Photoluminescence and positron annihilation measurements of nitrogen doped CVD diamond

A range of techniques have been used, to determine the nitrogen levels in a series of polycrystalline diamonds. The crystals were grown by the chemical vapour deposition (CVD) technique and EPR measurements indicate that they have a single substitutional nitrogen (N-s)concentration between 10 ppb and 50 ppm. Photoluminescence (PL) spectroscopy provides an extremely sensitive way of detecting the nitrogen vacancy complex, in both the neutral (N-V)(0) and negative charge states (N-V)(-), down to the 10 ppb range and below. It has been observed that for diamonds with a single substitutional nitrogen content of approximately (0.5-2.0) ppm, the nitrogen vacancy complex exists in both charge states, with an almost equal abundance. Below this level, the complex exists with greater regularity in the neutral charge state, with the (N-V)(-) centre dominating at (N-s) levels greater than 2 ppm. Electron irradiation of so-called 'high nitrogen' films, with an (N-s) value in excess of 15 ppm, show an incredibly high abundance of the interstitial related 3H centre. Such samples show little sign of the neutral vacancy, GR1, although some absorption is evident in the tail of the (N-V)(-) centre. No evidence of the negative vacancy, the ND1 centre, was observed in PL spectra. For equivalent electron doses on 'low N' samples, typically with an (N-s) level of below 1 ppm, the GR1 centre completely dominates the PL spectra. For all diamonds the intensity of the nitrogen vacancy complexes are seen to reduce within the irradiated region. Annealing studies of high N samples reveal that by 900 degreesC all the vacancies are annealed with many being trapped at the nitrogen to form a large concentration of the (N-V)(0) centre. Positron annihilation measurements have been performed on the samples. As positron annihilation is sensitive to vacancy concentrations, it is expected to yield lifetime measurements that are strongly dependent on the bulk (N-V) concentration. Doppler broadening spectra have been recorded as the samples were illuminated with light varying from 325 to 785 nm. The shape 'S' parameter was found to increase upon illumination and relax back to its ground state, post-illumination. The relaxation time was found to vary between 30 and 200 hours and be strongly dependent on both the nitrogen concentration and the wavelength of light used. Work is currently on-going, and includes angular correlation of annihilation experiments. Lifetime measurements show clear evidence of the formation of positronium in the voids that are present in the CVD material. (C) 2002 Elsevier Science B.V. All rights reserved.