For a successful fabrication of hetero-MOSFETs (HMOSFETs) the compatibility of the epitaxial growth of the heterostructures with the standard CMOS process is an important factor. For this reason, we demonstrate for the first time the growth of relaxed SiGe buffer layers with a Ge content up to 30% by differential molecular beam epitaxy (MBE) after the definition of the active device areas of n-channel HMOSFETs by lateral oxide isolation. The influence of lateral dimensions and the composition of the epitaxial layer stacks on the surface topography, relaxation and defect density has been investigated. Scanning electron microscopy (SEM) and differential interference contrast (DIC) micrographs were applied to characterize the quality of the differential epitaxy and the surface topography. The degree of relaxation in the patterned areas was determined using micro-Raman spectroscopy. The analysis of the dislocations was carried out by transmission electron microscopy (TEM). The reduction of the surface cross-hatch with decreasing lateral dimensions was clearly seen. Further, we found, that the increase of the relaxation corresponds to a higher density of misfit dislocations; confined at the interface SiGe buffer/Si buffer. To our surprise, we also observed a strong influence of the pure Si channel (10 nm tensile-strained Si layer) on the SiGe buffer layer. This indicates that the investigation of the quality of the relaxed SiGe buffer layers should be only done with the whole epitaxial layer stack of the device structure.