Conduction in ion-implanted single-crystal diamond

We have implanted sodium, phosphorus and arsenic into single-crystal, type IIa diamond as possible n-type dopants. Particular emphasis was placed on the implantation of sodium at different temperatures and doses; combined implantation energies of 55, 80 and 120 keV were used to provide a uniformly doped layer over a depth of approximately 100 nm. The implanted layers exhibited a semiconducting behavior with a single exponential activation energy between 0.40 and 0.48 eV, as determined by temperature-dependent resistance measurements. A sample implanted to a concentration of 5 × 10¹⁹ Na⁺ cm^-3 at 550°C exhibited a single activation energy of 0.415 eV over a temperature range from 25 to 500°C. Thermal annealing above 900°C was found to remove implantation damage, as measured by optical absorption and Rutherford backscattering-channeling. However, concomitant increases in the resistance and the activation energy were observed. Implantation of ²²Ne was used to introduce a damage density equivalent to the ²³Na implant, while not introducing an electrically active species. The activation energy and the electrical resistance were similar but higher than those produced by implantation with sodium. We conclude that the electrical properties of the sodium-implanted samples were at least partly due to electrically active sodium, but that residual implantation damage was still important.