Study of Electronic, Structural, Optical, Dynamic and Thermodynamic Properties of Zinc-blende and Wurtzite Cadmium Chalcogenides(CdX (X = S, Se and Te)) Using Density functiona theory

Abstract

Advisor: Dr.Menberu Mengesha(Assco.Prof.) Abstract Zinc blende and wurtzite structure of CdX(X= S, Se and Te) are analyzed using Density functional theory within LDA, PBE, GGA+U, and PBE0. To assure the accuracy of calculation, the convergence test of total energy with respect to energy cutoff and k-point sampling was performed. The relaxed atomic position for the CdX in zb and wz structure was obtained by using total energy and force minimization method following the Hellmann Feynman approach. The structural optimization and electronic band structure properties of CdX were investigated. Analysis of the results shows that LDA and PBE underestimate the bandgap due to their poor approximation of exchange-correlation functional. However, the DFT+U and the PBE0 approximation give a good bandgap value which is comparable to the experimental result. Moreover, Optical properties: complex and real parts of dielectric function, static dielectric function, energy-loss spectrum, absorption-coefficient and refractive index of CdX in both zb and wz phase were studied within LDA, GGA and DFT+U to characterize its optical nature. The dynamic properties like: phonon dispersion curve, phonon density of state, frequency along high symmetry points, static and dynamic polarizability are calculated. Thermodynamic: internal energy, Helmholtz free energy, entropy, electron chemical potential and constant-volume specific heat were calculated within LDA, PBE and DFT+U approximations and analyzed. Accuracy is found for the calculated values within DFT+U and PBE0 approximation. It was found that the wz-CdX exhibits anisotropy in two directions (in basal-plan and z-axis). Key words: DFT, CdX, electronic, structural, optical, dynamic and thermodynamic.