Wurtzite-Derived Quaternary Oxide Semiconductor Cu2ZnGeO4: Its Structural Characteristics, Optical Properties, and Electronic Structure.

The quaternary I2-II-IV-O4 semiconductor, Cu2ZnGeO4, with a wurtz-kesterite structure and 1.4 eV energy band gap has been synthesized for the first time via ion exchange of precursor Na2ZnGeO4. Its crystal structure was refined by Rietveld analysis, and the structural distortion was quantitatively evaluated based on the cation tetrahedral tilting and angle distortion indexes. The tetrahedral distortion in Cu2ZnGeO4 was smaller than in Ag2ZnGeO4 but larger than in β-CuGaO2, suggesting an indirect band gap of Cu2ZnGeO4. Density functional theory calculations using the functional of the local density approximation with corrections for on-site Coulomb interactions indicated that Cu2ZnGeO4 is an indirect semiconductor as expected from its structural feature. However, the energy difference between the direct and indirect band gaps is very small, suggesting that Cu2ZnGeO4 shows strong light absorption near the band edge.