Using first-principles density functional theory calculations of chemical reactions between the dominant precursor (the SiH3 radical) for plasma deposition of hydrogenated amorphous silicon (a-Si:H) thin films and different hydrogen-terminated crystalline silicon surfaces, we show that SiH3 insertion into strained Si-Si bonds is barrierless. This reaction, together with barrierless hydrogen abstraction and chemisorption reactions, account for the temperature-independent reaction probability of the SiH3 radical with a-Si:H surfaces. In addition, molecular-dynamics simulations of a-Si:H thin-film growth confirm that the same reactions take place on the amorphous surface and the probability for Si incorporation into the a-Si:H film is independent of temperature. (c) 2005 Elsevier B.V. All rights reserved.