In this paper we describe the intimate relation between the surface morphology of epitaxial Si and Si1-xGex layers and the pressure during growth. All the experiments were done at a temperature of 625-degrees-C and the pressure was varied between 10(-4) and 2 x 10(-1) Torr. The layers were grown in an ultrahigh vacuum very low pressure chemical vapour deposition reactor (UHV-VLPCVD) with a gas mixture of SiH4 and GeH4 without the addition of H-2. Based on the pressure and the growth-initiating conditions, we distinguish two different growth modes, based on a different surface morphology and growth rate. The first mode (mode I) is characterized by a high growth rate and good surface morphology, whereas in mode II the growth rate is significantly lower and the surface morphology substantially rougher. We present also a qualitative explanation for these observations in terms of hydrogen coverage and impingement rate of precursor molecules on the growing crystal surface.