Identification and modulation of electronic band structures of single-phase β-(Al_xGa_1-x)₂O₃ alloys grown by laser molecular beam epitaxy

Understanding the band structure evolution of (Al_xGa_1-x)₂O₃ alloys is of fundamental importance for developing Ga₂O₃-based power electronic devices and vacuum ultraviolet super-radiation hard detectors. Here, we report on the bandgap engineering of β-(Al_xGa_1-x)₂O₃ thin films and the identification of compositionally dependent electronic band structures by a combination of absorption spectra analyses and density functional theory calculations. Single-monoclinic β-phase (Al_xGa_1-x)₂O₃ (0 ≤ x ≤ 0.54) films with a preferred (-201) orientation were grown by laser molecular beam epitaxy with tunable bandgap ranging from 4.5 to 5.5 eV. The excellent fitting of absorption spectra by the relation of (αhν)^1/2 (hν-E) unambiguously identifies that β-(Al_xGa_1-x)₂O₃ alloys are indirect bandgap semiconductors. Theoretical calculations predict that the indirect nature of β-(Al_xGa_1-x)₂O₃ becomes more pronounced with increased Al composition due to the increased eigenvalue energy gap between M and Cyrillic capital letter GHE points in the valence band. The experimentally determined indirect bandgap exhibits almost a linear relationship with Al composition, which is consistent with the theoretical calculation and indicates a small bowing effect and a good miscibility. The identification and modulation of (Al_xGa_1-x)₂O₃ band structures allows rational design of ultra-wide bandgap oxide heterostructures for the applications in power electronics and solar-blind or X-ray detection.