A possible solution to the problem of compositional change with ion‐bombarded oxides

Abstract

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 O. 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 O content was distinctly lower than that after Ar+ impact, with the extra O 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O bonds with MN bonds is disfavoured by thermodynamics and this, combined with the already mentioned tendency for the extra O 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 O 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. Copyright © 1992 John Wiley & Sons Ltd.