Energetic, Electronic, and Optical Behavior of Intrinsic Charge Carrier-Trapping Defects in Ge-Doped ZnGa2O4: Insights from a DFT Study

Wide band gap semiconductors with high dielectric constant and good thermal dissipation are very popular for a wide range of optical and electronic devices. In recent studies, ZnGa2O4 has been projected as an alternative to Ga2O3. The structural simplicity (face-centered-cubic spinel structure) results in isotropic electronic and optical properties for ZnGa2O4 in comparison to the large anisotropic properties for the β-monoclinic variety of Ga2O3. Motivated by recent experimental observations on the enhancement of optical properties of ZnGa2O4 due to doping with Ge, the author has carried out a detailed systematic calculation using density functional theory to find out the origin. The present study investigates the structure, formation energies, and electronic properties of different intrinsic vacancy defects in Ge-doped ZnGa2O4 under various growth conditions. The doping of Ge is found to be energetically favored at both the tetrahedral sites and octahedral sites under oxygen-rich conditions. It has been revealed that the optical behavior of the undoped ZnGa2O4 is attributed to the electron trap centers induced by oxygen vacancy and hole trap centers induced by gallium vacancy, zinc vacancy, and antisite defect. Interestingly, the relative population of these intrinsic defects changed in the presence of Ge. It has been observed that antisite defects are no longer dominating defects in the presence of Ge. More interestingly, gallium vacancy, which is the dominating defect in the presence of Ge, can reduce the band gap significantly without introducing localized midgap states, which is very much important for achieving enhanced efficiency. Thus, the present study will be helpful for understanding the effects of native point defects and external Ge doping on ZnGa2O4 and provides important guidelines to design its band gap with the desired range by tuning the synthesis conditions.