Electronic structure of Cu2ZnGeSe4 single crystal: Ab initio FP-LAPW calculations and X-ray spectroscopy measurements

Abstract

High-quality Cu2ZnGeSe4 single crystal has been successfully grown by a solution-fusion method and the crystal structure of the compound is refined within tetragonal kesterite-type (space group 14¯), with the unit cell parameters a=5.6112(1) Å and c=11.0473(3) Å. X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces of the Cu2ZnGeSe4 crystal are measured. Our X-ray photoelectron spectroscopy (XPS) data indicate that the Cu2ZnGeSe4 single crystal surface is very rigid with respect to Ar+ ion-irradiation. The electronic structure of the Cu2ZnGeSe4 compound has been theoretically studied using first principles calculation employing the full potential linearized augmented plane wave (FP-LAPW) method. Calculations of the total and partial densities of states of atoms constituting Cu2ZnGeSe4 reveal that the dominant contributors to the valence band of the compound studied are the Cu 3d states, which contribute mainly at the top of the band, while the central portion of the band is composed mainly from the Cu 3d and Se 4p states in almost equal proportion. Additionally, the present FP-LAPW calculations indicate that the bottom of the valence band is dominated by the Zn 3d states, while the bottom of the conduction band is composed mainly from the unoccupied Ge 4s and Se 4p states. The data of the present FP-LAPW calculations are confirmed by comparison on a common energy scale of the X-ray emission bands representing the energy distribution of the valence Cu d, Zn d, Ge p and Se p states and the XPS valence-band spectrum of the Cu2ZnGeSe4 single crystal.