Absorption Edge of Impure Gallium Arsenide

The exponential absorption edge of GaAs may be interpreted as reflecting the presence of tails of states extending both intrinsic bands into the energy gap. The distribution of these tail states seems exponential, varying with energy as $\exp \left(\frac{E}{E_0}\right)$ where $E_0$ increases with doping. $E_0$ starts rising rapidly with doping at lower concentrations in $n$-type material than in $p$-type GaAs. The temperature dependence of the absorption edge is larger than that of the energy gap; this can be attributed to a strongly temperature-dependent shift of the Fermi level in agreement with the model of tailing of states.