A POSSIBLE SOLUTION TO THE PROBLEM OF COMPOSITIONAL CHANGE WITH ION-BOMBARDED OXIDES

We have explored a new type of experiment in which the beam-induced surface changes of MgO, SiO2, Al2O3, TiO2, V2O5 and MgAl2O4 are determined by XPS after ion bombardment at 2.5 keV with Ar+ or N2+. The initial bombardment, typically with Ar+, led to a combined surface cleaning and loss of 0. Although the relative extents of these were difficult to establish, the result was in every case to define the composition in an N-free situation. After a subsequent bombardment with N2+ the 0 content was distinctly lower than that after Ar+ impact, with the extra 0 deficiency in each case made up almost exactly by the N addition. For example, Al2O3 evolved from a nominal Al2O2.74 to a nominal Al2O2.20N0.48. The chemical state of this N was predominantly nitride-type, indicative of M - N bonds (M, metal). The observed replacement of M - 0 bonds with M - N bonds is disfavoured by thermodynamics and this, combined with the already mentioned tendency for the extra 0 loss to be closely matched by the N addition, together constitute a strong indication that the disturbed lattice is to some extent relaxing randomly. Interestingly, however, the extent by which N replaces 0 increases as the energy change calculated for this replacement decreases. This confirms, in agreement with a large body of earlier work, that the changes in the oxides are to some extent also chemically guided. The similarity to ion beam mixing and to ion-beam-induced grain growth is worth noting, where again one finds a mixture of ballistic (i.e. random) and chemically guided behaviour.