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 t emperatures. 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 subsequ ent isochronal annealing steps. As diamond is a metastable form of car bon, it is possible to create enough lattice damage by ion implantatio n to initiate a thermally stable phase change. The structure of the la yer generated in this way has been termed ''amorphous'' in the literat ure, and the object of this study was to illucidate the mechanisms inv olved for its formation. Accordingly the implantation doses were chose n 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 th is value, the phase change was found to be ''triggered'' at a suitable annealing temperature: the higher the ion dose, the lower the anneali ng temperature to effect the structural change. Evidence has been foun d to indicate that this ''amorphisation'' process is driven, at least partly, by the strain in the damaged lattice.