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

Ga ₂O ₃ 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) ₂O ₃ ternary alloys are fabricated on c -plane sapphire via magnetron co-sputtering of Ga ₂O ₃ and Bi ₂O ₃. The bandgap of Ga ₂O ₃ 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 Bi ₂O ₃ causes the up-shift of the valence band. Concurrent temperature-resolved cathodoluminescence (CL) measurements of the Ga ₂O ₃ and (Bi _0.04Ga _0.96) ₂O ₃ 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 Bi ₂O ₃ lowers the self-trapping energy for holes. Temperature-dependent electrical measurements reveal the conductivity of the (Bi _0.04Ga _0.96) ₂O ₃ film is one order of magnitude lower than that of the undoped Ga ₂O ₃ 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) ₂O ₃ can be attributed to the compensation of shallow donors by acceptors activated by the Bi ₂O ₃-induced upward shift of the valence band edge. The frequency-dependent conduction and ionic polarization mechanisms up to 10 ⁷ Hz are found to be identical for Ga ₂O ₃ and (Bi _0.04Ga _0.96) ₂O ₃, indicating that the conduction in this ternary alloy does not involve the activation of Bi acceptors.