A research project undertaken by Dr. GP Srivastava and Dr. Stephen Jenkins, at Exeter University, funded by the EPSRC.
The snag is that it turns out to be somewhat difficult to grow Ge on Si in a controlled layer-by-layer manner. After a few smooth layers have been grown (epitaxial growth) the growth then proceeds by the formation of clumps, or islands. This is known as the Stranski-Krastanov growth mode. It has been found, however, that deposition of a single monolayer of arsenic (As) or antimony (Sb) onto the silicon surface before growth of Ge begins can overcome the problem. The Ge then grows epitaxially, while the As or Sb atoms apparently rise up through the growing Ge layers and "float" on the top (i.e. segregation occurs). This process, by which the Ge is encouraged to spread out across the Si surface, has been likened to the action of surfactants in causing fluids to wet solid surfaces. In fact, the mechanism is unrelated, but the term "surfactant mediated growth" has stuck. As or Sb atoms are the surfactants in this case, while the growing Ge "wets" the Si surface.
With EKSETER, our ab initio LDA plane-wave pseudopotential code, we have the ability to theoretically determine the structure of the Si(001) surface at an atomic scale. We can then study the modification of the geometry due to the adsorption of a surfactant layer. Finally we can examine precisely how the surfactant (a) causes Ge to wet the surface and (b) rises through the Ge layer, ready to perform the same function for the next layer.Our first publication on this subject will shortly appear in the journal Surface Science. The Web-version of our poster, presented at ECOSS-15 (Lille, September 1995), is now available.
Another process of interest to us is growth on the gallium arsenide (GaAs) (001) surface. Our paper entitled "Theoretical studies of GaAs(001)-Ge(2x1) and (1x2) structures" will shortly appear in the journal Surface Science, and an investigation of the detailed geometry of the clean GaAs(001)-beta2(2x4) surface will appear in Physical Review B on 15th May 1996.