We have applied both sinusoidal and pulse-shaped rf substrate biasing techniques to the expanding thermal plasma deposition of hydrogenated amorphous silicon. Spectroscopic ellipsometry and Fourier transform infrared spectroscopy data demonstrate that both methods of substrate biasing can result in improved film properties at deposition rates ranging from 1.4 to 16 nm s¿1, at relatively low substrate temperatures (200 °C), as demonstrated by an increase in refractive index, and a decrease in the microstructure factor (R*). An extra plasma forms in front of the substrate upon the application of both bias techniques, but optical emission spectroscopy data show that the emission intensities are significantly greater with the sinusoidal rf bias. Although the application of sinusoidal rf bias results in an initial improvement of the materials, the H content and R* values both increase at higher rf-induced substrate bias voltages (¿Vbias >~ 70 V). The same bias conditions that result in increased H content and R* correspond to the conditions where the additional plasma in front of the substrate undergoes both a sharp increase in emission, and a decrease in measured ion current, suggesting an ¿ to ¿' transition.@en