Self-trapped holes and polaronic acceptors in ultrawide-bandgap oxides

Abstract

Although Ga2O3 is widely believed to be one of the most promising ultrawide-bandgap semiconductors, its inability to be p-type doped hampers its future applications. Other oxides have recently emerged as potential competitors to Ga2O3, but their propensity for hole conductivity is less well known. Here, the stability of hole polarons is examined in pristine material and in the presence of impurities for a set of ultrawide-bandgap oxides (Ga2O3, Al2O3, ZnGa2O4, MgGa2O4, LiGaO2, and GeO2). Holes spontaneously self trap in all oxides investigated here. Acceptor impurities (such as group-I elements, N, and F) further stabilize these trapped holes, leading to large acceptor ionization energies. Hole trapping also leads to characteristic distortions and distinct optical transitions, which may explain some experimentally observed signals. These results indicate that achieving p-type conductivity in any of these oxides is unlikely, with the possible exception of GeO2.