Hole conductivity through a defect band in $\mathrm{ZnG}{\mathrm{a}}_2{\mathrm{O}}_4$

Abstract

Semiconductors with 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 $\mathrm{ZnG}{\mathrm{a}}_2{\mathrm{O}}_4$ may be suitable for these applications, standing out as an alternative to $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. The simple face centered cubic spinel structure of $\mathrm{ZnG}{\mathrm{a}}_2{\mathrm{O}}_4$ results in isotropic electronic and optical properties, in contrast to the large anisotropic properties of the ${\hat{I}}^2$-monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. In addition, $\mathrm{ZnG}{\mathrm{a}}_2{\mathrm{O}}_4$ 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 $\mathrm{ZnG}{\mathrm{a}}_2{\mathrm{O}}_4$, focusing on the possibility for $n$- and p-type conductivity. We find that the cation antisite $\mathrm{G}{\mathrm{a}}_{\mathrm{Zn}}$ 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 excess of Zn, forming $\mathrm{Z}{\mathrm{n}}_{(1+2x)}\mathrm{G}{\mathrm{a}}_{2(1-\mathrm{x})}{\mathrm{O}}_4$ alloys 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.