Surface diffusion experiments with STM: equilibrium correlations and non-equilibrium lowtemperature growth

Abstract

Measurements of surface diffusion depend on the state of the system whetherthe state is equilibrium versus non-equilibrium. Equilibrium experimentscarried out in 2-d overlayers measure the collective diffusion coefficientDc and can test theoretical predictions in two-dimensional statistical mechanics. Growthexperiments typically carried out at low temperatures and/or high flux rates probesystems under non-equilibrium conditions where novel diffusion mechanisms canpotentially exist. The use of STM to study both types of measurements is discussed.Dc can be measured from the autocorrelation of time-dependent tunneling current fluctuationsgenerated by atom motion in and out of the tunneling area. Controlled experiments asfunction of temperature, coverage and tip–surface separation confirm that the signal isdiffusive. For growth experiments the unusually uniform height island (for Pb/Si(111)In/Si(111)) has revealed a novel and intriguing type of diffusion at low temperaturesthat accounts for the high degree of the self organization. By monitoring theevolution of the stable islands out of a mixture of stable and unstable islands theunusual role of the wetting layer surrounding the growing islands is revealed.