Homoepitaxial chemical vapour deposition (CVD) on high pressure high temperature (HPHT) synthetic diamond substrates allows production of diamond material with controlled point defect content. In order to minimize the extended defect content, however, it is necessary to minimize the number of substrate extended defects that reach the initial growth surface and the nucleation of dislocations at the interface between the CVD layer and its substrate. X-ray topography has indicated that when type IIa HPHT synthetic substrates are used the density of dislocations nucleating at the interface can be less than 400 cm-2. X-ray topography, photoluminescence imaging and birefringence microscopy of HPHT grown synthetic type IIa diamond clearly show that the extended defect content is growth sector dependent. <111> sectors contain the highest concentration of both stacking faults and dislocations but <100> sectors are relatively free of both. It has been shown that HPHT treatment of such material can significantly reduce the area of stacking faults and cause dislocations to move. This knowledge, coupled with an understanding of how growth sectors develop during HPHT synthesis, has been used to guide selection and processing of substrates suitable for CVD synthesis of material with high crystalline perfection and controlled point defect content.

Studies carried out at the DTC Research Centre have shown that single-crystal CVD synthetic diamond is clearly distinguishable from natural diamond. This article presents information about the CVD process, the history of its development, the different kinds of CVD synthetic diamond material that can be produced, and properties that differentiate them from natural diamond. The authors studied more than a thousand CVD synthetic diamond samples that were grown for research purposes using a range of different process conditions. Absorption, photoluminescence, and cathodoluminescence spectra of these CVD synthetic diamond samples showed a range of different impurity-related features not seen in natural diamond. Photoluminescence imaging is also useful in identifying CVD synthetic diamond, and X-ray topography may give supportive evidence. The effectiveness of the Diamond Trading Company Diamond Verification Instruments for identifying CVD synthetic diamond is also described.

Topographic infrared and photoluminescence spectroscopy of synthetic single-crystal diamonds revealed that a higher growth rate increases the concentration of Ni and Co impurities in {111} growth sectors and of N impurities in {001} growth sectors. Also, the change in the concentration of Ni or Co in {111} growth sectors, caused by changing the rate of growth, of HPHT synthetic diamonds grown with Ni or Co catalysts is inversely related to the change of the concentration of N.

Electron paramagnetic resonance (EPR) and optical spectroscopy have been used to determine the structure and electronic state of nickel-nitrogen centres in natural diamonds and in synthetic diamonds enriched in 13C. The latter were grown in an Fe-Ni-C solvent/catalyst system at 1750 K, under stabilizing pressure, by the temperature gradient method and afterwards treated at high temperature and pressure. The parameters and directions of the 13C hyperfine structure (HFS) tensors for the NE1 centre were found to confirm the previously proposed model for this defect, with a nickel ion at the centre of a double semivacancy as the basic structural unit. In this unit the nickel atom has six atoms in its coordination shell. The NE1 centre has C2h symmetry, and the two equivalent nitrogen atoms in the coordination shell lie in the symmetry plane. New data on the HFS of 14N and 13C for the NE5 centre, also with C2h symmetry, indicated the same structural unit, but the two equivalent nitrogen atoms (and two equivalent carbon atoms) lie out of the symmetry plane and are related to one another by reflection in it. A new paramagnetic centre was found, labelled NE8, also with C2h symmetry, with four equivalent nitrogen atoms in the coordination shell all lying out of the symmetry plane. This centre is responsible for the 793.6 nm vibronic system in absorption and luminescence spectra. The new data have allowed a reinterpretation of the HFS tensors for the NE2 centre, which has C1 symmetry, suggesting that it has the same structure as NE1 but with one additional nitrogen atom in the coordination shell. The electronic states of these nickel-containing centres are discussed using the approach of Ludwig and Woodbury to transition metal ions in covalent crystals.

Infra-red (IR) absorption results on irradiated and annealed synthetic diamond are presented which confirm an earlier proposal that a component found in the defect-induced one-phonon region of some diamonds arises from positively charged single-substitutional nitrogen (N+). The concentration ratio of N+ to neutral substitutional nitrogen (N-0) centres may be changed by shining light of various energies onto the examined samples. By correlating changes in absorption of the IR component associated with N-0 centres with changes in the N+ component, and using a previously determined relation between the concentration of N-0 centres and peak absorption coefficient at 1130 cm(-1), the relationship between peak absorption at 1332 cm(-1) and concentration of N+ centres has been derived, namely 1 cm(-1) of absorption is produced by (5.5 +/- 1) ppm N+ centres. Other defects may also give rise to absorption at 1332 cm(-1), but the N+ component is uniquely identified by further peaks at 1046 and 950 cm(-1).The significance of this component is demonstrated by the bet that some samples can contain in excess of 80 ppm N+ centres, and this must consequently be accounted for when assaying the total nitrogen concentration in such samples.Using the above relationship useful parameters relating the concentration of neutral vacancies, negative vacancies and negatively charged nitrogen-vacancy centres to their respective zero-phonon line integrated absorptions have been derived.

The effect of nickel and cobalt impurities on nitrogen aggregation in high-pressure, high-temperature (HPHT)-grown synthetic diamonds has been investigated. Samples have been grown from solvent-catalysts of different nickel-iron ratios to vary the nickel content. Experimentally, a marking method has been developed to allow accurate repositioning of samples for infra-red absorption measurements at each stage of the annealing, and samples have been carefully prepared to exclude the possibility of growth-sector overlap and to minimise effects due to inhomogeneity within a growth-sector. The results indicate an enhancement in {111} growth sectors due to the preferential incorporation of nickel, with the degree of enhancement increasing with increasing nickel content. A marked deviation from simple second order kinetics was found for the enhanced cases Similar results were found for {111} sectors of cobalt/iron-grown samples. Features in infra-red and visible absorption spectra allow a model for the aggregation enhancement via interstitial atoms to be proposed.

More than 400 blue diamonds were studied at the GIA Gem Trade Laboratory over several years to reveal additional information about the relationships between their color, clarity, and other gemological properties.

Topographic infra-red spectroscopy has been used to investigate the inhomogeneity of the aggregation of nitrogen in {111} growth sectors of synthetic diamonds. The aggregation rate constant was established to be different even within the same {111} growth sector. In particular, changes in the growth temperature, most likely changing the impurity content, are shown to result in changes in the aggregation rate constant; namely an increase in the growth temperature, in diamonds grown with a Co catalyst, is found to reduce the nitrogen aggregation rate constant, and vice versa. These results indicate that the presence of Co impurities in the {111} growth sector influences the aggregation rate. It is found that the aggregation process follows second order kinetics as first established by Chrenko et al. [Nature, London, 270 (1977) 141]. The activation energy of the aggregation process in {111} growth sectors is measured as 5.5 +/- 0.7 eV.It is also found that increasing the applied pressure during the annealing process reduces the aggregation rate constant, such that there is a significant difference between annealing at ambient pressure and annealing at 10 GPa. (C) 1997 Elsevier Science S.A.

Long-lived phosphorescence has been investigated in high-pressure synthetic boron-doped diamond. The spectrum exhibits two broad bands at peak energies of 2.1 and 2.5 eV after ultra-violet light excitation. The typical lifetime for both the bands shows a monotonous decrease from 50 s to 300 ms as the temperature rises from 200 to 400 K. The integrated intensities of the 2.1 and 2.5 eV bands increase as the temperature rises to 350 and 360 K, respectively. Beyond these temperatures, the intensities decrease rapidly as temperature rises. A model based on a recombination process involving both a shallow acceptor and a deep donor has been examined by theoretical fitting with the temperature dependence of the lifetime and the integrated intensity. It is concluded from the analysis that a shallow acceptor with an activation energy of 0.35 eV, which can be assigned as that of boron, plays a key role in the phosphorescence.

This article presents evidence that cobalt forms a series of optically active defect centers in diamond grown by high-temperature, high-pressure synthesis. Photoluminescence (PL) studies reveal that the newly observed vibronic systems with zero-phonon energies at 1.989, 2.135, 2.207, 2.277, 2.367, and 2.590 eV appear only in samples grown using a cobalt-containing solvent–catalyst. Results of an annealing study, carried out in the temperature range 1500 to 1800 °C, establish that many of the new bands appear during the temperature regime of nitrogen aggregation. It is therefore proposed that nitrogen forms complexes with cobalt to produce optically active centers, in a manner analogous to that of nickel point defects in diamond. Detailed radiative decay time measurements and temperature dependence measurements show that all but one of the bands which are here associated with nitrogen–cobalt complexes have long radiative decay times (∼100 μs), and this again is a characteristic of the PL centers arising from nickel–nitrogen complexes. All of the vibronic bands observed by PL may also be produced by electron-beam excitation (cathodoluminescence). In this case it is necessary to use a low beam current density (≤10 mA cm−2), otherwise the spectra are dominated by emission from optical centers with much shorter decay times (∼20 ns). Only one vibronic band, with a zero-phonon line at 1.852 eV, has been detected in absorption measurements, and the center responsible for this system does not give rise to luminescence.

Diamond crystals have been grown by the temperature gradient method under high temperature and pressure conditions, using Co and Ni catalysts with and without the use of Ti as a nitrogen getter. For each growth run the growth temperature has been changed during the run in order to observe the effect which temperature has on the incorporation of the impurities nitrogen, boron and nickel.The concentration of nitrogen in the diamonds grown from both Co and Ni without the use of a nitrogen getter decreased with increases in temperature. On the other hand, when the getter was added the nitrogen concentration increased with increasing temperature. This suggests that the effect of the nitrogen getter decreases with increasing temperature.In the case of boron-doped crystals grown from Co and Ni, the yellow colour changed to blue with increases in the growth temperature. The change was reversed when the nitrogen getter was added. Based on the knowledge that the depth of blue colour is proportional to the concentration of uncompensated acceptor, N-boron - N-nitrogen, the Change in colour is reasonably understood if the concentration of nitrogen changes in the same manner as it does for boron-free crystals.A distinct green or brown coloured stripe caused by incorporation of nickel appeared in sample regions formed during decreases in the growth temperature, but not in regions corresponding to increases in temperature. This suggests that the concentration of nickel depends on growth rate rather than on the growth temperature itself, in contrast to the case for nitrogen and boron.

The nature of an UV cathodoluminescence band, which can appear in both boron-doped high-pressure high-temperature (HPHT) synthetic diamond and boron-doped diamond grown by chemical-vapor-deposition (CVD) techniques, is investigated. The band has a peak energy at 4.6 eV (270 nm), a full width at half-maximum of ∼0.4 eV and at low temperatures (∼130 K) can represent the overwhelmingly dominant cathodoluminescence (CL) from selected regions of a given sample. The band has been examined from boron-doped HPHT diamond grown from different solvent catalysts and from boron-doped CVD diamond grown under a variety of deposition conditions. Low-temperature spatially resolved CL imaging, using a scanning electron microscope with CL attachment, has revealed a clear growth-sector dependence of the 4.6 eV band in HPHT diamond. Using this technique an investigation of the relationship between this band and other commonly observed CL bands has been carried out. The band has an interesting temperature dependence which is investigated over the temperature range 77–300 K and explained in terms of competition between exciton capture and exciton release at some unidentified trapping center.

Many papers have been published recently concerning the incorporation of nickel in diamond. In this paper we present results of an annealing study, carried out in the temperature range 1500–1900°C, of nickel-related optical centres in high-pressure synthetic diamond. We establish the temperature regime in which the well known 1.883 eV and 2.51 eV systems anneal out, and observe the concurrent growth of an array of structure which extends throughout the visible region of the absorption spectrum and gives the previously bright-yellow coloured diamonds a rich golden-yellow colour. By carrying out the annealing sequence on diamonds grown using various solvent-catalysts, we establish the conditions under which the annealed structure is maximized and propose that it involves nickel. Both the content and configuration of nitrogen present in diamond are shown to be critical in determining the stable form of nickel which results.

Diamonds grown by the temperature-gradient method using a nickel catalyst and sufficient nitrogen getter to produce barely detectable nitrogen absorption in the defect-induced one-phonon region have a deep brown colour which we show results from continuum absorption whose threshold lies around 1.7 eV. In these diamonds we observe, for the first time, the presence of zero-phonon structure lying between 1.2 and 1.25 eV in the near-infrared spectral region of the absorption spectrum. This structure exhibits interesting photochromic effects and we observe a correlation between partial decay of the 1.40 eV system (whose transition has previously been ascribed to nickel) and growth of the new zero-phonon structure. Like the 1.40 eV system, the new zero-phonon structure shows strong polarization effects and is confined exclusively to the {111} growth sectors of diamond. From these results we propose that the new structure results from a different charge state of the 1.40 eV nickel system.

Results of an annealing study, carried out in the temperature range 1500–1900 °C, of nickel-related optical centers in high-pressure synthetic diamond are presented. It is established that the well-known 1.883 and 2.51 eV systems anneal out during the temperature regime of nitrogen aggregation and the concurrent growth of an array of structure, which extends throughout the visible region of the absorption spectrum, and gives the previously bright-yellow-colored diamonds a rich golden-yellow color, is observed. By carrying out the annealing sequence on diamonds grown using various solvent catalysts a correlation is found between preanneal 1.883 eV absorption and maximized absorption of some of the annealed structure. From the results it is proposed that centers associated with nickel and nitrogen are produced and their possible natures are speculated on. It is found that the defect-induced one-phonon spectra of the diamonds examined may be satisfactorily decomposed into three components. To account for certain changes in the infrared spectra during the annealing sequences, and using previously reported results, it is proposed that one of these components may result from nitrogen in a positive charge state.