Real-space investigation of fast diffusion of hydrogen on Si(001) by a combination of nanosecond laser heating and STM

Abstract

The rearrangement of silicon dangling bonds induced by pulsed laser heating of monohydride-covered Si(001) surfaces has been studied by means of scanning tunneling microscopy (STM). The initial configurations, which were created by laser-induced thermal desorption, consist of isolated pairs of dangling bonds at two adjacent dimers and represent an excited state of the surface. Hydrogen diffusion causes this arrangement to change during only a few nanosecond laser pulses into the equilibrium configuration with most of the dangling bonds being paired up at a single dimer. STM images taken after different numbers of heating pulses show snapshots of the surface configuration frozen at various stages of the fast equilibration process. In this way, hydrogen diffusion associated with rates as high as ${10}^{8}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ could be monitored with atomic resolution. Comparison with Monte Carlo simulations of the diffusion processes allows for the precise determination of both the diffusion rates and the pairing energies between dangling bonds at high surface temperature.