We have analyzed low-energy electron microscopy observations of the equilibrium fluctuations of steps on Si(001) in the temperature range 640–1170 °C. By examining the wavelength dependence of the time constants of the fluctuations, we find that the step motion is limited by the rate of random attachment and detachment of adatoms at the step edges. From the values of the time constants, we determine the step mobility which in principle governs how fast a step responds to being out of equilibrium. This mobility is the same, within experimental uncertainty, for ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ steps. By studying the decay of nonequilibrium rough step profiles, we explicitly show that the step motion is curvature driven, and that the mobility deduced from the thermal fluctuations quantitatively accounts for step smoothing rates. From the amplitude of the equilibrium fluctuations, we determine the stiffnesses of the ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ steps as a function of temperature. © 1996 The American Physical Society.

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