Characterization of low-temperature (LT) Si and SiGe on the LT-Si grown by molecular beam epitaxy was carried out. Positron annihilation spectroscopy showed that the size of point defects in LT-Si grown at 400 degrees C was much larger than divacancies. These vacancy clusters were considered to play an important role in strain relaxation of SiGe layers on the LT-Si. Although Si0.7Ge0.3 layer grown on the LT-Si had excellent qualities compared with the graded buffer Si1-xGex layer, it was found to degrade when Ge content was increased up to x similar to 0.5 but be improved again above this content. The surface morphology also changed at this Ge content from crosshatch pattern to smooth surface with increasing Ge content. p-type modulation doped structures with pure-ae channels were fabricated on the LT-buffers and the transport properties were found to significantly depend on the surface morphology. The reason that the rough surface gave rise to rather high mobility was discussed in the terms of roughness scattering.