Concentration dependences of electronic band structure of CdSe 1– x S x thin films

In this study, we report for the first time the results of ab initio calculations of the electronic energy spectra and optical functions for the thin films of solid-state solution CdSe 1– x S x ( x = 0, 0.25, 0.50, 0.75 and 1.00). A case of interaction of these films with CO molecules is also considered, which is practically important in relation to the problem of developing new CO sensors. Concentration dependences of the energy gaps associated with the main optical transitions in CdSe 1– x S x , Г 8 v –Г 6 c and Г 7 v –Г 6 c (levels in the Г point of Brillouin zone; v and c subsymbols correspond to the valence band and conduction band, respectively), are studied and a spin–orbit decoupling is evaluated. It is demonstrated that the concentration dependences of these energy gaps are quadratic, with a characteristic ‘bending’ parameter equal to 0.13 and 0.12, respectively, for the Г 8 v –Г 6 c and Г 7 v –Г 6 c transitions. Interaction of the thin films with CO imposes a tendency to increasing bandgap and transforming its concentration dependence to linear one. It is also revealed that the above interaction corresponds to exothermic adsorption processes and the corresponding adsorption energy is determined. The concentration dependences of optical properties are mainly studied on the example of refractive index at the light wavelength 500 nm. Moreover, we elucidate the influence of adsorption of CO gas occurring on the surface of CdSe 1– x S x thin films on their optical functions. Finally, the electronic band structure and the main optical properties obtained theoretically in this work are compared with the experimental data known from the literature, which reveals a fair agreement with the experiment.