Microstripline Transient Photocurrents: A Probe of Carrier Thermalization and Band-Tail State Densities in Amorphous Semiconductors

Abstract

Much progress has been made concerning the general features of carrier transport and thermalization mechanisms in amorphous semiconductors from time-resolved studies of photocurrents, photoluminescence and photoinduced absorption. A common feature of all of these measurements is the observation of anomolous dispersion in the carrier dynamics characterized by a power-law time decay which is generally thought to derive from an exponential distribution (energy) of band-tail states in which thermalization procedes by multiple trapping (MT). However, an exponential energy distribution of hopping sites will also give rise to a power-law decay. Therefore, a determination of the transport and thermalization mechanisms operative in amorphous semiconductors is a difficult problem compounded by uncertainty regarding the actual energy distribution of band-tail states near the band edges and whether or not any structure exists. The purpose of this paper is to present new transient photocurrent measurements in a-Se and a-Si:H with improved experimental time resolution which allows one to examine carrier thermalization and the nature of the distribution of states close to the band edges.