Surface-heating phenomenon by the radiation from high density plasma during growth of microcrystalline silicon (mu c-Si: H) thin films at high rate (> 5 nm/sec) is one of the crucial issues to be solved for obtaining high quality intrinsic-layer material for solar cells. We have utilized an optical emission spectroscopy (OES) in the plasma to observe the time evolution of gas temperature during film growth as well as the film-growth rate under mu c-Si:H deposition conditions at high rate. Gas temperature has been successfully controlled by changing total flow rate of monosilane (SiH(4)) / hydrogen (H(2)) gas mixture, leading to a drastic improvement of optoelectronic properties in the resulting mu c-Si:H. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim