Why hydrogen on silicon?

Hydrogen on silicon is in many ways the prototypical system for studies of gases on semiconductor surfaces. Silicon is the most well-studied surface, while hydrogen is the simplest of gases. However, even such a simple system has thrown up many interesting phenomena. Two different reconstructions are seen, (2x1) and (3x1) and the adsorption/ desorption behaviour of molecular hydrogen seem to follow different pathways.
I was looking at two aspects of hydrogen: one was its adsorption on different parts of the surface - the dangling bonds, defects, step edges etc.; and the second was the dynamic features such as diffusion of the hydrogen atoms across the surface at around 600 K. This was both for its own value, and also to provide crucial information about the gas-source growth of silicon using disilane ( which is a compound of silicon and hydrogen).
I have produced several papers on this subject, in collaboration with two modelling colleagues (Dave Bowler and Chris Goringe) which may be found on my publications page. The first was a study of the diffusion of isolated H atoms, the second the diffusion of pairs of H atoms, and the last brings all my work together, and much modelling, and ties in with my silicon work, where we have found an interesting interaction between H atoms and step edges.

What does it look like?

The Si(001) surface is comprised of long rows of silicon dimers, each with a dangling bond sticking up into vacuum. Hydrogen atoms can attach to these dangling bonds, and make very strong bonds to the surface. This makes it difficult to attach other atoms such as silicon, so that one of the problems of gas-source growth is getting rid of the surface hydrogen. Images of the Si(001) surface with adsorbed hydrogen are shown below.

At low coverages, hydrogen adsorbs at one end of a dimer, which becomes dark. The energy of the silicon orbital at the clean end of the dimer shifts upward, and so it appears bright. One of these bright dots is indicated with a circle. At high coverage, hydrogen adsorbs onto both ends of the dimer to form a monohydride dimer. These can be seen as dark rows of dots in the background. A clean dimer, which may be thought of as a vacancy in the hydrogen layer, appears bright relative to the background (circled). You can see more about this on a separate page.