W-SINGLE ADATOM DIFFUSION ON W-SURFACES

We have studied the self-diffusion of W single adatoms on the W(110), W(211) and W(321) surfaces, using a modified fourth moment approximati on to tight-binding theory. The binding sites of adatoms on those surf aces are determined and they are in good agreement with experimental o bservations. The activation energy and pre-exponential factor of singl e W adatom diffusion on these surfaces have been calculated, and they are in reasonable agreement with FIM (field ion microscopy) results. T he exchange mechanism is also investigated for all surfaces and it is found to be unfavorable on the (110) and (211) surfaces. For the (321) surface, the exchange mechanism is unfavorable for adatom diffusion f rom a most stable site, but the activation energy is much smaller for an adatom on the meta-stable site replacing a channel atom.