Type IIa Diamond Research Articles

A study of radiation dosimeters based on synthetic HPHT diamond

We investigated the applicability of synthetic diamond crystals routinely produced by the HPHT (High Pressure High Temperature) technique for the detection of ionising radiation. Split-sphere-type multi-anvil equipment was used to produce these HPHT diamonds. Both as-grown type Ib and HPHT treated type IaA diamonds were examined at the Centre of Oncology in Kraków using a therapeutic Co-60 γ-ray beam. Detectors fabricated from type IaA diamonds showed, on average, better performance than those based on type Ib diamonds. IaA- type synthetic diamonds containing up to 150-200 ppm nitrogen showed sensitivity up to 160 nC/(Gy mm 3), charge collection distance up to 180 μm and close to linear dose-rate response. Considerable variations in the detecting characteristics were observed for the devices containing similar concentrations of nitrogen. From our preliminary qualitative analysis we conclude that the observed response variations may be associated with varying concentration of the nickel impurity present in these diamond crystals.

Fabrication of diamond p– i– p– i– p structures and their electrical and electroluminescence properties under high electric fields

Diamond p– i– p– i– p device structures with highly insulating ( i) and p-type ( p) regions were laterally fabricated using a microscopic etching process of high-pressure/high-temperature-synthesized (HPHT) Ib-type (N-included) diamond with focused ion beams and a homoepitaxial chemical-vapor-deposition (CVD) process. The i regions (at room temperature (RT)) were composed of thinned HPHT diamond with a thickness of 0.5 or 1 μm while the p regions were composed of boron-doped CVD diamond films with a thickness of 0.2 or 0.5 μm. Thus, high electric fields up to ≈ 1 × 10 7 V/cm were successfully applied to the thinned i regions. When the electrical property of the p– i– p– i– p structures was measured at RT either in a vacuum (5 × 10 − 5 Pa) or in 1-atm ambient N 2 and Ar gases, dramatic current increases were observed for all the specimens examined at the voltages, V, slightly above the threshold voltages, V th, depending on the i region thickness. For V > V th, we observed electroluminescence whose spectra well corresponded to cathodoluminescence spectra taken from the devices. Furthermore, in the measurements in the gases, additional current increases appeared at V substantially higher than V th due to gas discharges under high electric fields existing outside the diamond devices. These observed phenomena are discussed in relation to an avalanche current amplification process and the band structure of the p– i– p– i– p devices.

Investigation of the unit cell parameter and dislocation structure of polycrystallinediamond films

The values of the unit cell (UC) parameter were measured and elements of the internalstructure were determined in polycrystalline diamond films (PDFs) using x-ray diffraction.It was established that the values of the UC parameter were connected with the dislocationdensities in mosaic PDF crystallites. The dislocation density was calculated from measuredmicrodistortions and dispersion in the blocks based on the surface stretch forces thatarise on the boundaries between blocks. This allowed more precise computing ofthe dislocation density. A similar connection between the UC parameter anddislocation density also exists in epitaxial diamond films and in plastic deformednatural type IIb and Ic diamonds. A regular relationship between physical valuesof the UC parameter and dislocation density in mosaic diamond materials wasdetermined using mathematical modelling. An increase in the dislocation densityresults in a decrease in the UC parameter from 0.356 689 nm in more pure andperfect type IIa diamonds to a limiting value of 0.356 42 nm in PDF samples.

Deformation microstructure of high-quality synthetic diamond crystal subjected to Knoop indentation

The deformation microstructures of high-quality synthetic type IIa diamond crystal around a Knoop indentation formed at room temperature were investigated. The atomic force microscopy observation showed that there are smooth deformation areas without cracks or fractures around the indentation. In addition, many dislocation lines which lie on {111} planes and have Burgers vectors in ⟨110⟩ directions were observed beneath the Knoop indentation in transmission electron microscopy investigations. These results reveal that plastic deformation in diamond is possible beneath Knoop indentations even at room temperature, and that the deformation is mainly ascribed to {111}⟨110⟩ slip systems.

Origin of brown coloration in diamond

Measurements of the absorption spectra of brown natural type IIa diamond as well as brown nitrogen-doped CVD diamond are reported. These are largely featureless and increase almost monotonically from about 1--$5.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is argued that the brown coloration is due to an extended defect and not to a point defect. First principles modeling studies demonstrate that the spectra could be attributed to vacancy disks lying on ${111}$ planes. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

TEM and DSC studies on the synthetic diamond grown from Fe–Ni–C–B system under HPHT

Diamond was synthesized by the static pressure catalyst method may contain many kinds of impurities [1], which can affect the color, hardness, strength, thermal conductivity, optical absorption, and other physical properties of diamonds [2]. Evergrowing requirements imposed on thermal management materials in applications such as microelectronics; optical communications and semiconductor processing equipment drive the development of new materials with enhanced thermal conductivity [3]. The boron and nitrogen were two important impurities that could control the electronic properties of diamond through playing roles of donor and acceptor [4]. The reports on boron in the synthetic diamond grown at high-pressure high-temperature (HPHT) has not been found compared to that on nitrogen, which was ever studied extensively [5, 6]. Besides the effect on the electronic resistance of diamond, born could influence the outset temperature of oxidation resistance, i.e., the thermal stability of diamond crystals [7]. Further studies of the boron effect on thermal stability of diamond would have great significance in mechanical industry. In present paper, investigation was focused on both the microstructure and chemical composition of boron compounds in the diamond and their influence on the thermal stability of diamond crystals synthesized using Fe–Ni–B metal catalyst under HPHT conditions. In addition, we examined the effect of boron compounds on the outset temperature of oxidation resistance of synthetic diamond grown using boron-contained catalyst by different scanning calorimetry (DSC) techniques. Four alloys: Fe–Ni, Fe–Ni–0.01wt%B, Fe–Ni– 0.1 wt%B and Fe–Ni–0.4 wt%B, sampled as A, B, C, and D, respectively, were used as metal catalysts. They were placed alternately with graphite discs (purity 99.9%) to synthesize four types of boron-content diamond. In order

The vacancy as a probe of the strain in type IIa diamonds

The width of the vacancy related photoluminescence from diamond is used to qualitatively measure the strain in type IIa diamonds of a range of brown colours and to measure the effect on the strain of high pressure high temperature annealing of the brown diamonds. The results indicate that for untreated type IIa diamonds the strain in colourless diamonds is generally less than that observed in brown ones. Annealing to remove the brown colour brings about a measurable reduction in the strain as assessed via the luminescence line width; but a dependence on the depth of the original brown colour is retained with the value remaining in most cases above 1.70 meV, a width below which 92% of the untreated colourless diamonds lie.

Thermally activated deformation under Knoop indentations in super-hard directions of high-quality synthetic type-IIa diamond crystals

The deformation resistance at room temperature against a Knoop indentation in (001)<110> (the<110> direction on the (001) plane) of high-quality synthetic type-IIa diamond is known to be extremely high. The behavior of deformation in the hard direction activated thermally by heating was investigated, using super-hard Knoop indenters prepared from high-quality diamond crystals by taking the tip orientation to (001)<110>. Indentation tests in (001)<110> with a load of 4.9 N revealed that the formation of normal Knoop impressions arises suddenly at a threshold temperature of 200–240 °C, whereas no impressions are observed up to 200 °C. The hardness values derived from the impressions in (001)<110> formed above the threshold temperatures are as low as 50–60% those in (001)<100> at the same temperatures. The anisotropy in the Knoop hardness at such high temperatures is consistent with the nature of anisotropy predicted by an effective resolved shear stress model for a {111}<110> slip deformation.

Electronic properties of boron-doped {111}-oriented homoepitaxial diamond layers

p-type diamond single crystal layers were grown by Microwave Plasma-enhanced Chemical Vapor Deposition (MPCVD) on {111}-oriented Ib-type diamond substrates. The variation of the gas-phase boron-to-carbon atomic concentration ratio from 0.1 ppm to 10 4 ppm led to solid state incorporations of boron ranging from the 8 × 10 15 cm − 3 to 3 × 10 21 cm − 3 . Low temperature cathodoluminescence spectra in the bandgap region are shown to yield a reliable estimate of the quality of the layers and of the neutral acceptor concentration up to the metal–insulator transition. Temperature-dependent resistivity and Hall effect measurements allow a discussion of the relevance of the various charge carrier scattering mechanisms.

Gas-phase characterization in diamond hot-filament CVD by infrared tunable diode laser absorption spectroscopy

In hot-filament CVD the gas-phase composition is a vital parameter for diamond coating results. Concentrations of carbon-containing species have significant influence on growth rate, quality and morphology of deposited diamond. To learn more about the correlations between process parameters and gas species concentrations we applied the highly sensitive infrared tunable diode laser absorption spectroscopy (IR-TDLAS) technique. With a sophisticated compact IR-TDLAS unit, relative and absolute concentrations of CH 4, C 2H 2 and CO were simultaneously measured. Also, the absolute concentration of the methyl radical was determined in dependence on process parameters. Concentrations of CO 2, C 2H 6 and HCN were investigated but found to be lower than the detection limit. The influence of the typical diamond CVD-process parameters on various species concentrations is discussed. The applicability of IR-TDLAS for hot-filament process monitoring is evaluated. In context with diamond growth results, information for CVD process refinement was deducted from the IR-TDLAS measurements.

Optical Properties of Various Types of Diamond

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Natural, untreated diamonds showing the A, B and C infrared absorptions (“ABC diamonds”), and the H2 absorption

Unusual brown to yellow diamonds of mixed type Ib/IaAB were analyzed. The occurrence of nitrogen in a combination of A, B and C centers directly detectable by IR spectroscopy in natural diamonds is considered to be extremely rare. We propose to call such stones ABC diamonds for short. These diamonds are characterized by a color, luminescence, and anomalous double refraction distribution which results from mixed growth: a center core formed by cuboid growth, rich in nitrogen, covered by an outer rim of “normal” octahedral growth, with much less nitrogen. Nitrogen in the core is present in a more aggregated state than in the rim, which is practically pure type Ib. In the infrared spectra, besides A, B and C 1-phonon absorptions a large number of previously undescribed, tentatively H-related features were identified in some samples; these proposed H-related features were only present in the periphery of the core and the rim of the diamonds, in the core practically no hydrogen was detected. The low temperature Vis/NIR spectra were mainly characterized by two defect-induced centers: the H2 center, and the apparently new 905 nm vibronic absorption with phonon-side bands at 880, 867, 847 nm plus a feature at 806.5 nm. The 905 nm absorption can possibly be attributed to a hydrogen-related center. Although these diamonds present a combination of spectroscopic characteristics (A + B + C center absorptions, H2 absorptions) deemed characteristic of certain HPHT-treated type Ia diamonds, they cannot be confused for such treated diamonds on the basis of their color, color distribution and some spectral peculiarities.

The characterization of synthetic and natural single crystal diamonds by X-ray diffraction

Natural and synthetic single crystal diamonds (types Ia, Ib, IIa and IIb) have been characterized by measurements of the X-ray integrated scattering intensity, lattice parameter and diffuse scattering. We found a drastic increase of X-ray integrated scattering intensity for natural and B-doped diamond crystals. Synthetic type IIa diamond was also measured for comparison. Measurements of the lattice parameters have been made at room temperature by using the X-ray bond method. We found that the lattice parameter is smallest for natural diamond and largest for the B-doped diamond. Diffuse scattering experiments were performed using a four circle diffractometer at room temperature. We measured the diffuse scattering for (4 0 0) Bragg reflection for the four types of crystals. The scattering intensity of the natural crystals diffuses asymmetrically to form a streak along the [1 0 0] direction parallel to the reciprocal lattice vector. These results suggest the existence of the nitrogen atom platelets on (1 0 0) plane in the natural diamond. We also found the diffuse streaks along the [ 1 ˙ 0 0 ] direction for (4 0 0) Bragg reflection for the B-doped crystal, suggesting that boron atoms are likely to form precipitates on the (1 0 0) plane.

Infrared absorption studies of neutron-irradiated type Ib diamond

Infrared (IR) absorption measurements were performed at room temperature on neutron-irradiated type Ib diamonds which contain around 100 ppm substitutional nitrogen atoms in single form. It was observed that the 1332 cm −1 absorption peak appears by irradiation but vanishes again at the dose region higher than 2×10 18 n/cm 2. This phenomenon is explained by using two-step charge compensation model. Furthermore three remarkable peaks are observed at 1530 and 1570 cm −1 and at 1450 cm −1. Relations between absorption intensity and neutron dose of these peaks were studied. Intensity of these peaks increase linearly with dose; however, the 1450 cm −1 peak intensity saturates around 2×10 17 n/cm 2 in contrast with the others. These results are consistent with the interstitial models proposed previously.

Assessment of percolation thresholds for channel system in MFI type zeolite based on the MFI and diamond lattice equivalence

The critical percolation probability for both bond and site percolation in partially blocked MFI channel system was determined. The evaluation of the percolation threshold was based on topological equivalence of the channel system in MFI type zeolites and diamond lattice where the chemical bonds correspond to MFI channels and carbon atoms to MFI cross-sections.

Root-like structure at the nanowire/substrate interface in GaAs nanowires

We report the observation of a root-like structure at the interface between GaAs nanowires and the (100) single-crystal GaAs substrate. These nanowires were grown via the vapor-liquid-solid mechanism using metalorganic vapor phase epitaxy. The root-like structure extends from the base of the nanowires into the substrate and has been investigated in detail using transmission electron microscopy and high-resolution electron microscopy. While the nanowires predominantly exhibit the zinc-blende type diamond cubic structure with the growth axis parallel to ⟨111⟩ and growth twins perpendicular to the growth axis, the root regions have a CdTe type orthorhombic structure that has been reported to occur in GaAs only under high-pressure conditions.

Implantation of labelled single nitrogen vacancy centers in diamond using N15

Nitrogen-vacancy (NV−) color centers in diamond were created by implantation of 7 keV N15(I=1∕2) ions into type IIa diamond. Optically detected magnetic resonance was employed to measure the hyperfine coupling of single NV− centers. The hyperfine spectrum from NV−15 arising from implanted N15 can be distinguished from NV−14 centers created by native N14(I=1) sites. Analysis indicates 1 in 40 implanted N15 atoms give rise to an optically observable NV−15 center. This report ultimately demonstrates a mechanism by which the yield of NV− center formation by nitrogen implantation can be measured.

A Review on High-Speed Machining of Titanium Alloys

Titanium alloys have been widely used in the aerospace, biomedical and automotive industries because of their good strength-to-weight ratio and superior corrosion resistance. However, it is very difficult to machine them due to their poor machinability. When machining titanium alloys with conventional tools, the tool wear rate progresses rapidly, and it is generally difficult to achieve a cutting speed of over 60m/min. Other types of tool materials, including ceramic, diamond, and cubic boron nitride (CBN), are highly reactive with titanium alloys at higher temperature. However, binder-less CBN (BCBN) tools, which do not have any binder, sintering agent or catalyst, have a remarkably longer tool life than conventional CBN inserts even at high cutting speeds. In order to get deeper understanding of high speed machining (HSM) of titanium alloys, the generation of mathematical models is essential. The models are also needed to predict the machining parameters for HSM. This paper aims to give an overview of recent developments in machining and HSM of titanium alloys, geometrical modeling of HSM, and cutting force models for HSM of titanium alloys.

Grow Large High-Quality Diamonds with Different Seed Surfaces

Large high-quality type Ib diamond crystals have been grown withdifferent seed surfaces by temperature gradient method at 5.5 GPa,1500–1600 K, with NiMnCo alloy as the metal solvent. Compared with{100} as the growth surface, the growth region of large high-qualitydiamond crystals with {111} as the growth surface at a higher growthrate shifts markedly from lower temperatures (suitable for{100}-facet growth) to higher temperatures (suitable for{111}-facet growth). However, regardless of different growthsurfaces, {100} or {111}, the grown crystals of sheet-shaped shapeare most difficult for metal inclusions to be trapped into, and whetheror not matched growth between the seed surfaces and the growthtemperatures determines the crystal shapes. In view of the growthrates, large high-quality diamond crystals of sheet-shaped shapes canbe grown at a growth rate of above 2.5 mg/h, while the growth rate oflarge high-quality diamond crystals should not be beyond 1.5 mg/h fortower-shaped crystals.

Structural Transition in Nanostructured Eu2O3 Under High Pressures

We report here studies on the effect of high pressure on the structural properties of nano-sized Europium sesquioxide (Eu2O3) up to a pressure of about 16.4 GPa. At ambient conditions, the starting sample was found to be predominantly cubic type Eu2O3 or in Eu3+ state with a trace of Eu2+. The presence of Eu2+ state is assumed to be arising due to the non-stoichiometric Eu(1-x)O phase which is obtained from XPS studies by the deconvolution of the Eu 3d-core levels. The Raman studies at ambient show a strong peak at about 333 cm(-1), which is known to occur due to the Fg mode of cubic Eu2O3 and in a similar way, the XRD data shows major peaks corresponding to the cubic phase of Eu2O3. A Mao-Bell type diamond anvil cell (DAC) was used to generate high pressures for XRD and Raman spectroscopy studies. It was observed that the material undergoes a structural change from cubic to monoclinic structure with an on set transition pressure at around 2 GPa and completes at around 8 GPa. This has been inferred from the fact that above about 2.0 GPa pressure, Raman studies show the emergence of a new peak corresponding to the monoclinic phase which increases in intensity and shifts further with increase in pressure, while the XRD studies show that above about 2.0 GPa, the peaks corresponding to monoclinic phase emerge, which show a slight increase in preferred orientation as the pressure is increased. A detailed discussion has been provided to explain this fact.