Field effect in chalcogenide glasses

Abstract

It is generally believed that chalcogenide glasses contain large densities of defect centers characterized by a negative effective correlation energy. If so, the Fermi level should be pinned and at most a small field effect should be observed. Nevertheless, there have been several reports of significant field effects in chalcogenide glasses. In this work, we report the results of new field-effect measurements on amorphous ${\mathrm{Te}}_{39}$${\mathrm{As}}_{36}$${\mathrm{Si}}_{17}$${\mathrm{Ge}}_{7}$${\mathrm{P}}_{1}$. We find that the response is dominated by very slow transients which take several hours to reach steady state at room temperature. The steady-state field effect is consistent with the concept of defects characterized by a negative effective correlation energy, but the transient response is difficult to understand. We show that first-order kinetic model of carrier trapping by valence alternation pairs explains the transient effects observed in a natural way. We also demonstrate that field-effect measurements as a function of time and temperature can be used to evaluate qualitatively both the defect densities and energies. This analysis further enables us to understand apparent inconsistencies in the previously reported results. The defects investigated appear to be characteristic of all Te-As based glasses.