Thermalization of excited carriers through band states studied by photoluminescence in As₂Se₃ and Se single crystals

Abstract

The role of thermalization of carriers, excited with light of excess energy, in luminescence and photoconductance has been investigated. In As₂Se₃ crystals, as a consequence of fast scattering during thermalization, predominantly geminate pairs are formed which can be separated by electric fields. At low temperatures, this field-enhanced separation of pairs occurs during thermalization and not during subsequent diffusion of the thermalized carriers. Thermalization determines the shape and temperature dependence of the excitation spectra of luminescence, which in both As₂Se₃ and trigonal Se arise from recombination of geminate pairs. At high excitation energy, decreasing temperature facilitates carrier separation by electric fields, observed as a quenching of the luminescence intensity, and indicates phonon-controlled scattering rates down to 30 K. A quantitative comparison of the prediction of a thermalization model with luminescence quenching, points, in this range of temperatures, to a decrease in the mobility with increasing field, apparent also in a sublinear rise of photocurrents with applied voltage.