Phase formation and amorphisation processes under high-energy ion bombardment

To describe and moreover to predict the formation of crystalline, possibly metastable or amorphous, phases in the near-surface region of ion bombarded systems is a very complicated problem. In this work we first summarise the atomistic segregation charge transfer (SCT) model to interpret phase nucleation in binary alloy films bombarded under conditions favouring the formation of dense collision cascades. A non-equilibrium compositional profile develops at the cascade-matrix interface as a consequence of preferential migration to the interface of one of the film components. Such a local stoichiometry alteration is associated with the development of an electronic density profile, which can be a non-equilibrium one over the typical cascade quenching timescale. Relaxation to (meta)stable equilibrium involves charge transfer reactions that give rise to the formation of dimers of an effective compound. The energy cost of introducing into the matrix an effective alloy dimer is calculated; comparing surface and thermochemical properties of starting and effective alloys, qualitative differences are found between systems undergoing amorphisation or crystalline phase formation under ion bombardment. The model was then applied to the analysis of fifteen alloys, out of which eight were amorphised by ion beams. The validity of the correlation between the parameters adopted in the SCT model to describe global and surface electronic energy changes in alloys is illustrated with reference to a large set of systems, demonstrating the ability of the model both to interpret and to predict phase formation under ion irradiation. (orig.)