Bipolar self-doping in ultra-wide bandgap spinel ZnGa2O4

The spinel group is a growing family of materials with general formulation AB2X4 (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa2O4) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (∼5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa2O4 intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa2O4 epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (ZnGa) defects, while up to a (n > 1019 cm−3) donor concentration is obtained due to the hybridization of the Zn–O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole conduction (at high temperature) for p-ZnGa2O4 while for n-ZnGa2O4 a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ∼3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa2O4 exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics.