# Surface oxidation and oxygen incorporation in ScAlN films grown by molecular beam epitaxy

Valentina Gallardo-Mödinger, Frédéric Georgi, Xavier Wallart, Ileana Florea, Philippe Vennéguès, Yvon Cordier, Maxime Hugues

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Valentina Gallardo-Mödinger, Frédéric Georgi, Xavier Wallart, Ileana Florea, Philippe Vennéguès, et al. Surface oxidation and oxygen incorporation in ScAlN films grown by molecular beam epitaxy. Journal of Applied Physics, 2025, 138 (24), pp.245701. 10.1063/5.0303670. hal-05440883

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# Surface oxidation and oxygen incorporation in ScAlN films grown by molecular beam epitaxy

Cite as: J. Appl. Phys. **138**, 245701 (2025); doi: **10.1063/5.0303670**

Submitted: 23 September 2025 · Accepted: 6 December 2025 ·

Published Online: 23 December 2025

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Valentina Gallardo-Mödinger,<fnref n="1" />,ª ID Frédéric Georgi,<fnref n="2" /> ID Xavier Wallart,<fnref n="3" /> ID Ileana Florea,<fnref n="1" /> ID
Philippe Vennéguès,<fnref n="1" /> ID Yvon Cordier,<fnref n="1" /> ID and Maxime Hugues<fnref n="1" /> ID

## AFFILIATIONS

## ABSTRACT

Recently, there has been much interest in the ScAlN alloy, thanks to its various potential applications in high-frequency high-power, acoustoelectric, and ferroelectric devices. Nonetheless, these applications could be impaired by the presence of oxygen in the alloy and the formation of a surface oxide layer due to the high affinity of scandium with such impurity. A current trend is to increase the Sc content and most of the time it is accompanied by a lowering of the growth temperature. As it probably strongly influences the incorporation of impurities, we have carefully studied the effects of growth temperature and Sc content in the surface oxide and oxygen content of ScAlN films grown by ammonia source molecular beam epitaxy. The oxygen concentration and bonding configuration have been determined by combining normal-incidence, angle resolved, and ion-etching x-ray photoelectron spectroscopy.

© 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0303670

## INTRODUCTION

The $Sc_xAl_{1-x}N$ alloy has attracted great interest in the last years due to its large piezoelectric and spontaneous polarization coefficients<fnref n="1" />⁻<fnref n="3" /> as well as the possibility to be grown lattice matched to GaN.⁴⁻⁷ Consequently and considering the necessity for high crystal quality, several groups have developed the epitaxial growth of $Sc_xAl_{1-x}N$ to benefit from these properties for realizing high electron mobility transistors (HEMTs) and acoustic wave devices.⁸,⁹ In addition, the discovery of its ferroelectric behavior in 2019¹⁰ has boosted even more the interest for this alloy with the perspective to develop ferroelectric memories,¹¹ ferroelectric field effect transistors,¹² and the possibility to co-integrate $Sc_xAl_{1-x}N$ with other III-nitrides.¹³

However, scandium exhibits a large affinity with oxygen, approximately three times larger than that of aluminum, which leads to strong surface oxidation of the $Sc_xAl_{1-x}N$ films.¹⁴,¹⁵ Furthermore, due to the relatively low purity of the scandium source, substantial oxygen incorporation within the film is observed, with concentrations typically ranging from $10^{19}$ to $10^{20} \text{ cm}^{-3}$.¹⁴,¹⁶ High oxygen concentrations have been reported to

deteriorate the crystalline quality of the material,¹⁷,¹⁸ reduce the bandgap,¹⁹ and thus consequently, increase the electrical leakages.²⁰ In addition, the presence of oxygen has been correlated to a degradation of piezoelectric properties,²⁸ as well as a potential origin of the ferroelectric cycling fatigue.²¹

In this context, the high oxygen concentration will have a detrimental impact on overall device performances, while the presence of a native oxide will affect their process control. The formation of a native oxide on the surface of $Sc_xAl_{1-x}N$ layers is a common phenomenon, regardless of the deposition technique,¹⁴,¹⁵,²² necessitating external mitigation strategies, such as the addition of a surface passivation layer.²³ Furthermore, the precise nature of this native oxide layer (thickness, chemical composition) is still lacking. In contrast to sputtering, epitaxial techniques such as molecular beam epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) have demonstrated a reduction in oxygen incorporation in $Sc_xAl_{1-x}N$ layers through the use of high-purity scandium sources and precursors, respectively.¹⁶,²²

Driven by new application requirements and the need for enhanced device performance, current research trends in $Sc_xAl_{1-x}N$

<footnote n="1">¹Université Côte d'Azur, CNRS, CRHEA, rue B. Gregory, 06560 Valbonne, France</footnote>

<footnote n="2">²Mines Paris, PSL University, Center for Material Forming (CEMEF), UMR CNRS, 06904 Sophia Antipolis, France</footnote>

<footnote n="3">³University of Lille, CNRS, University Polytechnique Hauts de France, Junia-ISEN, UMR 8520-IEMN, F-59000 Lille, France</footnote>

<footnote>ªAuthor to whom correspondence should be addressed: vgm@crhea.cnrs.fr</footnote>
focus on increasing the scandium content as much as possible.²⁴,²⁵ This approach aims to maximize the piezoelectric response in acoustic wave-based devices²⁶ and to lower the coercive field in ferroelectric structures.²⁵ However, maintaining good crystalline quality at higher scandium concentrations generally requires a reduction in the growth temperature.²⁶,²⁷

Growth conditions have a strong impact on the ScₓAl₁₋ₓN microstructure²⁶ and the former vary depending on the chosen growth technique.²⁸ Most works focused on epitaxial growth of ScₓAl₁₋ₓN have used either plasma-assisted molecular beam epitaxy (PA-MBE)⁴,⁸ or MOCVD²²,²⁹ and, despite its recency, have been able to demonstrate high-performance RF-HEMTs²⁹,³⁰ and surface acoustic wave resonators.³¹ Recently, ammonia source molecular beam epitaxy (NH₃-MBE) has been explored as an option for epitaxial growth of ScₓAl₁₋ₓN³² and has already been used to fabricate RF-HEMTs.³³

These considerations motivated an in-depth investigation of the surface oxide layer and oxygen incorporation in ScₓAl₁₋ₓN thin films grown by NH₃-MBE. Particular attention was given to the influence of growth temperature and scandium content, in light of the current trends toward higher Sc content. The samples were characterized by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), and angle resolved x-ray photoelectron spectroscopy (AR-XPS).

## MATERIALS AND METHODS

Two sample series of ScₓAl₁₋ₓN films were grown on GaN by NH₃-MBE in a Riber Compact 21 T reactor. Effusion cells for gallium, aluminum, and scandium (99.999% pure) were used, while ammonia was supplied via an injector at a flow rate of 200 SCCM. The beam-equivalent pressures for Al and Sc were 1.1 × 10⁻⁷ and 1.15 × 10⁻⁸ Torr, respectively, resulting in a growth rate of 0.15 µm/h.³⁴ The heterostructure consists of a 400–800 nm thick GaN layer grown on a MOCVD GaN-on-sapphire template, followed by a 10–25 nm thick ScₓAl₁₋ₓN layer. A 1 nm AlN interlayer was systematically inserted to improve electron mobility in these HEMT heterostructures. In the first series, the ScₓAl₁₋ₓN layer was grown with a fixed scandium composition of x = 0.15 and a thickness of 25 nm, while the growth temperature was varied between 620 and 800 °C. In the second series, the Sc composition in the ScₓAl₁₋ₓN

layer was varied from x = 0.15 to x = 0.30, while the growth temperature was fixed at 650 °C and the film thickness maintained at 10 nm (see Table I for a summary of the mentioned growth parameters).

The elemental characterization was performed by XPS and AR-XPS using a Thermo Scientific K-Alpha XPS system and a Physical Electronics model 5600 spectrometer, both equipped with a monochromatized aluminum x-ray source. The binding energies of the XPS experimental setup were calibrated using Cu, Ag, and Au standards. Peak decomposition to identify the specific bonding configurations was performed with the Avantage and Multipak softwares, applying the standard criteria for doublet peaks [energy separation, intensity ratio, and full width at half maximum (FWHM)] and using symmetrical peaks consisting of a 30% Lorentzian and 70% Gaussian mix. Structural analysis, particularly the evaluation of the surface oxide thickness, was conducted using transmission electron microscopy with a ThermoFisher Titan SPECTRA 200 microscope.

## RESULTS AND DISCUSSION

As previously mentioned, ScₓAl₁₋ₓN films typically exhibit a native oxide layer at the surface. To investigate this oxide, TEM observations were performed. Figure 1(a) clearly reveals an amorphous oxide layer with a thickness of approximately 3 nm. In Fig. 1(b), the abundant presence of oxygen at the surface is confirmed by the EDS elemental mapping. Following this confirmation of the presence of an oxide layer, we focused on its elemental characteristics and its dependence on the growth temperature and Sc content. For each sample, a full XPS survey was performed, complemented by high-resolution measurements of the Sc 2p, N 1s, O 1s, Al 2p, C 1s, and Ga 3d core-level peaks. Figure 2 presents a representative XPS spectrum acquired from the surface of the samples. A pronounced O 1s peak is observed, indicating the presence of a considerable amount of oxygen at the surface. Additionally, a C 1s peak is detected, which is attributed to surface contamination. As this carbon signal has no physical relevance to the material system under investigation, it is excluded from all subsequent quantitative analyses. The presence of the C 1s peak is probably influenced by the time elapsed between sample growth and XPS analysis. In this study, samples grown two years prior were used to ensure that the surface oxidation had stabilized over time. For comparison, a fresh reference sample was grown and

<table><caption>TABLE I. Growth parameters of the different samples used in this study. Samples 1–5 correspond to the T series and samples 6–8 to the Sc series.</caption><thead><tr><th>Sample #</th><th>Growth temperature (°C)</th><th>Sc composition (%)</th><th>Thickness (nm)</th><th>Al effusion cell temperature (°C)</th><th>Sc effusion cell temperature (°C)</th></tr></thead><tbody><tr><td>1</td><td>620</td><td>15</td><td>25</td><td>1070</td><td>1200</td></tr><tr><td>2</td><td>670</td><td>15</td><td>25</td><td>1070</td><td>1200</td></tr><tr><td>3</td><td>720</td><td>15</td><td>25</td><td>1070</td><td>1200</td></tr><tr><td>4</td><td>760</td><td>15</td><td>25</td><td>1070</td><td>1200</td></tr><tr><td>5</td><td>800</td><td>15</td><td>25</td><td>1070</td><td>1200</td></tr><tr><td>6</td><td>650</td><td>15</td><td>10</td><td>1070</td><td>1200</td></tr><tr><td>7</td><td>650</td><td>22</td><td>10</td><td>1070</td><td>1220</td></tr><tr><td>8</td><td>650</td><td>30</td><td>10</td><td>1070</td><td>1240</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>FIG. 1. (a) TEM image of a representative sample and (b) its corresponding EDS measurement. The oxide layer is marked with a yellow arrow.</figcaption></figure>

characterized exactly one week after its growth. In this sample, the intensity of the C 1s peak was greatly reduced, while the pro- nounced O 1s peak was maintained at a similar level. An XPS study reported by another group has shown that surface oxidation occurs even within one day after growth.¹⁴ The Al 2s and Sc 2s peaks are also observed at 118.1 and 499.6 eV, respectively. However, they were excluded from quantitative analysis due to overlapping loss structures in the case of Al 2s and the low signal intensity of the Sc 2s peak.

<figure><img src="image_6.png" /><figcaption>FIG. 2. XPS survey of a Sc<sub>x</sub>Al<sub>1-x</sub>N sample grown at 720 °C with a scandium concentration of 15%. The principal peaks are labeled.</figcaption></figure>

Quantitative elemental compositions were determined by cal-
culating atomic percentage based on the XPS signals of O 1s, Sc 2p,
N 1s, and Al 2p, considering the corresponding sensitivity factors
for each element. The N 1s peak has been used as reference to
correct some slight binding energy shifts caused by charging effects
in the samples. Figures 3(a)–3(c) show the XPS spectra of Sc 2p, N
1s, Al 2p, and O 1s for the different growth temperatures investi-
gated, along with their corresponding peak decompositions used to
calculate the atomic percentage (at. %). The resulting compositional
data are summarized in Fig. 3(d). In the case of the O 1s region,
two distinct components were observed. However, this spectral
region includes multiple overlapping oxygen-related bonds that are
difficult to unambiguously differentiate.³⁵,³⁶ Therefore, all quantita-
tive analyses were performed using the integrated intensity of the
entire O 1s envelope, but removing the oxygen linked to carbon
obtained through the C 1s peak. Based on the results shown in
Fig. 3, a clear correlation is observed between the growth tempera-
ture and the oxygen concentration. As the growth temperature
increases, the amount of oxygen decreases, reaching a minimum at
720 °C. Beyond this growth temperature, the oxygen content
slightly increases once again. Previous studies on the same sample
series³⁴ have shown that the highest crystalline quality is achieved
for growth temperatures in the range of 670–720 °C. Since minimal
surface oxidation is also observed within this temperature range, a
possible correlation between the crystalline quality of the film and
the extent of surface oxidation cannot be excluded. However, more
<figure><img src="image_7.png" /></figure>

a)

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b)

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c)

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d)

Binding energy (eV)

Binding energy (eV)

Binding energy (eV)

Temperature (°C)

<caption>FIG. 3. XPS spectra and decomposition of (a) Sc 2p (in blue), N 1s (in red), (b) Al 2p (in orange) and (c) O 1s (in gray), for each growth temperature studied. The black dots represent the experimental data and the red line the resulting fit. (d) Atomic percentage for each growth temperature studied, calculated with the decompositions shown in (a), (b), and (c).</caption>

06 January 2026 08:43:19

investigations are required to confirm this assumption. In addition, a correlation between N and O concentrations was also observed, with N content decreasing as O content increases. This trend suggests that O is primarily substituting for N in the scandium bond environment. This assumption is supported by theoretical predictions indicating that O incorporation at the nitrogen site (O_N) is energetically favorable.<sup>37-39</sup>

To investigate the bonding configuration in detail, we focused on the analysis of the Sc 2p and N 1s peaks, following the criteria for doublet peaks mentioned in the Materials and Methods section. This analysis enabled the detection of the N-O contribution

otherwise hidden within the Sc 2p envelope<sup>40,41</sup> and confirmed the absence of metallic Sc.<sup>36,42</sup> An energy separation of 4.7 and 4.37 eV was obtained between the Sc 2p<sub>3/2</sub> and Sc 2p<sub>1/2</sub> components for the Sc-N and Sc-O bonds, respectively (see Fig. S1 and Table S1 in the **supplementary material** for details). Figure 4(a) shows a representative decomposition of the Sc 2p peaks illustrating the contributions from Sc-N and Sc-O bonds. Additional decompositions of the Sc 2p peak for each growth temperature are provided in Fig. S2(a) in the **supplementary material**. Figure 4(b) displays the decomposition of the N 1s peaks showing the N-Al, N-Sc, and N-O bonds. Since the N-Al and N-Sc components occur at similar binding energies,
<figure><img src="image_11.png" /><figcaption>FIG. 4. Decompositions of the spectra recorded on the sample grown at 720 °C. (a) Sc 2p peak, showing each component of the doublet for the Sc-N and Sc-O bonds; (b) N 1s peak, showing the N-(Al, Sc) and N-O contributions. The black dots represent the experimental data and the red line the resulting fit. A summary of the bond contribution for the (c) Sc 2p<sub>3/2</sub> and (d) N 1s peaks.</figcaption></figure>

06 January 2026 08:43:19

they are represented by a single combined peak (red area). Figures 4(c) and 4(d) present the relative bond contributions to Sc and N contents, respectively. For the at. % calculations of Sc, the Sc 2p<sub>3/2</sub> peaks were used. To validate the fitting procedure, the

same analysis was performed using the Sc 2p<sub>1/2</sub> peak as shown in Fig. S2b in the *supplementary material*. A minimum of oxygen content is observed at 720 °C for both at. % calculations. This trend is consistent with the global oxygen content previously extracted
from the full XPS survey (Fig. 3), reinforcing the reliability of the spectral decomposition approach. The convergence of both methods, total atomic percentage analysis and bond-specific fitting, supports the validity of the observed minimum in oxygen incorporation at 670–720 °C. Regarding the bonding configuration, a clear decrease in the Sc–N, N–Al, and N–Sc bond contributions is observed, while the Sc–O and N–O ones increase. This trend is even more noticeable for the Sc-related bonds, probably due to the prevalence of oxygen substituting nitrogen sites (O_N). However, at the lower growth temperatures (620 and 670 °C), which correspond to the higher overall oxygen content, the N–O contribution is

noticeably higher compared to the other growth temperatures. This could suggest that as the oxygen content increases, different types of defects in the bonding configuration become more relevant. Density functional theory (DFT) studies have shown that, at lower O concentrations, the O_N defect is energetically favored, while as the O content rises different types of defects become more energetically favorable.¹⁸

XPS measurements were also performed on samples with different scandium concentrations. Figures 5(a)–5(c) show the Sc 2p, N 1s, Al 2p, and O 1s core-level spectra for each scandium content. The associated peak decompositions, used to calculate at.%, are

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

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**FIG. 5.** XPS spectra and decomposition of (a) Sc 2p (in blue), N 1s (in red), (b) Al 2p (in orange) and (c) O 1s (in gray), for each Sc composition studied. The black dots represent the experimental data and the red line the resulting fit. (d) Atomic percentage for each Sc composition studied, calculated with the decompositions shown in (a), (b), and (c).</figcaption></figure>
presented in Fig. 5(d). A clear increase in oxygen content is observed with raising the scandium concentration. Similar to the effect of the growth temperature, the nitrogen content decreases, probably due to the O<sub>N</sub> defect. Once again, particular attention is

given to the bonding configurations of the Sc 2p and N 1s peaks. Figure 6(a) shows the same decomposition analysis as previously applied, now performed for each sample of the Sc concentration series. A noticeable increase in the Sc–O bond component is

<figure><img src="image_16.png" /><figcaption>FIG. 6. (a) Decompositions of the Sc 2p spectra for the 3 samples with different Sc content, showing each component of the Sc doublet for the Sc-N and Sc-O bonds. The black dots represent the experimental data and the red line the resulting fit. A summary of the bond contribution for the (b) Sc 2p<sub>3/2</sub> and (c) N 1s peaks.</figcaption></figure>
observed as Sc content increases, accompanied by a corresponding decrease in the Sc-N bond contribution. The at.% calculations, shown in Fig. 6(b), confirm this trend. Focusing on the N-related bonding configurations, Fig. 6(c) shows the corresponding at.% values. Similar to the observations made for the varying growth temperatures, an increase in the N-O bond is observed alongside a decrease in both N-Sc and N-Al bonds. As mentioned before, more complex defects appear as O content increases.

Given the relatively small XPS probed area (400 × 800 µm²), it was essential to verify that the measurements, and thus the analyses, were not affected by potential oxidation inhomogeneities across the sample surface. For that purpose, a series of measurements was performed along the radius of one of the samples, as shown in Fig. S3(a) in the **supplementary material**. For the Sc 2p, N 1s, Al 2p, and O 1s core levels, the measurements reveal a high degree of at.% homogeneity [Figs. S3(b) and S3(c) in the **supplementary material**], thereby confirming the reliability of the analyses performed on the sample series. A slight increase in oxygen content is observed toward the edge of the sample. This trend is consistent with the results obtained from the series of samples grown at different temperatures, as the growth temperature is known to be lower near the sample edges within the reactor. Finally, the spectral maps of the N 1s and O 1s core levels, presented in Figs. S3(d)-S3(e) in the **supplementary material**, clearly illustrate the opposite evolution of nitrogen and oxygen distributions.

In addition to surface measurements, depth profiling was performed by successive argon ion etching to obtain the oxygen profiles along the ScₓAl₁₋ₓN layer thickness. Considering that oxygen is a light element, it can be subject to both preferential sputtering and knock-on effects. In consequence, the oxygen concentrations obtained after etching should be regarded qualitative rather than quantitative. Figure 7(a) shows the complete depth profile for a representative sample. It is important to emphasize that the apparent gradient observed between 16 and 26 nm does not reflect a compositional gradient within the layers, but rather results from the finite probed depth of XPS. The probed depth was estimated as three times the inelastic mean free path (IMFP) of electrons in the probed material. Using the TPP-2M model¹³ for ScₓAl₁₋ₓN, the IMFP ranges from 2.1 to 2.4 nm for 900–1050 eV electrons (O 1s, Sc 2p, and N 1s) and from 2.9 to 3.1 for 1350–1450 eV electrons (Al 2p and Ga 3d). Therefore, the effective probing depth varies between 6.3 and 9.3 nm depending on the element. For simplicity, a fixed probed depth of 10 nm is used in all the following analyses. Because of this probed depth, the GaN layer begins to contribute to the signal around 10 nm before the ScₓAl₁₋ₓN and AlN layers are completely removed by etching and after this, influences the calculation of the N content due to the presence of Ga Auger peaks in the same area that are difficult to separate. Taking this into account, the data analysis was limited to the first 100 s of etching for the growth temperature series and 40 s for the Sc series, prior to the point at which the GaN layer begins to influence the measurements. After each etching step (10 s), multiple O 1s measurements were performed over time to evaluate the timescale of the reoxidation process in the XPS analysis chamber, which was under ultra-high vacuum at 5 × 10⁻⁷ mbar under an argon flow. Figure 7(b) shows the reoxidation curves (i.e., the variation of the oxygen at.% as a function of the time) recorded after five etching steps (i.e., 50 s

of etching) for the samples grown at different temperatures. The data indicate that reoxidation within the first two minutes post-etching is minimal and relatively stable. Consequently, the O 1s spectra acquired 1 min after etching were selected for analysis as they offer improved spectral resolution compared to those recorded at 15 s. Beyond two minutes, a linear relationship between time and amount of oxygen is observed. The reoxidation rate appears nearly identical across all samples with different growth temperatures, thus indicating an insignificant influence of the crystalline quality on the reoxidation process. A similar trend was obtained for the samples with different Sc concentrations.

Figure 7(c) shows the O content depth profiles for each growth temperature studied. To facilitate the comparison, a logarithmic scale was used. After the first etching step, corresponding to approximately 1.4 nm of etching, a pronounced drop in oxygen content is observed. This suggests the presence of an oxygen-rich surface layer, as previously observed in our TEM images. While this trend is consistent across all samples in the series, the rate of oxygen content decrease becomes more rapid with increasing growth temperature. After the third etching steps, corresponding to a depth of approximately 4–4.5 nm, the sample grown at higher temperatures reaches a stable O content of approximately 2%. At this stage, it is not possible to determine whether the observed saturation level corresponds to the bulk oxygen concentration incorporated during growth, potentially originating from intrinsic oxygen contamination in the scandium source, from the small post-etching oxidation, or if it reflects the detection limit of our XPS setup. However, the latter explanation has been confirmed as secondary ion mass spectrometry (SIMS) measurements on a representative sample (Fig. S4 in the **supplementary material**) indicate a much lower oxygen concentration in the bulk. Indeed, the oxygen concentration decreases from 6 × 10²¹ cm⁻³ at the surface (corresponding to 10%–15% oxygen), down to approximately 3 × 10¹⁹ cm⁻³ inside the layer. As for Sc concentration,²⁸ an excellent agreement in surface oxygen concentrations is obtained from both XPS and SIMS measurements. The surface oxygen concentration deduced from the XPS and SIMS measurements are in good agreement with the value reported for sputtered layers.²¹,⁴⁴ For bulk oxygen concentration, the resolution of the XPS system is limited (≈10²¹ cm⁻³) but the concentrations measured in our samples by SIMS (10¹⁹–10²⁰ cm⁻³) are consistent with the values reported by other groups using plasma-MBE.¹⁶,²⁰,⁴⁵ Despite the resolution limit of the XPS, the oxygen depth profiles are similar, and the difference in concentration values does not affect the overall observation: both the oxygen concentration in the surface layer, and its penetration into the bulk, increase as the growth temperature decreases.

Figure 7(d) shows the O content depth profiles for the samples with different Sc concentrations, plotted on a logarithmic scale for clarity. As previously observed for the surface, a consistent trend is evident throughout all etching depths: the oxygen content increases with increasing scandium concentration, which can be attributed to the high O affinity of Sc. When compared with the O content vs growth temperature profiles shown in Fig. 7(c), it becomes apparent that, for both conditions corresponding to maximum oxygen incorporation (i.e., 620 °C for growth temperature and 30% for Sc content), the decrease in O content after etching is less abrupt than for the other cases. This behavior may
<figure><img src="image_17.png" /><figcaption>FIG. 7. (a) Concentration profile measured during Ar ion etching of the sample grown at 720 °C. The zone where the probed depth includes signals from the ScₓAl₁₋ₓN layer and the underlying GaN simultaneously is marked with black dotted lines. (b) O content depending on the time after etching, showing the reoxidation of each sample of the T series. O content in the logarithmic scale, depending on the etch time for each sample of the (c) T series and (d) Sc series. Every etching step corresponds approximately to 1.4 nm of etching.</figcaption></figure>

be explained by diffusion tails of the oxygen atoms from the surface layer into the bulk of the material. However, further investigation is required to determine whether this diffusion occurs naturally during the oxidation process or is induced by the etching procedure.

It is important to note that the at. % does not directly represent the atomic composition of the ScₓAl₁₋ₓN alloy, as this

nominal formula does not account for the presence of point defects within the layer. By correlating the measured at. % with the elemental composition, we found that the alloy can be reasonably approximated as ScₓAl₁₋ₓNᵧO₁₋ᵧ for the bulk of the material. However, at the surface, the combined amount of oxygen and nitrogen exceeds 50%, indicating a deviation from the expected stoichiometry, likely due to surface oxidation and/or defect-related
<figure><img src="image_18.png" /><figcaption>FIG. 8. Decompositions of the Sc 2p spectra for the three samples of the Sc series. The rows indicate the angle of measurement (25°, 45°, or 75° with respect to the sample surface) and the first, second, and third columns indicate the samples with 15%, 22%, and 30% of Sc content, respectively. The contribution of each component of the Sc doublet, the oxidation peaks, and the bond between the N-(Al, Sc) are shown. The black dots represent the experimental data and the red line the resulting fit. The diagrams in the inset represent the positions of the x-ray beam and the detector, as well as the probed depth corresponding to each angle of measurement. The probed depth is calculated by 10*sin(θ) nm.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>FIG. 9. (a) O content for the three samples of the Sc series, measured at 25°, 45°, and 75°. (b) O content depending on the effective probed depth for the three samples with different Sc content. The triangles represent the data obtained by AR-XPS, while the dots represent the data obtained by XPS during Ar ion etching. For the AR-XPS values, the effective probing depth is calculated considering the probed depth corresponding to each angle of measurement, as shown in Fig. 8. For the XPS data, the calculation considers the standard 10 nm of probed depth plus the depth etched on each step.</figcaption></figure>

enrichment. Therefore, this compositional model is not suitable for describing the surface region. Based on this analysis, it can be concluded that the O<sub>N</sub> defects are predominant in the bulk, while the higher oxygen concentration at the surface gives rise to a broader variety of oxygen-related defects, as previously discussed.<sup>18,46</sup>

Finally, to gain further insight into the surface oxide layer, AR-XPS measurements were performed for three different electron takeoff angles on the sample series with different Sc contents. Figure 8 shows the Sc 2p and N 1s core-level spectra for three scandium contents, 15%, 22%, and 30% (from the left to the right) and 25°, 45°, and 75° electron takeoff angles with respect to the surface plane (from the top to the bottom). The same peak decomposition procedure was applied to extract the different bonding contributions. Figure 9(a) shows the O content as a function of the Sc content for the three detection angles. It is worth noting that the observed increase in oxygen content with increasing scandium concentration is consistent with the one obtained under normal-incidence conditions (Fig. 6). As the detection angle decreases, thereby increasing the surface sensitivity, a clear increase in the oxygen content is observed, along with a corresponding increase in the Sc-O and N-O bonds. Once again, this is accompanied by a pronounced decrease in the nitrogen content. Under normal detection conditions, the typical probing depth for this material is ~10 nm. In contrast, measurements performed at 25° probe a shallower depth of about 4.2 nm, which is comparable to the oxide layer thickness observed in TEM measurement. Therefore, we can estimate that this measurement provides a good estimation of the chemical composition of the surface oxide layer. Figure 9(b) presents the oxygen content as a function of the estimated effective

probed depth combining the angle resolved and the Ar etching measurement results. As mentioned before, the probed depth is dependent on the takeoff angle. To estimate the O content throughout the full thickness of the film incorporating XPS and AR-XPS results, an effective probed depth has been established. The effective probed depth was defined as the bottom of the first third of the corresponding XPS probed range as this region accounts for approximately 65% of the detected signal. As the measurements become more surface-sensitive, primarily probing the oxide layer, the oxygen content reaches up to 55%, underscoring the pronounced surface oxidation that can occur in Sc<sub>x</sub>Al<sub>1-x</sub>N films with high Sc concentrations.

These measurements were also used to estimate the thickness of the surface oxide layer.<sup>47</sup> This estimation confirmed the values obtained by TEM, indicating an oxide layer thickness of approximately 3–4 nm. A slight increase in the oxide thickness with rising Sc concentration was also observed (see Table S2 in the *supplementary material*), in agreement with the trend reported in Fig. 7(d).

CONCLUSIONS

In conclusion, a clear correlation between oxygen content, growth temperature, and scandium concentration exists. We have been able to observe and accurately characterize the surface oxide layer of Sc<sub>x</sub>Al<sub>1-x</sub>N films grown by NH<sub>3</sub>-MBE. This oxide layer presents a higher O content at lower growth temperatures and higher Sc concentrations. These results could provide a path to minimize the amount of oxygen incorporation when growing Sc<sub>x</sub>Al<sub>1-x</sub>N layers, thus potentially improving the future device performances
and reliability. These results also highlight the crucial role of surface passivation, especially considering the current trend of increasing scandium content to meet application demands.²⁶,²⁷ This increase in scandium concentration is generally accompanied by a reduction in growth temperature, thereby rendering the issue of surface passivation even more critical.

## SUPPLEMENTARY MATERIAL

See the supplementary material for additional details on the peak decompositions, the bond contribution for the Sc 2p₁/₂ peak, the homogeneity study, SIMS measurements, and the thicknesses of the oxide layer.

## ACKNOWLEDGMENTS

The authors acknowledge financial support from the French National Research Agency (ANR) through the project TWINS (No. ANR-23-CE51-0011), the "Investissements d'Avenir" program GaNeX (Grant No. ANR-11-LABX-0014), and the French technology facility network RENATECH. This work has been also carried out in the framework of the European Project GaN4AP (Gallium Nitride for Advanced Power Applications) under Grant Agreement No. 101007310.

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author Contributions

**Valentina Gallardo-Mödinger:** Data curation (equal); Formal analysis (equal); Investigation (supporting); Methodology (equal); Visualization (equal); Writing – original draft (lead). **Frédéric Georgi:** Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Writing – review & editing (equal). **Xavier Wallart:** Formal analysis (supporting); Investigation (equal); Resources (equal); Writing – review & editing (equal). **Ileana Florea:** Formal analysis (supporting); Investigation (equal); Writing – review & editing (supporting). **Philippe Vennéguès:** Formal analysis (supporting); Investigation (equal); Writing – review & editing (supporting). **Yvon Cordier:** Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal). **Maxime Hugues:** Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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