The self-diffusion of thermally generated W atoms near to (123) and (257), on the (011)-(112) zone, and near to (023), on the (011)-(001) zone, of a W field emitter was investigated by using the field-emission fluctuation method. A rectangular probe was used to investigate diffusion anisotropy. In agreement with earlier findings, the diffusion of single W atoms along, and across, (011) terraces separated by (011) steps (step edges running along [111]) was essentially isotropic and could be described by:

D (cm2/s) = 0.0001 exp[-16(kcal)/RT]

The crossing of (001) oriented steps was described by:

D (cm2/s) = 0.01 exp[-35(kcal)/RT]

Slow diffusion parallel to steps, which had previously been attributed to kink motion, was also seen along the (011)-(112) zone. Distinct dips in the slow regime diffusion coefficients occurred at 910K. This was somewhat higher than the previously reported dip onset temperature (875K) and was suggested to reflect roughening effects. Slow diffusion perpendicular to steps was also seen in this zone. It was thought to arise from some step components which were always perpendicular to the short slip dimensions, or to correspond to more complicated surface configurations than the step and terrace pattern of an ideal emitter surface.

D.S.Choi, S.K.Kim, R.Gomer: Surface Science, 1990, 234[3], 262-72