The diffusion of oxygen on W(001)

Abstract

The diffusion of oxygen on the W(001) plane has been studied by the field emission fluctuation method as function of coverage, temperature, preanneal and direction of diffusion. Diffusion is complex, showing a number of maxima and minima at a given coverage, and depends on preanneal. It is reversible without anneal only to 600 K. These findings suggest oxygen-induced reconstruction, in agreement with other work. Diffusion is also anisotropic, indicating the existence of ordered reconstructed and unreconstructed domains of preferred orientation. The activation energies E vary from 2 to >8 kcal mol −1 and prefactors D 0 are generally very small, as low as 10 −10 cm 2 s −1. E and D 0 increase with coverage for unannealed layers, suggesting that the low values correspond to substrate assisted hopping, with stiffening of the lattice resulting from increases in O coverage. Such stiffening removes low E, low probability hopping events. Prediffusive “flip-flop” characterized by exponentially decaying correlation functions was also observed, with fast and slow components, down to temperatures as low as 78 K, suggesting a tunneling component.