3 Diffusion and Desorption

As discussed previously, silicon atoms have four possible bond sites. Normally silicon bonds with four other silicon atoms, however in the case of surface atoms it has two traditional bonds, one bond with another surface atom forming a dimer row, and one available bond. Hydrogen has one available bond site and joins with the silicon above it on the surface. This allows both of the atoms to use all of their bond sites. This is a much more stable situation in comparison to one or two dangling bonds.

Like the self-diffusion of silicon, hydrogen diffuses along the silicon surface with increased diffusion rates along the dimer rows while retarded along the surface perpendicular to the rows. This directional dependence of diffusion is another byproduct of the dimer rows. The energy required to diffuse, or jump, within the dimer rows, where the atoms are the closest together, is significantly less than the energy required to jump out of the dimer row and onto the next one. Due to this the diffusion along the dimer rows is increased while the diffusion perpendicular to the dimer rows is decreased.

The hydrogen positions itself above the silicon when they bond. When the hydrogens bond with the silicons in the dimer rows, the positions of the hydrogen mimic those of the silicon dimer rows. So the geometry of the silicon places the hydrogen in pairs. This is significant for the desorption of hydrogen. The desorption of hydrogen happens when two hydrogen atoms form $\mbox{H}_2$and gain enough energy to desorb. The dimer rows place the hydrogens in pairs relieving them of the responsibility of finding a mate with whom to form $\mbox{H}_2$. Eventually the two hydrogens beside each other on the dimer row will have enough energy to leave the surface and desorb. It has been hypothesized by Zhang et al. that the steps are a better desorption site for the hydrogen than the surface [12]. For our project we are going to compare the desorption rates for hydrogen at the surface and on each of the step types.



Subsections
Chris Siefert and Molly Moore 2002