EVOLUTION OF SURFACE-MORPHOLOGY AND STRAIN DURING SIGE EPITAXY

We have studied the morphology of x = 0.20-0.26 Si1-xGex alloy layers during vapour phase epitaxial (VPE) growth at 610-750-degrees-C using in situ laser light scattering (LLS) and ex situ atomic force microscopy (AFM). A rippled surface morphology is a clearly defined intermediate form in the roughness evolution, preceding misfit dislocation generation. Transmission electron microscopy (TEM) shows that microscopic strain fluctuations are associated with the thickness variations. Using a simple one-dimensional sinusoidal model for the form of the strain variations, we demonstrate that a flat uniformly strained layer can be energetically unstable with respect to this rippled topography when the ripple period exceeds a critical value. Data from the literature concerning roughness on pseudomorphically strained InGaAs/GaAs, SiGe/Si and Ge/Si epitaxial layers shows an inverse trend between the lateral roughness scale and the mismatch strain, which is consistent with the model. An undulating topography therefore provides a means of partial elastic strain relaxation, which is general for strained-layer epitaxy. Finally, we discuss the role of surface diffusion in enabling the changes in Si1-xGex surface morphology observed by AFM.