Ab-initio investigation of structural, electronic and optical properties for three phases of ZnO compound

The complex density-functional theory (DFT) calculations of structural, electronic and optical proper-ties for the three phases: wurtzite (B4), zincblende (B3) and rocksalt (B1) of ZnO compound have been reported using the full-potential linearized-augmented plane-wave (FP-LAPW) method as implemented in the WIEN2k code. We employed both the local-density approximation (LDA) and the generalized-gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also, we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band-structure calculations. The 3d orbitals of the Zn atom were treated as the valence band. The calculated structural properties (equilibrium lattice constant, bulk modulus, etc.) of the wurtzite and rocksalt phases are in good agreement with experiment. The B4 structure of ZnO is found to transform to the B1 structure with a large volume collapse of about 17%. The phase transition pressure obtained by using LDA is about 9.93 in good agreement with the experimental data. B1-ZnO is shown to be an indirect bandgap semiconductor with a bandgap of 1.47 eV, which is significantly smaller than the experimental value (2.45 +/- 0.15 eV). While B3 and 131 phases have direct bandgap semiconductors with bandgaps 1.46 and 1.57 eV, respectively. Also, we have presented the results of the effective masses. We present calculations of the frequency-dependent complex dielectric function epsilon(omega) and it zero-frequency limit epsilon(1)(0). The optical properties of B4 phase show considerable anisotropic between the two components. The reflectivity spectra has been calculated and compared with the available experimental data. (C) 2007 WTLEY-VCH Verlag GmbH & Co. KGaA, Weinheim.