Oxygen adsorption on Ni₃Al(001) by low-energy ion beams

The interaction of O with the Ni₃Al(001) surface has been studied using low-energy He⁺ scattering. Oxygen adsorption at an exposure of 10 L and a temperature of 700°C results in the total disappearance of the Ni ISS signal, indicating the formation of a monolayer AlO_x overlayer. This oxide AlO_x overlayer has been examined by measuring the azimuthal angle dependence of the scattered ion yield at fixed scattering angle under different coverages of oxygen. The aximuthal variations of the Ni and Al ISS signals from the O-covered surface are compared with those of Ni and Al from the clean surface, and the results used to derive a value for the fractional coverage of oxygen on the surface. The analysis supports the model that the oxide initially grows in the form of islands, with no influence on remaining areas of the crystal. No evidence of the ordering of the oxide islands was observed. Further evidence of the growth of the AlO_x islands has been obtained by comparing the incident angle ISS scan from the clean surface to that from the partially covered AlO_x surface. The results indicate that the remaining (uncovered) Ni₃Al surface retains the crystal structure of a clean surface. The rate of growth of the AlO_x islands on the Ni₃Al(001) surface was also found to be related to temperature.