Temperature dependence for the metastable phase formation in Au50Ni50 thin films under ion bombardment

Abstract

The transformation of crystalline (111) oriented Au50Ni50 thin films produced by ion beam mixing of Au/Ni multilayers was studied under krypton ion irradiation in the temperature range from 4 to 100 K. The volume fraction of the amorphous phase was determined as a function of the ion fluence from in situ resistivity measurements, while ex situ x-ray diffraction was used to analyze the residual crystalline fraction. The experimental results indicate that the total Kr+ fluence necessary for complete amorphization of a metastable Au50Ni50 solid solution exponentially increases with increasing bombardment temperatures up to a critical value Tc of approximately 100 K, above which an ion induced amorphization is no longer possible. Furthermore, within the amorphization regime, two different mechanisms can be distinguished: A direct amorphization process via dense cascades at very low temperatures, while close to Tc overlapping of sequentially damaged regions is required to obtain the amorphous phase. The fact that amorphization is never observed under light ion irradiation even at 4 K demonstrates that the idea of a lattice collapse by defect accumulation cannot explain the amorphization behavior in this system. This points to the importance of energetic dense cascades for the amorphization process. Additional experiments on nanocrystalline Au50Ni50 films suggest that the lack of structural relaxation and/or the reduction of crystalline regrowth at amorphous/crystalline interfaces are necessary conditions for the stabilization of the amorphous clusters formed within the cores of dense cascades. Thus, the observed kinetic behavior is explained by considering the balance between dynamic annealing and amorphous phase formation. The importance of interface processes for crystalline growth is corroborated by studying the ion induced crystallization of an amorphous film.