Xu et al. Nanoscale Research Letters 2011, 6:113 http://www.nanoscalereslett.com/content/6/1/113 Page 2 of 7 Figure 1 Model of group IV semiconductor NW growth by MBE. The NW has a length L and a diameter D. Elemental compounds are evaporated with a rate J and impinge on the surface as well as on the Au seed particle, positioned at the top of the NW. Once adsorbed, the adatoms diffuse on the substrate and on the NW sidewalls with diffusion lengths ls and lf, respectively. Au adatoms can also diffuse away from the seed particle. <111> and <110>-oriented NWs, we are able to explain why <110>-oriented NWs grow longer. Experimental details The SiNWs were fabricated by the MBE method using gold droplets. The gold droplets were formed directly by gold deposition on a heated Si(111) surface in UHV at a gold deposition pressure of 6 × 10-10 mbar. The density and the diameter of the gold droplets are determined by the Au evaporation rate and the temperature of the samples via the Ostwald ripening process [6]. In the next step, the growth of the NWs was achieved from the sublimation of Si or Ge at a deposition pressure of 10-9 mbar. In order to grow the NWs, the Si(111) sur- face was heated either at 550°C for the growth of Si NWs or at 350°C to obtain Ge NWs. The evaporation rate of elemental Si and Ge was measured from the thickness of the two-dimensional film grown on the substrate during the NW growth. The morphology of the NWs was investigated by elec- tron microscopies: scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). To perform the HRTEM experiments, the NWs were cleaved with micromanipulators in the chamber equipped with a focus ion beam machine and transfer into holey grids covered with a very thin carbon layer. Results and discussion Tilted views of the post-growth Si(111) surfaces are shown in Figure 2. When Si is sublimated, the majority of the Si NWS are found to be perpendicular to the Si (111) surface (Figure 2a). Their growth axis is thus along the [111] direction, in agreement with previous observations [4,5]. In contrast, the growth of Ge NWs leads to two different kinds of growth directions (Figure 2b). The shortest NWs usually show the Au seed particle just above the overgrown Ge film. These NWs appear normal to the surface when observed in top view SEM images such as the one shown in the inset of Figure 3. They thus grow along the [111] direc- tion, likewise the Si NWs. As for the second type of Ge NWs, these NWs are much longer and point at 54.7° from the surface plane. In the top view SEM image of Figure 3, these inclined Ge NWs are found to grow along three different directions only. When projected in the (111) surface plane, the directions make an angle of 60°. Therefore, these Ge NWs are oriented along one of the equivalent <110> directions, in agreement with the Figure 2 SEM images of (a) Si NWs and (b) Ge NWs grown on a Si(111) surface by MBE. The orientations of the NWs are indicated in the SEM images. The growth times were 2 and 1 h for the Si and Ge NWs, respectively. The scale bars correspond to 400 nm.