We have investigated structural properties of GaAs grown on top of a pseudomorphic Si interlayer deposited on a GaAs substrate by x-ray reflectivity and Bragg scattering. The surface and interface roughness of the 500-Å thick GaAs overlayer depends critically on the thickness of the Si interlayer. The surface roughness increases from roughly 10 to 40 Å when the Si thicknesses increases from 3 to 5 Å, whereas the interface roughness remains roughly constant for Si thicknesses larger than 9 Å. The Laue oscillation of the GaAs(004) Bragg reflection together with the asymmetry of the intensity allow a precise determination of the pseudomorphic Si interlayer. ©1997 American Institute of Physics.