Band-to-band transitions, and ultraviolet dielectric functions of unintentionally-doped (x = 0 . . . 0.3) and Silicon doped (x = 0 . . . 0.25) single crystal (100) β-(Al_xGa_1−x)₂O₃

The monoclinic beta phase of gallium oxide is an ultra-wide bandgap semiconductor that has been widely studied for potential use in high power switching applications. Advances in crystal growth techniques enable us to investigate high quality β-(Al_xGa_1−x)₂O₃ films and bulk substrates. For the first time, the properties of unstrained bulk substrates are investigated, permitting for the decoupling of the effects of strain and the native properties of the lattice. Understanding the fundamental properties of the substrates also permits the investigation of compressive and tensile strain of epitaxial β-(Al_y Ga_1−y )₂O₃ or β-Ga₂O₃ on a different solid solution β-(Al_xGa_1−x)₂O₃ crystal substrate, which has not yet been done.

Here, we study β-(Al_xGa_1−x)₂O₃ (x = 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) and Silicon doped (x = 0.05, 0.1, 0.15, 0.2, and 0.25) both with (100) surface orientation. Bulk β-(Al_xGa_1−x)₂O₃ crystals were grown by the Czochralski method using Ir crucibles and oxidizing atmosphere. We present model dielectric functions in the near ultraviolet to the vacuum ultraviolet. We investigate the changes in the optical bandgap and band-to-band transitions associated with the increasing aluminum concentration. We compare the results to our previous work done on an additional sample set of pseudomorphically strained β-(Al_xGa_1−x)₂O₃ films on (010) β-Ga₂O₃, with aluminum molar content up to 21%. We contrast the data obtained from the doped samples to the unintentionally-doped samples to resolve the effects of doping on free charge carrier concentration, bandgap, and higher photon energy band-to-band transitions.