Natural Single Crystal Diamond Research Articles

Thermal conductivity of pink CVD diamond: Influence of nitrogen-related centers

Thermal conductivity κ(T) of single-crystal CVD diamond lightly doped (about 3 ppm) with nitrogen has been measured at temperatures from 5.7 to 410 K. The sample was carefully characterized by optical absorption and photoluminescence spectroscopy for the presence of impurities. Nine different optically active defects related with nitrogen, hydrogen, and silicon impurities have been identified and quantified. This pink-tint crystal showed a high thermal conductivity of 24.0±0.5 W cm−1 K−1 at room temperature, which is very close to the highest value ever measured at about 25 W cm−1 K−1 for diamonds of natural isotopic composition. At the same time, the κ(T) of the crystal showed strong suppression >10% at temperatures 6<T<120 K with a maximum decrease of 2.7 times at ≈40 K compared to high purity diamonds. This behavior of the conductivity is attributed to a phonon scattering by charge carriers bound to nitrogen-related impurity centers, which is ineffective, however, at room and higher temperatures. The κ(T) has been calculated within the model based on the Callaway theory taking into account the elastic phonon scattering off charge carriers (holes and electrons) in the ground states of doping centers, and a very good agreement between the measured and theoretical data has been achieved. The model also gives a good approximation to the experimental data for κ(T) given in the literature for synthetic and natural single-crystal diamonds.

A Review of Binderless Polycrystalline Diamonds: Focus on the High-Pressure-High-Temperature Sintering Process.

Nowadays, synthetic diamonds are easy to fabricate industrially, and a wide range of methods were developed during the last century. Among them, the high-pressure–high-temperature (HP–HT) process is the most used to prepare diamond compacts for cutting or drilling applications. However, these diamond compacts contain binder, limiting their mechanical and optical properties and their substantial uses. Binderless diamond compacts were synthesized more recently, and important developments were made to optimize the P–T conditions of sintering. Resulting sintered compacts had mechanical and optical properties at least equivalent to that of natural single crystal and higher than that of binder-containing sintered compacts, offering a huge potential market. However, pressure–temperature (P–T) conditions to sinter such bodies remain too high for an industrial transfer, making this the next challenge to be accomplished. This review gives an overview of natural diamond formation and the main experimental techniques that are used to synthesize and/or sinter diamond powders and compact objects. The focus of this review is the HP–HT process, especially for the synthesis and sintering of binderless diamonds. P–T conditions of the formation and exceptional properties of such objects are discussed and compared with classic binder-diamonds objects and with natural single-crystal diamonds. Finally, the question of an industrial transfer is asked and outlooks related to this are proposed.

Fourier-transform infrared spectroscopy for analysis of diamond materials of different origin

The study of impurity composition of diamond materials is an important element of their classification for use in various fields of science and technology. Many physical (thermal and electrical conductivity, optical purity), mechanical (hardness and strength) and aesthetic (color grade and clarity) properties of diamond depend on the presence of impurities in their structure; therefore, it is important to quickly and reliably determine each impurity in diamond. In this work, the method of FTIR spectroscopy is proposed as an express method for determining the impurity composition of diamonds. We examined three types of diamond materials: single crystal natural diamonds (before and after heat treatment), single crystal synthetic HPHT-diamonds and synthetic polycrystalline diamond CVD-films. We have established that the used technique allows to determine impurities in single crystals of natural and synthetic origin reliably and effectively, while the study of polycrystalline thin films is best performed using spectrophotometry.

The use of spectroscopy methods for structural analysis of CVD diamond films, polycrystalline and single-crystal diamonds

For the work results correct interpretation, it is important to study initial materials that scientists have to deal with. Currently, there are a large number of different diamond substrates. Comparison of materials among themselves allows you to determine which material you are dealing with. In this work, the methods of infrared (IR) spectrometry, Raman spectroscopy and spectrophotometry are used to study four types of diamond materials: diamond polycrystalline CVD-films; natural single-crystal diamonds; synthetic polycrystalline HPHT-diamonds (such as DSPC – diamond synthetic polycrystal by GOST 9206-80); polycrystalline CVD-diamonds CDM manufactured by E6. In work it was shown that the Raman spectroscopy allows to measure the effect of heat treatment on changes in the diamond structure, even if it is such highly advanced diamond materials as natural diamonds. Heat treatment affects the perfection of diamond crystal structure by reducing stresses and the number of defects in it due to graphitization process. The IR spectrometry method is effective for determining the shape and amount of nitrogen inclusions in diamond structure. To study polycrystalline CVD-films, the spectrophotometry method turned out to be the most effective, because it made possible to determine a small number of nitrogen defects and draw conclusions about the quality of the films. The investigation of polycrystalline diamonds CDM and DSPC demonstrated that, despite their coarse-crystalline structure, diamond crystallites consist of a highly defective diamond phase; in addition, DSPC-diamonds were studied using this method in the first time.

Ultrafast femtosecond laser micro-marking of single-crystal natural diamond by two-lens focusing system

The inscription of unique logo and security marking on diamonds and gemstones is in high demand by worldwide manufacturers and businesses for anti-counterfeiting purposes and traceability. Short pulsed lasers enable marking of transparent materials, challenge remains to produce digital security micro-features on thin facets of the natural diamond in a non-intrusive manner. We propose the design and demonstration of a novel two-lens focusing system to inscribe diamond at microscale and high throughput by an ultrafast laser scanning process. A threshold laser fluence of ∼2.5 J/cm2 and a scan speed of 2 mm/s realized writing of high-contrast data matrix code and serial number without inducing defects and cracking in the diamond. Characterization revealed smooth ablation depth profile with distinct laser-induced periodic surface structures (LIPSS) in the laser inscribed regions determined through scanning electron microscopy (SEM) and 3D optical microscopy. Raman spectroscopy revealed diamond cubic structure dominating with mixing of graphitic structure in the laser markings at various scanning speeds. Comparing with the 0.05 NA f-theta lens, the two-lens focusing system offered 7x improvement in the marking resolution (3 μm in line-width) in addition to its simplicity and add-on flexibility to industrial laser marking systems.

Characterization of electronic properties of natural type IIb diamonds

Precision admittance spectroscopy measurements were carried out over wide temperature and frequency ranges for a set of natural single crystal type IIb diamond samples. Peaks of conductance spectra vs. temperature and frequency were used to compute the Arrhenius plots, and activation energies were derived from these plots. The capacitance-voltage profiling was used to estimate the majority charge carrier concentration and its distribution into depth of the samples. Apparent activation energies between 315 and 325meV and the capture cross section of about 10−13cm2 were found for samples with uncompensated boron concentrations in the range of 1 to 5×1016cm−3 (0.06–0.3ppm). The obtained boron concentrations are in good coincidence with FTIR results for the samples. Also, a reason for the difference between the observed admittance activation energy and the previously reported ionization energy for the acceptor boron in diamond (0.37eV) is proposed.

Investigation of focused ion beam induced damage in single crystal diamond tools

In this work, transmission electron microscope (TEM) measurements and molecular dynamics (MD) simulations were carried out to characterise the focused ion beam (FIB) induced damage layer in a single crystal diamond tool under different FIB processing voltages. The results obtained from the experiments and the simulations are in good agreement. The results indicate that during FIB processing cutting tools made of natural single crystal diamond, the energetic Ga+ collision will create an impulse-dependent damage layer at the irradiated surface. For the tested beam voltages in a typical FIB system (from 8kV to 30kV), the thicknesses of the damaged layers formed on a diamond tool surface increased from 11.5nm to 27.6nm. The dynamic damage process of FIB irradiation and ion–solid interactions physics leading to processing defects in FIB milling were emulated by MD simulations. The research findings from this study provide the in-depth understanding of the wear of nanoscale multi-tip diamond tools considering the FIB irradiation induced doping and defects during the tool fabrication process.

Ion beam sharpening of a single-crystal diamond knife without facet and ripple formations by swinging motion of knife

Natural single-crystal diamond has unique properties including an ultrahigh strength, ultrahigh hardness, and very low friction coefficient, making it an ideal source material for knives used to make ultrathin and smooth sections for electron microscopy. Facets at the tip and ripple formations on the cutting face are common problems in sharpening diamond knives with small apex angles. In this paper, we propose a method that involves swinging the sample during ion beam sharpening. Diamond knives with an apex angle of 60° (5-μm tip radius) were used as specimens and were machined with 1-keV Ar+ or 0.5-keV O+/O2+ ions using an electron-cyclotron-resonance-type ion beam machining apparatus. We show that the diamond knives can be sharpened in a given swinging angle without the formation of facets or ripples. Swinging the knife through an angle of ±30° during processing resulted in a very sharp diamond knife with a tip radius of 50nm and an apex angle of 57°. The machining time with the O+/O2+ beam was about four times faster than that with the Ar+ beam. We also calculated the change in the profile of the diamond knife during the processing. The calculated results coincided with the experimental results, which enables prediction of the knife shape and calculation of the machining time.

Effects of disorder state and interfacial layer on thermal transport in copper/diamond system

The characterization of Cu/diamond interface thermal conductance (hc) along with an improved understanding of factors affecting it are becoming increasingly important, as Cu-diamond composites are being considered for electronic packaging applications. In this study, ∼90 nm thick Cu layers were deposited on synthetic and natural single crystal diamond substrates. In several specimens, a Ti-interface layer of thickness ≤3.5 nm was sputtered between the diamond substrate and the Cu top layer. The hc across Cu/diamond interfaces for specimens with and without a Ti-interface layer was determined using time-domain thermoreflectance. The hc is ∼2× higher for similar interfacial layers on synthetic versus natural diamond substrate. The nitrogen concentration of synthetic diamond substrate is four orders of magnitude lower than natural diamond. The difference in nitrogen concentration can lead to variations in disorder state, with a higher nitrogen content resulting in a higher level of disorder. This difference in disorder state potentially can explain the variations in hc. Furthermore, hc was observed to increase with an increase of Ti-interface layer thickness. This was attributed to an increased adhesion of Cu top layer with increasing Ti-interface layer thickness, as observed qualitatively in the current study.

Experimental Study on the Surface Integrity and Chip Formation in the Micro Cutting Process

The micro cutting mechanism is very different from the macro cutting process, where the workpiece is not considered as isotropic solid. Hence the effect of microstructure on the micro cutting mechanism needs to be further studied. In this paper, pure copper with two different grain sizes was machined by the natural single crystal diamond cutting tool, in order to study the effect of microstructure on the machined surface integrity and chip formation, during the micro cutting process. According to the experimental results, the hardness of original pure copper was higher than the annealed pure copper due to the grain boundary strengthening. The grain boundary can be found in the machined surface of annealed pure copper. As for original pure copper, the grain boundary is absent and the surface had only traces of tool path. In the grain boundary, the height of peak-valley is larger than that inside grain. The following indicates that the reduction of grain size will reduce the effect of microstructure on the surface integrity in the micro cutting process. With the decreasing of feed rate, cutting chip's shape changed from continuous to segmented at the same depth of cut (DOC) in both original and annealed pure copper. The chip of annealed pure copper is more segmented than the original copper at some cutting conditions.

Elastic properties of transparent nano-polycrystalline diamond measured by GHz-ultrasonic interferometry and resonant sphere methods

The sound velocities and elastic moduli of transparent nano-polycrystalline diamond (NPD) have been determined by GHz-ultrasonic interferometry on three different bulk samples, and by resonant spectroscopy on a spherically fabricated NPD sample. We employ a newly-developed optical contact micrometer to measure the thickness of ultrasonic samples to ±0.05μm with a spatial resolution of ∼50μm in the same position of the GHz-ultrasonic measurements, resulting in acoustic-wave sound velocity measurements with uncertainties of 0.005–0.02%. The isotropic and adiabatic bulk and shear moduli of NPD measured by GHz-ultrasonic interferometry are KS0=442.5 (±0.5)GPa and G0=532.4 (±0.5)GPa. By rotating the shear-wave polarization direction, we observe no transverse anisotropy in this NPD. Using resonant sphere spectroscopy, we obtain KS0=440.3 (±0.5)GPa and G0=532.7 (±0.4)GPa. For comparison, we also measured by GHz-ultrasonic interferometry the elastic constants of a natural single-crystal type-IA diamond with about one-half the experimental uncertainty of previous measurements. The resulting Voigt–Reuss–Hill averaged bulk and shear moduli of natural diamond are KS0=441.8 (±0.8)GPa and G0=532.6 (±0.5)GPa, demonstrating that the bulk-elastic properties of transparent NPD are equivalent to natural single-crystal diamond as calculated from polycrystalline averaging of its elastic constants.

Muonium in synthetic diamond

Behavior of the muonium in two polycrystalline CVD diamond samples and in the sample composed of few single-crystal synthetic diamonds was studied by transverse-field μSR measurement. The hyperfine interaction constants for muonium in the synthetic samples are in agreement with those in natural diamond. The relaxation rate of muon spin and the amount of the MuT fraction in the synthetic samples are close to those observed in IIa- and IIb-type single-crystal natural diamonds.

Fabrication and characterizations of large homoepitaxial single crystal diamond grown by DC arc plasma jet CVD

Homoepitaxial diamond layers have been grown on HPHT synthetic type Ib (100) single crystal diamond plates brazed on a Φ65mm Mo holder using a commercial type 30kW dc arc plasma jet CVD with rotating arc root and operating at gas recycling mode with gas mixture of Ar/H2/CH4. The effects of substrate temperature and CH4/H2 ratio on the surface morphology, the growth rate and the quality of the synthesized diamond have been studied using optical microscopy and Raman spectroscopy. The growth rate up to 17.4μm/h has been obtained in the single crystal diamond sample deposited at 1000°C with CH4/H2=0.813%, exhibiting relatively smooth surface morphology with the typical feature of step-flow growth, without any non-epitaxial diamond crystallites. Under the optimum synthesis conditions, large size (7.5mm×7.5mm) polished freestanding single crystal diamond plate up to 1.03mm thickness has been produced and characterized by UV–vis–IR absorption, photoluminescence and Raman spectroscopy, as well as high-resolution X-ray diffraction. It was demonstrated that the quality of the as-grown CVD single crystal diamond was close to that of the natural type IIa single crystal diamond. It was shown that the epitaxial growth of high quality CVD single crystal diamonds could be achieved over the entire Φ65mm Mo holder on which a large quantity of CVD single crystal diamonds could be grown simultaneously. This is of economical importance in the development of high power DC arc plasma jet CVD systems for fabricating large size, high quality CVD single crystal diamond.

Structure and hardness of octahedral natural diamond single crystals depending on the HPHT treatment conditions

The structure evolution of octahedral natural diamond single crystals has been studied depending on the HPHT treatment using Raman scattering and infrared spectroscopy. It has been found that the formation of a polycrystalline diamond capsule around a single crystal at p = 8 GPa and T = 1500°C gives rise to a combined structural-stressed state in the single crystal due to its plastic strain. This state has been manifested by a significantly (more than double) broadening of the characteristic line of diamond (1332 cm−1) in the Raman spectrum and the increase of a single crystal hardness from 105 to 120 GPa.

Ion beam forming of a shell type natural single crystal diamond stylus and sharpening of it by swinging a conical type diamond stylus

Natural single crystal diamond is used as a source material for diamond stylus employed in instruments for surface roughness measurement, and also used as diamond probe for AFMs due to ultra-high strength, ultra-high hardness, very-low friction coefficient, and so on. During the sharpening of diamond tools, facet formations at the tip and ripple formations at the cutting face (or at the tip's face) become problematic. We have successfully employed ion beam processing for the sharpening of diamond tools with small apex angles without any formation of facet or ripples. However, the sharpening conditions are limited to small areas only. Therefore, we introduced a new technique where a sample is swung during ion beam sharpening process to expand the processed area. We applied the technique for sharpening of a conical type diamond stylus and forming of a shell type diamond stylus by 1keV Ar+ ion beam at the swing angles of ±60° and ±90° respectively. Result shows that the facet or ripples were not formed on the processed surface. A simulation method is also developed for predicting the profile changes of diamond tools during ion beam machining at different swing angles.

Bulk femtosecond laser marking of natural diamonds

Point-by-point multi-shot femtosecond laser writing of micro-scale linear damage tracks and symbolic logo information was performed inside single-crystal natural diamonds in transversal writing geometry at variable basic operational parameters (numerical aperture of focusing optics, depth of focusing inside samples and peak laser pulse powers). The filamentary character of femtosecond laser writing of buried damage tracks in this material at the supercritical peak laser powers was revealed.

Surface morphology analysis of natural single crystal diamond chips bombarded with Ar + ion beam

We have studied the surface morphology of natural single crystal diamond chips machined by 0.5–3.0 keV Ar + ion beam irradiation at ion incidence angles of 0°, 30°, 45°, 60°, and 80° with ion doses from 3.4 × 10 18 ions/cm 2 to 6.8 × 10 18 ions/cm 2. The surface of diamond chips machined with 0.5 and 1.0 keV Ar + ion beam, at angles of ion incidence from 0° to 45° can be made smooth. Results show that the machined surface at ion dose of 6.8 × 10 18 ions/cm 2 and beam energy of 0.5 and 1.0 keV become ultra-smooth (surface roughness SR = 0.1 nm rms) compared with unprocessed surface (SR = 0.15–2.1 nm rms). Results also confirm the ripple formation on diamond surface at ion incidence angles of 60°–80° by 0.5–3.0 keV Ar + ion beam. Therefore, the technique of smoothing by choosing ion beam irradiation parameter can be applicable to nano-finishing of diamond tools without ripple formation. This technique can also be applicable in mass production if the diamond surface is mechanically pre-finished.

Thermal Conductivity of Diamond Composites

A major problem challenging specialists in present-day materials sciences is the development of compact, cheap to fabricate heat sinks for electronic devices, primarily for computer processors, semiconductor lasers, high-power microchips, and electronics components. The materials currently used for heat sinks of such devices are aluminum and copper, with thermal conductivities of about 250 W/(m·K) and 400 W/(m·K), respectively. Significantly, the thermal expansion coefficient of metals differs markedly from those of the materials employed in semiconductor electronics (mostly silicon); one should add here the low electrical resistivity metals possess. By contrast, natural single-crystal diamond is known to feature the highest thermal conductivity of all the bulk materials studied thus far, as high as 2,200 W/(m·K). Needless to say, it cannot be applied in heat removal technology because of high cost. Recently, SiC- and AlN-based ceramics have started enjoying wide use as heat sink materials; the thermal conductivity of such composites, however, is inferior to that of metals by nearly a factor two. This prompts a challenging scientific problem to develop diamond-based composites with thermal characteristics superior to those of aluminum and copper, adjustable thermal expansion coefficient, low electrical conductivity and a moderate cost, below that of the natural single-crystal diamond. The present review addresses this problem and appraises the results reached by now in studying the possibility of developing composites in diamond-containing systems with a view of obtaining materials with a high thermal conductivity.

Planarization of natural single crystal diamond polishing lines by microwave hydrogen plasma

AbstractThe effect of microwave hydrogenated (MW‐H) plasma exposure on the morphology of mechanically polished natural single crystal (100) oriented diamond type 2a surfaces is reported. It is shown that the surface morphology is very sensitive to plasma power and exposure time. Under appropriate plasma exposure conditions the diamond surfaces smooth out as reflected in the decrease in the number and depth of the polishing lines. A systematic study of the influence of hydrogen MW plasma power and exposure time on the diamond surface morphology is presented. The morphology of the diamond surfaces at the different stages was monitored with sub‐nano‐metric resolution by atomic force microscopy and scanning electron spectroscopy.

Machining characteristics of complex prism pattern on electroplated roll by copper

The BLU (back light unit) is the core component of the LCD for notebook, mobile-phone, navigation, as well as large sized TV, PID (public information display), etc. In order to enhance optical efficiency of LCD, optical films with the uniform prism patterns have been used for BLU by stacking two films up orthogonally. In this case, light interference-phenomenon occurred such as Morie, wet-out, u-turning, etc. It caused several problems such as low brightness, spots and stripes in LCD. Recently, the high-luminance micro complex prism patterns are actively studied to avoid the light interference-phenomenon and enhance the optical efficiency. In this study, the roll master to manufacture complex micro prism pattern film was machined by using the high precision lathe. The machined patterns on the roll master were 50, 45, 40, 35, 30, 25, 20, 15, 10 and 5 μm in the pitch with 25.0, 22.5, 20.0, 17.5, 15.0, 12.5, 10.0, 7.5, 5.0 and 2.5 μm in the peak height, respectively. The roll was 2 000 mm in length and 320 mm in diameter. The electroplated roll by copper and the natural single crystal diamond tool was used for machining the patterns. The cutting force was measured and analyzed for each cutting condition by using the dynamometer. The chips and the surfaces after being machined were analyzed by SEM and microscope.