We report on two experimental studies carried out to reveal insight into the interaction of SiH3 radicals with the a-Si:H surface as assumed essential in the a-Si:H growth mechanism. The surface reaction probability β of SiH3 on the a-Si:H has been investigated by spectroscopic means as a function of the substrate temperature (50 - 450 °C) using the time-resolved cavity ringdown technique. The silicon hydrides -SiHx on the a-Si:H surface during deposition have been studied by the combination of in situ attenuated total reflection infrared spectroscopy and argon ion-induced desorption of surface hydrogen. For SiH3 dominated plasma conditions, it is found that the surface reactivity of SiH3 is independent of the substrate temperature with β = 0.30±0.03 whereas the silicon hydride composition on the a-Si:H surface changes drastically for increasing substrate temperature (from -SiH3 to = SiH2 to ≡SiH). The implications of these observations for the a-Si:H growth mechanism are addressed.