Effects of Strain Relaxation in SiGe Growth on Uniquely Oriented Si Substrates

Abstract

The growth of SiGe epitaxial layers on Si has become the model system for studies of strain layer epitaxy. In this research solid source molecular beam epitaxy was employed to prepare the epitaxial films, and we have employed methods to quantitatively measure the strain in the film (Raman spectroscopy), the interface misfit structure (transmission electron microscopy), and the surface morphology (atomic force microscopy). This thesis explores the formation and subsequent relaxation of strain in SiGe films grown on substrates, which were identified to have unique stable surface structures. The substrates, with crystallographic surface orientations between (001) and (111), are characterized by atomic steps and facets aligned with the [-110] direction. Specifically, the substrates studied have surface normals rotated by 10 and 22 degrees from [001] representing a (118), and (114) - (113) faceted surface respectively. The interface misfit array on the (001) surface that is characterized by two orthogonal arrays was found to form in three unique directions on substrates with a large off-axis tilt. These dislocations align with the predicted intersections of the [111] planes with the substrates, and we derived an expression that describes the observations. We estimated the relaxation of a layer by determining dislocation densities in the TEM images. Notably, the relaxation will vary depending on the Burgers vector chosen to represent the dislocation arrays. A direct correlation was found with the pattern of misfit dislocations and the surface morphology of relaxed films grown on these off-axis substrates. The long range surface corrugations align well with the same three directions as the dislocations indicating that the formation of the misfit dislocations cause this alignment. Our studies of the strain showed that the strain relaxation increased linearly with film thickness indicating a kinetic process as opposed to a sudden plastic relaxation by misfit dislocation formation. In combining these results, we find that only a fraction of the observed strain relaxation can be accounted for with the observed density of misfit arrays. The organization and presence of the surface morphology indicates that the surface morphology also significantly contributes to the strain relaxation both in the coherently strained films and in the partially relaxed structures.