The interest in virtual substrates, consisting of standard Si substrates overgrown with SiGe strain-relaxed buffers (SRBs), is mainly driven by applications in MOSFETs. They require thin SiGe layers with a high Ge content, a high degree of relaxation, smooth surfaces, and a low threading dislocation density. To fabricate such SRBs, we have developed a two-step MBE method. The first step is carried out at low temperature (LT) and introduces a supersaturation of point defects. In the second step, growth continues at a conventional MBE temperature. During this stage, the point defects introduced in the first step enable a high degree of strain relaxation, a smoother surface, and a lower defect density. The most critical process parameter is the growth temperature during the first (LT) step. If the temperature is too high, sufficient relaxation will not occur because the concentration of point defects is insufficient. When the temperature is too low, the concentration of point defect becomes too high and causes structural disorder. We have found a process window in which a high degree of relaxation and good crystal quality are achieved in sub-100 nm layers. To characterize the surface morphology and defects in the layers, various methods of optical, atomic force, and transmission electron microscopy have been employed.