Surface segregation during molecular beam epitaxy: The site-blocking effects of surfactant atoms

The two-site exchange model for surface segregation is extended to take account of the site-blocking action of foreign atoms on the surface, either those applied deliberately as a surfactant, or those occurring as reaction products of the growth process. The model is applied to segregation of Ge during the growth of a Si him on a SiGe surface by gas-source molecular beam epitaxy, using Si2H6 as the Si precursor. It is shown how the reaction product, hydrogen,which is adsorbed on the growing surface, acts to reduce the Gibbs energy of segregation. The trends for the blocked-site concentration by hydrogen as a function of temperature and Si2H6 flux derived from the Ge segregation data show excellent agreement with reported results of surface hydrogen coverage obtained by completely different techniques.