# Multi-techniques characterisation of anti-reflective Ta2O5 and TaOxNy thin films deposited by reactive sputtering: coupling X-ray photoelectron spectroscopy, scanning/transmission electron microscopy and ion beam analysis

Florian Chabanais, Babacar Diallo, Aissatou Diop, Angélique Bousquet, Thierry Sauvage, Béatrice Plujat, Sébastien Quoizola, Audrey Soum-Glaude, Laurent Thomas, Éric Tomasella, et al.

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Florian Chabanais, Babacar Diallo, Aissatou Diop, Angélique Bousquet, Thierry Sauvage, et al. Multi-techniques characterisation of anti-reflective Ta2O5 and TaOxNy thin films deposited by reactive sputtering: coupling X-ray photoelectron spectroscopy, scanning/transmission electron microscopy and ion beam analysis. Thin Solid Films, 2025, 825, pp.140725. 10.1016/j.tsf.2025.140725. hal-05172348

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Submitted on 21 Jul 2025

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*Multi-techniques characterisation of anti-reflective Ta<sub>2</sub>O<sub>5</sub> and TaO<sub>x</sub>N<sub>y</sub> thin films deposited by reactive sputtering: coupling X-ray Photoelectron Spectroscopy, Scanning/Transmission Electron Microscopy and Ion Beam Analysis*

Florian Chabanais<fnref n="1" />*, Babacar Diallo<fnref n="2" />, Aissatou Diop<fnref n="3" />,<fnref n="4" />,<fnref n="5" />, Angélique Bousquet<fnref n="5" />, Thierry Sauvage<fnref n="2" />, Béatrice Plujat<fnref n="3" />,<fnref n="4" />, Sébastien Quoizola<fnref n="3" />,<fnref n="4" />, Audrey Soum-Glaude<fnref n="3" />, Laurent Thomas<fnref n="3" />,<fnref n="4" />, Éric Tomasella<fnref n="5" />, Antoine Goullet<fnref n="1" /> and Mireille Richard-Plouet<fnref n="1" />*

**Key words:** TaON, sputtering, Plasma processes, TEM, XPS, IBA, Concentrated Solar Thermal

**Abstract**

TaOₓNᵧ thin films were elaborated by reactive magnetron sputtering of a Ta target in an Ar/N₂/O₂ plasma mixture. These layers are intended to be used as antireflective coatings in systems for concentrated solar applications (operating temperature around 500°C in air). The investigation of these layers using a set of complementary techniques, including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ion beam analysis (IBA), revealed the impact of nitrogen flux during deposition on the overall morphology and elemental composition. These characterizations also highlighted the various chemical environments constituting the thin films, ranging from oxidized environments (Ta₂O₅) to oxynitride and nitride environments (TaOₓNᵧ and Ta₃N₅). It was also shown that with a low nitrogen flow, nitrogen is present only near the substrate and is absent in the rest of the layer. The IBA technique was also carried out to quantify the concentration of light elements as a function of depth, in particular H, which is not detected by the other techniques used in this paper (XPS and TEM). It has been shown that the majority of hydrogen atoms is incorporated into thin films during deposition as hydroxyl groups or only close to the surface, depending on the samples.

<footnote n="1">¹ Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes, France</footnote>

<footnote n="2">² CNRS, CEMHTI UPR 3079 (Conditions Extrêmes et Matériaux : Haute Température et Irradiation), Université d'Orléans, 3A rue de la Férollerie 45071 Orléans, France</footnote>

<footnote n="3">³ PROMES-CNRS UPR 8521 (Laboratory of PROcess, Materials, Solar Energy), Rambla de la Thermodynamique 66100 Perpignan, France / 7 rue du Four Solaire 66120 Font-Romeu-Odeillo-Via, France</footnote>

<footnote n="4">⁴ Université de Perpignan Via Domitia, 52 Avenue Paul Alduy 66860 Perpignan, France</footnote>

<footnote n="5">⁵ Université Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, 24 Avenue Blaise Pascal 63178 Aubière, France</footnote>

<footnote>*Corresponding author - E-mail address: florian.chabanais@liu.se</footnote>

<footnote>*Corresponding author - E-mail address: mireille.richard@cnrs-imn.fr</footnote>
# 1) Introduction

On the one hand, metal oxides are widely used in various applications such as microelectronics (transistors, LEDs), energy production and battery manufacturing. On the other hand, metal nitrides also exhibit interesting properties and are being studied in fields such as microelectronics (e.g., GaN) [1], hard protective coatings [2] or corrosion barrier coatings (e.g., CrN) [3]. Nowadays, transition metal oxynitrides are attracting a great deal of interest in the field of materials science due to their remarkable properties, and wide potential applications, stemming from their ability to possess characteristics of both metal oxides and metal nitrides. By adjusting the atomic composition between oxygen and nitrogen it is possible to control their physical, chemical and functional properties [4], [5], [6]. These materials are of interest for optics [7], [8], supercapacitors [9], solar-driven photocatalytic processes [10] and biomedical applications [11].

Within the category of metal oxynitrides, tantalum oxynitride remains relatively unexplored. Tantalum nitride demonstrates commendable mechanical properties and corrosion resistance [12] while tantalum pentoxide presents tunable optical properties. Hence depending on the N/O ratio, tantalum oxynitride band gap ranges from 2.4 to 4.5 eV [13], [14], [15]. A low extinction coefficient and a high refractive index in the UV-Vis domain are also reported for tantalum oxynitride [16], [17]. The substitution of nitrogen in the metal oxide structure is a way to control the optical behaviour of the film (band gap, absorbance, optical index). Moreover the decrease in the band gap of TaON with respect to $Ta_2O_5$ has boosted its interest as photocatalyst for water splitting into $H_2$ and $O_2$ [18], [19].

Due to its transparency in the visible range and its chemical stability, tantalum oxynitride can be used for antireflective coatings [20]. More recently, we also demonstrated by optical simulation that tantalum oxynitride thin films deposited on W/W-SiCH absorbing layer could increase the solar flux absorption by reducing reflectance in the visible range [21]. There are numerous methods for preparing $TaO_xN_y$ thin films, including electron beam evaporation, physical vapor deposition, sol-gel process, pulsed laser deposition and reactive sputtering [22], [23], [24], [25], [26], [27], [28]. The latter is one of the most widely used methods for depositing such thin films. It allows a simple control of the nitrogen and oxygen percentages in the sample by modifying the flow rates of the reactive gases in the discharge.

In this paper, the primary objective is to perform a thorough structural characterization of different magnetron-sputtered $TaO_xN_y$ thin film, deposited in conditions that could be up-scaled to industrial process, without specific storage cautions after deposition. Hence in case those films are used as anti-reflective coatings, they are expected to be contacted with air, humidity and therefore we make our
studies having realistic conditions of storage. XPS technique was employed to get insight into the evolution of the chemical bonds as a function of the deposition parameters, providing a better understanding of the TaOₓNᵧ thin film structures. Transmission Electron Microscopy (TEM) measurements were performed to investigate the morphology and microstructure of thin films and Ion Beam Analysis (IBA) was used to obtain the in-depth elemental composition with some eventual concentration gradients.

## 2) Experimental

### 2.1) Film deposition

TaOₓNᵧ thin films were prepared by reactive magnetron sputtering in a PLASSYS vacuum chamber from a 99.99% pure tantalum target with a diameter of 3 inches and a mixture of argon, oxygen and nitrogen. Before deposition, the system was kept under a vacuum near 10⁻⁶ mbar. First, the target, isolated from the substrate by shutter, was cleaned by sputtering it in a pure Argon plasma. The complete target cleaning is followed thanks to self-bias voltage, whose value in known in this condition for a metallic tantalum surface. Then, thin films were deposited in various Ar/N₂/O₂ atmospheres at a constant total flow rate of 17 sccm, corresponding to a 1 Pa pressure. The gases flow rates are controlled by gas mass flowmeters. They are fixed at 15 sccm for Ar and 2 sccm for the reactive gases (O₂ + N₂). We noted $R_F$ the flux ratio which corresponds to the ratio between the flow of O₂ and the total flow of reactive gases (N₂ + O₂): $R_F = Φ_{O₂}/(Φ_{O₂} + Φ_{N₂})$. The radiofrequency power applied to the target is at 200 W. The distance between this latter and the substrates is fixed at 15 cm. During deposition, the substrate holder rotates at 4 rotations/ minute. It was not intentionally heated, nor biased. The previous measurements showed that in these conditions the substrate temperature does not exceed 70°C. The deposition durations were adapted to obtain layer thicknesses around 200 nm, except in the case of RF1 where the deposition rate is very low due to the poisoning of the target by the oxygen injected into the reactor.

Table 1 presents the different samples obtained according to the reactive gas flow rates.

<caption>Table 1: Flow rate parameters.</caption>
<table><thead><tr><th>Sample</th><th>O<sub>2</sub> flux (sccm)</th><th>N<sub>2</sub> flux (sccm)</th><th>Ar flux (sccm)</th></tr></thead><tbody><tr><td>R<sub>F</sub>=1</td><td>2.0</td><td>0.0</td><td>15.0</td></tr><tr><td>R<sub>F</sub>=0.75</td><td>1.5</td><td>0.5</td><td>15.0</td></tr><tr><td>R<sub>F</sub>=0.4</td><td>0.8</td><td>1.2</td><td>15.0</td></tr><tr><td>R<sub>F</sub>=0</td><td>0.0</td><td>2.0</td><td>15.0</td></tr></tbody></table>

## 2.2) Characterization techniques

XPS measurements were carried out using a Kratos Axis Nova operating at 1486.6 eV (Al source). After recording survey spectra at a low resolution with pass energy of 160 eV to check for any spurious elements and with a resolution in energy of 0.56 eV (Fermi edge of silver foil used as reference), the pass energy and the step scan to measure the regions of interest were fixed to 40 eV (resolution in energy of 0.44 eV) and 0.1 eV, respectively. The probed depth was 10 nm over a 700 × 300 µm² zone. The dwell times and number of sweeps were chosen such as the highest XPS peak of the selected zone exhibits, at least 10000 counts. Charge neutralization was used when acquiring the spectra. Binding energies (BE) were calibrated assuming that the C 1s peak of C–C bonds present in hydrocarbon contamination appears at 285.0 eV. Using CasaXPS software [29] was used for peak fitting, after removal of a Tougaard-type background or simply a linear background (Ta 4p<sub>3/2</sub>). A Gaussian/Lorentzian (GL(p), p representing the percentage of Lorentzian) line-shape was used for peak fitting (GL(30) for C 1s, O 1, Ta 4f and GL(80) for Ta 4p<sub>3/2</sub> since the Ta 4p<sub>3/2</sub> band is broader at the edges). The relative elemental analysis is performed using the sensitivity factor method (RSFs) supplied for the spectrometer and directly implemented in CasaXPS. To remove the surface pollution, etching using conventional Ar⁺ ion gun (2 kV, 100 µA as extracting current) was conducted according to 3 × 3 mm² raster for 60 s only, in order to avoid beam damages and differential sputtering effect between the elements that could modify chemical information.

Scanning / Transmission Electron Microscopy (S/TEM) observations were performed using a Themis Z G3 Cs-probe corrected (from Thermo Fisher Scientific) operating at 300 kV. High-angle annular dark field (HAADF) images were acquired with a 30.5 mrad convergence angle and 50–200 mrad collection angles. The azimuthally averaged line profiles of SAED (Selected Area Electron Diffraction) were made using the Interactive Rotational Profile plugin for Gatan Digital Micrograph©. Thin cross sections for TEM were prepared using a standard FIB preparation (lift out procedure) with Ga⁺ ions accelerated at 30 kV. A final clean-up procedure with Ga⁺ ions at 2 kV was performed.

The film's chemical composition was determined by Ion Beam Analysis (IBA) techniques with the Pelletron accelerator facility at CEMHTI. The tantalum, oxygen, nitrogen and hydrogen contents were
determined by Rutherford Backscattering Spectroscopy (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection Analysis (ERDA) techniques. TaON specimens with thickness ranging between 130 and 240 nm were analysed in first by ERDA with a 2.8 MeV alphas ($^4$He) beam to quantify the hydrogen concentration. The alphas incident beam impinged the sample at a 15° grazing incidence angle and the recoiled protons from the $^1H(^4He, ^1H)^4He$ elastic nuclear reaction were collected at a forward-scattering angle of 30° by a PIPS (Passivated Implanted Planar Silicon) detector. Five spectra were successively collected to observe possible hydrogen desorption under analysis beam. A cumulated charge of 5 µC is sufficient to achieve a good $^1H$ counting statistic and provide a precise hydrogen concentration value. The Ta concentration and its in-depth homogeneity in films were measured by RBS with a 2 MeV $^4He$ beam. The backscattered alpha particles were detected with a 25 mm² PIPS detector located at a 166° angle with respect to the beam axis. The thin films characterization was completed by NRA to determine the oxygen and nitrogen contents through the $^{16}$O(d, $\alpha_0$)$^{14}$N and $^{14}$N(d, $p_5$)$^{15}$N nuclear reactions [30], [31], using 0.9 MeV deuterons in the same detection geometry than RBS analyses. The TaON film stoichiometry with eventual concentration gradient was obtained by simulating the RBS, NRA and ERDA spectra using the SIMNRA software with a depth resolution close to 30 nm [32]. The accuracy of our ERDA and NRA methodology in determining H, O and N contents was tested on standards, an a-Si(H$^+$) hydrogen standard [33], a polycrystalline silicon wafer with a top layer of 213 nm SiO$_2$ on Si substrate and a Si$_3$N$_4$ /Si film.

### 3) Characterisations of surface compositions by XPS

#### 3.1) Tantalum oxide

The XPS survey spectrum associated with the $R_F=1$ (flow rate $N_2$ = 0% and flow rate $O_2$ = 100%) sample is reported in Figure 1.a. The expected species are present, namely peaks associated with tantalum (Ta 4s, 4p, 4d, 4f) and associated with oxygen (O 1s). In addition, we observe peaks allotted to C 1s and spurious Si 2p. The results presented in Figure 1.b, c, d, e show the spectra obtained for the core levels O 1s, Ta 4p, C 1s, and Ta 4f. The components give us the different chemical environments encountered in the films. Parameters associated with the decomposition are summed up in Table 2.
<figure><img src="image_2.png" /><figcaption>Figure 1: (a) XPS survey spectrum of as-deposited $R_F=1$ sample and experimental high-resolution spectra, different components and total fitted envelop for (b) O 1s, (c) Ta 4p, (d) C 1s and (e) Ta 4f.</figcaption></figure>
<table><caption>Table 2: Summary of the main peak decomposition parameters.</caption><thead><tr><th>Photoelastic peaks</th><th>Assignment</th><th>BE (eV)</th><th>FWHM (eV)</th><th>References</th></tr></thead><tbody><tr><td rowspan="2">Ta 4f<sub>7/2</sub></td><td>Sub-oxide</td><td>24.4</td><td>1.5</td><td>[15], [34], [35], [36]</td></tr><tr><td>Ta<sub>2</sub>O<sub>5</sub></td><td>26.3</td><td>1.2</td><td>[15], [34], [35], [36]</td></tr><tr><td rowspan="2">Ta 4f<sub>5/2</sub></td><td>Sub-oxide</td><td>26.3</td><td>1.5</td><td>[15], [34], [35], [36]</td></tr><tr><td>Ta<sub>2</sub>O<sub>5</sub></td><td>28.2</td><td>1.2</td><td>[15], [34], [35], [36]</td></tr><tr><td colspan="2">EELS peak</td><td>42.0</td><td>2.8</td><td>[37]</td></tr><tr><td rowspan="4">C 1s</td><td>C-C</td><td>285.0</td><td>1.4</td><td>[38], [39]</td></tr><tr><td>C-O</td><td>286.2</td><td>1.4</td><td>[38], [39]</td></tr><tr><td>(C=O=-N)</td><td>287.6</td><td>1.4</td><td>[39], [40]</td></tr><tr><td>O-C=O</td><td>289.1</td><td>1.4</td><td>[39], [40]</td></tr><tr><td>N 1s</td><td>N=C</td><td>399.9</td><td>2.3</td><td>[38]</td></tr><tr><td rowspan="2">O 1s</td><td>O-Ta</td><td>530.7</td><td>1.3</td><td>[34], [35]</td></tr><tr><td>O-C/O-H</td><td>531.7</td><td>1.3</td><td>[34], [35]</td></tr></tbody></table>

The binding energy (BE) at 530.7 eV and 531.7 eV can be assigned to O-Ta bonds (from Ta₂O₅) and O-C/O-H bonds (from surface contamination) [34], [35] (Figure 1.b). A slight bump in the Ta 4p spectrum was also observed at 399.9 eV, assigned to N-(C=O) bond [29] (Figure 1.c). C 1s signal needs to be decomposed into 4 components at 285.0, 286.2, 287.6 and 289.1 associated to C-C, C-O, (C=O)-N, O-C=O, respectively [39], [40] (Figure 1.d). The decomposition of Ta 4f₇/₂ peak shows Ta-O bonds at 28.2 eV and 26.3 eV which correspond to Ta₂O₅ and sub-oxide (Ta 4f₅/₂ at 26.3 and 24.4 eV, respectively), respectively. At higher energy, we can note the presence of Ta 5p peaks (5p₃/₂ and 5p₁/₂) at 45.1 and 36.9 and eV and Ta 4f EELS peak at 42.0 eV [15], [34], [35], [36], [37]. We have also determined presence of O 2s at 22.6 eV [35], [36], [39] (Figure 1.e).
## 3.2) Effects of etching

The objective of etching is to get rid of possible surface pollution, in particular hydroxide/oxide and carbon originated from atmospheric air exposure of samples.

Figure 2.a presents C 1s spectra before and after etching (60 s), showing that almost all of the carbon peak has disappeared after etching. On the other hand, O 1s spectra (Figure 2.b) reveal that the same components are required to account for the experimental signals, i.e. associated to O-Ta (Ta₂O₅) and O-C/O-H bonds. The difference between spectra is the proportion one to the other. Before etching, the contamination signal represents 48.9% of the total O 1s peak (0 s spectra), whereas after 60 s of etching, the contamination component represents 35.1%. This hydroxylated surface (O-H bond presence after etching) mainly comes from residual water vapor in the XPS vacuum system reacting with the new etched surface to produce a small concentration of surface hydroxyls. Moreover, we observe by ERDA technique the presence of hydrogen in the TaON film volume. Thus, we cannot exclude that part of O-H XPS signal after etching can be due to the hydroxyl groups formed by water dissociation during film deposition.

<figure><img src="image_3.png" /><figcaption>Figure 2: High-resolution spectra before and after etching for 60 s (a) C 1s and (b) O 1s.</figcaption></figure>

The analysis of the main components present in the sample allows extracting the compositions gathered in Table 3 (after etching, O-C/O-H component was excluded). After etching, the O/Ta ratio obtained is equal to 2.4, which is close to the expected one for Ta₂O₅.
<table><caption>Table 3: Composition before and after etching for 60 s.</caption><thead><tr><th>Samples</th><th>Ta (%)</th><th>O (%)</th><th>N (%)</th><th>C (%)</th></tr></thead><tbody><tr><td>R<sub>F</sub>=1 before etching</td><td>15.8</td><td>61.6</td><td>0.6</td><td>21.9</td></tr><tr><td>R<sub>F</sub>=1 after etching</td><td>28.8</td><td>70.2</td><td>0</td><td>0</td></tr></tbody></table>

### 3.3) Tantalum oxynitride

Tantalum oxynitride films synthesized with RF <1 (N₂ flow rate > 0) were studied by XPS, whose spectra are shown in Figure 3 and compared to RF= 1 ones. The energy range shown in Figure 3.a reveals the characteristic peaks corresponding to Ta 4p₃/₂ and N 1s (Table 2). All the samples show a peak at around 404.7 eV, which is therefore attributed to Ta 4p₃/₂. At lower energies, specifically 396.4 eV, we observe the emergence of N 1s (Ta-N bond [15], [36]) peaks for the R<sub>F</sub>=0 and R<sub>F</sub>=0.4 samples (Figure 3.c). For sample R<sub>F</sub>=1, to the absence of N-Ta peak complies with the deposition conditions since the N₂ flow rate was fixed at 0 sccm. In contrast, for sample R<sub>F</sub>=0.75, the N₂ flow rate was fixed at 0.5 sccm (equivalent to 25.0% of the total N₂+O₂ gas flow rate) and still no Ta-N bond was detected. These observations are corroborated by the Ta 4f spectra presented in Figure 3.b. The spectra for R<sub>F</sub>=1 and R<sub>F</sub>=0.75 exhibit similarities, and the data analysis reveals the exclusive presence of Ta₂O₅, (Ta₂O₅ 4f₅/₂ and 4f₇/₂ at 28.2 and 26.3 eV). In the spectrum obtained for R<sub>F</sub>=0.4 the components associated to Ta₂O₅ 4f (28.3 and 26.4 eV) are discernible and we also observe a component associated to TaOₓNᵧ, 4f (27.8 and 25.9 eV and FWHM equal to 1.5 eV [15], [36], [41], [42]). For R<sub>F</sub>=0, the peaks at 26.9 and 25.0 eV (FWHM = 1.0 eV) have been associated with Ta₃N₅ [15], [41], [42], [43]. Even though the O flow rate is set at 0 sccm, we still observe a large Ta₂O₅ component for R<sub>F</sub>=0 sample. Using these XPS analyses, we have been able to determine that R<sub>F</sub>=1 and R<sub>F</sub>=0.75 exhibit stoichiometry close to Ta₂O₅ while R<sub>F</sub>=0.4 and R<sub>F</sub>=0 compositions are close to the global chemical formulation (Ta₂O₅)₀.₃₉(TaO₀.₇₈N₁.₀₁)₀.₆₁ and (Ta₂O₅)₀.₅₅(Ta₃N₅)₀.₄₅ (Table 4). To obtain these stoichiometries, we used the composition obtained by accounting for the sensitivity factors of the three elements and the ratio of the different components used to fit the Ta 4f peak. A comparison of the stoichiometry is interesting, revealing a diminution in the Ta₂O₅ content when the N₂ flow rate is increased i.e., first in favour of an oxynitride, for R<sub>F</sub>=0.4 and then in favour of a nitride (Ta₃N₅), for R<sub>F</sub>=0. Additionally, for R<sub>F</sub>=0.4, the peak positions at 27.8 and 25.9 eV obtained for the TaO₀.₇₈N₁.₀₁ components are close to the positions found in the literature for TaON stoichiometry [15], [41].
<table><caption>Table 4: Surface composition and stoichiometry for each sample.</caption><thead><tr><th>Samples</th><th>Ta (%)</th><th>O (%)</th><th>N (%)</th><th>Stoichiometry</th></tr></thead><tbody><tr><td>R<sub>F</sub>=0</td><td>31.7</td><td>37.3</td><td>31.0</td><td>(Ta<sub>2</sub>O<sub>5</sub>)<sub>0.55</sub>(Ta<sub>3</sub>N<sub>5</sub>)<sub>0.45</sub></td></tr><tr><td>R<sub>F</sub>=0.4</td><td>32.6</td><td>47.5</td><td>19.9</td><td>(Ta<sub>2</sub>O<sub>5</sub>)<sub>0.39</sub>(TaO<sub>0.78</sub>N<sub>1.01</sub>)<sub>0.61</sub></td></tr><tr><td>R<sub>F</sub>=0.75</td><td>28.3</td><td>71.7</td><td>0</td><td>Ta<sub>2</sub>O<sub>5</sub></td></tr><tr><td>R<sub>F</sub>=1</td><td>28.8</td><td>71.2</td><td>0</td><td>Ta<sub>2</sub>O<sub>5</sub></td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>Figure 3: a) and c) High-resolution spectrum of Ta 4p<sub>3/2</sub> and N 1s (N-(C=O) and N-Ta bonds). b) and d) High-resolution spectrum of Ta 4f, Ta<sub>2</sub>O<sub>5</sub>, TaO<sub>x</sub>N<sub>y</sub> and Ta<sub>3</sub>N<sub>5</sub> are discriminate.</figcaption></figure>
To summarize XPS results, with only a single flow of O₂ into the chamber during sputtering, the study of chemical environment revealed a layer composed entirely of tantalum oxide (R<sub>F</sub>=1), as expected. With the addition of a 25 % N₂ (R<sub>F</sub> = 0.75), we also find, on the surface, a layer composed solely of tantalum oxide. At a higher N₂ flow rate (R<sub>F</sub> = 0.4), part of the film is composed of an oxynitride, Ta₂O<sub>0.78</sub>N<sub>1.01</sub>, and part is oxidized as Ta₂O₅. Finally, for films deposited with pure N₂ gas flow, this oxidation still occurs (probably due to residual oxygen in the chamber during deposition and/or exposure to atmospheric conditions after deposition) but the nitridation of the film is more efficient with the formation of Ta₃N₅. Surprisingly the Ta₂O₅ is detected in greater proportion for R<sub>F</sub>=0 than for R<sub>F</sub>=0.4, while the O₂ flow rate is zero. However as expected since a nitride is formed additionally to the oxide, instead of an oxynitride, the total amount of O atoms incorporated in the film is higher for R<sub>F</sub>=0.4.

4) TEM Results

In this part, we aim to conduct a more in-depth characterization to obtain a comprehensive
understanding of morphologies, spatial localisation of the elements and amorphous or crystalline
nature of the different phases identified by XPS along the growth direction of the different films.

4.1) STEM-HAADF and STEM-EDX

Firstly, the thin films were observed by STEM-HAADF, and the cross-views of these layers are shown in
Figure 4. When dioxygen is introduced in reactor, the R<sub>F</sub>>0 thin films appear to be homogeneous, with
no apparent distinct microstructures. However, in the case of the R<sub>F</sub>=0 sample prepared in nitrogen
plasma, darker linear contrasts were observed spanning the entire thickness of the layer. We did not
detect any notable composition variation in relation to HAADF intensity (Figure 5). We can note,
however, that the small variations in the O and N signals are correlated: when the O signal decreases,
the N signal increases (and vice versa). This fluctuation in contrast might be attributed to the presence
of less dense or possibly porous regions within the R<sub>F</sub>=0 film. It is possible that the formation of the
thin films through a columnar growth mode may have led to less dense regions.

The EDX spectra corresponding to the full thickness of the samples are shown in Figure 6. The profiles
associated with R<sub>F</sub>=1 and R<sub>F</sub>=0.75 are relatively similar with around 71 ± 2 at. % O in the layer, 27 ± 2
at.% Ta and around 2 at.% N for R<sub>F</sub>=1. As it is well-known that the detection of EDX signals for elements
such as O and N is not very accurate, these results, leading to a stoichiometry close to Ta<sub>2</sub>O<sub>5</sub>, are in
agreement with XPS measurement. At R<sub>F</sub>=0.75, the layer also exhibits a stoichiometry close to Ta<sub>2</sub>O<sub>5</sub> as
deduced from XPS. However, an in-depth analysis of the layer shows that close to the substrate the N
content increases gradually from 1 at.% to reach 10 at.%. The same phenomenon is observed for sample R<sub>F</sub>=0.4. In the core of the layer and at its surface, the N content reaches around 20% and then increases up to 30% close to the substrate. An initial layer containing N is formed, and the N becomes increasingly difficult to incorporate into the film until, after around 50 nm of growth, a stable layer is obtained (we will discuss the causes later in the IBA section). Finally, for sample R<sub>F</sub>=0, the overall composition is constant throughout the layer thickness (in average: 20 at. % Ta, 34 at. % O and 46 at. % N). The layer produced is favourable to N integration throughout growth.

<figure><img src="image_5.png" /></figure>

a) R<sub>F</sub>=1

<figure><img src="image_6.png" /></figure>

b) R<sub>F</sub>=0.75

<figure><img src="image_7.png" /></figure>

c) R<sub>F</sub>=0.4

<figure><img src="image_8.png" /></figure>

d) R<sub>F</sub>=0
Figure 4: STEM-HAADF images, a) R<sub>F</sub>=1: observed thickness = 135 nm, b) R<sub>F</sub>=0.75: observed thickness = 172 nm, c) R<sub>F</sub>=0.4: observed thickness= 240 nm, d) R<sub>F</sub>=0: observed thickness = 175 nm. The layer of interest is deposited on a Si substrate and is covered by a protective layer (Pt/C) for the FIB preparation.

<figure><img src="image_9.png" /></figure>

Figure 5: a) STEM image of R<sub>F</sub>=0 sample (first 90 nm) and b) Lateral EDX profile of the topmost surface (first 10 nm) along the white (45 nm long) arrow in the STEM image in (a). Atomic compositions in Ta, O, N are plotted as well as HAADF Intensity (right hand side y-axis). c) High magnification STEM image of R<sub>F</sub>=0 sample and d), e) and f) are EDX mapping of Ta, O and N.
<figure><img src="image_10.png" /><figcaption>Figure 6: EDX profile corresponding to the full thickness of the samples and over a width of 200 nm, Ta, O, N are plotted. a) R<sub>F</sub>=1; b) R<sub>F</sub>=0.75; c) R<sub>F</sub>=0.4 and d) R<sub>F</sub>=0.</figcaption></figure>

## 4.2 Selected area electron diffraction

The Selected area electron diffraction (SAED) patterns are presented (Figure 7.a, b, c, d) and confirm the amorphous nature of the films with the occurrence of diffuse rings only. By integrating the intensity of these diffuse rings (blue, magenta and green rings (i.e. Figure 7.a, b, c, d), we were able to obtain signals corresponding to each sample.

Both patterns for R<sub>F</sub>=1 and R<sub>F</sub>=0.75 (Figure 7.e - red and blue lines, respectively) show two broad humps located at 1.8 and 3.6 Å, distances that can be allotted to (002) and (001) indexation planes of Ta<sub>2</sub>O<sub>5</sub> (JCPDS No. 25-0922), respectively. This is in agreement with XPS results again. It has to be noted that if these two features can be detected for R<sub>F</sub>=0.4 (magenta curve) and R<sub>F</sub>=0 (green curve), the general shapes are different. Actually, for R<sub>F</sub>=0.4 (magenta curve), a broad peak account for three TaON plane indexations: (111), (-111) and (110) at 2.74, 3.07 and 3.51 Å (JCPDS No. 20-1235). This can be explained. Then, for R<sub>F</sub>=0 (green curve), a peak is located at 2.87 Å which is coherent with (203/(-203) indexations of Ta<sub>3</sub>N<sub>5</sub> (JCPDS No. 89-5200).
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

Figure 7: SAED patterns of the different films deposited by reactive sputtering a) R<sub>F</sub>=1, b) R<sub>F</sub>=0.75, c) R<sub>F</sub>=0.4 and d) R<sub>F</sub>=0. The blue dotted circles correspond to the main diffraction planes of Ta<sub>2</sub>O<sub>5</sub>. The magenta circle corresponds to TaON signal, and the light green one corresponds to Ta<sub>3</sub>N<sub>5</sub> signal. e) represents the rotational sum of intensities of the two most intense rings where origin corresponds to the center of the SAED pattern.
The different chemical environments of elements in TaON films at their surface and depth were highlighted by XPS, SAED and EDX analyses with the extraction of approximative phases proportions.

To collect more quantitative values of elemental concentrations and support our previous interpretations, we implemented the IBA techniques with an additional value concerning the measurement of hydrogen depth profiles.

### 5) IBA Results

A main characteristic of the IBA techniques is their capabilities to provide depth global composition analysis, with a high accuracy for light elements such as H, N and O. Elemental depth profiles of TaON films derived from IBA techniques (RBS, NRA and ERDA) are presented in Figure 8 and their mean stoichiometries in table 5. Ar gas being used as vector (flow rate at 15 sccm) for N₂ + O₂ reactive gases (2 sccm) introduction in the sputtering chamber, a low incorporation of argon (0.5 at.%) during deposition is observed for all samples. Additionally, H is present at low atomic concentration (less than 10%). The sources of the hydrogen presence in TaON films are multiple, such as the OH or H₂O degassing of sputtering chamber walls under plasma, the contamination of vector/reactive gases or the high TaON surface reactivity with H₂O during its air exposition after deposition. The latter argument would explain the enhanced concentration in H near the surface for all samples. Actually, for R<sub>F</sub>=0, STEM-HAADF image of this particular sample reveals a columnar microstructure hence the constant concentration of H in the sample RF= 0 could be the signature of the high H diffusivity within this
sample

along

preferential

diffusion

paths.

<figure><img src="image_13.png" /><figcaption>Figure 8: Elemental depth profiles derived from total-IBA analysis (i.e. combining various beam conditions, RBS, NRA and ERDA), (symbols were connected by lines to better track the observed trends). a) R<sub>F</sub>=1, b) R<sub>F</sub>=0.75, c) R<sub>F</sub>=0.4, d) R<sub>F</sub>=0.</figcaption></figure>

The IBA analysis (Figure 8) results reveal that samples R<sub>F</sub>=1 and R<sub>F</sub>=0.75 exhibit a O/Ta ratio of (2.7 ± 0.1), that means a value slightly higher than the Ta<sub>2</sub>O<sub>5</sub> one due to the presence of hydroxyl in the film volume. The impact of incorporating a N<sub>2</sub> flow rate during the deposition of R<sub>F</sub>=0.75 is evident with a mean N concentration value of 3.3 at.%. In the case of the R<sub>F</sub>=0.4 sample, concentration gradients are observed for both O and N. Specifically, N concentration decreases from 30.0% to 22.0% towards the surface, while O exhibits an increase from 37% to 47% and Ta decreases from 30.0% to 27.0%. Lastly, the composition of sample R<sub>F</sub>=0 remains constant throughout the layer, with average concentrations of 28.1% Ta, 40.0% N, and 21.3% O.

The important takeaway emerged from the IBA elemental depth profile is the decrease of N content towards the surface for R<sub>F</sub>=0.75 and R<sub>F</sub>=0.4 samples. Effectively, when a high reactive gas as O<sub>2</sub> is introduced during the sputtering process, it interacts with the Ta target, forms an oxidized compound layer at its near surface and influences the sputtering parameters. The progressive poisoning in O of the Ta target
would change the chemical composition of the coating and explain the N concentration depletion in film along the sputtering time.

<table><caption>Table 5: Mean elemental stoichiometries measured by IBA techniques</caption><thead><tr><th>Samples</th><th>Ta (at. %)</th><th>O (at. %)</th><th>N (at. %)</th><th>H (at. %)</th><th>C (at. %)</th><th>Ar (at. %)</th></tr></thead><tbody><tr><td>R<sub>F</sub>=1</td><td>25.3 ± 0.6</td><td>69.3 ± 2.4</td><td>1.0 ± 0.1</td><td>3.9 ± 0.2</td><td>&lt; 0.1</td><td>0.50 ± 0.02</td></tr><tr><td>R<sub>F</sub>=0.75</td><td>24.4 ± 0.7</td><td>65.1 ± 2.3</td><td>3.0 ± 0.2</td><td>6.1 ± 0.4</td><td>1.4 ± 0.1</td><td>0.50 ± 0.02</td></tr><tr><td>R<sub>F</sub>=0.4</td><td>28.3 ± 0.9</td><td>43.5 ± 1.7</td><td>24.0 ± 1.7</td><td>2.2 ± 0.1</td><td>1.0 ± 0.1</td><td>0.50 ± 0.02</td></tr><tr><td>R<sub>F</sub>=0</td><td>28.1 ± 1.1</td><td>21.3 ± 1.0</td><td>40.0 ± 2.9</td><td>8.7 ± 0.6</td><td>1.4 ± 0.1</td><td>0.50 ± 0.02</td></tr></tbody></table>

From the elements concentration extracted from IBA spectra and their chemical environments informed by XPS and SAED, we can establish consolidated assumptions (Table 6) concerning the mixture of the different phases formed during our plasma deposition conditions. We have considered that if OH groups could be incorporated during film synthesis, some contamination due to the high TaON surface reactivity with H₂O during its air exposition after deposition could also occur, as shown in H depth profiles and the oxynitride of tantalum as stoichiometric [15], [41]. The IBA results corroborate in broad line the first interpretations extracted from XPS measurements, but with a lower relative proportion of the Ta₂O₅ phase for all R<sub>F</sub> values due to OH amount introduction. Additionnaly, it is important to mention that XPS technique probes only a depth of 10 nm, certainly suroxidized.

<table><caption>Table 6: Relative proportions of phases in TaON films</caption><thead><tr><th>Samples</th><th>Stoichiometry</th></tr></thead><tbody><tr><td>R<sub>F</sub>=1</td><td>(Ta<sub>2</sub>O<sub>5</sub>)<sub>0.92</sub>(OH)<sub>0.08</sub></td></tr><tr><td>R<sub>F</sub>=0.75</td><td>(Ta<sub>2</sub>O<sub>5</sub>)<sub>0.80</sub>(TaON)<sub>0.08</sub>(OH)<sub>0.12</sub></td></tr><tr><td>R<sub>F</sub>=0.4</td><td>(Ta<sub>2</sub>O<sub>5</sub>)<sub>0.22</sub>(TaON)<sub>0.74</sub>(OH)<sub>0.05</sub></td></tr><tr><td>R<sub>F</sub>=0</td><td>(Ta<sub>2</sub>O<sub>5</sub>)<sub>0.21</sub>(Ta<sub>3</sub>N<sub>5</sub>)<sub>0.62</sub>(OH)<sub>0.17</sub></td></tr></tbody></table>

The determination of TaON films thicknesses in at.cm⁻² from IBA analyses and in cm from STEM-HAADF images (Figure 4) allows to calculate their density in at.cm⁻³ or g.cm⁻³. The films densities are presented in table 7 and compared to the Ta₂O₅ and Ta₃N₅ bulk ones. The ratios of R<sub>F</sub>=1 and R<sub>F</sub>=0.75 samples
density and Ta₂O₅ bulk one is (0.93 ± 0.03) and (0.97 ± 0.03), respectively. The porosity yield of R<sub>F</sub>=1 sample estimated to 7 % is too low to detect less dense regions by STEM-HAADF or the porosity is formed by nanoporosities, which their sizes cannot be detected. The ratio of R<sub>F</sub>=0 sample density and Ta₃N₅ bulk one is (1.00 ± 0.04) and is in coherence the high abundance of this phase as shown in table 6. The density of R<sub>F</sub>=0.4 sample has an intermediate value between Ta₂O₅ and Ta₃N₅ ones and correlates the XPS characterizations highlighting the various chemical environments constituting the thin film, from oxide, oxynitride and nitride environments.

<table><caption>Table 7: Absolute and relative densities of TaON films calculated from IBA and STEM. The material nature to calculate the relative densities is mentioned in brackets.</caption><thead><tr><th>Samples</th><th>Density ρ (g.cm<sup>-3</sup>)</th><th>Relative density</th></tr></thead><tbody><tr><td>R<sub>F</sub>=1</td><td>7.6 ±0.2</td><td>0.93 ± 0.03 (Ta<sub>2</sub>O<sub>5</sub>)</td></tr><tr><td>R<sub>F</sub>=0.75</td><td>7.9 ±0.2</td><td>0.97 ± 0.03 (Ta<sub>2</sub>O<sub>5</sub>)</td></tr><tr><td>R<sub>F</sub>=0.4</td><td>8.4 ± 0.3</td><td>-</td></tr><tr><td>R<sub>F</sub>=0</td><td>9.9 ±0.4</td><td>1.00 ± 0.04 (Ta<sub>3</sub>N<sub>5</sub>)</td></tr><tr><td>Ta<sub>2</sub>O<sub>5</sub> bulk reference</td><td>8.2</td><td>-</td></tr><tr><td>Ta<sub>3</sub>N<sub>5</sub> bulk reference</td><td>9.9</td><td>-</td></tr></tbody></table>

## 6) Conclusion

In this work, TaOₓNᵧ thin film were deposited by reactive magnetron sputtering of a Ta target in an Ar/N₂/O₂ plasma mixture.

Firstly, it has been shown that with a N flow rate of 25.0% (i.e 75.0% oxygen), N does not incorporate throughout the entire layer; it is only present at the beginning of growth, close to the substrate. This result was demonstrated by 2 different techniques: STEM-EDX and by IBA. XPS experiment also confirmed the absence of nitrogen at the surface (<10 nm).

Moreover, XPS was used to identify the chemical environments present at the very surface of the thin films. Ta₂O₅ is the only chemical environment present for the R<sub>F</sub>=1 and R<sub>F</sub>=0.75 samples. In addition to Ta₂O₅, a peak related to TaOₓNᵧ was identified for the R<sub>F</sub>=0.4 sample (60.0% N₂ flow rate during deposition). Considering the composition and proportion of the Ta₂O₅ and TaOₓNᵧ peaks, this chemical environment was determined to be slightly lacunar in oxygen. Finally, for the R<sub>F</sub>=1 sample, the Ta₃N₅ phase and Ta₂O₅ were clearly identified by XPS. These results were confirmed by TEM-SAED, where the
same chemical environments were revealed, this time throughout the layer. The quantification of light
elements concentrations as a function of depth by IBA techniques allowed to bring consolidated
assumptions about the phases present in tantalum oxynitride films. IBA analyses show that the
majority of hydrogen atoms are embedded in thin films during air-exposure after deposition as
hydroxyl groups for all samples.

Finally, the coupling of the IBA and STEM-HAADF results were used to calculate the densities (g.cm⁻³) of films versus RF values. This parameter is a key data in the choice of the synthesis conditions of the tantalum oxynitride coating. This coating deposited on absorbers for concentrating solar power (CSP) technologies has to act as antireflective layer and as barrier layer against oxygen sorption and diffusion towards the absorber. A barrier against oxygen transport has to present no preferential diffusion path. The R<sub>F</sub>=0 sample is excluded from the final material selection, because its columnar microstructure. A dense material with a low nanoporosity concentration seems to us to be a relevant choice and the material with a relative density close to 1 is the RF=0.75 sample. To demonstrate their potential as antireflective coatings for concentrated solar technologies, the evolution of microstructure and stoichiometry of R<sub>F</sub>=0.75 samples in air at 500°C will be studied and reported in a forthcoming paper.

This study highlights how important it is to gain insight in the structural characterisations beyond a
global chemical formulation in order to assess the reactivity of a film with respect with its morphology,
especially in the targeted application, as anti-reflective coatings where the films are in contact with air
and humidity.

Acknowledgments

This work was financed by the French National Research Agency (ANR), as part of the NANOPLAST
project (nanoplast-project.cnrs.fr) under contract ANR-19-CE08-0019. Measurements were
performed using the IMN's characterisation platform, PLASSMAT, Nantes, France.
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