Incorporation of aluminum in (Al_xGa_1−x)₂O₃alloys

Monoclinic β-Ga₂O₃ is a wide-gap (4.8 eV) semiconductor that exhibits a high breakdown field. Alloying Ga₂O₃ with Al₂O₃ can further increase the band gap. Epitaxial growth of (Al_xGa_1−x)₂O₃ is important for forming heterojunctions to Ga₂O₃ for high power electronics applications. Density functional theory (DFT) studies have found that Al atoms prefer to occupy the octahedral sites in bulk (Al_xGa_1−x)₂O₃ alloys.¹ However, recent experimental studies have demonstrated that Al can occupy tetrahedral sites in concentrations almost as high as the octahedral sites. In this work, we perform DFT calculations to explore the co-adsorption of Al, Ga, and O adatoms on the Ga₂O₃ (010) surface. We find that Al can adsorb on tetrahedral sites in most of the reconstructions. The migration barrier of an Al adatom escaping from a tetrahedral site to an octahedral site is 1.81 eV, indicating that Al diffusion is much more restricted than Ga diffusion on the surface. Our findings indicate that kinetic limitations are responsible for Al occupying both octahedral and tetrahedral sites in (Al_xGa_1−x)₂O₃ thin films.
[1] H. Peelaers, J. B. Varley, J. S. Speck, and C. G. Van de Walle, Appl. Phys. Lett. 112, 242101 (2018).