Gas-phase molecular composition in silane (pure and H-2-diluted) plasma was studied using a mass spectrometry under various deposition conditions of hydrogenated amorphous silicon (a-Si:H). Our aim was to correlate the gas-phase species with the film property after light-induced degradation and to find the ideal deposition conditions to achieve high-rate growth of stable a-Si:H films. A parallel-plate plasma enhanced chemical vapour deposition (PECVD) system with an excitation frequency of 13.56 MHz (RE) was used. Both the film deposition and mass spectrometry were performed under deposition conditions of a-Si:H giving growth rates ranging from 2 to 20 Angstrom/s. We observed the signal intensities of SiH2+ (m/e = 30) and Si2H4+ (m/e = 60) as the most abundant ions in the mass fragmentation related to monosilane and disilane molecules, respectively. The film property of a-Si:H films after light-induced degradation, monitored by the fill factor (FF) of photo current - voltage characteristics in n + crystalline Si/a-Si:H/Ni Schottky cells after light soaking, deteriorated with the increase in the growth rate. The contribution of gas-phase disilane related radicals to the film growth, represented as the disilane fraction ([m/e = 60]/[m/e = 30]), showed a good correspondence with the FF after light soaking. It is suggested that the suppression of gas-phase higher-order silane-related radicals as well as short-lifetime radicals is a clue for obtaining stable a-Si:H solar cells at high growth rate.