SURFACE-DIFFUSION MEASUREMENTS FROM STM TUNNELING CURRENT FLUCTUATIONS

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