Enhanced impurity solubility and diffusion near surfaces

Abstract

There is, quite generally, a strain-mediated interaction between a defect and a free surface. In this talk, I discuss how this interaction can have important implications for impurities and other defects in semiconductors. (1) It can lead to reduced defect formation and/or migration energies, and hence enhanced diffusion, for a few layers near the surface. (2) It can lead to a reduced energy for impurities, e.g. dopant atoms, near the surface, and hence an enhanced solubility there. (3) In epitaxial growth, e.g. by MBE or CVD, with reasonable assumptions regarding growth kinetics, the near-surface layers with high impurity concentration may be buried by subsequent growth, without segregation, leading to a high impurity supersaturation throughout the volume of the material grown. These general points are made concrete with calculations for two systems, using a model interatomic potential. To illustrate the enhancement of diffusion, I calculate the activation energy, i.e. the energy of the presumed saddle-point configuration, for one particular mechanism, vacancy diffusion near Si(001). Enhanced impurity solubility is illustrated by the case of substitutional carbon near Si(001). In both cases, the enhancement is several orders of magnitude for the first four layers, at typical growth temperatures. Relevance for dopant impurities in Si and GaAs is also discussed.