TABLE OF CONTENTS An implication of the terrace ledge migration (step flow) growth mode is that it occurs in such a way that no excess vacancies are introduced into the lattice of the growing film. Conversely, it seems reasonable to expect that excess vacancies will be incorporated into the film in the oscillatory mode of producing planar ledges on the film surface. Indeed, Roland and Gilmer11 have found that adatoms do exchange with subsurface atoms in their molecular dynamic (MD) simulation of homoepitaxial growth in the RHEED regime. Such vacancies in the penultimate surface planes have the possibility of diffusing to the film surface before the latter is covered during deposition. This problem was treated in Chapter II, where it was found that the critical temperature for the freezing-in of subsurface vacancies in fee metals equals about 0.12T M. The lower critical temperature for RHEED oscillations in metals is about 0.125T M whereas the upper critical temperature is about 0.375T M. Thus, it appears unlikely that excess vacancies are frozen-in during the epitaxial deposition of fee metals at substrate temperatures in the RHEED oscillation regime.
The vacancy migration activation energy is not well established for semiconductors. Some investigators12 believe that it is high enough to immobilize vacancies. However, values measured by Watkins13 and assigned to the vacancy migration energy equal about 0.4 eV for Si. Further, a recent molecular dynamic simulation14 provides a value of about 0.4 eV...
