Microscopic modeling of irradiation-induced metastability in ceramic thin films

To understand, and moreover to predict, why the near surface region of an ion bombarded crystalline film is structurally stable, or undergoes radiation-induced amorphisation, is a formidable basic problem, with important technological implications in the choice of materials for applications under severe radiation conditions. The atomistic segregation-charge transfer model is applied to binary borides, carbides and nitrides, which are reported in the literature to withstand irradiation, or respectively to turn amorphous under ion bombardment. The stopping of a highly energetic, massive projectile results in the formation of dense collision cascades; the time evolution of such exiguous volumes of excited matter involves both non-equilibrium and thermodynamic processes, depending on the relevant timescale. Towards the end of collision cascade life, short-range atomic migration of one of compound constituents occurs at the matrix-cascade interface. Consequently a localised non-equilibrium profile, concerning both composition and electronic density results. Relaxation to (meta)stable equilibrium is simulated via elementary charge transfer reactions (CTR), each involving a pair of dissimilar atoms. In each CTR, an effective compound dimer is formed. This is analysed and compared to the elementary structural unit of the starting compound. We consider three structure stability parameters, namely the electronic energy cost associated with a CTR, the enthalpy difference between the starting and the effective compound and the local strain induced in the starting compound lattice when ions are formed by a CTR. Threshold parameter values with a clear physical interpretation are found that allow for separating the compounds vitrified from those crystalline upon ion bombardment