The crystallization behavior of polycrystalline silicon (Si) and germanium-silicon alloys (GexSi1-x) from SiH4 and GeH4, where x is in the range of 0-0.32, has been investigated for thin film transistor (TFT) applications. Furnace anneals as well as rapid thermal anneal (RTA) and combinations of these two techniques have been used to crystallize amorphously deposited thin (less than or equal to 100 nm) films. The effects of time and temperature for the furnace anneals and time, temperature and pulse rate for the RTA have been investigated. Smooth Si and GexSi1-x layers with a surface roughness less than or equal to 0.6 nm have been obtained using an initial Si layer for the GexSi1-x material, since GexSi1-x shows a nucleation problem on oxide surfaces which influences the resulting surface roughness and grain size. For TFT applications the optimal film properties cannot be obtained with a single crystallization anneal. Conventional furnace crystallization results in smooth layers with Si furnace crystallized films exhibiting small grains with many intra-grain defects. An average grain size of approximately 300 nm for Ge0.25Si0.75 and slightly larger grains for Ge0.32Si0.68 with less defects is obtained at lower temperature. RTA results for Si and GexSi1-x in fine grained material with lower defect density.