Elucidations of electronic and optical properties of the w-ZnCh (Ch=O, S and Se) compounds: Insights from Ab-initio calculations and spectroscopy measurements

The electronic and optical properties of IIB-VIA Zn-based monochalcogenides w-ZnCh (Ch: O, S and Se) were reported. The band structures using the FP-LAPW method at zero pressure with the Tran–Blaha-modified Becke–Johnson potential evidence that w-ZnO, w-ZnS and w-ZnSe reveal direct band gaps (Γ–Γ) of 2.72, 3.87, and 2.88 eV, respectively. The complex dielectric function ε(ω), refractive index n(ω), extinction coefficient k(ω), absorption coefficient α(ω), reflectivity R(ω), energy-loss function L(ω) and complex conductivity σ(ω), are determined. The microscopic origin of the observed fine-structure peculiarities in the spectra of the imaginary part of the dielectric function (ε₂(ω)) is identified. An intensive X-Ray PhotoElectron Spectroscopy (XPES) investigation shows the effect of controlled UHV treatment (Ar⁺ ionic bombardment followed by gradual heating) as a method of cleaning and/or re-crystallising the upper layers. XPES and XAES (Auger Electron induced by X-Ray Excitation) transitions substantiate successfully the UHV in situ cleaning. The valence band structure was studied. The PLS technique employing UVvis excitation source of a 325 nm (He-Cd) gas laser unveils that ZnO, zinc sulfide and ZnSe luminescence mechanism concerns the energy band gap. The results of the experimental studies are found to be in good agreement with the theoretical predictions.