We report Raman scattering and high-resolution X-ray diffraction measurements performed on a series of MBE-grown Si / Gen / Si quantum wells with n varying from 3 to 6. Our results are consistent with a gradual evolution of the Si / Ge interface which starts with Ge segregation and formation of terraces for low Ge coverage, to smooth 2D Ge layers, bounded by interfacial alloy layers, for coverages superior to 4M Ls.