Ultrafast carrier dynamics in microcrystalline silicon probed by time-resolved terahertz spectroscopy

Abstract

We present the results of optical-pump--terahertz probe experiments applied to a set of thin-film microcrystalline silicon samples, with structures varying from amorphous to fully microcrystalline. The samples were excited at wavelengths 800 and 400 nm and studied at temperatures down to 20 K. The character of nanoscopic electrical transport properties markedly change on a subpicosecond time scale. The initial transient photoconductivity of the samples is dominated by hot free carriers with a mobility of $\ensuremath{\sim}70\text{ }{\text{cm}}^{2}/\text{Vs}$. These carriers are rapidly (within 0.6 ps) trapped into weakly localized hopping states. The hopping process dominates the terahertz spectra on the picosecond and subnanosecond time scales. The saturated high-frequency value of the hopping mobility is limited by the sample disorder in the amorphous sample and by electron-phonon interaction for microcrystalline samples.