A DETERMINATION OF THE CRITICAL DAMAGE DENSITY REQUIRED FOR AMORPHIZATION OF ION-IMPLANTED DIAMOND

A series of damaged layers were generated in natural type IIa diamond by implanting iso-chemical carbon ions over a range of doses (0.5 to 7 x 10(15) ions/cm2) while maintaining the diamond at liquid nitrogen temperatures. RBS-based ion beam channelling using 1.2 MeV He+ ions was used to monitor the buildup of damage in the three major channelling directions ([110], [111] and [100]), as a function of dose and subsequent isochronal annealing steps. As diamond is a metastable form of carbon, it is possible to create enough lattice damage by ion implantation to initiate a thermally stable phase change. The structure of the layer generated in this way has been termed ''amorphous'' in the literature, and the object of this study was to illucidate the mechanisms involved for its formation. Accordingly the implantation doses were chosen so as to bracket the critical ion dose (almost-equal-to 5.2 x 10(15) cm-2) needed for this transformation to occur. For ion doses above this value, the phase change was found to be ''triggered'' at a suitable annealing temperature: the higher the ion dose, the lower the annealing temperature to effect the structural change. Evidence has been found to indicate that this ''amorphisation'' process is driven, at least partly, by the strain in the damaged lattice.