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p-type diamond

Boron exists in natural type IIb diamond as an acceptor on a substitutional site. The activation energy of the boron acceptor is 0.37 eV above the valence band. Boron has been therefore widely studied as p-type dopant in diamond, and the results obtained by ion implantation are good enough, so that no further investigation on other group III potential dopants has been undertaken till now. It has to be mentioned that very old results were also published on aluminium implantation in diamond, but these were unreliable and the research was not pursued.

Boron can be introduced into diamond during CVD growth, and a p-type conductivity is obtained with the same activation energy as that found in natural diamond. However, hydrogen present in the ambient plasma was found to prevent the doping control of diamond [58].

Boron doped p-type diamond was also obtained by ion implantation. In a early experiment carried out by Vavilov et. al. [59], the depth profile of boron in diamond, after annealing at 1720 K, was found to extend into the crystal to a depth of $\sim$1.5 $\mu$m. The activation energy measured was 0.3 eV, but the mobility was relatively low ($\sim$ 400 cm2 V-1 s-1, compared to 1600 cm2 V-1 s-1), due to residual defects not completely annealed.

To prevent the formation of extended defects, Prins [32] proposed to implant the dopant in a cold-implantation-rapid-annealing process (CIRA), widely used now in doping experiment by ion implantation. The last stage in the process consits in annealing at high temperature ($\sim$ 1700 K) to reduce the damage and increase the electrical activation of the dopants. Although the efficiency of the intermediate rapid heating to increase the mobility of the carriers is controversial, the need of a final high temperature annealing is now well established, to increase the carrier concentration and their mobility, and to reduce the compensation ratio. Fontaine et. al. [60] applied the CIRA technique with a final high temperature annealing of 1450 C. A mobility of 400 cm2 V-1, a hole concentration of 6 $\times 10^{13}$ cm -3 and a donor-acceptor ratio of 0.005 were obtained in this experience.

A new method proposed by Prawer et. al. [34] improves the characteristics of the boron doped diamond. In this technique, the implantation is carried out at very high ionic kinetic energy, of the order of MeV, which creates deeply buried boron layers. Because of the very high pressure exerted by the diamond matrix, less defects are formed during the implantation itself, and the defect annealing is more efficient. In the samples obtained, a high mobility of 600 cm2V-1 s-1 and a compensation ratio of 5 % were measured [31].


next up previous contents
Next: n-type diamond Up: Impurities and defects in Previous: Impurities and defects in
David Saada
2000-06-22