Electronic structure and optical properties of Cu2CdGeS4: DFT calculations and X-ray spectroscopy measurements

Abstract

We report on measurements of X-ray photoelectron core-level and valence-band spectra for pristine and Ar+ ion-bombarded surfaces of Cu2CdGeS4 single crystal. Electronic structure of the Cu2CdGeS4 compound is studied from both theoretical and experimental viewpoints. Density functional theory (DFT) calculations employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package are used to explore the total and partial densities of states in Cu2CdGeS4. The calculations reveal that the Cu 3d and Cd 4d states are the dominant contributors to the valence band: they contribute mainly at the top and at the bottom of the band. Additionally, the calculations indicate that the bottom of the conduction band is composed mainly from the unoccupied Ge 4s and S 3p states. Data of the present DFT calculations are confirmed experimentally by comparison on a common energy scale of the X-ray emission bands representing the energy distribution of the valence Cu d, Cd d, Ge p and S p states and the XPS valence-band spectrum of the Cu2CdGeS4 single crystal. Main optical characteristics of the Cu2CdGeS4 compound are explored by the first-principles calculations.