New Phonon Dynamics in a Diamond Crystal during Post-Annealing

A new mesoscopic response in a crystal during post-annealing (PA) after ion implantation has been discovered. PA is an indispensable procedure for activating the implanted dopant center by retrieving the lost crystallinity of the host crystal, which usually elapses over periods longer than a few milliseconds. By using an empirical molecular-dynamics simulation, we examined what actually happened during extended PA for up to one nanosecond in case of sub-keV N2 beam implantation into bulk diamond at room temperature with annealing at 1000 K. While observing the changes in the long-range-order (LRO) parameter, a notable phase transition from the crystalline to the amorphous state (CA transition) occurred synergistically at approximately a few picoseconds (ps). After the CA transition, PA was initiated. A few tens of ps later, a sequence of restoration spikes was observed in LRO parameter with a regular time interval of 70–80 ps. Each sharp peak showed the spontaneous restoration of “amorphous -> crystal -> amorphous” transitions. This process may trigger a “critical state” that occurs during the slow and steady recovery of the lost crystallinity in the early stages of PA. It was found that annealing not only enhances the vibration amplitude of the individual atoms in the bulk but also causes phonon-assisted restoration in a mesoscopic space.