A scanning tunneling microscopy and spectroscopy study of the role of hydrogen desorption in epitaxial film growth on the Si(100) surface is presented. Following disilane adsorption, epitaxial growth is shown to be driven by the rebonding of the disilane fragments induced by H2 desorption. This requires the decomposition of only the higher surface hydrides and occurs between 640 and 670 K. The epitaxial layer formed in this manner has a 2×1 monohydride structure. Continuous exposure to disilane at 690 K resulted in multilayer epitaxial growth, the surface of which remains largely H passivated. This latter growth is in part due to a direct reaction between disilane and the monohydride surface. © 1991 The American Physical Society.