Bandgap narrowing and hole self-trapping reduction in Ga2O3 by Bi2O3 alloying

Ga₂O₃ is an emerging material with attractive electrical properties for improving the performance of high-voltage power electronics, and it is widely accepted that engineering its band edge energies will expand its applications, especially for bipolar devices. In this work, monoclinic phase (Bi_xGa_1−x)₂O₃ ternary alloys are fabricated on c-plane sapphire via magnetron co-sputtering of Ga₂O₃ and Bi₂O₃. The bandgap of Ga₂O₃ is found to decrease from 4.97 to 4.78 eV by alloying with a Bi x-fraction of 0.04, consistent with the theoretical prediction that alloying with Bi₂O₃ causes the up-shift of the valence band. Concurrent temperature-resolved cathodoluminescence (CL) measurements of the Ga₂O₃ and (Bi_0.04Ga_0.96)₂O₃ are employed to investigate their intrinsic ultraviolet (UV) luminescence and defect-related visible bands with increasing temperature from 80 K. The CL results reveal a more rapid thermal quenching of the self-trapped hole (STH) emission with an activation energy of only 32 meV, suggesting that alloying with Bi₂O₃ lowers the self-trapping energy for holes. Temperature-dependent electrical measurements reveal the conductivity of the (Bi_0.04Ga_0.96)₂O₃ film is one order of magnitude lower than that of the undoped Ga₂O₃ counterpart; however, both possess an electrical activation energy of ∼ 26 meV, likely associated with an impurity-related shallow donor. The low electrical conductivity of (Bi_0.04Ga_0.96)₂O₃ can be attributed to the compensation of shallow donors by acceptors activated by the Bi₂O₃-induced upward shift of the valence band edge. The frequency-dependent conduction and ionic polarization mechanisms up to 10⁷ Hz are found to be identical for Ga₂O₃ and (Bi_0.04Ga_0.96)₂O₃, indicating that the conduction in this ternary alloy does not involve the activation of Bi acceptors.