Self-trapped holes and polaronic acceptors in ultrawide bandgap oxides

Although Ga₂O₃ is widely believed to be the among most promising ultrawide-bandgap semiconductors, its inability to be p-type doped hampers future applications. Recently, other oxide materials have emerged as potential competitors to Ga₂O₃, but their propensity for hole conductivity is less well known. Here the stability of hole polarons in a set of ultrawide-bandgap oxides (Ga₂O₃, Al₂O₃, ZnGa₂O₄, MgGa₂O₄, LiGaO₂ and GeO₂) is examined and compared, both in pristine material and in the presence of acceptor impurities. Holes spontaneously self-trap in all oxides investigated, with varying stabilities. Acceptor impurities 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.