Coalescence of B ions during high-fluence implantation into a Si target

We have investigated the question, what is the driving force causing coales cence among implanted ions. A previous explanation was that the criterion o f clustering is an electrostatic interaction. That is if the formation enth alpy is positive, clustering starts. This is the case of B into Si. Moreove r experimentally it was observed, that real clustering started, if the B co ncentration reached 2 at.%. Nevertheless this is too low a concentration to assume an alloy model. because the encountering probability for B ions is unreasonably small. In fact using molecular dynamic (MD) simulations, we fo und that without energetic ion impacts and the consequent mixing, no cluste ring is observed at room temperature (RT), even if we start with a 4 at.% c oncentration of B in Si. This means that the electrostatic explanation seem s to support experiments, however, the encountering probability is elevated by heavy ion bombardment as MD proved. From our calculations we conclude, that ion impact and the consequent mixing processes in the lattice can trig ger B clustering, while the enthalpy criterion of clustering is not always fulfilled. (C) 2001 Elsevier Science B.V. All rights reserved.