The SiH4 dissociation products and their contribution to hydrogenated amorphous silicon (a-Si:H) film growth have been investigated in a remote Ar-H-2-SiH4 plasma which is capable of depositing device-quality a-Si:H at 10 nm/s. SiH3 radicals have been detected by means of threshold ionization mass spectrometry for different fractions of H-2 in the Ar-H-2-operated plasma source. It is shown that at high-H-2 flows, SiH4 dissociation is dominated by hydrogen abstraction and that SiH3 contributes dominantly to film growth. At low-H-2 flows, a significant amount of very reactive silane radicals, SiHx(x less than or equal to 2), is produced, as concluded from threshold ionization mass spectrometry on SiH2 and optical emission spectroscopy on excited SiH and Si. These radicals are created by dissociative recombination reactions of silane ions with electrons and they, or their products after reacting with SiH4, make a large contribution to film growth at low-H-2 flows. This is corroborated by the overall surface reaction probability which decreases from similar to 0.5 to similar to 0.3 with increasing H-2 fraction. The film properties improve with increasing H-2 flow and device-quality a-Si:H is obtained at high H-2 fractions where SiH3 dominates film growth. Furthermore, it is shown that at high-Hz flows the contribution of SiH3 is independent of the SiH4 flow while the deposition rate varies over one Order of magnitude.