Surface Morphology of SiGe Epitaxial Layers Grown on Uniquely Oriented Si Substrates
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Surface Morphology of SiGe Epitaxial Layers Grown on Uniquely Oriented Si Substrates Morgan E. Ware and Robert J. Nemanich North Carolina State University, Physics Department, Raleigh, NC 27695-8202, U.S.A. ABSTRACT The 4% lattice mismatch between Si and Ge creates strain in epitaxial layers of SiGe alloys on Si, and this strain can manifest itself in the morphological structure of the surface of the epitaxial layer. This study explores the relationship of the evolution of the surface morphology of SiGe layers grown on a range of Si surface orientations. We have grown thin, strained and thick, relaxed layers of Si0.7Ge0.3 by solid source molecular beam epitaxy on substrates with surface normals rotated from [001] towards [111] by angles of θ = (0, 2, 4, 10, 22) degrees. The surface morphology was investigated by atomic force microscopy, which showed considerable ordering of surface features on relaxed samples. These features evolve from hut-like structures at 0 degrees to large mesa-like structures separated by pits and crevices at 22 degrees. The organization of these features is also shown to vary with the substrate orientation. Each surface has characteristic directions along which features are aligned, and these directions vary continuously with the angle of rotation of the substrate. Transmission electron microscopy confirmed that misfit dislocations had formed along those same directions. The state of relaxation of each layer is quantified by Raman spectroscopy in order to make a direct correlation between residual strain and surface morphology. INTRODUCTION It is known that a strained epitaxial film can relax partially by forming corrugations at the surface [1-4]. The atomic planes running parallel to the growth direction spread near the top of the undulations thus partially relaxing that part of the surface. This is countered by a pinching of the planes near the bottom of the undulations. This latter effect extends over a shorter range on the surface, so a net relaxation is expected. It has been suggested, though, that this pinching and thus increased stress at the surface may influence the evolution of the film through nucleation of misfit dislocations at the interface [5]. In this study we have investigated qualitatively this interplay between the surface corrugations and misfit formation in SiGe layers on uniquely oriented Si substrates. The substrates used are described schematically in Figure 1. The figure displays a portion of the family of surfaces between the (001) and the (110) surfaces. Specifically the substrates used had surface normals which were misaligned from the [001] direction towards the [110] direction by 0, 2, 4, 10, and 22 degrees. We note that in this scheme the (111) surface is at approximately 54.7 degrees. It is relevant to note the large-scale features of the appropriate clean surfaces. On all the miscut substrates used the clean surfaces have either steps or facet edges that extend along the [-110] direction on the surface. The 2, 4 and 10 degree surfaces exhibit (001) terraces wi
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