The form of the edge emission band in CdS and ZnS crystals

Abstract

The temperature dependence of the shape of the edge cathodoluminescence band (conduction band to valence band recombination emission) in hexagonal CdS and both the hexagonal and cubic structures of ZnS has been measured between liquid nitrogen and room temperature. The intensity of the edge emission band can be expressed semiempirically as L(E)=L(E ₀) exp [g(E - E ₀)^β], where g and β are shape parameters, L is the intensity, E the photon energy and E ₀ is the photon energy at the maximum of the band. For the low energy region of the band β = 1 and the exponential Urbach tail is obtained while around the maximum β = 2 and the shape is Gaussian. The high energy tail is also exponential. Quantitatively similar exponential low-energy tails have been reported for the absorption edges of CdS and ZnS and a wide variety of other semiconductors and insulators and also in the edge emission band of GaAs. The interpretation of these results is discussed in relation to the Dow and Redfield (1972) internal electric microfield model for exponential edges.