eV and dielectric constant higher than 8. By experiment result (see Figure IV. 5), obtained optical bandgap is approx. 2.0 eV, and dielectric constant of ZnSnN2 was reported higher than 11 by T. Paudel et al [START_REF] Paudel | First-principles study of phonons and related ground-state properties and spectra in Zn-IV-N2 compounds[END_REF]. In addition, in the temperature range 135 -225°C, gold diffuses in tin by an interstitial mechanism, published by B. Dyson [START_REF] Dyson | Diffusion of gold and silver in tin single crystals[END_REF]. So, we can reasonably hypothesize that gold diffused into the ZnSnN2 layer during the annealing process.

VI.1.4 Effect of device structure and electrode distance

All along the parametric studies that are reported in Chapters II to IV, the photoelectric characterization was made on "simple" devices, i.e., the ones used for Hall measurements (see Figure II. 8). No particular electrodes were used, no technology steps used, only the material layer with some indium balls for contacting. This photoelectric characterization is not reported in those chapters since we never succeeded collecting the least photocurrent!

SUMMARY AND OPENING

SUMMARY

The first and major part of this work has been dedicated to a parametric study of the deposition conditions of ZnSnN2 material, using sputtering technique, on its electrical and optical properties. The main tools that have been used are XRD for the crystallography monitoring, SEM for the surface topology observation, Hall Effect measurements for the electrical characterization, and UV-Vis spectroscopy for the optical properties assessment.

Except the material deposition, no particular technological steps were made and all characterization were made on thin films; in particular, electrical characterization was conducted in a longitudinal carrier transport scheme, i.e., in a direction perpendicular to the film deposition. The second part aimed at evaluating the optoelectronic properties. I mean that technology was deployed, first for the fabrication of transparent conductive oxides, and second for fabricating photodetectors. Device structure included then bottom and top electrodes, closer to that of a real solar cell implementation. Thus, the main difference with all previous studies was that carrier transport was then in transverse direction, i.e., in the direction of film growth.

Parametric study -Deposition atmosphere:

Starting by sputtering metallic targets in a pure argon atmosphere, the power supply of each target was determined in order achieving a stoichiometric compound. However, the deposited films shift to a Zn-rich composition when nitrogen is inserted into the sputtering chamber. Additionally, for all sputtering atmospheres containing argon, the increase in nitrogen concentration leads to decrease the deposition rate. In H2/N2 atmosphere, the lighter mass of hydrogen (1 u) does not modify the sputtering process that is governed by the nitrogen ions.

The XRD patterns of samples that were fabricated in N2/Ar atmosphere show all the characteristic peaks of ZnSnN2, reported in literature from simulated or experimental