PHASE-FORMATION AND AMORPHIZATION PROCESSES UNDER HIGH-ENERGY ION-BOMBARDMENT

To describe and moreover to predict the formation of crystalline, poss ibly metastable or amorphous, phases in the near-surface region of ion bombarded systems is a very complicated problem. In this work we firs t summarise the atomistic segregation charge transfer (SCT) model to i nterpret phase nucleation in binary alloy films bombarded under condit ions favouring the formation of dense collision cascades. A non-equili brium compositional profile develops at the cascade-matrix interface a s a consequence of preferential migration to the interface of one of t he film components. Such a local stoichiometry alteration is associate d with the development of an electronic density profile, which can be a non-equilibrium one over the typical cascade quenching timescale. Re laxation to (meta)stable equilibrium involves charge transfer reaction s that give rise to the formation of dimers of an effective compound. The energy cost of introducing into the matrix an effective alloy dime r is calculated; comparing surface and thermochemical properties of st arting 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 fiftee n alloys, out of which eight were amorphised by ion beams. The validit y of the correlation between the parameters adopted in the SCT model t o describe global and surface electronic energy changes in alloys is i llustrated 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.