Low-Temperature Optical Investigation of the Urbach Tail in AmorphousGex Te1−xAlloys in the Light of the Anderson Model

Abstract

The recent Anderson model of the electronic structure of the amorphous chalcogenides predicts formation of paired localized singlet states through large electron-phonon interaction. We show that the interpretation of the absorption in the Urbach-tail region is completely consistent with modulation of band edges due to random internal electrical field associated with these bipolarons. They arise from electron transfer between Te dangling bonds, $x\le 0.33$, or Ge dangling bonds, $x>\mathrm{}0.33\mathrm{}$.