Analysis of time-of-flight photocurrents in a-Si:H deposited by expanding thermal plasma

The drift mobility of electrons and holes was studied in a standard time-of-flight (TOF) experiment, as a function of temperature and applied electric field, for several series of a-Si:H samples that were deposited at high growth rate in an expanding thermal plasma (ETP) at substrate temperatures near 400°C. Room-temperature electron mobilities are somewhat lower than for standard plasma-enhanced chemical vapor deposited (PECVD) material, but the hole mobility is more than one decade higher. The dispersion of the photocurrent transients is comparable to the one in PECVD samples, but both electron and hole mobilities of the ETP material are remarkably insensitive to the magnitude of applied electric fields in the measured range of 0.1 to 5.0 × 10⁴ V/cm. The temperature dependence of the mobilities, on the contrary, is high, with resolved activation energies of ∼0.24 eV for electrons and ∼0.40 eV for holes. Standard multiple-trapping analysis of the above results in terms of the underlying density of localized states suggests rather wide distributions of tail states on both sides of the gap. To account simultaneously for the weak field dependencies it is necessary to consider non-negligible recombination or deep trapping of the carriers. Measurements and analysis of the post-transit TOF signals do support the presence of a strong deep-trapping component in the transient signal.