Hole conductivity through a defect band in ZnGa2O4

Semiconductors with a wide band gap (>3.0 eV), high dielectric constant (>10), good thermal dissipation, and capable of n- and p-type doping are highly desirable for high-energy power electronic devices. Recent studies indicate that ZnGa2O4 may be suitable for these applications, standing out as an alternative to Ga2O3. The simple face-centered-cubic spinel structure of ZnGa2O4 results in isotropic electronic and optical properties, in contrast to the large anisotropic properties of the beta-monoclinic Ga2O3. In addition, ZnGa2O4 has shown, on average, better thermal dissipation and potential for n- and p-type conductivity. Here we use density functional theory and hybrid functional calculations to investigate the electronic, optical, and point defect properties of ZnGa2O4, focusing on the possibility for p- and n-type conductivity. We find that the cation antisite Ga-Zn is the lowest-energy donor defect that can lead to unintentional n-type conductivity. The stability of self-trapped holes (small hole polarons) and the high formation energy of acceptor defects make it difficult to achieve p-type conductivity. However, with an excess of Zn, forming Zn(1+2x)Ga2(1-x)O4 alloys, this compound can display an intermediate valence band, facilitating p-type conductivity. Due to the localized nature of this intermediate valence band, p-type conductivity by polaron hopping is expected, explaining the low mobility and low hole density observed in recent experiments.