A range of techniques was used to determine the N levels in a series of polycrystalline diamonds. The crystals were grown by using the chemical vapor deposition technique, and electron paramagnetic resonance measurements indicated that they had a single substitutional N (Ns) concentration of between 10ppb and 50ppm. It was noted that photoluminescence spectroscopy provided an extremely sensitive means for detecting the N vacancy complex, in both neutral (N–V)0 and negative charge states (N–V)-, to below the 10ppb range. It was observed that, for diamonds with a single substitutional N content of some 0.5 to 2.0ppm, the N vacancy complex existed in both charge states; with almost equal abundance. Below this level, the complex existed more often in the neutral charge state; with the (N–V)- center predominating at Ns levels greater than 2ppm. Electron irradiation of so-called high-N films, with an Ns value above 15ppm, exhibited a very high abundance of the interstitial-related 3H center. Such samples showed little sign of the neutral vacancy (GR1), but some absorption was seen in the tail of the (N–V)- center. No evidence was found for the negative vacancy (ND1 center) in photoluminescence spectra. For given electron doses, and low-N samples (with a typical Ns level of less than 1ppm), the GR1 center completely dominated the photoluminescence spectra. For all samples, the intensity of the N vacancy complexes decreased within the irradiated region. Annealing studies of high-N samples revealed that, by 900C, all of the vacancies were annealed; with many being trapped at the N to form high concentrations of the (N–V)0 center. Positron annihilation measurements were expected to yield lifetime measurements that were strongly dependent upon the bulk (N–V) concentration. Doppler broadening spectra were recorded as the samples were illuminated with light (325 to 785nm). The shape parameter was found to increase upon illumination, and relaxed back to its ground state after illumination. The relaxation time was found to vary from 30 to 200h, and was strongly dependent upon the N concentration and upon the wavelength of the light used. Lifetime measurements revealed clear evidence of the formation of positronium in voids in CVD material.
Photoluminescence and Positron Annihilation Measurements of Nitrogen Doped CVD Diamond. A.Wotherspoon, J.W.Steeds, B.Catmull, J.Butler: Diamond and Related Materials, 2003, 12[3-7], 652-7