We have applied real-time in situ spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) to investigate a-Si:H nucleation process on substrate that affects the resulting a-Si:H/substrate interface structure significantly. The analyses of these real-time measurements show the formation of a similar to 30 Angstrom thick H-rich interface layer having an average hydrogen content of similar to 20 at.% on a c-Si substrate covered with native oxide (30 Angstrom). This interface layer formation is primarily caused by the H-rich three-dimensional island growth on the substrate. We found a weak dependence of interface layer properties on a-Si:H deposition conditions. This result suggests that the interface layer formation is controlled by the nucleation site density of a-Si:H islands on the substrate, rather than plasma conditions. (C) 2001 Elsevier Science B.V. All rights reserved.