Optical Absorption, Reflectivity, and Electrical Conductivity in GeAs and GeAs2

Single-crystal samples of two germanium arsenides were prepared: monoclinic-type GeAs and orthorhombic-type Ge${\mathrm{As}}_{2}$. Measurements are described for optical absorption and reflectivity in the neighborhood of the fundamental absorption edge at 300 \ifmmode^\circ\else\textdegree\fi{}K and electrical resistivity over the temperature range 77-400 \ifmmode^\circ\else\textdegree\fi{}K. Analysis of the optical data indicates the following: GeAs has a possible indirect band gap at 0.65 eV and direct band gaps at 1.01 and 1.65 eV; Ge${\mathrm{As}}_{2}$ has a possible indirect band gap at 1.06 eV and direct band gaps at 1.77 and 1.10 eV. The tailing of the absorption edge in both materials in the low-absorption region prevents a clear understanding of band structure details at the minimal band gap. Analysis of the electrical measurements for Ge${\mathrm{As}}_{2}$ indicates an activation energy of 0.36 eV which is associated with extrinsic behavior, carrier concentrations of the order ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and mobilities of 60 ${\mathrm{cm}}^{2}$/V sec at 300 \ifmmode^\circ\else\textdegree\fi{}K. All measurements are correlated with crystallographic orientation in these anisotropic materials.