Influence of Group-III-metal and Ag adsorption on the Ge growth on Si(111) and its vicinal surfaces

Abstract

In the framework of this thesis the surfactant-mediated heteroepitaxial growth of Ge on different Si surfaces has been investigated by means of low-energy electron microscopy, low-energy electron diffraction, spot-profile analyzing low-energy electron diffraction, x-ray standing waves, grazing-incidence x-ray diffraction, x-ray photoemission electron microscopy, x-ray photoemission spectroscopy, scanning tunneling microscopy, scanning electron microscopy, transmission electron microscopy, and density functional theory calculations. As surfactants gallium, indium, and silver were used. The adsorption of Ga or In on the intrinsically faceted Si(112) surface leads to a smoothing of the surface and the formation of (Nx1) reconstructions, where a mixture of building blocks of different sizes is always present. For both adsorbates the overall periodicity on the surface is strongly dependent on the deposition temperature and the coverage. For the experimental conditions chosen here, the periodicities are in the range of 5.2<N<6.5 and 3.4<N<3.7 for Ga and In, respectively. The (Nx1) unit cells of Ga/Si(112) and In/Si(112) are found to consist of adsorbate atoms on terrace and step-edge sites, forming two atomic chains along the [1-10] direction. In the Ga-induced structures two Ga-vacancies per unit cell (one in the terrace and one in the step-edge site) are found and a continuous vacancy line on the surface is formed. In the In/Si(112) structure only one vacancy per unit cell in the step-edge site exists and, thus, a continuous adsorbate chain on the terrace sites is present. The adsorption of Ga or In on Si(112) strongly influences the subsequent Ge growth. Ge deposition on the Ga-terminated Si(112) surface leads to the formation of Ge nanowires, which are elongated along the Ga chains and reach lengths of up to 2000nm for a growth temperature of 600°C. On In-covered Si(112), both small dash-like Ge islands and triangularly shaped islands are found, where the latter ones are terminated by (111), (013) and (103) side facets as well as a (112) top facet. Ga and In have a contrary impact on the Ge diffusion on the Si(112) surface. While Ga reduces the diffusion for Ge atoms, compared to the growth on the bare Si(112) surface, In increases the diffusion. For the first time Ag was employed as surfactant material for Ge growth on Si. On a completely (sqrt(3) x sqrt(3))-R30°-Ag covered Si(111) surface a drastic increase of the diffusion length for Ge and, thus, the growth of huge Ge islands (sizes of several µm) is observed. Their density is about three orders of magnitude lower as compared to the growth on the clean Si(111) surface. At a coverage of around 90 bilayers (layer thickness of around 28 nm) the islands are coalesced and a closed Ge film is formed. Ag deposition on both, Si(112) and Si(113), induces the formation of a regular array of nanometer-scale facets along the [1-10] direction, the sizes of which are dependent on the growth temperature and a maximum periodicity perpendicular to [1-10] of 55nm is determined. On Si(112) the array consists of alternating (111) and (113) facets, whereas on Si(113) alternating (111) and (115) facets are found. Subsequently deposited Ge grows nicely along the direction of the facets. Thereby, Ge nanowires with lengths of up to 600nm and aspect ratios of up to 10:1 are formed.