W dimer diffusion on W(110) and (211) surfaces

Tungsten (W) dimer self-diffusion on the W(110) and (211) surfaces is investigated using a fourth moment approximation to tight-binding theory. For the (110) surface, we find the optimal diffusion mechanism is a concerted jump, i.e., adatom pairs hopping simultaneously on the surface. The linear displacement of a W dimer in the 〈111〉 direction is energetically favored over orientation changes. On the (211) surface, there are two different diffusion mechanisms. For a dimer in one channel along the 〈111〉 direction, the mechanism is a concerted jump with a high activation energy. For adatoms located in adjacent channels, dimers diffuse mainly via individual adatom jumps, and such dimer diffusion is highly correlated. For the present calculations, the activation energies for W dimer migration are similar to those for monomer diffusion, in good agreement with field ion microscopy (FIM) observations. The diffusion mechanism and diffusion anisotropy can be understood with a simple bond coordination model.