Band-edge properties of layered germanium dichalcogenides

Abstract

The band-edge characteristic of germanium dichalcogenides $\mathrm{Ge}{(\mathrm{Se},\mathrm{S})}_{2}$ has been characterized using thermoreflectance (TR) and resistivity measurements at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Two band-edge excitonic transitions denoted as ${E}_{A}$ and ${E}_{B}$ are found in each TR spectrum of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ $(0\ensuremath{\le}x\ensuremath{\le}1)$. A Rydberg excitonic series that contains two excitonic levels of $n=1$ and $n=2$ are simultaneously detected in the ${E}_{A}$ and ${E}_{B}$ transitions of $\mathrm{Ge}{\mathrm{Se}}_{2}$. For the other layered crystals, only the ground-state level $(n=1)$ is observed. Angular-dependent and polarization-dependent TR measurements of $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ were carried out to identify the transition origins of the band-edge transitions. The experimental TR results show that $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ with $x=0$, 0.2, 0.4, 0.6, and 0.8 are crystallized in the high-temperature crystalline phase, while the $\mathrm{Ge}{\mathrm{S}}_{2}$ belongs to the high-temperature and low-temperature mixed phases. The high in-plane resistivities of the $\mathrm{Ge}{({\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x})}_{2}$ indicate the high purity of the layer crystals.