High-electric-field transport ina-Si:H. I. Transient photoconductivity

The drift mobility of electrons at low temperatures in amorphous hydrogenated silicon is investigated by time-of-flight and charge-collection measurements. The experiments performed on p-i-n junctions in the temperature range 40<T<200 K reveal the strong influence of the electric field on carrier propagation. At 40 K, where thermally induced transport can be neglected, the drift mobility of electrons is enhanced by many orders of magnitude up to values of ${\mathrm{\mu }}_{\mathit{D}}$>${10}^{\mathrm{-}2}$ ${\mathrm{cm}}^2$/V s. The mobility has a superlinear dependence on the electric field and shows a time and thickness dependence indicative of dispersive transport. The high-field μτ product measured at 40 K is three orders of magnitude larger than the low-field μτ product calculated from dc-photoconductivity experiments. A model of field-enhanced band-tail hopping is developed to interpret the data. We show that the electric field creates a quasimobility edge in the tail region at an energy level ${\mathit{E}}_{\mathit{t}}$ which depends on the field strength and the tail state distribution. The hopping mobility as well as the energy level of the quasimobility edge are strongly dependent on the electric field. Ballistic capture of carriers into traps below the quasimobility edge is identified as the origin of dispersion. We introduce transport equations to calculate the drift of an ensemble of nonequilibrated carriers, and find reasonable agreement with experimental data assuming an exponential distribution of band-tail localized states with characteristic energy of 25 meV and localization length of 6≤α≤9 Å. We also discuss the introduction of an effective temperature as a substitute for the electric field.