Anomalous Urbach tail in GaAs

The Urbach tail in crystalline GaAs and Si due to thermal vibration is evaluated in the temperature range 0--810 K. Recent experiments suggest that there is no thermally induced Urbach tail in undoped GaAs. This is most unexpected since thermally induced tails at the absorption edge having the Urbach form and widths E₀(T) linear in T are observed in a wide range of crystalline and amorphous semiconductors. Clear understanding of the tail is important in device applications. In our calculations, we obtain an absorption tail having a simple exponential energy dependence (Urbach form) in GaAs at all temperatures with E₀(T) proportional to T. However, due to a very weak electron-phonon interaction, the magnitude of E₀(T) is unusually small, much smaller than the temperature independent, structurally induced component E₀{}10 meV. As a test of the method, the tail is similarly evaluated in Si providing an E₀(T) value in agreement with experiment.