Step edge barriers versus step edge relaxation in GaAs:Sn molecular beam epitaxy

Sn strongly segregates to the surface during MBE growth. We have previously proposed a microscopic mechanism for this process in which Sn can incorporate into either a substitutional site or into a surface interstitial site. These two sites compete with each other resulting in two main effects: (1) strain causes an initial surface roughening that has a characteristic length at 1000 nm and a corrugation height of about 5 ML and (2) a subsequent enhanced layer growth mode due to the strain driven formation of small islands. As growth proceeds, 2D islands nucleate on terraces, grow in size, and then coalesce at layer completions. In situ STM and ex situ AFM on quenched GaAs:Sn surfaces show that above descending step edges during growth there are zones denuded of islands. The focus of this paper is to consider the implications and causes of these denuded zones. We examine whether these denuded zones could be due to (1) the removal of step edge barriers or (2) an inhibition of nucleation due to strain. During growth on vicinal surfaces in the presence of Sn, the specular reflection high-energy electron diffraction (RHEED) intensity oscillates at a frequency which can be up to 15% faster than the monolayer growth rate. We propose that the relative change in the apparent growth rate is proportional to the ratio of the denuded length to the terrace length. We further demonstrate that the incoherent addition of oscillations in the diffracted intensity with different frequencies gives rise to the very strong beats observed during growth. The results are compared to Monte Carlo calculations. Of the mechanisms considered we conclude that only nucleation inhibited by strain can explain the data.