We report on the formation of Si and SiGe nanowires (NW) by molecular beam epitaxy (MBE) initiated via gold droplets. The MBE growth behavior essentially differs from the classical vapor-liquid-solid mechanism (VLS) observed in the case of NW growth by chemical vapor deposition (CVD). From a thermodynamic point of view, the driving force for the NW growth by MBE is related to the supersaturation determined by relaxation of elastic energy generated in Si substrate due to Au intrusion. Adding Ge decreases the growth rate of Si NW or can even result in its dissolution. This effect can be interpreted in terms of elastic energy accumulation because of differences in atomic radii of Si and Ge. This effect has a general influence on the formation of NW heterostructures. (c) 2006 Elsevier B.V. All rights reserved.