Hydrogen incorporation in amorphous hydrogenated silicon produced by the expanding thermal plasma is investigated as a function of substrate temperature at three different deposition rates of 0.3, 3 and 11 nm/s. The increase of the refractive index with increasing substrate temperature is attributed to decreasing hydrogen concentration. The latter result is explained by a model which assumes a thermally activated hydrogen cross-linking step immediately after the chemisorption of a silyl radical. The activation energy for this process is about 150 meV. For growth rates larger than 1 nm/s the hydrogen content is significantly growth-rate dependent.