Electronic transport in microcrystalline silicon controlled by trapping and intra-grain mobility

Time-resolved microwave conductivity (TRMC) and diffusion-controlled time-resolved microwave conductivity (DTRMC) were used on various microcrystalline silicon samples to investigate the mechanisms which control the mobility in these heterogeneous materials. First, in the diffusion mode, electron μ_e and hole μ_h mobilities were measured: their similar value excludes control of the transport by the amorphous interstitial tissue, if it exists. Next it is shown that TRMC is insensitive to the grain boundary barrier, while DTRMC is sensitive to grain boundary barrier effects. The once more similar values of the mobility obtained by the two measurements suggest that grain boundary barriers do not control the mobility. Finally, a detailed analysis of the TRMC transients as a function of the carrier density and deposition conditions reveals the contribution of recombination and trapping below and in the 0.1-0.2 eV energy range. We conclude that the grain quality, including low trapping inside or outside, is an essential factor in increasing the microcrystalline silicon mobility.