The first involves cooperative dimer diffusion where one of the O atoms drags the other into a Y-lid configuration, i.e. both atoms are in a y-lid configuration at the same time (a Si-O-Si-O square is formed). So far, we have failed to find a low energy route by this mechanism. If the constraints are chosen so that each O atom lies in a Y-lid configuration, this means that the O atoms bond to the Si atoms that are second neighbours to each other, and these two Si-O bonds have equal length. The energy of this configuration is 2.4 eV above that of the stable dimer. One reason for the high energy is that there is a large tensile stress acting on the dimer although this is offset by the absence of any broken Si bonds. It may be possible that there are low energy migration paths by this mechanism but they must involve other configurations.
An alternative diffusive mechanism occurs when the dimer partially dissociates in the (011) plane. This involves a diffusive jump where one O atom of the dimer moves to its Y-lid configuration and the other O atom is free to remain at its BC site. This configuration was found by imposing a pair of constraints so that the two arms of the Y-lid are equal and the Si atom at the base of the Y-lid is equidistant from the arms (see Figure 6.9). Remarkably, the energy of this structure is only 1.1-1.5 eV above that of the symmetric dimer, i.e. 1.36-1.76 eV above the stable puckered dimer. Again the uncertainty in energy is due to the different numbers of bond centres employed. This is in good agreement with the experimental formation energy for thermal donors of 1.7 eV, believed to be the dimer migration barrier. The crucial result is that the dimer energy is now lowered by the presence of the other O atom.