Studies of the tungsten-oxygen surface reaction by means of reflexion high energy electron diffraction
The reactions between both (100) and (110) surfaces of tungsten and oxygen have been studied in an ultra-high vacuum environment by means of reflexion mode high energy electron diffraction. Particular attention has been paid to changes in interfacial geometry owing to faceting which occurs on the (100) surface, and oxide nucleation which occurs on both faces. The faceting of the (100) face is shown to be more complicated than had previously been supposed, the faceted surface being composed of {211} planes which are themselves faceted into {110} planes. The activation energy for the degradation of facets in a vacuum has been measured as 6.5 ± 1.5 eV, greatly in excess of values reported for the formation of facets. It is suggested that faceting arises from the evaporation of oxide molecules. At temperatures below 1025 K tungsten trioxide nuclei form on both the (100) and (110) surfaces when exposed to oxygen. The exposures needed to form nuclei are much greater for the (110) surfaces than for the (100). Nuclei also form on (100) surfaces which have been previously faceted by heating in oxygen at temperatures above 1025 K. In this case the exposures needed to produce nuclei are characteristic of the (110) surface. The epitaxial relationships between the oxide and the metal have been determined. A simple relation has been found to hold. It is postulated that the metal plane at the oxide interface is not necessarily that which was originally exposed to the gas phase.