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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.

We have studied the coalescence mechanism of impurity atoms implanted into a crystalline target at low temperatures, paying attention to the dimer formation mechanism. A classical molecular-dynamics (MD) calculations was used in the case of 1 keV B ion implantation at 100 K into a crystalline silicon (c-Si) target that was supersaturated with pre-embedded B atoms at a concentration of 3 at. %. The initial phase of dimer formation was investigated in terms of the temperature distribution and the degradation of the long-range order (LRO) parameters defined in the pixel mapping (PM) method. One result was the locally high temperature and big temperature gradient that revealed the acceleration of collisional diffusion of light impurity atoms in the host medium. The other finding was the decrease of the LRO parameters. This fact associated with the number of self-interstitial atoms (SIAs) implies a deformed potential-field in a crystal under ion irradiation. These results indicate that dynamically enhanced diffusion or collisional-diffusion is the origin of dimer formation.