Theoretical Analysis of the Effect of Defects on Physical Properties of II-VI Semiconductors

Abstract

The ability to obtain tunable properties in semiconductor devices with compositional variation makes multi-ternary semiconductor compounds extremely useful for a variety of industrial applications such Solar Cells, Light Emitting Diodes (LED), X-rays and Gamma ray detectors. II-VI semiconductors have attracted great attention due to their interesting applications in everyday life and research. In this thesis, the physical properties of II-VI pure and their multi-ternary compounds with the zinc-blende phase are explored. To calculate the physical properties of ZnO1-xSx, ZnSe1-xSx and CdSe1-xSx by introducing defects using plane wave pseudo-potential method with Local Density Approximation (LDA) and LDA+U. The obtained results of each individual compound are given below. In ZnO1-xSx crystal, Sulfur is replaced by Oxygen in the step of 0.25 from 0 to 1. The variance in the structural, electronic, and thermodynamic properties of ZnO1-xSx semiconductors with the change in doping concentration of Sulfur are calculated. It is observed that the LDA+U method gives better improvement in the results instead of LDA. At various concentrations the ground-state properties are determined for bulk materials ZnO, ZnS, and their tertiary alloys in cubic zinc-blende phase. From the results, a minor difference has been observed in lattice parameters from Vegard‟s law and other calculated results which may be due to the large mismatch of lattice parameters of binary compounds ZnO and ZnS. A small deviation in the bulk modulus from linear concentration dependence has also observed for these alloys. The thermodynamic properties including the phonon contribution to Helmholtz free energy ΔF, phonon contribution to internal energy ΔE, and specific heat at constant-volume CV are calculated within quasi harmonic approximation based on the calculated phonon dispersion relations. The structural, electronic, and elastic properties of ZnSe1-xSx for zinc-blende structure with different composition are calculated. The exchange-correlation potential is treated with the pz-LDA for these properties. Moreover, LDA+U approximation is employed to treat the “d” orbital electrons properly. A comparative analysis of band gap calculated within both LDA and LDA+U schemes is presented. The analysis of obtained results shows considerable improvement in the band gap. The inclusion of compositional disorder increases the values of all elastic constants. The results have shown that elastic Page ix constant C11, C12, and C44 are mainly influenced by compositional disorder and found to be in well agreement with literature. The results also reflect the ductile nature of these pure and defected alloys. The ground-state properties are resolute for the bulk materials CdS, CdSe, and their alloys. Very small deviation of the lattice constant from Vegard‟s law is observed for CdSe1–xSx alloys at different concentrations but a little larger deviation of the bulk modulus from linear concentration dependence (LCD) is observed for considered alloy with downward bowing. The effect of Sulfur composition on electronic properties using LDA and LDA+U is explored. It is found that with LDA+U method, the band gap for CdSe1–xSx is very close to experimental value. Elastic constants such as C11, C12, C44, Young Modulus, and Shear Modulus are also calculated and these results revealed that CdSe1–xSx compound is ductile in nature.