The electronic structures of β-phase (Al_xGa_1-x)₂O₃ studied using DFT calculations

Band gap engineering by alloying Ga₂O₃ with Al₂O₃ can expand the accessible bandgap for power electronics and high electron mobility transistors. For such applications, it is important to know the evolution of the electronic structure with composition and the respective band offsets between β-(Al_xGa_1-x)₂O₃ and Ga₂O₃. In this work, we use first-principles calculations to investigate the crystal structure, band gap and electronic structure of β-(Al_xGa_1-x)₂O₃ alloys for various Al compositions. We found that the band gap of β-(Al_xGa_1-x)₂O₃ alloys increases from 4.86 eV (x = 0) to 5.80 eV (x = 0.5). In β-(Al_xGa_1-x)₂O₃ alloys, the conduction band minimum (CBM) shifts upward with increasing Al content, due to the contribution of Al 3s states replacing Ga 4s states at the conduction band edge. In contrast, the valence band maximum (VBM) exhibits a relatively small downward shift. This is primarily attributed to the weakened Ga 3d-O 2p interaction at the VBM, where the p-d repulsion typically acts to push the VBM upward. We observe that β-(Al_xGa_1-x)₂O₃ alloys show type I (straddling) band alignment with β-Ga₂O₃. The effective mass of β-(Al_xGa_1-x)₂O₃ alloys increases with higher aluminum content, which can be explained by the less delocalized Al 3s orbitals in place of the more delocalized Ga 4s orbitals at the conduction band minimum.