Crystallographic analysis of amorphization caused by ion irradiation

Ion irradiation often causes amorphization in a crystal. We have presented a new crystallographic analysis that defines a new type of order parameter, which we call pixel mapping (PM). PM can describe algebraically to what extent and how the crystallinity has changed under ion bombardment. In other words, PM describes the long-range-order (LRO) interactions, based on the crystallography. PM can be effectively used, when it is incorporated in a classical molecular dynamics (MD) calculation. In the case of B ions implanted into a Si crystal, we observed crystal to amorphous (CA) transitions under energetic ion bombardment at low temperature. The PM profiling was more effective to reveal the CA transition than other atomistic methods of analyses as radial distribution function g(r) or vacancy mapping Nv. PM could distinguish between perfect crystalline states, transition states, and random states. Moreover, PM revealed that the lattice reaction was cooperative even in a mesoscopic volume, e.g. in a cube of 60 A × 60 A × 60 A, indicating the importance of LRO interactions.