Thermoreflectance study of the electronic structure ofGe(Se1−xSx)2

Abstract

The band structure of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ series solids was probed experimentally by thermoreflectance (TR) measurements in the energy range of $2--6.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The TR measurements were done with the layered samples put in air and operated at room temperature. The TR spectra of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ series layers exhibit derivative-like spectral features in the vicinity of band edge as well as higher-lying interband transitions. The energies and broadening parameters of the interband transition features of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ are analyzed by detailed line-shape fits to the TR spectra. Compositional dependences of the transition energies of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ are analyzed. The presence of higher-lying interband transitions as well as the analysis of line-width broadening of the direct band gap indicates that the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ series should not be amorphous but belong to the polycrystalline solids with medium-range order. Based on the experimental results of TR measurements together with referring to previous band-structure calculations of crystalline and glassy $\mathrm{Ge}{X}_{2}$ $(X=\mathrm{S},\mathrm{Se})$, a probable experimental band structure near the fundamental edge of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ is proposed.