The effect of various growth conditions has been studied in order to modelize and optimize the SiC buffer layers obtained by reactive chemical‐vapor deposition (RCVD) on Si(100) substrates. First, thermodynamic calculations have been carried out on the Si–H2–CxHyClz system to simulate the RCVD process and to foresee the nature and the evolution of the deposit with varying parameters. The experiments have confirmed some of the thermodynamic results, such as the carbon deposition in specific conditions, and brought out complementary information on the kinetics point of view. Due to our low heating up rate, no SiC island formation was characterized at the early stage of growth. The obtained SiC layers are ultrathin with a very particular morphology. We propose a new model of growth based on atomic force microscopy observations to explain the resulting morphology. The optimal conditions have been deduced to elaborate ultrathin, smooth, and monocrystalline β‐SiC buffer layers. © 1996 American Institute of Physics.