Atomic force microscopy is used to measure surface morphology of modulation doped Si/SiGe heterostructures. Three components in the surface roughness are observed : mu m-scale roughness arising from misfit dislocations formed to relieve strain, 1000-Angstrom-scale roughness believed to be associated with three-dimensional growth of the electron or hole channel layers, and atomic-scale roughness with wavelengths of 10-100 Angstrom. Detailed Fourier spectra of the roughness are obtained and used as input to a scattering computation for determining mobility. The results are compared with other mobility-limiting mechanisms, including scattering from ionized impurities and from dislocations.