Subnanosecond transient photocurrents in a-Si:H: A probe of thermalization within shallow band-tail states

Abstract

The measurement of transient photocurrents in amorphous semiconductors has been shown to be a powerful way of exploring the thermalization of excess carriers within the manifold of localized states adjacent to the band edges. The extent to which thermalization within the shallowest states can be resolved in controlled by the experimental observation time. Reported here are measurements of transient photocurrents, using high-speed stripline techniques, in glow-discharge a-Si:H carried out in the time regime (0.6 ↔ 100 nsec) and the temperature range 150 ↔ 320K. We find an effective electron drift mobility of < μ D( t)> = 1–3 cm 2/ Vsec at t=0.6 nsec and T=300K and a time-dependent activation energy, ϵ(t), for the photocurrent magnitude. The initial photocurrent decay (t < 100 nsec) can be fit to a power-law, t −n, and the temperature dependence of the dispersion parameter, α=1−n, is given by α= α 0+ βT. In view of these results the nature of the band-tail near the conduction band edge is discussed.