SURFACE-DIFFUSION MEASUREMENTS FROM STM TUNNELING CURRENT FLUCTUATIONS

We have used the STM to study surface diffusion from the time dependence of the tunneling current. Results were obtained for the power spectrum, W(f), of oxygen adsorbed on stepped Si(111). The measured W(f) for clean Si(111) shows no temperature dependence and it is smaller by at least two orders of magnitude than the spectrum for oxygen-covered Si(111) which broadens with temperature. W(f) can be fitted to the expected theoretical form for a single-diffusion process and thus rules out simultaneous diffusion on the surface and on the tip. We have extracted an activation energy of E(a) = (0.92 +/- 0.15) eV which is lower than expected for such strongly bound system, possibly because of inhomogeneity in the binding sites.