We report on the growth and properties of high mobility polycrystalline silicon films grown at low temperatures (400-500 degrees C) on amorphous substrates using a controlled reactive plasma beam deposition. The technique consists of carefully controlling the H radical flux from an electron cyclotron resonance (ECR) plasma to promote both nucleation and grain boundary passivation during growth. Using electrical and optical spectroscopy of the ECR plasma, we find that the density of H radicals impinging on the surface is one of the most important parameter controlling crystallinity of the film, and that by changing this flux, we can controllably alter the structure of the film from amorphous to polycrystalline. Unlike traditional deposition techniques for polysilicon, which requires either high deposition temperatures (650 degrees C) or a low temperature deposition followed by a post deposition anneal to achieve good mobilities, our technique produces films with mobilities of the order of 30-40 cm2/Vs in as grown films at considerably lower temperatures. We have produced undoped and highly doped films using this technique.