Atomic-scale observation of dynamic processes

The STM allows us to study growth processes in UHV with atomic resolution. This has previously been done by exposing the surface to room temperature, annealing to a certain temperature, and then observing the result after the surface has cooled to room temperature. In our experiments, we have exposed the surface to disilane and imaged at elevated temperatures, which has allowed us to make new observations. In addition, the STM data has been complemented by atomistic modelling, to help identify some of the structures that we have observed, and to study the details of reactions which happen too fast for the STM. We have been able to image the rotation of an ad-dimer at 400 K. At this temperature it rotates about once every 18 seconds.

The Si square

A novel structure, which has not been seen before, is the square feature shown below. This structure has not been seen by the MBE groups. A similar structure has been seen after adsorption of silane and annealing. The square sits between two dimer rows, and between two dimers. In the empty states picture (left), the square looks similar to a pair of epitaxial dimers, but the distinctive nodes at 45 degrees to the dimer rows in the filled states image (right) make it clear that this is a different structure. A modelled structure, relaxed in DFT, is shown below. This structure is more stable than two separate ad-dimers, and can easily be formed from two ad-dimers. The rhomboidal shape can flip easily, at a similar rate to substrate dimers, which would give the square shape seen in STM. Modelling details about these features may be found at Dave Bowler's page.