Defect-free Stranski-Krastanov growth of strained Si 1- xGe x layers on Si

Abstract

Strained Si 1- x Ge x epitaxial layers on silicon are known to exhibit a non-planar surface under certain growth conditions. In the present work, combined transmission electron microscopy and atomic force microscopy (AFM) are employed to reveal the appearance and growth of surface undulations which form in diluted Si 1- x Ge x alloys. On the basis of these experiments and theoretical considerations, the possible causes of such a surface rippling are reviewed and discussed. It appears that the partial relaxation of the elastic energy, permitted by the undulations themselves, is the driving force of this phenomenon, what is referred as defect-free Stranski-Krastanov growth mode. AFM measurements permit one also to quantify the kinetics of appearance and growth of the undulations occuring during deposition. It is found that the amplitude of the undulations follows an exponential variation with the deposition time/film thickness. Finally we propose a simple model based on partial elastic relaxation (driving force) and on surface diffusion (limitative mechanism) to describe our experimental kinetics of the surface ripples during the very first steps.