Ultrasmooth strain-relaxed SiGe buffer layer with rms roughness of 0.4 nm was obtained by chemical mechanical polishing (CMP). The strained Si modulation-doped structure grown on the planarized buffer layer showed mobility enhancement by a factor of 6 at low temperatures. Thermal stability of the strained Si layer was also found to be improved by CMP, which may come from the reduction of dislocation nucleation sites related to the surface undulation. (C) 2002 Elsevier Science B.V. All rights reserved.