Koen Vanormelingen

H2S exposure of a (100)Ge surface : Evidences for a (2×1) electrically passivated surface

The experimental study of the bonding geometry of a (100)Ge surface exposed to H2S in the gas phase at 330°C shows that 1 ML S coverage with (2×1) surface reconstruction can be achieved. The amount of S on the Ge surface and the observed surface periodicity can be explained by the formation of disulfide bridges between Ge–Ge dimers on the surface. First-principles molecular dynamics simulations confirm the preserved (2×1) reconstruction after dissociative adsorption of H2S molecules on a (100)Ge (2×1) surface, and predict the formation of (S–H)–(S–H) inter-Ge dimer bridges, i.e., disulfide bridges interacting via hydrogen bonding. The computed energy band gap of this atomic configuration is shown to be free of surface states, a very important finding for the potential application of Ge in future high performance integrated circuits.

Quantitative characterization of the surface morphology using a height difference correlation function

A height difference correlation function was defined for the analysis of experimentally obtained real space images of a surface morphology. Using scanning tunneling microscope images of two different surfaces, the Si(111)-7×7 reconstruction and hyperthermally deposited thin Co films on Si(111), we demonstrate the advantages of this characterization procedure. Parameters such as the grain size and the roughness at short length scale, which are difficult to determine, especially for surfaces exhibiting randomly distributed closely packed grains, can be easily obtained from an appropriate fit of the height difference correlation function. This fit, based on the theory of kinetic roughening, simultaneously provides quantitative information on the roughness at short (Hurst parameter) and large length scales and surface in-plane correlation length of the film. The results for the overall surface roughness are consistent with the values which can be directly obtained from scanning tunneling microscope measuremen...

The influence of a Cu buffer layer on the self-assembly of iron silicide nanostructures on Si(111)

The role of a Cu buffer layer on the formation of iron silicide nanostructures is investigated using scanning tunneling microscopy and Mossbauer spectroscopy. The deposition of 1A Fe on the Si(111)-7×7 and the Si(111)-5×5-Cu surfaces results in the self-organization of nanoscale islands. Increasing the deposition temperature (300-600°C) leads to an exponential decrease in island density and to an increase of the average island size. At 475°C, the preferential nucleation site changes from the terrace to the step edges, i.e., step flow growth is observed. The self-assembled nanostructures exhibit the metastable CsCl–FeSi1+x structure. Due to the enhanced diffusion, nanodots formed on the 5×5 surface are significantly larger and more separated compared to growth on the bare 7×7 surface. These results show that a buffer layer provides an additional, experimentally controllable parameter, besides temperature, to tailor the size and distribution of nanodots.

Fe-silicide nanostructures on Si(111)-3×3-Ag

Scanning tunneling microscopy was used to investigate the influence of Ag-induced surface reconstructions on the formation of low-dimensional Fe-Si structures. The deposition of 1A Fe (i.e., 1.1 monolayer) at 300°C on the 3×1-Ag, the 3×3-Ag, and the 7×7 reconstructions of the Si(111) surface results in the self-assembly of small islands. For both Ag-induced reconstructions, these islands are significantly larger compared to those formed on the Si(111)-7×7 surface due to an increased surface diffusion. Moreover, on the 3×3 structure, these nanodots are well separated and in between, the initial reconstruction remains unchanged. In the presence of surface steps, these islands preferentially nucleate at the lower step edge, which can be used to grow long continuous nanowires for higher Fe coverage and vicinal surfaces. Furthermore, from the phenomena such as step retraction and island/hole combinations, it is concluded that these nanostructures consist of Fe-silicide.

The influence of surface steps on the formation of Ag-induced reconstructions on Si(111)

Using scanning tunneling microscopy, the influence of Si(111) surface steps on the formation of Ag-induced reconstructions was investigated. For low Ag coverage, both the 3×1 and the 3×3 structures form at the upper step edge while for increasing coverage, the 3×3 areas grow at the expense of the 3×1 and 7×7 regions. This growth critically depends on the height of the adjacent step. For a monoatomic step, the 3×3 patch grows uniformly (at the same level) over the upper and lower terrace resulting in a wandering of the step, while higher steps are splitted into two levels due to the formation of high and low Ag-covered areas. Furthermore, a quantitative description of the growth of the 3×3 patches is given, based on the shape evolution of the 3×3 regions and from the analysis of antiphase boundaries.