Optical Absorption In Gese0.75te0.25

Ge${\mathrm{Se}}_{0.75}$${\mathrm{Te}}_{0.25}$ is a new IV-VI compound which has electrical properties similar to those of the degenerate semiconductors GeTe and SnTe. Optical absorption has been measured in single crystals of Ge${\mathrm{Se}}_{0.75}$${\mathrm{Te}}_{0.25}$ grown by an iodine transport process. The measurements show a sharp absorption edge followed by a rapid rise in absorption at long wavelengths caused by the large free-carrier concentration (${10}^{20}$ holes ${\mathrm{cm}}^{\ensuremath{-}3}$). The absorption edge is interpreted as the fundamental absorption edge of a semiconductor. It is concluded that Ge${\mathrm{Se}}_{0.75}$${\mathrm{Te}}_{0.25}$ is a degenerate semiconductor which has a band gap considerably less than 0.82 eV.