Surface chemistry and growth modeling on epitaxy of Si-related thin films using hydride source gases are described. In particular, impacts of surface hydrogen on the kinetics of adsorption of source-gas molecules and of the hydrogen desorption on the growth kinetics are discussed in detail. This knowledge is used to construct a growth model that successfully accounts for the experimental growth rate and the hydrogen coverage in their temperature and pressure dependences. Impurity atoms added to the Si epitaxy affect both the adsorption and desorption kinetics, but their overall behaviors are given a unified understanding in terms of their bonding energies with the hydrogen atom.