An scanning tunnelling microscopy study of the diffusion of a single or a pair of atomic vacancies

A single or a pair of atomic vacancies have been created artificially with the scanning tunnelling microscope (STM) tip on the Ge(1 1 1) surface and their displacement across the surface studied. There appears to be no interaction between the two vacancies when they are separated by a long distance. However, when two vacancies met such that they occupied neighbouring sites on the Ge(1 1 1) surface, they were found to interact with each other. A time analysis of the diffusion showed that the pairing of vacancies gives rise to a stabilisation in energy of 0.03 and 0.02 eV depending on the exact location of the two vacancies on the surface. Thus, the pairing of two neighbour vacancies can be thought of as the formation of a pseudo-molecule. To obtain the necessary site dependent hopping rates of a single vacancy and a pair of vacancies, a new integration formalism was derived which takes into account the fact that the time interval between STM measurements is of the same order as the characteristic time of the dynamics under study.