First-principles density functional theory based electronic structure calculations of some zinc-oxide and zinc-sulphide polymorphs

Abstract

Recently, zinc oxide (ZnO) and zinc sulfide (ZnS) have drawn a resurgent attention in the research community due to their interesting properties with diverse potential applications. Wide bandgap, large exciton binding energy at room temperature, and small effective electron mass and piezoelectricity make ZnO a potential candidate for a variety of electronic and optoelectronic devices. ZnS possesses a direct bandgap of 3.6 eV at room temperature and appears to be a promising candidate for a broad range of technological applications including transparent conductors, visual displays and high-density optical memories. However, in order to realize the efficient utilization of ZnO and ZnS in blue, green and ultraviolet (UV) emitters with high efficiency, it is very important to modify these materials so that the full bandgap energy spectrum (from visible to UV) may be covered by the materials. Alloying of ZnO with sulfur (S) chalcogen reveals vivid changes in its electronic and optical properties due to the dramatic restructuring of electronic structure. In this thesis, the structural, electronic and optical properties of pure ZnO and ZnO1-xSx (x = 0, 0.25, 0.50, 0.75 and 1) alloys in wurtzite (WZ), sphalerite type, germanium phosphide (GeP) type, 5-5 type, nickel arsenide (NiAs) type, ß-beryllium oxide (BeO) type, and cesium chloride (CsCl) type are studied by using full-potential linearized augmented plane wave plus local orbital (FPLAPW + lo) method within density functional theory (DFT). The structural properties of pure ZnO and S-doped ZnO in seven crystal structures were calculated by using Perdew-Burke-Ernzerhof – generalized gradient approximation (PBE – GGA) exchange correlation whereas the calculations for electronic and optical properties were carried out by adding the mBJ potential to the PBE-GGA exchange correlation. The structural properties of S-doped ZnO in seven polymorphs reveal a small deviation from Vegard’s law which is consistent with the findings from previous literature. It was found that the replacement of the oxygen (O) atom by S produces interesting effects on the band structures of ZnOS alloys. The electronic bandgaps of ZnOS alloys in WZ structure, sphalerite type and BeO type were enhanced from 2.65 eV to 3.68 eV, 2.50 eV to 3.60 eV and 2.85 eV to 3.75 eV, respectively. The bandgap of 5-5 type ZnOS alloys decreases from 3.12 eV to 2.63 eV and the band structures of GeP type and NiAs type ZnOS alloys show different variations with different concentrations. On the other hand, CsCl type ZnOS alloys exhibit a metallic nature. The static dielectric constants of the seven considered polymorphs reveal that the polarization of the S doped ZnO increases by increasing the S concentration. The CsCl type ZnOS alloys with metallic character were found to have the highest value of static dielectric constant. The results for optical properties show that the incorporation of S atoms moves the maximum absorption, reflectivity and conductivity peaks towards low photon energies which reveal the potential of S doped ZnO. The static refractive indices of all considered ZnOS alloys were found to be increased by increasing the S content. The analysis of the absorption spectra shows that WZ structure, sphalerite type and BeO type ZnOS alloys are the promising candidates for visible and UV photoelectronic devices. The 5-5 type and NiAs type ZnOS ZnOS alloys were found suitable for visible light regime applications. On the other hand, GeP type ZnOS alloys are best for the applications corresponding to infrared to visible region.