Intrinsic Urbach Rule and Electron—Phonon Interaction in GaAs and Related III‐V Compounds

The optical absorption at the edge of the fundamental absorption band is investigated in highpurity and high‐perfection GaAs crystals. The experimental curves show an exponential decay of the absorption coefficient as a function of the photon energy (Urbach's rule) with temperature dependent slope. Since the impurity contribution can be neglected, the data are analyzed within the framework of the polaron model of phonon‐assisted transitions. The good agreement obtained between experimental and calculated curves makes it possible to conclude that the interaction of electrons with LO phonons plays a dominant role in shaping the intrinsic Urbach tail in GaAs. The same theoretical model is also applied successfully to InP and InAs using previously published experimental data.