The rearrangement of Si dangling bonds induced by pulsed laser heating of monohydride-covered Si(001) surfaces was studied by means of scanning tunnelling microscopy. The initial configurations, which were created by laser-induced thermal desorption, consisted of isolated pairs of dangling bonds at 2 adjacent dimers and represented an excited state of the surface. Hydrogen diffusion caused this arrangement to change during only a few ns laser pulses into the equilibrium configuration with most of the dangling bonds being paired up at a single dimer. Scanning tunnelling microscopy images, after various numbers of heating pulses, revealed snapshots of the surface configuration; frozen at various stages of the fast equilibration process. In this way, the H diffusion associated with rates as high as 108/s could be monitored at atomic resolution. Comparison with Monte Carlo simulations of the diffusion processes permitted the precise determination of both the diffusion rates and the pairing energies between dangling bonds at high surface temperatures.

Real-Space Investigation of Fast Diffusion of Hydrogen on Si(001) by a Combination of Nanosecond Laser Heating and STM. C.H.Schwalb, M.Lawrenz, M.Dürr, U.Höfer: Physical Review B, 2007, 75[8], 085439