Direct determination of surface diffusion by displacement distribution measurement with scanning tunneling microscopy

A scanning tunneling microscopy (STM) method for studying surface diffusion is developed based on measurements of the displacement distribution of adsorbates by ``image-anneal-image'' cycles which allow direct observation of the diffusion process while avoiding potential STM tip effects. The method is used to study the anisotropic diffusion of Sb dimers on Si(001). The energy barrier and the prefactor for the faster diffusion across the substrate dimer rows are measured. On the other hand, the diffusion observed by the ``image-while-hot'' method appears nearly isotropic and also much faster. It is shown that this discrepancy is due to the STM tip influence present in the latter method.