Mass transfer surface diffusion of noble gases

Abstract

Molecular dynamics simulations employing the Lennard-Jones potential have been performed for Ne, Ar, and Kr on Ar(111) to investigate the nature of surface diffusion at elevated temperatures. Despite the inapplicability of such a simple potential to materials other than noble gases, these simulations qualitatively reproduce the characteristic experimental features of high-temperature surface diffusion on metals and semiconductors. All species exhibit Arrhenius curves with discontinuities at temperatures ranging from 0.65–0.73 T m. Below this temperature, Ne does not island and diffuses by a standard site-hopping mechanism, whereas mobile adatoms of Kr and Ar are formed from the edges of immobile islands, yielding an activation energy which includes both an enthalpy of formation and an enthalpy of migration. Above the discontinuity in the Arrhenius curve at which the surface disorders, surface diffusion is dominated by mass transfer effects, with all surface species diffusing predominantly by a novel mechanism of adatom-vacancy pair formation, which sharply increases the number of mobile species. Perhaps surprisingly, this mechanism is operative for Ne surface diffusion at elevated temperatures despite surface segregation. In addition, high-temperature heterodiffusion is seen to decrease with increasing surface coverage as the adspecies quench surface disordering in the top substrate layer. These and other results demonstrate the importance of surface preparation during high-temperature surface diffusion measurements.