Extra-relaxation effects in ultra-thin Al ₂O ₃ films on metal substrates

The oxidation of NiAl(110) was investigated in the temperature range between 300 K and 1300 K by using Auger electron spectroscopy, X-ray photoelectron spectroscopy, and low energy electron diffraction. The adsorption of N ₂O on clean NiAl(110) at 300 K leads only to selective oxidation of the surface aluminium without any apparent surface reconstructions. Stepwise annealing of the heavily oxygen-doped samples from 600 K to 1300 K in ultra-high vacuum results firstly in the onset of randomly oriented oxide surface then finally fairly well-ordered Al ₂O ₃ films. The much wider energy separation between the core (or Auger transition) levels of metallic Al ⁰ and oxidic Al ³⁺ states for thicker oxide films is believed to be due to a lower degree of extra-relaxation by metallic electrons and, therefore, would imply better quality for oxide films with similar thicknesses.