Dependence of SiGe growth instability on Si substrate orientation

We present an experimental study of the influence of the Si substrate orientation on the growth modes and relaxation mechanisms of Si1-xGex for x ranging between 0 and 1 (misfit m up to 4%). At low misfits (m < 1%), strain is relaxed on both Si(001) and (111) orientations by the well-known tetragonal distortion. The latter is about twice as large on (001) as on (111), in good agreement with linear elastic theory. In this low stress regime, Si1-xGex/Si(001) heterostructures display surface morphology fluctuations due to the kinetic roughening phenomenon. With increasing thickness, this surface roughness orders into small isotropic undulations of large wavelength. At larger misfits (l% < m < 2.7%), in the regime of medium stress, the surface morphology dramatically changes with the substrate orientation. While a growth instability leads to undulation of Si0.7Ge0.3(001), the Si0.7Ge0.3(111) remains completely flat and strained. This indicates a total inhibition of the growth instability on Si(111), whatever the elastic strain energy. We suggest, in agreement with recent theoretical models, that the development of the growth instability is related also to atomic step interactions. This is consistent with our CBED, GIXRD and HREM analyses which demonstrate a negligible elastic relaxation of the Si0.7Ge0.3(001) in-plane parameter. Consequently, the elastic relaxation of the undulated layer currently invoked in the literature cannot be the single underlying mechanism for the development of the growth instability in Si1-xGex epilayers. (C) 1998 Elsevier Science B.V. All rights reserved.