# Influence of Growth Temperature and Scandium Concentration on the Surface Oxidation of ScAlN Films Grown by Molecular Beam Epitaxy

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

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Valentina Gallardo-Mödinger, Frédéric Georgi, Ileana Florea, Xavier Wallart, Philippe Vennéguès, et al. Influence of Growth Temperature and Scandium Concentration on the Surface Oxidation of ScAlN Films Grown by Molecular Beam Epitaxy. Wocsdice Exmatec 2025, Jun 2025, Cadiz, Spain. hal-05168319

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# Influence of Growth Temperature and Scandium Concentration on the Surface Oxidation of ScAlN Films Grown by Molecular Beam Epitaxy

**Valentina Gallardo-Mödinger**¹*, **Frédéric Georgi**², **Ileana Florea**¹, **Xavier Wallart**³, **Philippe Vennéguès**¹, **Yvon Cordier**¹ and **Maxime Hugues**¹.

¹Université Côte d'Azur, CNRS, CRHEA, rue B. Gregory, 06560 Valbonne, France, vgm@crhea.cnrs.fr

²Mines Paris, PSL University, Center for Material Forming (CEMEF), UMR CNRS, 06904 Sophia Antipolis, France

³University Lille, CNRS, Centrale Lille, Junia, University Polytechnique Hauts de France, UMR 8520-IEMN, F59000 Lille, France

## Abstract

Recently, there has been a large interest in the ScAlN alloy, thanks to its various applications in high-frequency high-power, acoustoelectric, and ferroelectric devices. Nonetheless, these applications could be impaired by defects present in the ScAlN alloy, such as oxygen impurities, a very prevalent defect due to the high oxygen affinity of scandium. Recent trends in ScAlN growth such as lowering growth and increasing Sc content could have an effect in the appearance of these impurities. Considering this, we have studied the influence of growth temperature and Sc content in the surface oxide and oxygen content of ScAlN films grown by ammonia source molecular beam epitaxy and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and angle resolved- X-ray photoelectron spectroscopy.

## Introduction

The ScₓAl₁₋ₓN alloy has attracted great interest in the last years due to its large piezoelectric and spontaneous polarization coefficients as well as the possibility to be grown lattice matched to GaN. Consequently, several groups have developed the epitaxial growth of ScₓAl₁₋ₓN to benefit of these properties for realizing high electron mobility transistor (HEMTs) and acoustic wave devices [1,2].

In addition, the discovery of its ferroelectric behavior in 2019 has boosted even more the interest for this alloy with the perspective to develop ferroelectric memory, ferroelectric field effect transistors and the possibility to co-integrate it with other III-Nitrides [3].

However, scandium possesses a large oxygen affinity, three times larger than aluminum, which results in an oxidation of the ScₓAl₁₋ₓN film surface but also to a high oxygen incorporation inside the layer (i.e. oxygen concentration ranging between 10¹⁹ to 10²⁰ cm⁻³) [3,4]. This native oxide and high oxygen concentration will impact the process reliability and the device performances, respectively.

These observations motivated us to study in-depth the surface oxide layer and the oxygen concentration in ScₓAl₁₋ₓN thin films grown by ammonia source molecular beam epitaxy (NH₃-MBE). Our 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).

## Experimental Details

Two sample series of ScₓAl₁₋ₓN on GaN were grown in a Riber Compact 21 T reactor. Effusion cells for gallium, aluminum and scandium were used, while ammonia was provided via an injector with a flow rate of 200 sccm [5]. The heterostructure consists of a thick GaN layer grown on sapphire substrate and a 10-25 nm thick ScxAl1-xN layer. For the first series, the growth temperature of the ScₓAl₁₋ₓN layer was varied between 620 °C to 800 °C, maintaining a Sc composition of x=0.15 and a thickness of 25 nm. For the second series, the Sc composition of the ScₓAl₁₋ₓN layer was varied between x=0.15 to x=0.30. A growth temperature of 650 °C and a thickness of 10 nm were kept constant.

The elemental characterization of the samples was performed by XPS and AR-XPS using a Thermo Scientific K-Alpha XPS system and a Physical Electronics model 5600 spectrometer, respectively. Decomposition of the peaks to observe the specific bonding configuration was performed using the Avantage and Multipak softwares. Structural analyses of a representative sample were performed using a ThermoFisher Titan SPECTRA 200 transmission electron microscope.

## Results and discussion

As mentioned, ScₓAl₁₋ₓN films generally present an oxide layer at the surface. To observe this oxide, we have performed TEM observation. On Fig. 1a an oxide layer of 3 nm can be clearly seen at the sample surface and the abundant presence of oxygen is confirmed in the EDS map.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) TEM and EDS images for a ScₓAl₁₋ₓN film. (b) XPS survey of a ScAlN sample grown at 720 °C with a scandium concentration of 15%. The principal peaks analyzed are labeled. (c) High-resolution XPS spectra and the decompositions for the same sample shown in (b). The black dots represent the experimental data, while the red line is the result of the fitting procedure.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

We performed a full XPS survey along with high resolution measurements of the Sc 2p, N 1s, O 1s, Al 2p, C 1s and Ga 3d peaks. Fig. 1b shows a representative spectrum for the surface of our samples. Here, we can clearly observe the large intensity of the O 1s peak, indicating the presence of a considerable amount of oxygen. Fig. 1c shows the spectra of the different elements with their corresponding decompositions. These decompositions were used to obtain quantitative information of the elemental composition of each sample by calculating the atomic percentage considering their sensitivity factors. In Fig. 2 we can observe the atomic percentage at the surface of the samples grown at different temperatures and with various Sc contents. We can observe two trends: (1) an increase of O concentration for the lower growth temperatures, (2) a significant increase of the O concentration as Sc content increases. Along with this, we can observe an inverse relation between the O and the N concentrations. Thus, we can assume that most of the O atoms replace N ones in the crystal. This assumption is supported by theoretical prediction of energetically favored O incorporation at the nitrogen site for ScN.[6]

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Atomic percentage calculations for each growth temperature and Sc content studied.</figcaption></figure>

Along with the surface measurements, samples were successively etched using an argon ion gun to obtain oxygen profiles along the ScₓAl₁₋ₓN layer thickness. In Fig. 3 we can observe the O depth profiles for each sample. For ease of observation, logarithmic scale has been used. Here, we can observe how the atomic percent rapidly decreases as depth increases, thus confirming the strong oxidation of the surface.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** O content depth profiles for each (a) growth temperature and (b) Sc content studied.</figcaption></figure>

To further examine the surface oxide layer, we performed AR-XPS measurements on the samples with 3 different Sc contents. Typical measurements for the sample with 15% Sc are reported in Fig. 4.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Decompositions of the Sc 2p and N 1s spectra for the sample with 15% Sc content, measured at 25°, 45° and 75°. The black dots represent experimental data while the red line is the result of the fitting procedure. The diagrams show the position of the X-ray and detector, along with the probing depth at the corresponding angle.</figcaption></figure>

As we decrease the scanning angle, meaning we increase the surface sensitivity, we can observe an increase in the Sc-O (green deconvoluted peaks) and N-O (grey peak) bonds, and thus an increase of the O concentration. Note, that at 25° we are only probing the first 4.2 nm of the sample, which would mainly correspond to the oxide layer thickness according to the TEM observation. Therefore, this measurement gives a quite accurate estimation of the surface oxide layer composition.

## Conclusions

In conclusion, a clear correlation between oxygen concentration, growth temperature and scandium concentration exists. We have been able to observe and accurately characterize the surface oxide layer of ScₓAl₁₋ₓN films. This oxide layer presents a higher O content at lower growth temperatures and higher Sc concentrations. These results clearly highlight the crucial role of surface passivation as the current trend is to increase the Sc content to match application requirements, which is generally combined to a lowering of the growth temperature.

## Acknowledgments

The authors acknowledge financial supports from the French National Research Agency (ANR) project TWINS (ANR-23-CE51-0011) and through the "Investissements d'Avenir" program GaNeX (Grant No. ANR-11-LABX-0014). 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.

## References

[1] Akiyama, M. et al., Adv. Mater., 21(5), 593-596 2009.

[2] Ambacher, O. et al., J. Appl. Phys., 130(4), 045102, 2021.

[3] Moram, M.A. et al., J. Mater. Chem. A, 2(17), 6042-6050, 2014.

[4] Dinh, D.V. et al., Appl. Phys. Lett., 122(15), 152103, 2023.

[5] Elias, C. et al., APL Mater., 11(3), 031105, 2023.

[6] Kumagai, Y. et al., Phys. Rev. Appl., 9(3), 034019, 2018.