GR1 Centre Research Articles

The Annealing Kinetics of Defects in CVD Diamond Irradiated by Xe Ions

The radiation-induced optical absorption at 1.5–5.5 eV (up to the beginning of fundamental absorption) has been analyzed in CVD diamond disks exposed to 231-MeV 132Xe ions with four fluences from 1012 to 3.8 × 1013 cm−2. The 5 mm diameter samples (thickness 0.4 mm) were prepared by Diamond Materials, Freiburg (Germany); the average grain size at growth site was around 80 μm; and the range of xenon ions was R = 11.5 μm. The intensity of several bands grows with ion fluence, thus confirming the radiation-induced origin of the defects responsible for these bands. The recovery of radiation damage has been investigated via isochronal (stepwise) thermal annealing procedure up to 650 °C, while all spectra were measured at room temperature. Based on these spectra, the annealing kinetics of several defects, in particular carbon vacancies (GR1 centers with a broad band ~2 eV) and complementary C-interstitial-related defects (~4 eV), as well as impurity-related complex defects (narrow bands around 2.5 eV) have been constructed. The experimental kinetics have also been analyzed in terms of the diffusion-controlled bimolecular reactions. The migration energies of tentatively interstitial atoms (mobile components in recombination process) are obtained, and their dependence on the irradiation fluences is discussed.

The near surface damage and recovery of low nitrogen diamond implanted with MeV phosphorus ions

The purpose of this work is to provide a novel perspective for investigating the damage of diamond surfaces implanted by MeV phosphorus (P) ions and recovery of the surfaces after high-temperature annealing. SRIM simulation and low-temperature luminescence spectroscopy were performed to analyze the properties of the samples comprehensively. The results revealed that a certain degree of diamond lattice damage was caused without graphitization by the irradiation of 1 MeV P ions with a fluence value of 1015 ions/cm2. The SRIM simulation revealed that the ions that were implanted into the lattice collided with carbon atoms. These ions stopped at a depth of approximately 0.55 μm and generated intrinsic diamond defects on the diamond near the surface. The simulated defects were confirmed by the appearance of neutral single vacancy (GR1) centers in the photoluminescence (PL) spectrum after annealing at 400 °C. In addition, the annealing behavior of optical centers at 900 °C indicated significant damage recovery, including the disappearance of GR1 centers, combined with the increased intensity, slightly blue-shifted peak position, and narrowed FWHM of the nitrogen-vacancy (NV) center.

Spectroscopy of GR1 centers in synthetic diamonds

We report on the spectroscopic characterization, absorption saturation, and excited-state dynamics of GR1 centers in synthetic diamonds. The non-linear optical measurements reveal an efficient bleaching of the GR1 center’s ground level under ns-pulsed 633 nm excitation. The maxima of absorption and emission cross sections were estimated to be 4.5 × 10−17 cm2 and 9 × 10−17 cm2 at 630 nm and 780 nm, respectively. The radiative lifetime of the excited level was estimated to be 8.5 ns. The 658 nm probe kinetics uncovered relaxation of 1T2 excited level going predominantly to a metastable state with a lifetime of 220 μs. An induced absorption detected with the use of a highly concentrated diamond sample could be due to up-conversion and photoionization processes in additional impurity-vacancy center with absorption at the short-wavelength tail of GR1. The results presented here indicate that synthetic diamonds with GR1 centers could serve as near infrared gain media or passive Q-switchers for laser cavities over the 633–750 nm spectral range. Optimization of the center concentration and the crystal parameters are required to minimize the induced absorption at the lasing wavelengths.

Photoluminescence studies of nitrogen-vacancy and silicon-vacancy centers transformation in CVD diamond

In this study, the low-temperature micro-photoluminescence (PL) technology was employed to investigate the transformation of nitrogen-vacancy (NV), silicon-vacancy (SiV) centers in diamond crystal. Results showed that the NV and SiV luminescence were controlled by electron irradiation followed by thermal annealing. Both centers vanished together with the emergence of neutral single vacancy (GR1 center) after 200 keV electron irradiation. Interstitial related defects and vacancies were activated to diffuse by annealing (above ∼400 and 700 °C, respectively). The vacancies migrated to be captured by N and Si atoms due to the strain fields around the atoms attracted vacancies, and the NV and SiV centers appeared again in the PL spectra. In addition, compared the annealing behavior with NV center, the new emission at 639.7 nm was attributed to the nitrogen combined with carbon interstitials.

Spectroscopic evidence of the origin of brown and pink diamonds family from Internatsionalnaya kimberlite pipe (Siberian craton)

New spectroscopic data were obtained to distinguish the specific features of brown and pink diamonds from Internatsionalnaya kimberlite pipe (Siberian craton). It is shown that pink and brown samples differ markedly in the content and degree of aggregation of nitrogen defects. Pink diamonds generally have higher nitrogen content and a lower aggregation state compared to brown samples, which often show significant variations in nitrogen content and aggregation state between different growth zones. The 491 and 576 nm luminescent centres, which are signs of deformed brown diamonds, are absent or of low intensity in pink diamonds implying that high nitrogen content predominantly in A form in the pink diamonds had stiffened the diamonds against natural plastic deformation. The GR1 centre, formed by a neutrally charged vacancy, was observed only in pink diamonds, which may be due to their formation and storage in the mantle at lower-temperature conditions. Mineral inclusions indicate peridotitic and eclogitic paragenesis for studied brown and pink diamonds, respectively. It is suggested that brown diamonds have been formed in a primitive mantle at higher temperatures and/or stored there much longer.

Temperature dependence of optical centres in ultrapure diamond after 200 keV electron irradiation

In this work, we conduct a temperature-dependent study of optical centres in ultrapure diamond after 200 keV irradiation by photoluminescence spectra, which are performed in a temperature range of 80–200 K. The zero phonon lines (ZPLs) include sizes of 550.3 nm, 593 nm, and 741 nm (GR1 centre). With an increase in measurement temperature, the ZPLs shift to the lower energy side, together with intensity quenching and an increase in the full width at half maximum. These results are noted in models of lattice contraction and electron–phonon coupling, and their inhomogeneous and homogeneous broadening mechanisms are also carefully distinguished by the Voigt function. The 550.3 nm and 593 nm emissions present harder bonds than the GR1 centre, close thermal quenching energies to the interstitial-related defect, a lower thermal softness than that of the ideal diamond, weak phonon bands at each lower energy side, and different broadening mechanisms with the GR1 centre, indicating that the 550.3 nm and 593 nm lines are possibly interstitial-related.

Origin of the asymmetric zero-phonon line shape of the silicon-vacancy center in nanocrystalline diamond films

The spectral line shape of the 1.68 eV (738 nm) emission line – usually associated with the negatively charged silicon-vacancy (SiV) center in diamond and promoted as a strong candidate for many quantum technology and nanobiology related applications – was studied by luminescence spectroscopy in different nanocrystalline diamond films prepared by MW CVD technique. An asymmetric line profile, expressed as a long tail on the low-energy side of the narrow luminescence band located around 1.68 eV, have been observed in all samples.By applying a multi-wavelength laser excitation, it was found that the asymmetric shape of the 1.68 eV emission line varies by changing the excitation energy and becomes more pronounced under red (1.95 eV) excitation, especially for nanodiamond film characterized by larger average grain size. Based on the observed excitation dependent line shape behavior and on the analysis of the fine-structured emission lines, recorded by photoluminescence mapping experiments, the asymmetry of the ZPL was assigned to another optically active defect, simultaneously presented in diamond nanocrystals, namely to the GR1 center. The common presence of the GR1 and SiV centers within the CVD diamond structures even in the case of high-quality microcrystals was supported by photoluminescence excitation measurements also.

Temperature dependence of optical centers in transmission electron microscope irradiated natural IIa diamond

Diamond, a wide band gap semiconductor material, has been attracting interest in several fields from electrics and optics to biomedicine and quantum computing due to its outstanding properties. These properties of diamond are related to its unique lattice and optically active defect centers. In this paper, we present a detailed study of the temperature-dependent linewidth and energy shift for two zero-phonon lines at 503.5 nm (3H center) and 741 nm (GR1 center). We also discuss the uses and properties of electron-photon coupling and zero-phonon lines. The results show that with the increasing temperature, both centers red-shifted and broadened, and the intrinsic defects of the GR1 center were mainly controlled by the homogeneous broadening mechanism, while the inhomogeneous broadening mechanism of the 3H center was probably caused by local stress.

Defect transformations in nitrogen-doped CVD diamond during irradiation and annealing

Nitrogen-doped CVD diamond treated with electron irradiation and subsequent annealing at temperatures from 860 to 1900 °C was studied using fluorescence imaging, optical absorption and photoluminescence. It was found that nitrogen impurity produces many optical centers active throughout the infrared and visible spectral ranges. The most prominent of them active in IR spectral range are the centers related to nitrogen–hydrogen complexes. They produce absorption lines at 2827, 2874, 2906, 2949, 2990, 3031, 3107, 3123 and 3310 cm−1. Two characteristic absorptions at wavenumbers 1293 cm−1 and 1341 cm−1 were tentatively ascribed to a modified form of nitrogen A-aggregates. In the visible and near IR spectral ranges, characteristic nitrogen-related centers have zero-phonon lines (ZPLs) at 457, 462, 489, 498, 722.5, 852.5, 865.5, 868.5, 908, 921.5 and 924.5 nm. Some of them, e.g. 457, 462 and 498 nm centers, are unique of CVD diamond. It has been confirmed that the brightest pink color of electron-irradiated nitrogen-doped CVD diamond is produced by annealing at temperatures about 1000 °C. Annealing at temperatures over 1600 °C destroys the irradiation-induced pink color. It was found that the center 489 nm is a major absorption feature in the visible spectral range of electron-irradiated, nitrogen-doped CVD diamond. Green color of electron-irradiated, nitrogen-doped CVD diamond is caused by combined absorption of GR1 center and 489 nm center. It has been confirmed that NV defects produced in CVD diamond during growth are very temperature stable. They survive heating at temperatures at least 2000 °C. In contrast, NV defects produced by irradiation may anneal out at temperatures as low as 1600 °C. This much lower thermal stability of the radiation-induced NV defects is the result of their interaction with other radiation defects produced in their vicinity. A conclusion has been made that in nitrogen-doped CVD diamonds nitrogen atoms may form clusters. These clusters are probably the origin of the broad band luminescence at wavelengths 360, 390, 535 and 720 nm and a strong broadening of ZPLs of many optical centers.

The characteristic photoluminescence and EPR features of superdeep diamonds (São-Luis, Brazil)

Photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) were used for the first time to characterize properties of superdeep diamonds from the Sao-Luis alluvial deposits (Brazil). The infrared measurements showed the low nitrogen content (>50 of 87 diamonds from this locality were nitrogen free and belonged to type IIa) and simultaneously the extremely high level of nitrogen aggregation (pure type IaB being predominant), which indicates that diamonds under study might have formed under high pressure and temperature conditions. In most cases, PL features excited at various wavelengths (313, 473, and 532 nm) were indicative of different growth and post-growth processes during which PL centers could be formed via interaction between vacancies and nitrogen atoms. The overall presence of the 490.7 nm, H3, and H4 centers in the luminescence spectra attests to strong plastic deformations in these diamonds. The neutral vacancy known as the GR1 center has probably occurred in a number of crystals due to radiation damage in the post-growth period. The 558.5 nm PL center is found to be one of the most common defects in type IIa samples which is accompanied by the EPR center with g-factor of 2.00285. The 536 and 576 nm vibronic systems totally dominated the PL spectra of superdeep diamonds, while none of “normal” diamonds from the Mir pipe (Yakutia) with similar nitrogen characteristics showed the latter three PL centers.

Electroluminescence from a diamond device with ion-beam-micromachined buried graphitic electrodes

Focused MeV ion microbeams are suitable tools for the direct writing of conductive graphitic channels buried in an insulating diamond bulk, as demonstrated in previous works with the fabrication of multi-electrode ionizing radiation detectors and cellular biosensors. In this work we investigate the suitability of the fabrication method for the electrical excitation of color centers in diamond. Differently from photoluminescence, electroluminescence requires an electrical current flowing through the diamond sub-gap states for the excitation of the color centers. With this purpose, buried graphitic electrodes with a spacing of 10μm were fabricated in the bulk of a detector-grade CVD single-crystal diamond sample using a scanning 1.8MeV He+ micro-beam. The current flowing in the gap region between the electrodes upon the application of a 450V bias voltage was exploited as the excitation pump for the electroluminescence of different types of color centers localized in the above-mentioned gap. The bright light emission was spatially mapped using a confocal optical microscopy setup. The spectral analysis of electroluminescence revealed the emission from neutrally-charged nitrogen-vacancy centers (NV0, λZPL=575nm), as well as from cluster crystal dislocations (A-band, λ=400–500nm). Moreover, an electroluminescence signal with appealing spectral features (sharp emission at room temperature, low phonon sidebands) from He-related defects was detected (λZPL=536.3nm, λZPL=560.5nm); a low and broad peak around λ=740nm was also observed and tentatively ascribed to Si-V or GR1 centers. These results pose interesting future perspectives for the fabrication of electrically-stimulated single-photon emitters in diamond for applications in quantum optics and quantum cryptography.

Photoluminescence studies of the neutral vacancy defect known as GR1 centre in diamond

The single isolate vacancy in diamond exists in three charged states, neutral, negative and positive; and many complicated defects such as di-vacancies, impurities-vacancy complexes could also be formed in diamond. In this paper, we investigate the optical properties of the irradiation-induced neutral vacancy in diamond by low-temperature micro-photoluminescence technology, which will play a guiding significant role in the further studies of the complex defects in diamond.

Radio-colouration of diamond: a spectroscopic study

We have undertaken a study of the common green or orange-brown spots at the surface of rough diamond specimens, which are caused by alpha particles emanating from radioactive sources outside the diamond. Richly coloured haloes represent elevated levels of structural damage, indicated by strong broadening of the main Raman band of diamond, intense strain birefringence, and up-doming of spots due to their extensive volume expansion. Green radio-colouration was analogously generated through the irradiation of diamond with 8.8 MeV helium ions. The generation of readily visible radio-colouration was observed after irradiating diamond with ≥1015 He ions per cm2. The accumulation of such a high number of alpha particles requires irradiation of the diamond from a radioactive source over long periods of time, presumably hundreds of millions of years in many cases. In the samples irradiated with He ions, amorphisation was observed in volume areas where the defect density exceeded 5 × 10−3 Å−3 (or 0.03 dpa; displacements per target atom). In contrast, graphitisation as a direct result of the ion irradiation was not observed. The green colouration transformed to brown at moderate annealing temperatures (here 450 °C). The colour transformation is associated with only partial recovery of the radiation damage. The colour change is mainly due to the destruction of the GR1 centre, explained by trapping of vacancies at A defects to form the H3 centre. An activation energy of ~2.4 ± 0.2 eV was determined for the GR1 reduction. The H3 centre, in turn, causes intense yellowish-green photoluminescence under ultraviolet illumination. Radio-colouration and associated H3 photoluminescence are due to point defects created by the ions irradiated, whereas lattice ionisation is of minor importance. This is concluded from the depth distribution of the colouration and the photoluminescence intensity (which corresponds to the defect density but not the ionisation distribution pattern). The effect of the implanted He ions themselves on the colour and photoluminescence seems to be negligible, as radio-colouration and H3 emission were analogously produced through irradiation of diamond with C ions. The photoluminescence emission becomes observable at extremely low defect densities on the order of 10−6 Å−3 (or 0.000006 dpa) and is suppressed at moderate defect densities of ~5 × 10−4 Å−3 (or ~0.003 dpa). Intensely brown-coloured diamond hence does not show the H3 emission anymore. Anneals up to 1,600 °C has reduced considerably irradiation damage and radio-colouration, but the structural reconstitution of the diamond (and its de-colouration) was still incomplete.

Discrimination between natural and HPHT-treated type IIa diamonds using photoluminescence spectroscopy

We have performed low-temperature (8 K) photoluminescence (PL) measurements on 71 natural and 12 high-pressure-and-high-temperature (HPHT)-treated type IIa diamonds. The GR1, NV 0, NV −, H4, and H3 defect center PL signals are compared. Some distinct differences in the PL lineshape, intensity, and appearance of side-band PL signals are observed. Furthermore, we processed 6 of the natural diamond samples with the HPHT treatment to investigate the effect of the treatment on the PL spectrum. By systematically analyzing the differences in the PL spectra, we developed a scheme to discriminate natural and HPHT-treated diamonds with 99% validity.

Non-linear optical properties of coloured diamonds: Observations of frequency up conversion and “whispering gallery-like” modes in photoluminescence

Four coloured diamonds, 1.7 MeV e-beam irradiated to a dose of 10 18e per sqcm and annealed around 900–930 °C, were investigated at room temperature using conventional photoluminescence (PL) and excitation spectra, and PL using 514.5 and 785 nm lasers. Interesting new observations are: (i) excitation spectra reveal energy transfer between NV 0 and NV − centers (ii) frequency up conversion, blue-green emission on excitation in 740–900 nm range, was observed in blue and lemon yellow samples. This appears to be due to two-photon absorption in GR1 centers and energy transfer to N3/H3 centres. (iii) Excitation with 514 nm line of Ar + laser, in pink and purple samples resulted in the appearance of an intense and broad emission at 267 nm with FWHM of 32 nm. This is rather close to 2 ν of the exciting line. It is identified as second harmonic generation (SHG) due to a synergistic effect of strain induced birefringence and resonant absorption which facilitated phase matching. The 785 nm excited anti-Stokes PL contained a frequency up converted band in 620 nm region having extensive closely spaced structure like in the case of ‘whispering gallery modes’ due to spheroid inclusions.

Creating diamond color centers for quantum optical applications

Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state qubits. This is because of their large dipole moment, convenient level structure and very long room-temperature coherence times. In general, a combination of ion irradiation and subsequent annealing is used to create the centers, however for the rigorous demands of quantum computing all processes need to be optimized, and decoherence due to the residual damage caused by the implantation process itself must be mitigated. To that end we have studied photoluminescence (PL) from NV −, NV 0 and GR1 centers formed by ion implantation of 2 MeV He ions over a wide range of fluences. The sample was annealed at 600 °C to minimize residual vacancy diffusion, allowing for the concurrent analysis of PL from NV centers and irradiation induced vacancies (GR1). We find non-monotic PL intensities with increasing ion fluence, monotonic increasing PL in NV 0/NV − and GR1/(NV 0 + NV 1) ratios, and increasing inhomogeneous broadening of the zero-phonon lines with increasing ion fluence. All these results shed important light on the optimal formation conditions for NV qubits. We apply our findings to an off-resonant photonic quantum memory scheme using vibronic sidebands.

Polarization of luminescence and site symmetry of the Xe center in diamond

We report on polarized photoluminescence studies of the Xe ion-related optical center in diamond. The sample was a single crystal CVD diamond film grown along the (0 0 1) crystallographic plane. The film was irradiated with 180 keV Xe + ions at room temperature at a dose 5×10 12 cm −2 and annealed at 1400 °C. The emission of the sample with laser excitation at 647 nm exhibits the first and second order Raman spectra of diamond, a broad band due to the phonon sideband of the GR1 center, and the Xe center-related features: two zero phonon lines at 813 nm and at 794 nm and a respective vibrational sideband. The Huang–Rhys factor for the Xe center was estimated to be exp(− S)=0.3 at room temperature. The polarization of the Xe center photoluminescence was studied as a function of the sample rotation. The center was established to be a 〈1 1 1〉 oriented defect with a circular dipole absorption transition at 647 nm, a circular dipole emission transition at 794 nm, and a linear dipole emission transition at 813 nm. Possible models of the center were discussed.

The effect of ion implantation on thermally stimulated currents in polycrystalline CVD diamond

Trapping levels in polycrystalline CVD diamond before and after ion implantation have been investigated using thermally stimulated currents (TSC). The activation energy of the dark current is reduced after ion implantation because of the GR1 centre and the energy levels formed by this defect. However, introduction of this additional defect does not effect the kinetic parameters of the dominant trap responsible for the observed TSC. Nevertheless, the implantation does effect the trapped carrier density suggesting that the GR1 centre acts as a deep trapping centre.

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 °C 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.

An oscillating visible light optical center in some natural green to yellow diamonds

Visible spectra taken at 72 K of a group of ten green-to-yellow-colored natural gem-quality diamonds show more than 30 sharp absorption bands between 13140∗ and 18420 cm−1 (1.63 and 2.28 eV), suggesting one or two defects that oscillate to visible light, a phenomenon not previously reported for diamond. Preliminary analysis of these spectra shows two oscillations, one dominating the lower energy end of this range with an energy spacing of 0.021 eV, and another appearing at the higher energy end of the range with a spacing of 0.025 eV. All samples show weak to moderate strength infrared absorptions typical of hydrogen, and of single substitutional nitrogen. The green color in nine of these diamonds is caused by the broad overlap at the base of these many absorption bands, not by a GR1 center, and thus forms a new cause of naturally occurring green color in diamond. Similar “many-banded” features have been observed by others in the low temperature cathodoluminescence spectra of diamond and in the low temperature luminescence spectra of minerals containing a molecular ion; perhaps the hydrogen or nitrogen (or both) play a role in causing these oscillations.