The overgrowth of Si on Ge/Si(0 0 1) islands in a UHV chemical-vapor deposition system is investigated by reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), transmission electronic microscopy (TEM), and photoluminescence. It has been shown that the capping process comprises three stages, firstly a shape transition from dome to pyramid induced by strain relaxation due to interdiffusion, followed by a pyramid-dome shape transition, before the smoothening of the surface. This study shows the existence of a strong surface roughness even if the Si cap thickness is higher than the island height. The roughness is then a parameter that should be considered in a multilayer system to explain the mechanism of vertical alignment. (C) 2003 Elsevier B.V. All rights reserved.