Influence of the step properties on submonolayer growth of Ge and Si at the Si(111) surface

The present work describes an experimental investigation of the influence of the step properties on the submonolayer growth at the Si(111) surface. In particular the influence of step properties on the morphology, shape and structural stability of 2D Si/Ge nanostructures was explored. Visualization, morphology and composition measurements of the 2D SiGe nanostructures were carried out by scanning tunneling microscopy (STM). The formation of Ge nanowire arrays on highly ordered kink-free Si stepped surfaces is demonstrated. The crystalline nanowires with minimal kink densities were grown using Bi surfactant mediated epitaxy. The nanowires extend over lengths larger than 1 µm have a width of 4 nm. To achieve the desired growth conditions for the formation of such nanowire arrays, a modified variant of surfactant mediated epitaxy was explored. It was shown that controlling the surfactant coverage at the surface and/or at step edges modifies the growth properties of surface steps in a decisive way. The surfactant coverage at step edges can be associated with Bi passivation of the step edges. The analysis of island size distributions showed that the step edge passivation can be tuned independently by substrate temperature and by Bi rate deposition. The measurements of the island size distributions for Si and Ge in surfactant mediated growth reveal different scaling functions for different Bi deposition rates on Bi terminated Si(111) surface. The scaling function changes also with temperature. The main mechanism, which results in the difference of the scaling functions can be revealed with data of Kinetic Monte-Carlo simulations. According to the data of the Si island size distributions at different growth temperatures and different Bi deposition rates the change of SiGe island shape and preferred step directions were attributed to the change of the step edge passivation. It was shown that the change of the step edge passivation is followed by a change of the preferred steps direction resulting into different islands shapes. The symmetry of the properties of the different step directions can determine the symmetry of the 2D islands. The growth shape of reconstructed 2D islands (nanostructures) on reconstructed surfaces can deviate from the internal symmetry of the substrate and the island. An analysis of the symmetry of the combined system of reconstructed substrate and island can deduce predictions for the island growth shape. It was found experimentally that the shape of two-dimensional (2D) Si or Ge islands has a lower symmetry than the threefold symmetry of the underlying Si(111) substrate if Bi is used as a surfactant during growth. Arrow-shaped or rhomb-shaped 2D islands were observed by scanning tunneling microscopy. This symmetry breaking was explained by a mutual shift between the surface reconstructions present on the substrate and on the islands. The mutual shift results into different step structure for initially symmetry related step directions. Using the kinematic Wulff construction the growth velocities of the steps could be determined from the island shape if the nucleation center had been located by a marker technique. The structural stability of 2D SiGe nanostructures was studied by scanning tunneling microscopy (STM). The formation of pits with a diameter of 2–30 nm in one atomic layer thick Ge stripes was observed. The unanticipated pit formation occurs due to an energetically driven motion of the Ge atoms out of the Ge stripe towards the Si terminated step edge followed by an entropy driven GeSi intermixing at the step edge. The pit formation can be also used for nanostructuring. Using conditions at which pit formation is enhanced the fabrication of freestanding GeSi stipes with single digit nanometer width is possible. Continuous ~ 8 nm wide freestanding GeSi wires have been fabricated by pit coalescence