Ge(001) before and after gold evaporation and annealing
Kernel of the 4-probe microscope (left), crushed tip on Si surface (middle), two STM tips over gold structures on Ge(001) surface

Investigation of growth of ultra-thin films and nanostructures by MBE.

Alkali halides are often considered as model ionic insulators and they have been studied extensively in the past both because of their interesting physical properties, and because of importance of insulators and insulator/semiconductor interfaces in all technologies for electronics. Alkali halide films have been grown on semiconductors, metals, and insulators. Recently, advances in non-contact atomic-force microscopy (nc-AFM) made it possible to study true dielectric surfaces with high (comparable with STM) resolution. Such studies deserve special attention now, since the science of epitaxial growth is considered one of the foundations of future nanotechnology.
It has been established that alkali halide films can be grown epitaxially on AIIIBV compound semiconductors. This is due to strong chemical bond between halogen ion and AIII atom on the substrate surface, and due to possibility of choosing AIIIBV compound with lattice (zinc-blende type) matched closely to the lattice of given alkali halide (rock-salt type). An example of closely matched alkali-halide/ AIIIBV adsorption system is KBr/InSb: InSb has the lattice constant 6.479 A and KBr has the lattice constant 6.586 A.
The group use scanning tunneling microscopy and non-contact atomic-force microscopy in ultra-high vacuum to study the growth of KBr films on indium rich, c(8x2) reconstructed InSb surface. The growth of insulating film is studied from very low (0.3 ML) to high (120 ML) coverages. Both initial stages of epitaxy and multilayer film surfaces are imaged with high resolution. Based on the images the interface structure and processes of epitaxial growth are analyzed.
    The most important results of these experiments are:
  • initially islands of monatomic thickness are formed. These islands are often cut along (100) crystallographic direction and the distribution of these islands on the substrate surface is anisotropic which reflects the anisotropic diffusion of KBr molecules during growth.

  • At 1-1.5 ML coverage a wetting single-atomic KBr film is formed and the deposited material in excess of 1 ML forms rectangular islands with edges oriented along (100) and (010) crystallographic directions.

  • For high (multilayer) KBr coverages the growth is basically a layer-by-layer type but, due to slow diffusion of KBr molecules down across steps, the ( )th layer starts to grow before the completion of nth layer. In result, pyramidal structures of rectangular bases are formed on the surface. These rough films can be, with thermal annealing, converted to flat films exposing large (0 0 1) terraces.

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