ESTIMATING SURFACE-DIFFUSION COEFFICIENTS

General guidelines for correlating surface diffusion parameters have e xisted up to now only in loose form and for specialized cases. Based o n a survey of over 500 systems (the largest currently in existence), w e propose new correlations for the activation energy E(diff) and pre-e xponential factor D-0 for thermally activated surface diffusion on met als, semiconductors and some types of insulators. Physical bases ratio nalizing these correlations are provided. The highlights of these resu lts follow, concerning two basic modes of surface diffusion: intrinsic , where the number of mobile particles remains constant with temperatu re, and mass transfer, where the surface releases mobile particles as the temperature increases. For intrinsic diffusion, E(diff) on metals depends most strongly on whether or not the adsorbate is metallic. On semiconductors and insulators with directional bonding, the most impor tant system property is the number of bonds an adsorbate forms with th e substrate. For mass transfer self-diffusion at lower temperatures, k inks act as sources of adatoms, whereas at higher temperatures the ter races serve as the primary source. Consequently, two regimes exist for E(diff) and D-0, arising from the enthalpy and entropy for adatom cre ation in each process. Related effects can sometimes exist for mass tr ansfer heterodiffusion. A clear increase in E(diff) and D-0 appears in the higher temperature regime in progression from metals to semicondu ctors to insulators. For predictive purposes, semiquantitative estimat es may be made for the enthalpy and entropy by analogy with bulk vacan cy behavior. The present work represents the most comprehensive overal l picture for surface diffusion yet available. Diffusion in general do es not seem to be perturbed significantly by the presence of an aqueou s ambient.