Chemical properties of a-Si:H interface layers on oxide-covered and hydrogen-terminated silicon

High quality a-Si:H films were deposited by pulsed VUV (157nm) laser CVD, allowing digital control of the deposition process. Nucleation and growth on native oxide-covered Si(100) and on H-terminated Si(lll) surfaces were studied in situ by FTIR transmission spectroscopy with sub-monolayer sensitivity. The film thickness was monitored simultaneously using a quartz crystal microbalance (QCM) with comparable resolution. The in situ spectra reveal that the nature of the substrate surface significantly influences the hydrogen bonding configuration in the interface region. In both cases the assumed duster growth during the initial stage is characterized by a band around 2100 cm(-1), which is assigned to SiHx (x = 1 - 3) surface modes. This band broadens until the clusters coalesce and band saturation occurs. At this time a second band starts to grow at 2000 cm(-1), which is attributed to bulk SiH bonds. Difference spectra calculated for layers at different depths with definite thickness and the deconvolution of the broad feature observed during the very first stage of film growth indicate the existence of a hydrogen-rich layer at the substrate surface whose composition and thickness depend on the properties of the substrate surface.