HPHT Synthetic Diamonds Research Articles

Spatial distribution of greenish blue phosporescence in HPHT synthetic diamond: The dual role of boron

The greenish-blue phosphorescence (GB-Phos) in high-pressure-high-temperature (HPHT) synthetic diamonds has attracted much interest since the 1960s. Nevertheless, why the intensity and lifetime of the GB-Phos are distributed spatially along the growth sectors remains unclear. In some cases, the lifetime of the GB-Phos lasts for seconds, which is quite unusual among recorded photoluminescence in diamonds. Understanding the spatial distribution characterization of the GB-Phos is crucial to unravel the mechanism by which the GB-Phos persists for such a long time, which makes diamond promising as a luminescent material. In this study, we systematically analyzed the photoluminescence in the {111} and the {100} growth sectors and found the GB-Phos in both sectors share similar spectral features, which indicates an identical mechanism in the {111} and the {100} sectors. Saturation intensity in the {111} sector is twice as strong as that in the {100} sector, whereas the lifetime in the {111} sector is five-folder larger. Based on the kinetics of luminescence theory, such discrepancies were mainly determined by traps, which were confirmed by a thermoluminescence curve in the {111} sector. The fitted trap depth (0.3 eV) and the spatial distribution of isolated boron suggested that boron is the most possible candidate for the trap. We highlighted the boron in directing the long persistence of the GB-Phos in HPHT synthetic diamond.

Thermal conductivity of type-Ib HPHT synthetic diamond irradiated with electrons

The effect of irradiation with 3 MeV electrons and subsequent high-temperature annealing on the thermal conductivity κT of synthetic HPHT diamond containing impurity nitrogen at a concentration of about 100 ppm was studied at temperatures from 5 to 410 K. At each stage of the experiment, the optical absorption and photoluminescence spectra, magnetization and thermal conductivity were measured on two plates from the same diamond single crystal. Color centers and their concentration in different locations in the samples were determined from the optical spectra. The concentration of paramagnetic impurities in the samples was determined from the temperature and field dependencies of the paramagnetic magnetization. Thermal conductivity is suppressed by two orders of magnitude at temperatures from 35 to 50 K after irradiation to a fluence of 5×1018 cm−2. Annealing at a temperature of 1800° C restored the thermal conductivity of diamond at moderate and high temperatures, but not at temperatures below 120 K. The measured data on thermal conductivity were analyzed within the framework of the phenomenological Callaway model in order to understand the role of various phonon scattering processes and how it changes during irradiation and annealing.

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.

Chinese Colorless HPHT Synthetic Diamond Inclusion Features and Identification

Chinese HPHT diamonds have improved dramatically in recent years. However, this brings a challenge in identifying type IIa colorless diamonds. In this study, eleven HPHT and three natural, colorless, gem-quality IIa diamonds were analyzed using magnified observation, Raman, PL and chemical element analysis. The results show that only HPHT samples possessed kite-like inclusions and lichenoid inclusions, as verified by their complex Raman spectra (100–750 cm−1). Through PL mapping, HPHT and natural IIa diamonds were distinguished by their growth environments, which were reflected by PL peaks at 503, 505, 575, 637, 693, 694 and 737 nm. The chemical components of HPHT IIa diamond carbide inclusions are mainly Fe, Co, Ni and Mn, but those of Natural IIa are mainly Fe and Ni. As a result, the chemical components can be used to distinguish a natural colorless IIa diamond from a synthetic diamond.

Photoluminescence Mapping of Optical Defects in HPHT Synthetic Diamond

Photoluminescence Mapping of Optical Defects in HPHT Synthetic Diamond

Distribution of H1a-centers in as-grown diamonds of Fe-Ni-C system: FTIR-mapping study

The distribution of main nitrogen defects and H1a-centers in HPHT synthetic diamond grown by temperature gradient method (system Fe-Ni-C) has been studied by FTIR-mapping technique. The IR-peak at 1450cm−1 of H1a-centers (di-nitrogen interstitials), appears only in some local areas of octahedral growth sectors where single substitutional nitrogen (C-defects) was partly aggregated into nitrogen pairs (A-defects). It was found that the disappearing of H1a-centers takes place simultaneously with the completion of C→A transformation. The observed distribution of nitrogen defects in diamonds with transition metals gives strong experimental evidence that the H1a-centers are not a by-product of nitrogen transformation, but are directly involved in the stepwise C→A nitrogen aggregation process. Prime novelty statementFor HPHT diamond crystals of Fe-Ni-C system, the distribution of H1a centers, studied by FTIR-mapping for the first time, provide a strong argument for the interstitial-related mechanism of nitrogen C→A aggregation.

The Spectral Characteristics and Identification Techniques for Colorless and Near-colorless HPHT Synthetic Diamonds

Melee-sized colorless HPHT synthetic diamonds mixed with natural diamonds in jewellery have been reported by gem testing laboratory. It was noted that the differences of the fluorescence and phosphorescence between natural and synthetic diamonds are the key features for quick screening in mixed diamonds. However, it is difficult to completely distinguish the two only by the differences of fluorescence and phosphorescence. In this paper, 5 colorless gem-quality HPHT synthetic diamonds were systematically investigated and characterized using mutiple spectroscopic techniques, together with traditional gem testing techniques. It was found that 5 samples display no, or weak, 270 nm absorption band at the UV-Vis spectra, and the lower the color grade, the stronger the 270 nm absorption band. Infrared absorption spectra showed that all samples contain boron. The photoluminescence spectra revealed that the colorless HPHT synthetic diamonds contain N, Ni or Si-related lattice defects. Under irradiation of the ultra-short wavelength light, all samples phosphoresced strongly with blue-green color. Typical octahedral and cubic growth patterns can be clearly observed under the DiamondViewTM imaging system. Although there are some differences between the samples, the HPHT synthetic diamonds can be accurately identified through careful analysis combining all features, particularly the spectral charateristics of phosphorescence and fluorescence, FTIR, UV-Vis, and photoluminescence spectra.

メレサイズの無色～ほぼ無色 HPHT 法合成ダイヤモンド

メレサイズの無色～ほぼ無色 HPHT 法合成ダイヤモンド

러시아의 뉴 다이아몬드 테크놀러지에서 생산된 보석용 합성 다이아몬드의 특성

러시아 상트페테르부르크에 소재한 뉴 테크놀로지 다이아몬드(NDT)에서 생산된 보석용 합성 다이아몬드의 보석학적, 분광학적 특성을 조사하였다. 컬러(무색은 극미량의 붕소를 함유하고 있고 블루는 미량의 붕소를 함유하고 있음)와 클래리티 등급은 천연 다이아몬드와 비교할 때 손색이 없었다. NDT 합성 다이아몬드는 자외선 단파에서 블루, 오렌지의 형광과 인광 반응이 관찰되었다. PL 분석에서 H3 센터와 NV 센터가 발견되었으며, H3 센터의 intensity는 천연 다이아몬드와 비교할 때 매우 약하게 존재하였다. NDT에서 생산된 합성 다이아몬드의 보석학적, 분광학적 특징들을 통해 천연 다이아몬드와 구분할 수 있다. Gemological and spectroscopic properties of HPHT synthetic diamonds from New Diamond Technology (NDT) company in St. Petersburg (Russia) were examined. Their color (colorless, near-colorless with some boron and Fancy blue with high boron content) and clarity (<TEX>$VVS-SI_1$</TEX>) grades were comparable to those of top natural diamonds. NDT synthetic diamonds fluoresced and phosphoresced blue or orange under SWUV light. Photoluminescence spectra revealed H3 center with very small intensity and NV centers. The intensity of H3 in NDT synthetic diamond has very weak in comparison with natural one. Using a combination of gemological and spectroscopic tests, gem-quality synthetic diamonds from NDT can be distinguished from natural diamonds of similar quality.

Near-Colorless HPHT Synthetic Diamonds from AOTC Group

Near-Colorless HPHT Synthetic Diamonds from AOTC Group

Effect of nitrogen impurity on the dislocation structure of large HPHT synthetic diamond crystals

The real structure of the {111} sectors of large single synthetic diamond crystals with different nitrogen contents has been studied by the selective etching method: (1) nitrogen-gettered diamonds with nitrogen impurity content at a level of 1–2ppm, (2) diamond crystals grown without additives with nitrogen impurity of 180–220ppm and (3) diamonds nitrogen alloyed during growth up to the concentrations of 550–600ppm. All diamond crystals have been grown by the temperature gradient method using the high pressure apparatus of “split-sphere” type (BARS) at T=1350°C, P=5.7GPa and (100) seed crystal. It is found that the density of dislocations increases from 85cm−2 to 4.0×103cm−2 and the total length of planar defects increases from 30 to 300μm/cm2 as the nitrogen content in the crystals increases up to 600ppm. It is shown that partial dislocations are the dominant dislocations (60–90%) in all the crystals studied. A proportion of perfect dislocations increases as the nitrogen content in crystals, and correspondingly, the dislocation density increase. Possible causes for the increase of the extended defects density in the synthetic diamond crystals with the nitrogen impurity concentration rise are discussed.

Photoluminescence studies of the 523.7 nm optical centre in HPHT synthetic diamond

The 523.7nm centre is very commonly observed in electron irradiated HPHT synthetic diamond, but not in CVD samples with high N content up to 50ppm. Absorption studies indicated that the 523.7nm centre was caused by nitrogen-interstitials complexes, and this hypothesis was supported by the insensitivity of its zero phonon line to stress. However, its high annealing temperature, strong broad phonon sideband and lack of high energy local vibrational modes suggest that the 523.7nm centre was more probably vacancy-related.

Conductive layers in diamond formed by hydrogen ion implantation and annealing

High conductivity is extremely difficult to obtain in diamond due to its wide band gap and low solubility of dopands. The goal of the investigation was to form a conductor inside HPHT synthetic diamond plates with initial high sheet resistivity ρs (∼1012Ω/sq) for 400μm thickness. We used metastable character of diamond structures relative to the graphitization of defective layers formed by 50keV hydrogen molecular ions at high fluence Φ=(1−13)×1016cm−2 ion implantation. High temperature (HT) (500–1600°C) and vacuum or high pressure (VP/HP) (3×10−3/4×109Pa) thermal annealing were chosen to provide the annealing regimes where the graphitic carbon is the most stable phase. Sheet resistance, dropped down up to nine orders of magnitude (ρs∼103Ω/sq), as well as Raman spectroscopy, and AFM measurements were used to determine electrical, optical and geometrical properties of multilayered heterostructures formed in the set of experiments. Temperature dependences of the conductivity show, that after highest fluencies and annealing temperatures the conductivity is quasimetallic and electronic system is above metal–insulator transition (MIT). At lower fluences and/or annealing temperatures the system is under MIT with the transport of charge carriers being well described by variable range hopping (VRH) mechanism with variable decay length of wave function for localized states. Two or three order of magnitude differences in the conductivity in VP and HP annealed samples are attributed with the higher dimensions of graphite nanocrystals in the case of vacuum annealing. This suggestion coincides with Raman spectra and optimum hopping length for carrier jumps in VRH model for conductivity in the buried layers.

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.

Nitrogen and Bias Effect on Homoepitaxial Diamond Growth by Hot-Filament Chemical Vapor Deposition

The development of homoepitaxial films for advanced device applications has been studied, but high growth rate and diamond film quality have not yet been explored. In the current study, high quality homoepitaxial diamond films were grown on type Ib (100) HPHT synthetic diamond substrate by hot-filament chemical vapor deposition. The reactant gases were mixed by CH4 and H2 with small amounts of N2 (500 to 3000 ppm). Besides, a bias system was used to assist diamond film deposition. The pyramidal crystals on diamond surface can be suppressed and high quality diamond film of FWHM (Full Width at Half Maximum) = 10.76 cm-1 with high growth rate of 8.78 ± 0.2 μm/ hr was obtained at the condition of adding 1000 ppm nitrogen. At the bias voltage of -150 V, the pyramidal crystals can also be suppressed and high quality diamond film of FWHM = 10.19 cm-1 was obtained. With nitrogen addition above 2000 ppm, diamond film was partly doped and some sp2 structures appeared. These homoepitaxial diamond films were characterized by optical microscopy and micro-Raman spectroscopy.

Radiation detector performances of nitrogen doped HPHT diamond films

AbstractThe remarkable physical properties of diamond such as tissue equivalence (atomic number Z = 6, close to that of human tissue), radiation and mechanical hardness and a wide band gap (5.5 eV) make it an ideal material for the fabrication of detectors for radiotherapy applications. However, the response to the radiation of such a kind of detector strongly depends on the bulk structure and in particular on the different impurities concentration inside the material.In the present paper we investigated the detector performances of two nitrogen doped HPHT synthetic diamonds grown at the IGM using a split‐sphere type high‐pressure apparatus. The samples were characterized using two different types of contacts: Au and Ti/Pt/Au configuration. The fabricated devices, studied as active detectors, presented totally different current response when they were exposed to the Co‐60 gamma irradiation. One sample in particular showed under irradiation strong value of current for which the photoconductive gain was found to be as high as 105. A model of trap‐induced sensitizing is suggested in order to explain the very high photoconductive gain obtained. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study on the HPHT synthetic diamond crystal from Fe-C(H) system and its significance

Investigations of crystal habit, micro-topographic imaging, micro-composition and micro-structural analysis of HPHT synthetic diamonds from the Fe-C(H) system indicate that most of them have an octahedral habit. The crystals grow mainly layer-to-layer from center to periphery. HPHT synthetic diamond is smaller in size than natural diamond because it only goes through nucleation and growth in the early stage. In the middle and late stages, due to the coalescence of diamond grains related to differences of surface energy, the growth of HPHT synthetic diamond is limited. The active energy (E) of transforming single nitrogen into a nitrogen-pair is lowered and the time of transforming single nitrogen into a nitrogen-pair is shortened because of the existence of hydrogen. Therefore, aggregate nitrogen (A-centers) may exist in synthetic diamond from HPHT and explosive detonation processes. It needs further discussion on a worldwide view that the time of natural diamond formation extracted from nitrogen aggregation is some hundred million years. Consideration of the way in which surface energy influences the growth of diamond can help to understand some of the remaining issues (e.g. growth mechanism, etc.) in the HPHT synthetic process and effectively explain the formation of natural diamond in terms of HPHT thermodynamic theory. Especially, it is important to pay more attention to the influence of hydrogen on surface energy in that hydrogen may be a “bridge” for explaining the formation of HPHT synthetic and natural diamond.

A new approach to investigation of nickel defect transformation in the HPHT synthetic diamonds using local optical spectroscopy

The analysis of spatial photoluminescence distribution along a plate from synthetic diamond as grown at 1500 °C allowed us to study a sequence of formation of nitrogen–nickel optically active centers and to build maps in some cases. Several groups of vibronic systems with specific spatial distribution along the sample were separated: nitrogen‐vacancy pairs in the thin near‐ surface layer, low‐temperature centers with blue emission, nickel–nitrogen complexes with a single nickel ion and those with several nickel ions.

Pulse height defect in pCVD and scCVD diamond based detectors

Polycrystalline (pCVD) and single crystal (scCVD) diamond films grown from Chemical Vapour Deposition (CVD), if sufficiently pure at Raman analysis, are very good materials for beam or flux monitors inside accelerators or nuclear reactors. This is because they are very hard to damage in high radiation fields and very resistant to high temperatures. Films of pCVD diamond are, however, not so good as spectroscopy detectors due to inhomogeneities induced by their growth in grains with the consequent presence of grain boundaries which worsen their energy resolution. The latter can be significantly improved by growing scCVD diamond films onto HPHT synthetic diamond substrates. We have shown that it is possible to measure the density of defects inside diamond specimens using as probes suitable penetrating nuclear radiations. With the preliminary results reported here we'll show that, bombarding CVD diamond films grown at Roma “Tor Vergata” with energetic protons and 4He, 6Li and 12C ions produced in the accelerators of Catania laboratories, the pulse height defects are higher than those in silicon detectors and likewise well described by a power law in the deposited energy. Furthermore, we'll show that pulse heights for the same particles seem to depend on the duration of the measurement, thus exhibiting a sort of depolarization of the insulator when exposed to the electric voltage which makes it a particle detector.

Freestanding (100) homoepitaxial CVD diamond

The interest in freestanding, low defect density, single crystalline CVD diamond films for electronic applications led to several studies of homoepitaxial CVD diamond growth. The aim of our work is to obtain nearly atomically flat surfaces even for several hundred microns thick CDV diamond films by controlling the growth mechanism on (100) surfaces. The effect of pre-growth etching of the substrate with a O 2/H 2 plasma and the influence of a high methane concentration on the growth on type Ib (100) HPHT synthetic diamonds were investigated. Using a MW PE CVD ASTeX PDS17-5 kW reactor at a pressure of about 180 Torr and temperatures typically around 700 °C, CVD diamond films were grown. A precise control of the growth mechanism led to the preparation of optical quality freestanding monocrystalline CVD diamond films of 600 μm thickness, practically free of hillocks and unepitaxial crystallites, with a relatively high growth rate of 4 to 5 μm/h. These films were characterized by optical microscopy, AFM, SEM and Raman spectroscopy. It is shown that even the thickest films have a mean roughness (Rms) remaining as low as 1 nm for a scanning area of 5 × 5 μm 2.