Formation of Ion Tracks and Q-Silicon by Swift Heavy Ion Irradiation

We report the formation of ion tracks in amorphous and crystalline silicon by swift heavy ions of 100 MeV Au ions. Remarkably, these tracks contained new allotropes of amorphous silicon, namely high-density (HD) Q-silicon and low-density (LD) α-Silicon. This provided a direct evidence for polyamorphism in amorphous silicon. These ion tracks exhibited room-temperature ferromagnetism with tremendous potential for integrating spintronics with microelectronics for novel quantum devices. Using HR STEM-Z (HAADF), we determined that the tracks contained HD Q-Si and LD α-Si in both amorphous and crystalline silicon. The tracks were wider in amorphous silicon compared to crystalline silicon. Thermal properties of silicon host materials played a critical role in the formation and the size of tracks. We discuss a model based upon melting along the tracks and rapid quenching to explain the formation of Q-Si and α-Si based upon undercooling. The Q-Si with nanostructuring as rings and strings showed robust ferromagnetism at room temperature with a Curie temperature over 500K.