Influence of metal trapping on the shape of cavities induced by high energy He+ implantation

In He implantation induced cavities highly contaminated with metals (Au, Ni, Pt) we found that, when no three-dimensional structure is observed, the shape of the cavities can be strongly modified depending on the nature of the metal and on its trapped quantity. The equilibrium shape of cavities is the Wulff shape associated with the minimum surface energy which can be determined using the code WULFFMAN. On the basis of these computations the effect of a metal chemisorption may be accounted for. At very low coverage (far below 1%) there is no effect to be expected. At coverages between 1% and 10%, independent of the nature of the metal, a reduction of the specific surface energy of the vicinal surfaces may produce spherical cavities. Eventually for coverages close to one monolayer, the specific surface energy of the concerned metal will drive the cavities toward spherical or highly facetted shapes depending on whether the specific energy of the metal is smaller or higher than the vicinal one of silicon.