Chemical vapor deposition has been applied to the fabrication of a relaxed SiGe buffer on (100) Si substrates. Our structures consist of a 0%-x% graded layer and a uniform Si1-xGex capping layer, with x between 32% and 52%. First, the variation of the threading dislocation density with grading rate has been determined. Our results clearly show an abrupt transition between two domains : steep gradings correspond to a high dislocation density, and smooth gradings to low dislocation densities. Second, the surface morphology has been studied at different steps of the buffer fabrication. In addition to the classical [110] crosshatch morphology, we report for the first time [100]-oriented undulations. This new morphology, only found on steep gradings, is attributed to a Stranski-Krastanov growth mode caused by large surface strains. Finally, we propose a new picture of the role of the grading : a steep grading, with a large surface strain, induces very undulated growths; in this case, small dislocation loops and a high density of threading dislocation are generated. Our calculations allow us to derive a simple criterion : grading rates lower than 137% Ge/mum will guarantee a relaxation of the low mismatched type. Following this, very low threading dislocation densities (10(3) cm-2) were indeed achieved.