Temperature Dependence of the Fundamental Optical Absorption Edge in Stannic Oxide

Abstract

The anisotropic fundamental absorption edge spectra edge spectra of SnO₂ have been studied in a temperature range from 7 to 350°K. The spectra are quite different in the energy of absorption edge, in shape and also in temperature dependence for two polarized lights (E⊥c and E⁄⁄c). Two direct edges are observed for light E⊥c; at low temperatures, the spectra begin with second class exciton transitions at Γ-point, while at high temperatures, the spectra are interpreted in terms of Urbach’s rule, and are assumed to arise from transitions from the second valence band to the lowest conduction band at Γ-point. For light E⁄⁄c, it was found that (absorption coefficient)^2⁄7 is proportional to photon energy for the whole temperature range investigated, and that the shape of the spectra shows only slight temperature dependence. A possible explanation that the spectra arise from “fourth class” direct trantions at Γ-point is discussed.