Crystal growth mechanisms in chemical vapor deposition (CVD) of silane are studied in some detail. An atomic view reveals that the free energy change upon the formation of a small crystal on the (100) silicon surface in Si-CVD from SiH4 is always negative, due to the presence of hydrogen. Therefore, nucleation and step growth are unnecessary, in contrast to crystal growth from melt. Epitaxial growth in this case takes place through iterative adsorption and desorption reactions on the periodic surface sites of the substrate. Slow desorption reactions result in vacancies and distorted bond angles which hinder epitaxial growth. Based on this model, the role of H desorption in low temperature Si epitaxy from SiH4 is investigated. A thermodynamic analysis implies that H desorption is the limiting factor to low temperature epitaxy in the pressure range of 1 to 760 Torr, and the pressure dependence of minimum epitaxial temperature is derived as T-epi(degrees C) = 106 log P-tot(Torr) + 668. Below 1 Torr, adsorption rate becomes the limiting factor to low temperature epitaxy.