Criteria have been developed for assuring epitaxial film growth in the presence of trace amounts of water and oxygen for both low and high temperature regimes. These concise and quantitative relationships can be used as a tool for guiding experiments and explaining conflicting experimental results. The criterion for the low temperature regime is based on the fundamental adsorption/desorption behavior of hydrogen known to date. The criterion for the high temperature regime is synthesized in a similar manner. Flow effect is given along with the effect of deposition rate. A new concept for effective oxygen partial pressure is introduced.