Natural Type IIa Diamond Research Articles

Photoluminescence Spectra of Helium Ion-Implanted Diamond

Ion implantation in diamond crystals is widely used both for producing conducting microstructures in the bulk of the material and for creating isolated photon emitters in quantum optics, photonics, cryptography, and biosensorics. The photoluminescence (PL) spectra of helium ion-implanted diamonds are dominated by two sharp emission lines, HR1 and HR2 (from Helium-Related), at ~536 and 560 nm. Here, we report on PL studies of helium-related optical centers in diamonds. Experiments have been carried out on a (110) plate of natural single-crystal type IIa diamonds. The uniform distribution of radiation defects in a 700 nm-thick layer was obtained by ten cycles of multiple-energy (from 24 to 350 kV) helium ion implantation with a total dose of 5 × 1016 cm−2. The diamonds were annealed in steps in a vacuum oven at temperatures from 200 to 1040 °C. It is demonstrated that helium ion implantation in diamonds followed by annealing gives rise to more than a dozen various centers that are observed in the PL spectra in the range of 530–630 nm. The transformations of the PL spectra due to annealing are investigated in detail. The spectral shapes of phonon sidebands are determined for the HR1, HR2, and HR3 bands with ZPLs at ~536, 560, and 577 nm, respectively, and it is shown that these bands are attributed to interstitial-related centers in diamonds. The reported results are important for understanding the structure and properties of helium-related defects in diamonds.

Characterization on the occurrence state of nitrogen getter Ti in HPHT grown synthetic diamond crystals

Type IIa diamonds are the most promising gem grade, optical and electronic grade materials with high quality and low defects, however natural type IIa diamonds are rare and therefore the synthetic methods to obtain the high quality type IIa diamonds have been a hot issue. Nitrogen getters play a crucial role in the synthesis of type IIa high temperature high pressure (HPHT) diamonds crystals by reacting with the nitrogen in the system and effectively removing it from the growth process. However, the occurrence state of nitrogen getter introduced after diamond crystal growth remains to be investigated. In this study, a group of HPHT synthetic diamond crystals with different mass fractions of Ti as a nitrogen getter were examined by scanning electron microscopy coupled with energy dispersive spectrometer (SEM-EDS) and X-ray fluorescent spectroscopy (micro-XRF) to observe the occurrence state of the nitrogen getter Ti within the synthetic diamond crystal, it was found that at the end of crystal growth, There are two forms of Ti present in the process: (i) Ti reacts with C to form TiC, which was deposited on the surface of the diamond crystal in the dendritic pattern, and (ii) Ti enters the diamond crystal to form metallic inclusions. This study provides reliable data to support a comprehensive analysis of the effect of Ti as a nitrogen getter in the growth procession of HPHT synthetic diamond crystal, and it will have a positive effect on the development of synthetic diamond technology.

Identification of Natural Type IIa Colourless Diamond with 270 nm Absorption Peak of UV-Visible Spectrum

In this paper, the conventional gemmological and spectral characteristics of a type Ⅱa colourless diamond sample were analyzed. Gemmological characteristics of the diamond sample were tested by gem microscope; The DiamondViewTM observed the growth structure and photoluminescence of the diamond sample; UV-Vis spectrometer detected the UV-Visible absorption characteristics of the diamond sample; Fourier infrared spectrometer determined the type of the diamond sample; The confocal laser Raman spectrometer and PL-3000 portable laser Raman spectrometer analyzed the characteristic spectra of defects in the diamond sample. Based on the analysis of experimental data, it is concluded that the diamond sample is natural Ⅱa type colourless diamond with the characteristic peak at 270 nm in UV-Vis spectrum, which provided complete laboratory appraisal basis, and enriched the instruction and guidance for special type ultraviolet absorption type of natural type Ⅱa diamond for daily testing.

Point defects and interstitial climb of 90° partial dislocations in brown type IIa natural diamond

Multiple electron microscopy techniques have been used to study a brown type IIa natural diamond. Electron backscatter diffraction shows evidence of plastic deformation in the form of slip bands, while cathodoluminescence reveals a network of low-angle grain boundaries, also observed in transmission electron microscopy together with long straight dislocations and dislocation dipoles. Aberration-corrected scanning transmission electron microscopy shows interstitial absorption on the 90° partial of both dissociated dislocations and Z-type faulted vacancy dipoles, forming structures similar to that observed in other fcc materials. The observations indicate an interstitial concentration of 1017 to 1019 cm−3 and calculations of point defect concentrations produced by plastic deformation show that this can be produced by strains of the order of 1%. Brown coloration in diamond has been previously attributed to vacancies and vacancy clusters with concentrations around 1018 cm−3, which suggests that roughly equal numbers of interstitials and vacancies are generated in diamond via plastic deformation. Atomic resolution images of Z-type faulted dipoles allowed a stacking fault energy of 472 ± 38 mJ m−2 to be determined.

Comparison of gemological and spectroscopic features in type IIa and Ia natural pink diamonds

The majority of natural pink diamonds have a color origin due to absorption from a broad 550 nm band that has been associated with plastic deformation. One consistent feature in the photoluminescence spectra of these pink diamonds is a wide emission band extending from ~600 to 750 nm, with a series of smaller oscillations overlaid on the larger emission band. This “pink emission band” is seen in diamonds colored by the 550 nm absorption band; the absorption band often, but not always, shows similar oscillations at ~600 nm (called the 609 nm system by previous researchers). This emission band served as a proxy for the 550 nm absorption band as we performed spatial mapping to chronicle the differences between the uniform coloration in type IIa pink diamonds and the pronounced banding in type Ia pink diamonds. We also used Raman spectroscopy to identify the internal crystal inclusions present in type IIa pink diamonds and determined that the majority have a sub-lithospheric origin.

Anomalous Green Luminescent Properties in CVD Synthetic Diamonds

In this paper, the authors explore the spectral properties of a sharp emission line centered at 499.6 nm (reported previously as 499 nm) observed in the UV‐luminescence spectrum of synthetic CVD diamond gemstones. These gemstones also exhibit spectrally prompt broad blue luminescence centered at 435 nm which is similar in character to the 425 nm blue luminescence characteristic of type IIa natural diamonds. By comparing the results against synthetic CVD diamond material which has been subjected to high temperature anneals, the authors can speculate on the nature of the defect associated with this color center. Since the 499 nm spectral line has only been observed in synthetic CVD diamonds to date, the authors further propose that it can be helpful in the identification of synthetic diamonds, in particular those showing the blue luminescence signature usually associated with type IIa natural diamonds.

Emission from Crystals Irradiated with a Beam of Runaway Electrons

An investigation of the spectral and amplitude-temporal characteristics of emission from different crystals, promising in terms of their application as detectors of runaway electrons, is performed. This emission is excited by subnanosecond electron beams generated in a gas diode. It is found out that at the electron energies of tens–hundreds of kiloelectronvolts, the main contribution into the emission from CsI, ZnS, type IIa artificial and natural diamonds, sapphire, CaF2, ZrO2, Ga2O3, CaCO3, CdS, and ZnSe crystals comes from the cathodoluminescence; the radiation pulse duration depends on the crystal used and sufficiently exceeds the Cherenkov radiation pulse duration. It is demonstrated that the latter radiation exhibits low intensity and can be detected in the short-wave region of the spectrum in the cases where a monochromator and a high-sensitivity photomultiplier tube (PMT) are used.

A Study of the relationship between brown colour and extended defects in diamond using core-loss electron energy loss spectroscopy

There is strong evidence to support the theory that the vacancy cluster, a by-product of dislocation interaction, is the extended defect responsible for brown colour in natural type IIa diamonds. A characteristic of these open volume structures is the hybridised sp2 bonding of the 3-fold coordinated carbon atoms that line their cage-like surfaces. When present in sufficiently large concentrations the sp2 bonds alter the electronic and optical properties of type IIa diamond dramatically. Similar sp2 bond concentrations are found in dark brown CVD diamond, although the defects in this material appear much larger in size than in the natural diamond. In this study we employ core-loss electron energy loss spectroscopy (EELS) and aberration corrected scanning transmission electron microscopy (AC-STEM) to compare the relative sp2 content of brown coloured natural type IIa diamonds and dark brown CVD diamonds, using 100% sp2 bonded graphite as a reference. The effect of high-pressure high-temperature (HPHT) treatment on sp2 content is also investigated.

Time Dependence of Charge Transfer Processes in Diamond Studied with Positrons

We have developed a method called optical transient positron spectroscopy and apply it to study the optically induced carrier trapping and charge transfer processes in natural brown type IIa diamond. By measuring the positron lifetime with continuous and pulsed illumination, we present an estimate of the optical absorption cross section of the vacancy clusters causing the brown color. The vacancy clusters accept electrons from the valence band in the absorption process, giving rise to photoconductivity.

Phosphorescence in type IIb diamonds

Spectroscopic properties, including phosphorescence spectroscopy, were collected for 354 natural type IIb diamonds, 7 natural type IIa diamonds, along with 24 HPHT-treated diamonds and 18 HPHT synthetic diamonds. The only phosphorescence bands observed in the natural diamonds were centered at 500 and 660 nm. The relative intensity and half-life of these measured bands corresponded well with the boron concentration. None of the treated or synthetic diamonds showed the 660 nm band. However, these samples did have the 500 nm band; the synthetics had much higher intensity than the comparable natural diamonds. The phosphorescence spectra are correlated to data collected from infrared absorption and photoluminescence spectroscopy and these results provide additional evidence of the mechanism responsible for the observed phosphorescence.

An analysis of vacancy clusters and sp2 bonding in natural type IIa diamond using aberration corrected STEM and EELS

The link between brown colour and vacancy structures in natural diamond and has been studied through the techniques of aberration corrected scanning transmission electron microscopy (AC-STEM) and electron energy loss spectroscopy (EELS). Bright field (BF) and high angle annular dark field (HAADF) STEM imaging modes have revealed discrete patches of stronger contrast, of the order of 1nm in size, that are similar to simulated images of vacancy clusters. Core loss spectra show a pre-edge feature for brown diamond corresponding to π* states. This feature is absent for colourless and heat treated brown diamonds. Additional spectra acquired in regions containing dislocations for both brown and white (colourless) diamonds showed a second pre-edge feature indicating that dislocations have electronic states associated with them and are therefore optically active.

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.

Fast bolometer built in an artificial HPHT diamond matrix

A fast bolometer built in a plate of diamond grown at high pressure by the gradient growth method is developed and fabricated. The parameters of this structure are compared with these of the structures investigated earlier, which were fabricated based on chemical vapour deposited (CVD) diamond and natural type IIa diamond.

Non-resonant X-ray/laser interaction spectroscopy as a method for assessing charge competition, trapping and luminescence efficiency in wide band-gap materials

Using a conventional fast-shuttered laboratory X-ray source in combination with pulsed laser diode modules, the possibilities for undertaking X-ray/laser interaction spectroscopy in wide band-gap luminescent materials are explored. It is shown that in such materials, a variety of X-ray/laser timing sequences can extract complimentary information regarding the charge-carrier trapping, de-trapping and recombination processes. The effects on the luminescence are illustrated for six example materials (YPO 4:Ce,Sm, Lu 3Al 5O 12:Pr, Al 2O 3:C, natural sodium feldspar NaAlSi 3O 8, cubic BN and type IIa natural diamond). By ramping the temperature from 10 to 320 K during repeated X-ray pump/laser-probe activation cycles, a rapid assessment can be made of the important thermally dependent changes to the charge carrier trapping competition processes.

Loss characteristics in coplanar propagation waveguides fabricated on IIa diamond substrates

The properties of coplanar propagation waveguides (CPW) on various diamond substrates are investigated. We demonstrate that on such material, the physico-chemical surface treatments may be fundamental to obtain good microwave properties. CPW were processed on single crystal CVD diamond samples that were grown either as bulk substrate or as a thin epitaxial layer on a type IIa natural diamond substrate. These CPW exhibited losses around 0.15 dB/mm at 10 GHz and 0.25 dB/mm at 40 GHz. Temperature-dependant measurements imply that structural defects may involve parasitic current conduction. The loss angle tangents of both samples have then been extracted from a quasi-TEM numerical modelling, they are about 0.025 for each kind of substrate.

Etching and micro-optics fabrication in diamond using chlorine-based inductively-coupled plasma

The effect of Inductively-Coupled Plasma (ICP) etching on diamond using chlorine-based plasma has been investigated. The diamond materials studied include type IIa natural diamond, High Pressure and High Temperature (HPHT) diamond and Chemical Vapour Deposition (CVD) diamond. It was found that argon and chlorine (Ar/Cl 2) ICP plasma etching can improve the smoothness of the diamond surface. By using this method, a minimum root-mean-squared (rms) surface roughness of 0.19 nm has been achieved. To demonstrate optimized Ar/Cl 2 plasma etching, diamond spherical micro-lenses and micro-trenches were fabricated. Compared to argon and oxygen (Ar/O 2) plasma etching, Ar/Cl 2 plasma etching has a low selectivity with respect to the photo-resist mask, which enables an accurate control over the dimensions of the microstructures fabricated. The surface quality and profiles of these micro-lenses and micro-trenches were characterized by atomic force microscopy (AFM) and were shown to be better than those fabricated by Ar/O 2 ICP plasma.

MBE grown optically pumped semiconductor disk lasers emitting at 940 nm

We present the optimization and performance of high-power optically pumped semiconductor disk lasers emitting near 940 nm. The emission efficiency at this wavelength is typically low owing to the poor carrier confinement within the shallow GaInAs quantum wells especially at high injection levels. Our work was focused on studying the efficiency of three types of structures with barrier materials made of AlGaAs, GaAs and GaAsP, providing different band offsets. The structures were grown by solid-source molecular beam epitaxy and consisted of a 30.5 pair AlAs/AlGaAs distributed Bragg reflector and an active region comprising 10 groups of triple-quantum-wells. To improve the crystal quality we have applied rapid thermal annealing for each grown wafer. The gain chips were then capillary bonded with water to a type IIa natural diamond heat spreader. The bonded gain elements were tested in a laser cavity with a linear-cavity configuration. Using a 3% output coupler we have obtained a maximum output power of 3 W from the structure with GaInAs/GaAsP composition. The corresponding M 2 value was 1.5 and the optical-to-optical conversion efficiency was 10%.

Electron-beam-induced conductivity and color cathodoluminescence of natural diamond plates in scanning electron microscope

Surfaces of natural type-IIa diamond plates used in ionizing-radiation detectors are studied in a scanning electron microscope using techniques of electron-beam-induced conductivity at different bias voltages and real-color cathodoluminescence.

Electron irradiation and the formation of vacancy–interstitial pairs in diamond

A large number (>60) of nitrogen-containing type Ia natural diamonds, and nitrogen-free type IIa natural andsynthetic diamonds, have been electron irradiated and their optical absorption spectra have beenanalysed in detail. The analysis reveals that the concentrations of vacancies [V] and interstitials[I0 ] in type IaA diamonds increase linearly with the concentration[NA ] of the A-form of nitrogen. This increase of [V] and[I0]with [NA ] is attributed to the trapping of vacancies and interstitials in the strain field of nitrogen. Thetrapping produces ‘strained’ vacancies and partly immobilizes interstitials, thus reducing thevacancy–interstitial recombination during electron irradiation, thereby increasing [V] and[I0 ]. Those strained vacancies and interstitials recombine upon annealing at temperatures∼390–420 °C. In type IaB diamonds no correlation has been found between [V] and the concentration[NB ] of the B-form of nitrogen; furthermore[I0] was found todecrease linearly with [NB ]. In both type IaA and type IaB diamonds the width of theGR1 zero-phonon line, associated with the neutral vacancies[V0 ], increases with nitrogen concentration. A new, more appropriate and accurate formula to calculate[V0] is proposed based onmeasuring the height H of the GR1 absorption band at 2.0 eV.

HR‐TEM imaging and image simulation of vacancy clusters in brown diamond

AbstractDiamonds, although famous for their sparkling colourless nature, can exhibit many colours due to imperfections in the crystal lattice. Brown diamonds, while not the favourite variety, have attracted much attention and mystery in finding the origin of the colour. The defect, which is not impurity related, can be removed with HPHT treatment. Collections of vacancies have been suggested as the source of the colour by theoretical calculations [1] and positron annihilation experiments [2, 3]. Therefore, simulations have been carried out to determine whether the suggested sizes of vacancy clusters could be seen in HR‐TEM. Substantial contrast has been revealed in defocused images. In light of the simulations, experimental defocus series have been acquired in lattice resolution for brown and colourless, natural and synthetic type IIa diamonds. A small‐scale patchiness is apparent in the lattice images of brown but not of colourless diamond. Further analysis, including simulations of multiple vacancy clusters, suggests this contrast in brown diamond could be due to many irregularly distributed vacancy clusters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)