From cavity ring down spectroscopy and threshold ionization mass spectrometry measurements in a remote Ar-H-2-SiH4 plasma it is clearly demonstrated that the properties of hydrogenated amorphous silicon (a-Si:H) strongly improve with increasing contribution of SiH3 to film growth. The measurements corroborate the proposed dissociation reactions of SiH4 for different plasma settings and it is shown that film growth is by far dominated by SiH3 under conditions for which solar grade quality a-Si:H at deposition rates up to 10 nm/s has previously been reported.