Formation of Q-Silicon by Swift Heavy Ion Irradiation: Nanoscale Materials Modifications

Abstract

We reported earlier the formation of a new allotrope of Q-silicon by nanosecond pulsed laser annealing of amorphous silicon, which was produced by ion implantation [1]. Here we show that swift heavy ion (SHI) beams of 100 MeV ions can be used to produce nanoscale Q-silicon tracks in amorphous as well as crystalline silicon. These ion tracks exhibited room-temperature ferromagnetism with Curie temperature of over 500K, which has tremendous potential for integrating spintronics with microelectronics for novel quantum devices. Using HRSTEM-Z (HAADF), we determined that the tracks contained high-density Q-Si and low-density α-Si in both amorphous and crystalline silicon, providing direct evidence for polyamorphism in amorphous silicon. The swift heavy ion tracks were wider in amorphous silicon compared to crystalline silicon. Since melting and quenching are controlled by thermal properties, thermal conductivity of silicon host materials played a critical role in the formation and the size of tracks. We also show similarities and differences between microstructures and phases produced by nanosecond pulsed laser annealing and SHI irradiations, where quenching times are in picoseconds regime. Since the ion-solid interactions along the SHI tracks are confined to lateral nanodimensions, this technique provides controlled materials modifications in nanoscale regime. We discuss a model based upon melting along the tracks and rapid quenching to explain the formation of Q-Si and α-Si determined by the undercooling and the rate of quenching. We also discuss the Q-Si nanostructuring as rings and strings, which are formed through self-organization of silicon tertrahedra.

Keywords: New allotropes of silicon, Q-silicon and α-Silicon, Swift heavy ion tracks, rapidmelting and quenching, ferromagnetism in silicon, nanoscale modifications