Electronic and Optical Properties of the Spinel Oxides GeB2O4 (B = Mg, Zn and Cd): An Ab-Initio Study

We report ab-initio density functional theory calculations of the electronic and optical properties of the spinel oxides GeMg2O4, GeZng(2)O(4) and GeCd2O4 using the full potential linearized augmented plane-wave method. To calculate the electronic properties, the exchange-correlation interaction was treated with various functionals. We find that the newly developed Tran-Blaha modified Becke-Johnson functional significantly improves the band gap value. All considered GeB2O4 compounds are direct band gap materials. The band gap value decreases with increasing atomic size of the B element. The decrease of the fundamental direct band gap (Gamma-Gamma) when one moves from GeMg2O4 to GeZn2O4 to GeCd2O4 can be attributed to the p-d mixing in the upper valence bands of GeZn2O4 and GeCd2O4. The lowest conduction band, which is mainly originated from the s and p states of the Ge and B (B = Mg, Zn, Cd) atoms, is well dispersive, similar to that of transparent conducting oxides such as ZnO. The topmost valence band, which is originated from the O-2p and B-d states, is considerably less dispersive. Optical spectra in a wide energy range from 0 to 30 eV are provided and the origin of the observed peaks and structures are assigned. We find that the zero-frequency limit of the dielectric function epsilon(0) increases with decreasing band gap value.