Low Temperature Optically Induced Esr in a-Si:H and the Presence of Interface States

Abstract

ABSTRACTComparative studies of low-temperature (T ∼ 30 K) optically induced ESR using above gap red (633 nm) and below gap i.r. (1060 nm) light have been performed on several samples of a-Si:H. Experiments were performed on both films on quartz substrates and on powdered samples which were removed from the substrates. Excitation with red light always yields the well-known light induced ESR (LESR) spectrum which is ascribed to a superposition of two resonances: electrons trapped in conduction band-tail states and holes trapped in valence band tail states in a 1:1 ratio. On the other hand, excitation with i.r. light results in a different LESR spectrum with an enhanced feature overlapping with the signal attributed to trapped band-tail electrons. Although observed on all films studied on quartz substrates, this asymmetry is not observed in a powdered sample where only the band tail electron and hole resonances are observed in the ratio 1:1. This result suggests that interface states are responsible for the excess LESR spin density in the films on quartz substrates. Experiments performed as a function of film thickness confirm this interpretation.