The First-principles Calculations of the Electronic Structures and Optical Properties of II-III2-VI4 (II = Zn, Cd; III = In; VI = Se, Te)

The electronic structures and optical properties of II-III2-VI4 (II = Zn, Cd; III = In; VI = Se, Te) compounds are studied by the density functional theory (DFT) using the Vienna ab initio simulation package (VASP). Geometrical optimization of the unit cell is in good agreement with the experimental data. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at G resulting in a direct energy gap. The optical properties are analyzed, and the independent second harmonic generation (SHG) coefficients are determined. By an analysis of the band structure, we can get that SHG response of the system can be attributed to the transitions from the bands near the top of valence band that are derived from the Se/Te p states to the unoccupied bands contributed by the p states of In atoms.