Alan T. Collins

The characterisation of point defects in diamond by luminescence spectroscopy

Abstract Luminescence spectroscopy is a vitally important technique for characterising the defects present in diamond. In this paper the two major methods for generating the emission — photoluminescence and cathodoluminescence — are considered, and the defects responsible for the most significant luminescence bands are discussed in detail. Problems with the donor-acceptor pair model, conventionally used to interpret the broad blue and green emission bands in cathodoluminescence, are outlined, and the fundamental differences between photoluminescence and cathodoluminescence are presented. The paper concludes with a cathodoluminescence study of diamond grown by the combustion flame technique. Not only does this material exhibit intrinsic edge emission, but there is evidence of bound-exciton recombination at a currently unknown donor or acceptor centre.

Colour changes produced in natural brown diamonds by high-pressure, high-temperature treatment

Abstract Absorption and luminescence spectra have been measured for natural brown diamonds before and after high-pressure, high-temperature (HPHT) treatment at 1700–1800°C, and after HPHT treatment at 2025°C. The reduction of the brown colour noted in diamonds with low nitrogen concentration is attributed to the annealing of plastic deformation. In nitrogen-containing diamonds the vacancies released during this annealing are trapped to form N–V–N centres. In addition, the photoluminescence band with a zero-phonon line (ZPL) at 2.526 eV (490.7 nm), characteristic of decorated slip traces, is reduced in intensity during this process, confirming that changes are taking place at the slip traces which have resulted from the plastic deformation. At the lower annealing temperatures the N–V–N centres are in the neutral charge state, giving rise to the H3 absorption band, with a ZPL at 2.463 eV (503.2 nm) and resulting in a yellow colour. At the higher annealing temperature some nitrogen aggregates decompose, producing single substitutional nitrogen which is an electrical donor. Consequently some N–V–N centres are in the negative charge state, and give rise to the H2 absorption band with a ZPL at 1.257 eV (986.9 nm). The combination of absorption in the H2 and H3 bands gives the diamonds a yellow-green or green colour. Annealing at the highest temperature, at pressures very close to the diamond/graphite transition, seems to be effective at reducing the non-radiative recombination channels in the diamonds, and, in white light, many specimens exhibit bright green luminescence, produced by absorbed energy being re-emitted in the H3 band. To the eye these processed diamonds appear similar to the very rare, naturally-occurring, ‘green transmitters’. However, because the latter have resulted from much lower geological temperatures, acting over much longer periods of time, they contain negligible concentrations of single substitutional nitrogen and consequently have low H2 absorption.

THE ANNEALING OF INTERSTITIAL-RELATED OPTICAL CENTRES IN TYPE II NATURAL AND CVD DIAMOND

Abstract Optical absorption bands in diamond, associated with the carbon interstitial, show annealing stages at temperatures below that at which the vacancy becomes mobile. These bands include the 1.86 eV, TR12, R11 and 5RL systems. Although the vacancy is not mobile, the intensity of the GR1 band, which is due to transitions at the neutral vacancy, does decrease by about 15–20% during annealing at these relatively low temperatures (typically 420–540 °C). In this paper we show that the 1.86 eV and R11 systems anneal out completely, concurrently with the “rapid” annealing of the GR1 band, and we attribute the behaviour to the migration of the interstitial to the vacancy, followed by mutual annihilation. The process follows first-order kinetics, with an activation energy of 1.68±0.15 eV. The intensities of the 1.86 eV and R11 absorption bands are closely correlated during this annealing, and it is possible that the R11 absorption is produced by transitions to excited states of the 1.86 eV centre. The 5RL and the TR12 absorption bands increase in intensity, following annealing at 420–540 °C, but their annealing behaviours are not simply related to those of the GR1 and 1.86 eV centres.

Spectroscopic study of cobalt‐related optical centers in synthetic diamond

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 fr...

Optical centres related to nitrogen, vacancies and interstitials in polycrystalline diamond films grown by plasma-assisted chemical vapour deposition

The authors have used cathodoluminescence spectroscopy to examine point defects present in polycrystalline diamond films grown by the decomposition of a methane-hydrogen mixture in a microwave plasma. For films grown with low methane concentrations (0.3%) the bright-blue luminescence is predominantly due to donor-accepted pair recombination, together with some weak zero-phonon lines. As the CH4 concentration in the gas mixture is increased the cathodoluminescence spectra from the resulting films contain a number of additional zero-phonon lines. Some of these are unique to this type of diamond; other lines at 2.156 eV, 2.807 eV, 3.188 eV and 4.582 eV are associated with optical centres that have been studied in diamonds produced by high-pressure synthesis, and indicate that the diamond films contain carbon interstitials, nitrogen-vacancy and nitrogen-interstitial centres. The widths of the zero-phonon lines in the diamond films suggest that the material is heavily strained.

Relative increase in choline in the occipital cortex in chronic fatigue syndrome

Puri BK, Counsell SJ, Zaman R, Main J, Collins AG, Hajnal JV, Davey NJ. Relative increase in choline in the occipital cortex in chronic fatigue syndrome. Acta Psychiatr Scand 2002: 106: 224–226.

Determination of boron in natural semiconducting diamond by prompt particle nuclear microanalysis and Schottky barrier differential-capacitance measurements

Capacitance-voltage characteristics have been obtained for Schottky barrier diodes, formed on polished surfaces of natural p-type semiconducting diamond, before and after illumination with radiation capable of neutralizing ionized donors in the depletion layer. Plots of 1/C2 versus V for the two cases have yielded values for both the acceptor concentration NA and the donor concentration ND. Comparison of these data with those obtained from Hall effect measurements indicate that the latter overestimate the value of NA-ND by almost a factor of 2. The boron concentration has been determined in approximately the same surface layer as the capacitance measurements by detecting the alpha -particles emitted during irradiation with 700 keV protons using the reaction 11B(p, alpha 1)8Be* to 2 alpha 2. A quantitative correlation has been obtained between the boron and acceptor concentrations.

Spectroscopy of defects and transition metals in diamond

Abstract This article reviews the optical and electron paramagnetic resonance (EPR) studies that have been carried out on diamonds containing nickel-related or cobalt-related defect centres. It is shown that this work is now at a stage where a real understanding may be reached about the nature of the defects and the inter-relationships between the optical and EPR phenomena. The behaviour of the transition metals is strongly influenced by the presence of other defects, and the properties of nitrogen, boron, phosphorus, the vacancy and the self-interstitial are briefly considered as a background to the main review.

Cathodoluminescence studies of isotope shifts associated with localised vibrational modes in synthetic diamond

Sharp structure between 2.60 and 2.64 eV observed in the cathodoluminescence spectra of synthetic diamond, following radiation damage and annealing at 300 degrees C, has been identified as a series of local-mode phonon replicas of the zero-phonon line at 2.807 eV. By examining the isotope shifts of these replicas in diamonds grown using 13C (and containing natural nitrogen) compared with diamonds synthesised from natural carbon and doped with 15N, the authors conclude that the optical centre contains one nitrogen atom in association with the interstitial produced by the radiation damage. The local-mode structure is very similar to that observed for the system with a zero-phonon line at 3.188 eV, which has also been shown to be a single nitrogen plus interstitial centre. Coupling of the 2.807 eV electronic transition to lattice modes is, however, much weaker than for the 3.188 eV transition. In diamonds which have not been thermally annealed the intensities of these vibronic bands, initially zero, are observed to increase with increasing time under the influence of the electron beam-a phenomenon observed previously for the 2.156 eV optical centre. The absence of localised vibrational modes, the response of the zero-phonon line to random strain and the greatly increased luminescence intensity following annealing at 800 degrees C lead us to propose that the 2.156 eV centre involves single nitrogen with one or more vacancies.

Spectroscopic studies of carbon-13 synthetic diamond

Vibrational absorption spectra in the infrared, and vibronic absorption and luminescence bands in the visible and ultraviolet spectral regions have been studied for synthetic diamonds grown using 99% 13C. This is the first such study of any semiconductor for which the isotope of the host material has been totally changed. As expected, vibrational frequencies involving only the motion of carbon atoms are changed in the approximate ratio (12/13)12/, whereas those involving carbon and nitrogen are changed according to the square root of the ratio of the reduced masses. The frequency changes observed are consistent with the models proposed recently for the defects responsible for the vibrational absorption lines observed in the infrared spectral region. Furthermore the measurements have allowed the authors unambiguously to identify the 237 meV localised mode in the 5RL band (zero-phonon line at 4.582 eV) of electron-irradiated diamond with the vibration of a carbon atom.