Si1-xGex/Si/Si1-xGex heterostructures with a flat channel were successfully fabricated by molecular-beam epitaxy combined with solid-phase epitaxy. The solid-phase epitaxial growth completely suppressed Ge segregation between the channel layer and the doped layer. Cross-sectional transmission-electron-microscope images revealed that misfit dislocations were limited to the bottom part of the buffer layer and to the substrate, and that the heterointerfaces were very smooth and flat. The electron mobility for a sample with a graded buffer layer was nearly one order of magnitude higher than without such a layer. Ultrahigh mobility of 8.0×105 cm2V-1s-1 was obtained at 15 K for a sample with a graded buffer layer (x=0.2).