Nanoscale Phase Transitions in Amorphous Materials Under Swift Heavy Ion Irradiation

Analyzing the behaviour of amorphous materials under swift heavy ion irradiation by means of molecular dynamics simulations, we have discovered unexpectedly fast phase transition processes that take place on a nanoscale in the regions affected by the passing ion. The behavior is characteristic to a specific combination of pressure and temperature that may appear in the wake of a swift heavy ion passing through the material. We relate this behavior to the strong tetrahedral network in Si-based materials that also explains rich polyamorphism in these materials. We determine that the fine structure of the track is composed of two distinct amorphous phases that result from the different pressure levels in the core and in the shell during solidifiation. The mechanism involving ultrafast nanoscale phase transitions sheds the light on the origin of the core-shell fine structure observed in a-SiO2 and a-Si3N4, as well as the dense core in a-Si.<br/>On the other hand, in amorphous Ge fast phase transitions under swift heavy ion irradiation lead to nanoporosity that displays a peculiar self-organisation process. Low-density solid to high-density liquid transtions lead to formation of initially almost randomly distributed pores which grow with ion fluence and eventually self-organize in well-separated layers parallel to the sample surface. This self-organisation mechanism depends on the ion energy, thickness of the amorphous Ge layer and the angle of ion incidence and is potentially caused by the fragmentation of ion tracks into segments of characteristic length. Using molecular dynamics simulations we investigate the process of pore formation and the mechanisms leading to self-organization of the pores in a layered structure.