Nonideal anion displacement, band gap variation, and valence band splitting in Cu–In–Se compounds

Abstract

Polycrystalline thin films of ternary chalcopyrite CuInSe 2 and defect compounds CuIn 3Se 5 and CuIn 5Se 8 are prepared in vacuum by three-source coevaporation method. Structural and optical characterizations of the films are done using X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and optical absorbance spectra measurements. With variation in the composition of CuInSe 2, a change over from p-type to n-type conductivity is observed (as noted by the hot probe method). The deformation parameters and the anion displacements are calculated from the X-ray diffraction data, and the cation–anion bond lengths are deduced. The dependence of band gap variation on nonideal anion displacement in the ternary compounds and the effect of Se- p–Cu- d repulsion on band gap are studied. The threefold optical structure observed in the fundamental absorption region of the absorption spectra is analysed to extract the valence band splitting parameters. Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation is used to determine the crystal field splittings and spin orbit splittings, and the linear hybridization model is used to calculate the percentage of d-orbital and p-orbital contribution to hybridization in the compounds under consideration.