Polycrystalline Diamond Samples Research Articles

Formation of Ge-V color centers in poly- and monocrystalline CVD diamond: A comparative study

Germanium-Vacancy (Ge-V) color centers in diamond have narrow-band photoluminescence (PL) at room temperature (RT) and attract considerable attention due to their possible application in quantum information technologies, biomedicine, and local optical thermometry. In this work, we used microwave plasma chemical vapor deposition (MPCVD) to synthesize Ge-doped polycrystalline diamond (PCD) films and single-crystal diamond (SCD) epitaxial layers in hydrogen-methane-germane gas mixtures. Thick (>100 μm) layers of both types were grown to analyze their structure and luminescence characteristics. The first demonstration of the absorption of Ge-V centers in both PCD and SCD materials at low temperatures is presented. The sufficient narrowing of the Ge-V PL line for the SCD sample in comparison with the PCD sample was observed. On the other hand, the intensity of the Ge-V signal was an order of magnitude higher in the PCD sample compared to the SCD sample. A set of single Ge-V centers formed during CVD has been demonstrated. The results obtained can be used to manufacture and design various photonic devices based on Ge-V color centers.

Deuterium retention in CVD diamond: Combined experimental and computational study

Diamond’s intrinsic hardness, excellent thermal conductivity and low atomic number make it a highly promising candidate as a plasma facing material. However, as with the previously used graphite, concerns over tritium retention and resultant chemical etching have so far limited research interest in the use of synthetic diamond. In order to study tritium retention, the DELPHI facility at the Culham Centre for Fusion Energy was used to expose polycrystalline diamond samples to a deuterium plasma. Deuterium ions were accelerated to an energy of 0.2 keV to 1 keV for a 5 h exposure time, achieving a fluence of approximately 5.5 × 1021 D m−2. Exposed samples were analysed using Thermal Desorption Spectroscopy. Increasing implantation energy resulted in additional D2 release peaks observed in the 800–1100 K temperature range that were not seen at lower energies. These peaks were interpreted as an additional bonding mechanism, a likely candidate for which is inter-grain deuterium. Experimental work was complemented with molecular dynamics simulations on the University of Bristol’s high performance computer—Blue Crystal Phase 4. In these simulations, both varying implantation energy and the presence of grain boundaries were explored. A two-step etching mechanism was observed, in which the surface initially swelled before carbon removal. No significant differences could be observed on the inclusion of a grain boundary at the energies tested.

Formation mechanism and regulation of silicon vacancy centers in polycrystalline diamond films

Diamond silicon vacancy centers (SiV centers) have important application prospects in quantum information technology and biomarkers. In this work, the formation mechanism and regulation method of SiV center during the growth of polycrystalline diamond on silicon substrate are studied. By changing the ratio of nitrogen content to oxygen content in the growing atmosphere of diamond, the photoluminescence intensity of SiV center can be controlled effectively, and polycrystalline diamond samples with the ratios of SiV center photoluminescence peak to diamond intrinsic peak as high as 334.46 and as low as 1.48 are prepared. It is found that nitrogen promotes the formation of SiV center in the growth process, and the inhibition of oxygen. The surface morphology and photoluminescence spectrum for each of these samples show that the photoluminescence peak intensity of SiV center is positively correlated with the grain size of diamond, and the SiV center’s photoluminescence peak in the diamond film with obvious preferred orientation of crystal plane is higher. The distribution of Si centers and SiV centers on the surface of polycrystalline diamond are further characterized and analyzed by photoluminescence, Raman surface scanning and depth scanning spectroscopy. It is found that during the growth of polycrystalline diamond, the substrate silicon diffuses first into the diamond grain and then into the crystal structure to form the SiV center. This paper provides a theoretical basis for the development and application of SiV centers in diamond.

Femtosecond and nanosecond laser polishing of rough polycrystalline diamond

The laser polishing of rough (roughness of 5 μm) diamond plate using femtosecond and nanosecond pulses was realized for the first time. The effect of the angle of incidence, the number of passes, scanning speed, and laser fluence on the surface roughness was investigated and the optimal conditions were found. The initial roughness of the diamond plate was reduced by 5 times from 5 to 1 μm for both femtosecond and nanosecond laser sources. The results indicate the prospects of laser polishing of rough polycrystalline diamond samples.

Effect of GaN-on-diamond integration technology on its thermal properties

Two different integration technologies of GaN-on-diamond are explored, direct growth of polycrystalline diamond (PCD) on GaN (sample A) and deposition of GaN on PCD (sample B). The reversed evolution of PCD grain structure is assumed for each technology, resulting in an increasing/decreasing tendency of thermal conductivity with growth thickness. Simulation of AlGaN/GaN high electron mobility transistors on PCD substrate with capped diamond is made considering anisotropic and inhomogeneous thermal conductivities of GaN and PCD, different combinations of thermal boundary resistance (TBR) values at the top diamond/GaN and bottom GaN/PCD substrate are investigated. The results show that the effect of top TBR is limited and increasing the bottom TBR from 6.5 to 100 m2K GW−1 results in a temperature rise of 80.8 °C for sample A and 85.3 °C for sample B at power dissipation of 10 W mm−1. Enlarging the thickness of capped diamond helps to reduce the junction temperature, which is more effective at small top TBR under constant bottom TBR and at large bottom TBR with fixed top TBR. In addition, increasing gate pitch is a promising solution to reduce junction temperature and a weak dependence of junction temperature on gate width is revealed.

H-diamond MOS interface properties and FET characteristics with high-temperature ALD-grown HfO2 dielectric

The performance of hydrogen-terminated diamond MOSFETs with a large gate–drain interspace (LGD) was analyzed on a polycrystalline diamond sample. A 300 °C atomic layer deposition grown (ALD-grown) HfO2 dielectric (28 nm) was used the gate dielectric and device passivation layer. The HfO2/H-diamond interfacial valence band offset of 1.98 eV was determined by x-ray photoelectron spectroscopy, indicating that the high temperature grown HfO2 dielectric has great potential for accommodating a high density of holes on the H-diamond surface, and the high dielectric constant of HfO2 is also useful for improving the control capability of the gate on the device channel. A breakdown voltage of 657 V was achieved on the device. Baliga’s figure of merit of the device was calculated to be ∼2.0 MW/cm2, which is comparable to that of the H-diamond FETs with Al2O3 dielectrics that are more than 200 nm thick. The HfO2 dielectric shows great potential for use in H-diamond power devices.

Impact of surface fluorination on phase stability of nanodiamond particles and inter-granular bonding in polycrystalline diamond under HPHT conditions

Polycrystalline diamond (PCD) compacts, made by conventional high pressure-high temperature (HPHT) technology, contain Co catalyst in inter-granular spaces causing degradation of PCD mechanical strength and thermal stability during rock cutting due to thermal expansion and graphitization. To solve this problem a catalyst-free synthesis of PCD based on HPHT sintering of fluorinated nanodiamond (FND) and submicron size aluminum mixtures was explored in the present work. The feasibility of using FND has been established by evaluation of phase stability of FNDs of 5, 10, and 90 nm sizes under industrially favorable P-T conditions (7.5–8 GPa, 1500–1700 °C) for PCD synthesis. It was found that surface fluorine slows down the graphitization of nanodiamond particles. Enlargement of diamonds from 5 and 10 nm up to a 10 μm has been observed for FND/Al systems. This phenomenon can be associated with the Wurtz-type reaction under the HPHT conditions leading to formation of inter-granular carbon‑carbon bonds between the nanoparticles and formation of the fluid Al-C-F phases in the samples volume, accelerating the mass transport and re-crystallization of nanodiamond. PCD samples sintered from mixtures of fluorinated micro-diamonds, nano-diamonds FND-90 and 3 wt% aluminum exhibited enhanced up to 1200 °C thermal stability and as high wear resistance as leached commercial PCD.

Evaluation of the counting efficiency of a pcCVD diamond detector irradiated by 62 MeV/nucl. carbon beams

It is well known that synthetic diamond exhibits unique properties which make it an attractive material for a broad range of applications in the particle-detection field. In particular, thanks to its radiation hardness and availability in larger sizes, the polycrystalline (pc) diamond sample, grown by chemical vapour deposition (CVD), is especially suitable for online beam-monitoring applications, provided it demonstrates excellent counting performances. The present work reports the counting efficiency of a large 20.0× 20.0× 0.3 mm3 pcCVD diamond detector irradiated by 62 MeV/nucl. carbon beams. The parameter under study was estimated at different particle rates by employing a combination of different detector counters in a sandwich configuration. Results show a counting efficiency of ∼ 95% up to 0.7 MHz particle rate. The pcCVD diamond detector under study was selected among other prepared devices by studying their leakage current and photocurrent characteristics under X-ray irradiation.

Experiment Analysis Research on Electrical Discharge Machining Polycrystalline Diamond Tool

As the hardest material in the field of synthetic materials, polycrystalline diamond has been paid more and more attention in the field of tools, which embody the basic characteristics of "high efficiency, precision and flexibility" of modern advanced cutting technology and cleaner production. Electrical discharge machining is typically used in processing polycrystalline diamond tools, especially in the machining of special shape, thin and fine cutting tools. In this paper, 2 micrometer and 10 micrometer polycrystalline diamonds are taken as the research object, and the surface quality of the disc electrode and wire electrode is simply compared by means of process and material test. The effects of diamond particle size, electrode polarity and electrode speed on the surface quality and the material removal rate of precision electrical discharge machining polycrystalline diamond are studied. The results show that there is no porous structure and no selectivity in discharge etching when the negative polarity electrode machine is used. With the increase of electrode rotation speed, the removal amount of polycrystalline diamond increases gradually, while the surface roughness of polycrystalline diamond decreases first and then increases. When the electrode linear velocity reaches 80 m/min, the surface roughness of polycrystalline diamond sample reaches the minimum value, and the removal amount of polycrystalline diamond material tends to be stable.

Experiments Research on Electrical Discharge Grinding Polycrystalline Diamond Tool based on Surface Quality Analysis

As the hardest material in the field of synthetic materials, polycrystalline diamond has attracted more and more attention in the field of tools, which reflects the basic characteristics of ‘high efficiency, precision and flexibility’ of modern advanced cutting technology and clean production. Electrical discharge grinding (EDG) is often used in polycrystalline diamond tools, especially in special shape, thin and fine edge cutting tools. Taking two kinds of grain size of polycrystalline diamond as the research object, taking the machining test and material test as the analysis means, this paper simply compares the surface quality of disc electrode and wire electrode machining, and studies the influence of diamond particle size, electrode polarity and electrode rotation speed on the surface quality and material removal rate of precision electrical discharge grinding polycrystalline diamond. The results show that there is no porous structure, good surface quality and no selectivity in discharge removal when using the negative polarity machining. With the increase of electrode rotation speed, the removal amount of polycrystalline diamond material increases gradually, and the surface roughness of polycrystalline diamond material decreases first, then increases. When the electrode rotation speed reaches 80 m/min, the surface roughness of the polycrystalline diamond sample reaches the minimum value, and the removal amount of polycrystalline diamond material also tends to be stable.

Impact Analysis of Electrode Material on Electrical Discharge Grinding Polycrystalline Diamond Cutting Tools

The electrode material plays a very important role in the electrical discharge grinding (EDG), affects the electrical discharge machining efficiency and machining surface quality. Experiment testing and materials analysis as the primary method, affected layer thickness and workpiece surface roughness value as evaluation indexes. Polycrystalline diamond cutting tool samples were electrical discharge machined using copper electrodes and graphite electrodes respectively. After machining, the polycrystalline diamond cutting tool samples’ surfaces was analyzed by X-ray diffraction, Surfaces appearances were observed under the scanning electron microscope, and the surface roughness values were measured by the roughness measuring instrument. Polycrystalline diamond samples’ surface effect layer thickness values and surface roughness values, affected layer distribution structure was statically analyzed in typical EDG process conditions with different electrode materials. Test results demonstrate that the affected layer is inevitably presented on a polycrystalline diamond surface after electrical discharge grinding polycrystalline diamond. The principal cause of the affected layer is the diamond graphitization and the decomposition of binders. In the same EDG process parameters, the processing efficiency of the graphite electrode is clearly higher than that of the copper electrode; but the tool edge roughness value and affect layer thickness of the graphite electrode machining is greater than those of the copper electrode machining. This research provides an essential experimental basis for designing technics of precision and highly efficient electrical discharge grinding polycrystalline diamond cutting tools.

Single-crystal and polycrystalline diamond erosion studies in Pilot-PSI

Diamond is a promising candidate for enhancing the negative-ion surface production in the ion sources for neutral injection in fusion reactors; hence evaluation of its reactivity towards hydrogen plasma is of high importance. Single crystal and polycrystalline diamond samples were exposed in Pilot-PSI with the D+ flux of (4‒7)·1024 m−2s−1 and the impact energy of 7–9 eV per deuteron at different surface temperatures; under such conditions physical sputtering is negligible, however chemical sputtering is important. Net chemical sputtering yield Y = 9.7·10−3 at/ion at 800 °C was precisely measured ex-situ using a protective platinum mask (5 × 10 × 2 μm) deposited beforehand on a single crystal followed by the post-mortem analysis using Transmission Electron Microscopy (TEM). The structural properties of the exposed diamond surface were analyzed by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Gross chemical sputtering yields were determined in-situ by means of optical emission spectroscopy of the molecular CH A-X band for several surface temperatures. A bell-shaped dependence of the erosion yield versus temperature between 400 °C and 1200 °C was observed, with a maximum yield of ∼1.5·10−2 at/ion attained at 900 °C. The yields obtained for diamond are relatively high (0.5–1.5)·10−2 at/ion, comparable with those of graphite. XPS analysis shows amorphization of diamond surface within 1 nm depth, in a good agreement with molecular dynamics (MD) simulation. MD was also applied to study the hydrogen impact energy threshold for erosion of [100] diamond surface at different temperatures.

Nitrogen Desorption and Positron Sensitive Defect of CVD Diamond

The chemical vapor deposition (CVD) process can produce single or poly-crystalline diamond samples of high purity or with controlled doping concentrations. The defect type in the CVD diamonds can be changed by heating the samples. Controlling the defect type can be used to create devices for quantum diamond switches that could be used in radiation sensors and quantum information technology. Eight samples of CVD diamonds were analyzed with Doppler broadening of positron annihilation radiation (DBAR) before and after annealing in high vacuum with an electron gun. Between temperatures of 1700 - 1850 K, nitrogen was liberated from the diamond sample. At these high temperatures, the surface was graphitized and a change in the color and transparency of the diamond was observed. Some of the samples were analyzed with DBAR during periods with and without light. The defect properties were observed to change depending on the time exposure to the positron beam and were then regenerated by exposure to light. The DBAR data is compared to photoluminescence data and a time varying defect state is discussed for detector and optical grade type II CVD diamonds.

Diamond graphitization by laser-writing for all-carbon detector applications

The surface of a detector grade CVD polycrystalline diamond sample (5×5×0.05mm3) was irradiated by an ArF excimer laser (λ=193nm, τ=20ns) to produce graphitic conductive layers.In particular, two sets of four parallel graphitic strip-like contacts, with 1mm pitch, were created along the whole sample on the top and on the rear surfaces of the sample respectively. The two series of stripes lie normally to each other. Such a grid allows to obtain a segmented all-carbon device capable of giving bi-dimensional information on particle detection processes in nuclear applications.Afterwards, an extensive characterization of the samples was performed: SEM and micro-Raman investigations to study the morphological and structural evolution of the irradiated areas, EDS measurements to individuate any absorption phenomena from environment associated to laser treatment, and nanoindentation mapping to understand how the hard-soft transformation occurred depending on the locally transferred energy. Finally, current-voltage analyses were carried out checking the ohmic behavior of the diamond-graphite contact. By comparing the results of the different characterization analyses, a strong periodicity of the modified surface properties was found, confirming the reliability and reproducibility of the laser-induced graphitization process.The results demonstrate that the laser-writing technique is a good and fast solution to produce graphitic contacts on diamond surface and therefore represents a promising way to fabricate segmented all-carbon devices.

Surface smoothing of a polycrystalline diamond using an iron plate-H2O2 chemical reaction

In this study, the researchers demonstrated an ultraprecise polishing method that achieved an improved polycrystalline diamond (PCD) surface. This novel polishing technique uses hydroxyl radicals generated on an iron plate in a hydrogen peroxide (H2O2) solution. To demonstrate the feasibility of obtaining an ultrasmooth PCD surface, we conducted a polishing experiment using an iron plate in an H2O2 solution; the rotating speed of the polishing table/sample holder and the contact load were controlled. After polishing, we cleaned the PCD samples using two different methods and evaluated the surface morphology and chemical components using scanning white light interference microscopy, atomic force microscopy, and an electron-probe micro analyzer. The results showed that atomic-level smoothing of the topmost grain diamond (which constitutes the PCD substrate) could be realized using our proposed method. Moreover, cleaning with a mixed acid (H2SO4/H2O2) solution was more effective in removing residual contaminates than ultrasonic cleaning in acetone.

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm2 upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm2, sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.

Electrical discharge grinding of polycrystalline diamond – Effect of wheel rotation

ABSTRACTElectrical Discharge Grinding (EDG) is an advanced machining process that becomes popular in manufacturing of Polycrystalline Diamond (PCD) tools. This research investigated the effects of wheel rotation as well as debris flow direction on the quality of PCD tools based on a series of EDG experiments. Experimental results showed the debris that flowed toward the cutting edge could significantly affect the edge sharpness and symmetry of the tool, which were critical for the smaller edge apex angle. Evidence of spark concentration caused by the debris accumulation phenomenon were found through microscopic analysis on the eroded surfaces. This research also revealed the unexplained phenomenon associated with the undercut that normally formed beneath the PCD cutting edge after erosion. By examining PCD samples using scanning electron microscopy (SEM) and Raman spectroscopy, the formation of the heat-affected layer caused by the high-temperature erosion process in the EDG was analyzed. Results also proved that the surface finish of tungsten carbide (WC) and notch width of the PCD tools, particularly on the tungsten carbide WC/PCD interface, should not be taken as the index to measure PCD tool quality.

An effort in planarising microwave plasma CVD grown polycrystalline diamond (PCD) coated 4 in. Si wafers

Large area CVD grown diamond coatings should have very smooth surface in many of its applications, like microwave power transmission windows etc. Combination of mechanical and chemical forces during polishing helps to achieve desired surface roughness of the polycrystalline diamond (PCD) coatings. Authors report the variation of pressure, slurry feeding rate, addition of chemicals and the time of polishing to observe the efficiency of chemo-mechanical polishing (CMP) technique in planarising CVD diamond material grown over 100mm diameter silicon wafers. PCD were found to be polished by bringing down the as-grown surface roughness from 1.62µm to 46nm at some points on large areas. One coherence scanning interferometer was used to check the roughness at different stages of CMP. Raman spectroscopy was used to evaluate the polished PCD samples in terms of their quality, internal stress at different positions on the same wafer surface after CMP process. It was found that the well polished regions were of better quality than the less polished regions on the same wafer surface. But due to coating non-uniformity of the deposited PCD grown by microwave plasma CVD over large area, CMP could not produce uniform surface roughness over the entire 100mm diameter wafer surface. We concluded that CMP could effectively but differentially polish large area PCD surfaces, and further process improvements were needed.

Direct observation of electron emission from grain boundaries in CVD diamond by PeakForce-controlled tunnelling atomic force microscopy

A detailed investigation of electron emission from a set of chemical vapour deposited (CVD) diamond films is reported using high-resolution PeakForce-controlled tunnelling atomic force microscopy (PF-TUNA). Electron field emission originates preferentially from the grain boundaries in low-conductivity polycrystalline diamond samples, and not from the top of features or sharp edges. Samples with smaller grains and more grain boundaries, such as nanocrystalline diamond, produce a higher emission current over a more uniform area than diamond samples with larger grain size. Light doping with N, B or P increases the grain conductivity, with the result that the emitting grain-boundary sites become broader as the emission begins to creep up the grain sidewalls. For heavy B doping, where the grains are now more conducting than the grain boundaries, emission comes from both the grain boundaries and the grains almost equally. Lightly P-doped diamond samples show emission from step-edges on the (111) surfaces. Emission intensity was time dependent, with the measured current dropping to ∼10% of its initial value ∼30h after removal from the CVD chamber. This decrease is ascribed to the build-up of adsorbates on the surface along with an increase in the surface conductivity due to surface transfer doping.

Dielectric properties of single crystalline diamond wafers with large area at microwave wavelengths

In this paper, we present first measurements, carried out at microwave wavelengths, aiming to characterize the dielectric properties of large size single crystalline diamond (SCD) wafers. While the sizes of the SCD wafers are still not sufficient for practical use, we obtained good optical property results. The sample with both sides polished shows a dielectric loss tangent tanδ as low as or possibly lower than a polycrystalline diamond sample with high quality. Results show the importance of surface treatment, especially on the boundaries of the composed SCD chips even without any graphitic component in the diamond wafers.