Vacancy enhanced aggregation of nitrogen in diamond

Abstract

The aggregation of nitrogen in type Ib diamond, at 1500 degrees C in vacuo, has been investigated for natural and synthetic samples containing around 50 ppm and 500 ppm isolated substitutional nitrogen respectively. The rate constant for the aggregation process in untreated diamonds is very much less than that reported previously for samples subjected to a hydrostatic pressure of approximately 6 GPa, but may be increased by a factor of at least 50 if the diamonds are irradiated with approximately 10²² 2 MeV electrons/m². The vacancy concentration produced by such an irradiation is only approximately 5 ppm and is proposed that at high nitrogen concentrations the aggregation process involves the multiple release and retrapping of vacancies. Analysis of infrared absorption spectra before and after aggregation of the nitrogen suggests that the conversion factor normally used to calculate substitutional nitrogen concentrations from the strength of the 1130 cm^-1 absorption band is too low by a factor of almost two.