Sb-surfactant-mediated growth of Si and Ge nanostructures

We have used reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), and high-resolution electron microscopy (HREM) to investigate Sb surfactant mediated growth (SMG) of Ge on Si. We show that SMG of ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ promotes the two-dimensional (2D) nucleation regime and increases the critical thickness of the 2D-3D transition at 350 °C. At this temperature, thick defect free flat Ge layers can be epitaxied on 1 ML Sb-Si(001) without formation of Ge islands. At higher temperature (550 °C) Ge islands nucleate after the growth of several 2D monolayers; SMG induces a dramatic reduction of the islands lateral size on both Si(111) and Si(001). We show that the different SMG roles obtained at 350 and 550 °C are explained by the conjunction of thermodynamic and kinetic effects. First we explain the substantial increase of the 2D-3D transition critical thickness by the reduction of Sb induced surface energy. This surface energy reduction has a stabilizing effect of flat ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ layers (against island growth). Second, we attribute the dramatic reduction of islands size to a lower Ge diffusion length on Sb rich surface. We suggest that this reduced diffusion length is due to the competition between surface diffusion and exchange with subsurface Sb atoms. In addition, Ge:Sb SMG on Si(001) is successfully used for two applications: the growth of flat layers and the self-assembling of ultrasmall, dense and homogeneous Ge dots.