Surface diffusion of Au on Si(111): A microscopic study

Abstract

The direct evolution of submonolayer two-dimensional Au phases on the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ surface was studied in real time using the spectroscopic photoemission and low energy electron microscope located at the synchrotron radiation source ELETTRA. A finite area covered by 1 monolayer (ML) of gold with a steplike transition zone was prepared by evaporation in situ. Subsequent annealing resulted in the spread of the Au layer and the formation of laterally extended $\mathrm{Si}(111)\ensuremath{-}(5\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{Au}$ and $\mathrm{Si}(111)\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\mathrm{Au}$ surface reconstructions. At a temperature around 970 K, the boundary of the gold-covered region propagates on the clean $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ and exhibits a nonlinear dependence on time. The ordered $\mathrm{Si}(111)\ensuremath{-}(5\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{Au}$ plateau develops a separated front moving with constant velocity. Two values of the Au diffusion coefficients were estimated at a temperature of about 985 K: (1) ${D}_{7\ifmmode\times\else\texttimes\fi{}7}=5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{cm}}^{2}{\mathrm{s}}^{\ensuremath{-}1}$ as the average diffusion coefficient for Au on a clean $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ surface in the concentration range from 0.4 ML up to 0.66 ML and (2) ${D}_{5\ifmmode\times\else\texttimes\fi{}1}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}{\mathrm{cm}}^{2}{\mathrm{s}}^{\ensuremath{-}1}$ as the lower limit for the diffusion of single Au atoms on the $\mathrm{Si}(111)\ensuremath{-}(5\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{Au}$ ordered phase.