THE MULTI-ASPECTS OF ION-BEAM MODIFICATION OF INSULATORS

The complexity of the phenomena observed when insulators are bombarded with energetic ions is due to many different aspects. On the one hand ''insulators'' are generally polyatomic materials having a wide range of structure and bonding and thus different behaviors under electronic excitations or elastic atomic collisions. On the other hand, the strong localization of the defect creation by electronic or nuclear processes surrounding the particle trajectories leads to specific aspects, as the yield for defect creation can be very different in the anion or cation sublattices. In addition, the doping process by ion implantation occurs out of thermodynamic equilibrium, and then chemical bonds or charge states of the implanted species depend on their site location, local concentration, temperature and on the defects created in their vicinity. Current examples of these multi-aspects will be discussed with particular emphasis on the effects of electronic excitations at high density, leading to: (a) defect creation and defect diffusion at low temperature in refractory oxides well known as insensible to radiolysis; (b) abrupt changes of polymer properties illustrated by the sol-gel transition in polystyrene or the insulator-metal transition in polyimide, which can be interpreted by a simple model of percolation.