Step edge barriers versus step edge relaxation in GaAs : Sn molecular beamepitaxy

Sn strongly segregates to the surface during MBE growth. We have previously proposed a microscopic mechanism for this process in which Sn can incorpor ate 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 lengt h of 1000 nm and a corrugation height of about 5 ML and (2) a subsequent en hanced layer growth mode due to the strain driven formation of small island s. As growth proceeds, 2D islands nucleate on terraces, grow in size, and t hen 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 im plications and causes of these denuded zones. We examine whether these denu ded zones could be due to (1) the removal of step edge barriers or (2) an i nhibition of nucleation due to strain. During growth on vicinal surfaces in the presence of Sn, the specular reflection high-energy electron diffracti on (RHEED) intensity oscillates at a frequency which can be up to 15% faste r than the monolayer growth rate. We propose that the relative change in th e apparent growth rate is proportional to the ratio of the denuded length t o the terrace length. We further demonstrate that the incoherent addition o f oscillations in the diffracted intensity with different frequencies gives rise to the very strong beats observed during growth. The results are comp ared to Monte Carlo calculations. Of the mechanisms considered we conclude that only nucleation inhibited by strain can explain the data. (C) 1999 Pub lished by Elsevier Science B.V. All rights reserved.