Energy Gap inβ−Ag2Se

The energy gap of the modification of silver selenide stable below 133°C has been investigated. Studies of the optical-absorption spectrum at 80°K and at 5°K and of the temperature variation of the Hall coefficient and electrical conductivity have been made on polycrystalline samples. Analysis of the data on the electrical properties in the intrinsic region suggests the existence of two phases, not distinguishable by x-ray diffraction, with energy gaps $E_0$ at 0°K of 0.07±0.01 eV and 0.18±0.01 eV, respectively. The latter value agrees with that proposed for the ${\beta }_2$ phase of $\beta -{\mathrm{Ag}}_2\mathrm{Se}$ reported by Baer. The optical-absorption spectra suggest that the band-to-band transitions observed are direct. The extrapolated threshold of absorption is at 0.13 eV at 5°K and correlates best, because of the probable Burstein shift of the absorption edge, with the proposed value of $E_0=0.07\mathrm{}$ eV. It is also concluded that the energy gap increases as the temperature decreases.