Band offsets at metalorganic chemical vapor deposited β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ interfaces—Crystalline orientation dependence

The β-(Al_xGa_1−x)₂O₃ alloy represents an emerging ultrawide bandgap semiconductor material for applications in high-power electronics and deep ultraviolet optoelectronics. The recent demonstrations of orientation-dependent epitaxial growth of high quality β-(Al_xGa_1−x)₂O₃ films have unlocked prospects for the development of ultrahigh frequency β-(Al_xGa_1−x)₂O₃/Ga₂O₃ based transistors with high-power tolerance. To control the electronic and optical properties of β-(Al_xGa_1−x)₂O₃/Ga₂O₃ heterostructure-based devices, an understanding of the band offsets between β-(Al_xGa_1−x)₂O₃ and β-Ga₂O₃ is crucial. However, there have been no systematic experimental studies on the evolution of the band offsets between differently oriented β-(Al_xGa_1−x)₂O₃/Ga₂O₃ heterointerfaces as a function of Al compositions. This work presents the valence and conduction band offsets at (010) and $(2¯01)$ oriented β-(Al_xGa_1−x)₂O₃/Ga₂O₃ interfaces using x-ray photoelectron spectroscopy. β-(Al_xGa_1−x)₂O₃ films with x ≤ 0.35 and x ≤ 0.48 were grown by metalorganic chemical vapor deposition on (010) and $(2¯01)$ oriented β-Ga₂O₃ substrates, respectively. The determined band offsets reveal the formation of a type-II (staggered) band alignment at (010) oriented β-(Al_xGa_1−x)₂O₃/Ga₂O₃ and a type-I (straddling) heterojunction between $(2¯01)$ β-(Al_xGa_1−x)₂O₃ and β-Ga₂O₃. For both crystalline orientations, the valence and conduction band offsets are found to increase with increasing Al content with a much weaker variation in the valence band offsets as compared to the conduction band offsets. Among different orientations investigated, such as (010), $(2¯01)$⁠, and (100), the largest conduction band offset occurs at a β-(Al_xGa_1−x)₂O₃/Ga₂O₃ interface with (100) orientation, providing opportunities for excellent electron confinement with a high-density two-dimensional electron gas. Results from this study on the tunable and orientation-dependent band offsets with the variation of the Al alloy fraction will provide guidance for the design of heterostructures in future β-(Al_xGa_1−x)₂O₃/Ga₂O₃ based devices.