Bandgap Tailoring of Monoclinic Single-Phase β-(Al_xGa_1−x)₂O₃ (0 ≤ x ≤ 0.65) Thin Film by Annealing β-Ga₂O₃/Al₂O₃ Heterojunction at High Temperatures

Herein, bandgap tuning of monoclinic (−201)-oriented β-(Al_xGa_1−x)₂O₃ thin films is achieved through a β-Ga₂O₃/Al₂O₃ heterojunction by a feasible annealing process with an O₂ atmosphere. During the annealing process, Al atoms of the Al₂O₃ substrate outdiffuse easily into the β-Ga₂O₃ thin film deposited by ozone-assisted molecular beam epitaxy (OMBE). The Al compositions in the β-(Al_xGa_1−x)₂O₃ samples are tuned through adjusting the annealing temperature from 800 to 1300 °C and experimentally determined from the result of X-ray photoelectron spectroscopy (XPS) measurements combined with Vegard's law. Successive Al-composition-gradient β-(Al_xGa_1−x)₂O₃ thin films with controlled bandgap are constructed. On these bases, β-Ga₂O₃ thin films are deposited on β-(Al_xGa_1−x)₂O₃ (0 ≤ x ≤ 0.65) substrates through OMBE, yielding β-Ga₂O₃/β-(Al_xGa_1−x)₂O₃ (0 ≤ x ≤ 0.65) heterojunction structures, and the band offsets of this heterojunction are determined by XPS accordingly. This methodology to achieve high-quality β-(Al_xGa_1−x)₂O₃ thin films with adjustable Al composition and tunable band offsets of the β-Ga₂O₃/β-(Al_xGa_1−x)₂O₃ interface will provide guidance for potential strategies to develop and fabricate β-(Al_xGa_1−x)₂O₃-based deep-UV photodetectors and power devices.