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

The direct evolution of submonolayer two-dimensional Au phases on the Si(111)-(7 x 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 An layer and the formation of laterally extended Si(111)-(5 x 1)-Au and Si(111)-(root 3 x root 3)R30 degrees-Au surface reconstructions. At a temperature around 970 K, the boundary of the gold-covered region propagates on the dean Si(111)-(7 x 7) and exhibits a nonlinear dependence on time. The ordered Si(111)-(5 x 1)-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 x 7 = 5.2 x 10(-8) cm(2) s(-1) as the average diffusion coefficient for Au on a clean Si(111)-(7 x 7) surface in the concentration range from 0.4 ML up to 0.66 ML and (2) D-5 x 1 = 1.2 x 10(-7) cm(2) s(-1) as the lower limit for the diffusion of single Au atoms on the Si(111)-(5 x 1)-Au ordered phase.