Oxygen incorporation in hexagonal and amorphous nickel films

Abstract

A prominent increase of the integrated intensity of the O ls energy level after exposure of fcc nickel films — but not hcp and amorphous nickel films — to oxygen is clearly observed by means of ESCA. It seems that in hcp and amorphous films, a considerable amount of oxygen is incorporated from the beginning, causing these films to be nearly saturated with oxygen. When hcp and amorphous films are annealed in a 10 −3 to 10 −4 Pa vacuum at above 400°C, structural transformation from hcp or amorphous into fcc is found to occur. This would seem to indicate that excess oxygen works as a stabilizing agent of these metastable structures, and that under vacuum annealing condition this stabilizing oxygen escapes from these films. Moreover, the stoichiometric NiO component in O ls spectra shows little change after exposure on all of these films. Thus, incorporated oxygen of hcp and amorphous structures seems to be located at positions different from the octahedral coordination in NiO. The ESCA spectra of valence regions of hcp and amorphous films exhibit features similar to those of pure bulk nickel, which are different from the case of NiO films or single NiO crystals. This means that metastable hcp and amorphous nickel retain their metallic nature, even with the oxygen incorporated.