A new method to relax thin constant composition molecular beam epitaxy (MBE) grown SiGe buffer layers on silicon (100) substrates has been studied. A low energy plasma cleaning process (LEPC) using hydrogen prior to deposition can reduce the Si1-xGex layer thickness to approximately 10% of a standard graded buffer. The layers were characterised by secondary ion mass spectroscopy (SIMS), high resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM), Rutherford backscattering (RBS) and atomic force microscopy (AFM). The threading dislocation density is 10(5) cm(-2) (TEM measurement) and is comparable to or even lower than in standard buffers. Ge concentration and Si1-xGex layer thickness are limited by spontaneous relaxation during growth. The critical thickness on hydrogen prepared wafers is reduced to nearly 50% of that prepared in the standard manner. A complete post-epitaxial relaxation has been obtained for a 160-nm-thick Si0.83Ge0.17 layer at temperatures as low as 600 degreesC. AFM investigations of such samples show significantly reduced area-RMS values of 0.3-0.9 nm. By means of multistep growth the Ge content can be increased. In a two-step mode thin buffers up to 34% Ge were grown with a high crystal quality.