There has been a resurgence of interest in the growth characteristics of amorphous Se (a-Se) films due to their recently commercialized applications in x-ray imaging as x-ray photoconductors. Electronic quality a-Se films require the substrate temperature during vacuum deposition to be as high as possible but increasing the substrate temperature leads to amorphous films containing crystalline inclusions and eventually to polycrystalline films which have high conductivity and are therefore not useful as photoconductors. We prepared a variety of thick a-Se films on Al substrates by conventional vacuum deposition in which only the substrate temperature T-sb and boat temperature Tbt Were varied. Experiments indicated the appearance of crystalline zones (crystallites) in the amorphous Se film as the substrate temperature was increased. We identified an experimental critical T-bt-T-sb boundary between amorphous films with and without crystalline inclusions. A model has been developed for the critical relationship between T-sb and T-bt, i.e., T-bt=f(T-sb) based on two competing rate mechanisms: the rate of deposition (or material condensation on the substrate) and rate of crystal growth in the film assuming heterogeneous nucleation at favorable substrate sites. Over the narrow substrate temperature range, we approximated the Vogel-Tamman-Fulcher behavior of the crystal growth rate by an Arrhenius rate with an apparent activation energy E-cr that accounts for most crystallization studies on a-Se. The condensation rate was assumed to follow an Arrhenius rate with the activation energy determined by the enthalpy of evaporation, E-ev. The T-bt=f(T-sb) model was particularly sensitive to the E-cr/E-ev ratio. By using the presently accepted values for E-ev and E-cr, it was shown that the experimental results could be well fitted to the T-bt=f(T-sb) model. The results have a wider significance than a-Se and can be easily extended to apply to a number of other chalcogenide semiconductor films.