Rate constants for the reactions of SiH and SiH2 with SiH4 in a low-pressure SiH4 plasma

From the laser-induced fluorescence measurements of the decay rate of SiH2 and SiH densities in the afterglow of a RF SiH4-Ar discharge, the rate constants for the reactions of SiH2 and SiH with SiH4 have been determined at pressures below 1 Torr. The rate constants obtained at 70 mTorr for the SiH2+SiH4 and SiH+SiH4 reactions are 4.3*10-11 and 4.8*10-11 cm3 S-1 per molecule, respectively. The rate constants of the two reactions decrease with decreasing pressure in a manner consistent with the high-pressure (>or=1 Torr) data available in the literature, indicating that, despite the present low-pressure conditions, the three-body association reactions producing, respectively, Si2H6 and Si2H5 are the dominant reaction channels. Measurements were also carried out using a SiH4-He discharge at 200 and 400 mTorr, giving rate constants somewhat smaller than those obtained using a SiH4-Ar discharge, possibly because of incomplete thermalization of SiH2 and SiH. The SiHx (x=0-3) production frequencies in SiH4 plasmas are discussed on the basis of the measured reaction rate constants.