Brownian motion of steps on Si(111)

Step motion on surfaces can now be measured quantitatively. We present a formalism for analyzing equilibrium step fluctuations and apply it to real-time reflection electron microscope observations of step motion on Si(111). The time correlation functions of the step positions and of their Fourier components are compared with predictions from Langevin equations for two extreme mechanisms for step motion: edge diffusion and terrace exchange. At 900 °C, the dominant mechanism is terrace exchange with a time constant τ_a of ∼1 μs. The significance of τ_a for atomic mechanisms of surface mass transport is discussed.