On the detection of $U^-$ centers in $g$-As$_2$Se$_3$ films by thermal cycling measurements of electrical conductivity

Pinning of the Fermi level near the midgap is one of the basic properties of chalcogenide glassy semiconductors. It is assumed in most models that the pinning is due to the presence of charged defects ($U^-$ centers) that strongly polarize the lattice and have energy levels close to the Fermi level. $U^-$ centers are detected in this study by the thermal cycling method, which made it possible to markedly extend the time during which the charge released by these centers is recorded. The results of measurements of the electrical conductivity of As$_2$Se$_3$ films in successive cycles of sample cooling and heating are presented. This cycling leads to the appearance and evolution of two discrete peaks. A conclusion is made that at least one of these peaks is associated with negatively charged $U^-$ centers with levels near the Fermi level.