Improvements of crystallinity of single crystal diamond plates produced by lift-off process using ion implantation

A single crystal diamond substrate cut from a 9 mm thick ingot which was grown by chemical vapor deposition (CVD) was used to produce freestanding single crystal CVD diamond plates with improved crystallinity by the lift-off process using ion implantation. To reduce dislocations on the substrate surface, the ingot was sliced along the {100} plane parallel to the growth direction. In addition, the repeated lift-off processes reduced the surface damage on the substrate. These treatments were shown to improve the crystallinity of the CVD diamond plates produced by polarized light microscopy (PLM) and high-resolution X-ray.     

Epitaxial lateral overgrowth (ELO) of homoepitaxial diamond through an iridium mesh

In the present work iridium layers forming a mesh on diamond have been studied as potential candidates for buried electrodes or stopping layers in an ELO process for heteroepitaxial diamond. Thin iridium layers (∼ 15 nm) were deposited by e-beam evaporation at ∼ 700 °C on the facets of individual (001)-oriented CVD diamond crystallites and macroscopic Ib HPHT substrates with off-axis angles of several degrees. The heteroepitaxial iridium films formed a mesh with 10–200 nm large holes. These were penetrated by homoepitaxial diamond in a microwave plasma chemical vapour deposition process (MWPCVD) burying the iridium layer completely after 15 min of diamond growth. High resolution X-ray diffraction including reciprocal space mapping and Raman spectroscopy was used to characterize the structural properties of the diamond overlayer on the Ib HPHT substrate. It was monocrystalline with an FWHM of 0.03–0.05° of the X-ray rocking curve. Its lattice planes were tilted by ∼ 0.01° with respect to the substrate and showed a macroscopic strain of − 10^− 4 perpendicular to the surface. Besides the smaller lattice constant due to the lack of nitrogen the strain is mostly attributed to a tensile in-plane stress state. Strain and tilt can be attributed to the lateral overgrowth and the off-axis angle of the substrate.     

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH₄/H₂ ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm² mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.     

Heteroepitaxial nucleation and growth of graphene nanowalls on silicon

Heteroepitaxial nucleation of {0 0 2} graphene sheets on {1 1 1} facets of plasma treated (1 0 0) silicon by direct-current plasma enhanced chemical vapor deposition in methane–hydrogen gas mixtures is confirmed by high-resolution transmission electron microscopy. Lattice mismatch by 12% is compensated by tilting the graphene {0 0 2} with respect to silicon {1 1 1} and matching the silicon lattice with fewer graphene layers. The interlayer spacing of graphene sheets near the silicon surface is 0.355 nm, which is larger than that of AB stacked graphite and confirmed as AA stacked graphitic phase. Subsequent growth of standing graphene nanowalls is characterized by scanning electron microscopy and Raman scattering (633 and 514 nm excitation). The Raman peaks of D-band, G-band, and 2D-band are discussed in correlation with SEM images of graphene nanowalls. A strong Raman peak corresponding to silicon–hydrogen stretch vibration is detected by 633 nm excitation at the early stage of graphene nucleation, indicating the silicon substrate etched by hydrogen plasma. With these analyses, the growth mechanism is also proposed in this paper.     

The distribution of wrinkles and their effects on the oxidation resistance of chemical vapor deposition graphene

We present here a detailed study of the oxidation resistance of chemical vapor deposition (CVD) graphene. The results reveal that CVD graphene shows an excellent performance as a passivation layer below 200 °C, but the protection ability degenerates rapidly with increasing the air temperature. Our work demonstrates for the first time that the most adverse effect on the degeneration of oxidation resistance in high temperature air comes from wrinkles but not others, such as Cu grain boundaries, periodic surface depressions due to Cu surface reconstruction induced by the graphene overlay, graphene domain boundaries, which are always believed the primary factor for inferior quality of the CVD graphene at present. In addition, we found that the distribution of the wrinkles in CVD graphene depended on the Cu crystal structure, and the results of the Electron-backscatter diffraction indicate that the folded wrinkles always appear on Cu (0 0 1) facets, while the standing collapsed wrinkles appear more easily on the Cu (1 1 1) facets.     

Low resistivity p+ diamond (100) films fabricated by hot-filament chemical vapor deposition

By hot-filament (HF) chemical vapor deposition (CVD), heavily boron (B)-doped single-crystal diamond (100) films were fabricated and their structural and electrical properties were studied. We did not observe the soot formation, which is frequently observed and limits the performances in the case of microwave plasma (MWP) CVD. The B concentration was successfully controlled over the range from 10¹⁹ to 10²¹ cm^− 3. Hillock-free films were obtained, whose mean surface roughness measured by atomic force microscopy (AFM) was less than 0.1 nm. From the reciprocal space mapping (RSM) around 113 diamond reflection, it was revealed that the films possess the smaller lattice expansion than that expected from the Vegard's law. The room-temperature resistivity was decreased lower than 1 mΩ·cm for B concentration ~ 10²¹ cm^− 3. These results indicate that the HFCVD possesses large potential for fabricating the device-grade p⁺ diamond.     

Preferential {1 0 0} etching of boron-doped diamond electrodes and diamond particles by CO2 activation

The etching behavior of polycrystalline boron-doped diamond (BDD) electrodes and diamond particles with gaseous CO₂ at 800 and 900 °C was investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Polycrystalline BDD (800 ppm), composed of a mixture of cubic {1 0 0} and triangular {1 1 1} orientated planes, was used so as to pursue the possibility of preferential etching by high temperature CO₂ treatment. Nanometer sized pits were observed on the {1 0 0} planes while no change was observable for the {1 1 1} planes when the activation temperature was 800 °C. The difference in the etching behavior by CO₂ with regard to the different planes was clarified using diamond particles and comparing with steam activation. The results demonstrate that CO₂ activation leads to preferential {1 0 0} etching, whereas steam-activation results in preferential {1 1 1} etching.     

Analysis of diamond film creation by pulsed optical lattices

The ability of a chirped pulsed optical lattice to create diamond films using a molecular beam of fullerene molecules is numerically investigated. Two cases, high and low density, are considered. In both cases, the molecular beam was found to impact the substrate at velocities between 10 and 14 km/s. The proposed scenario for the diamond coating stems from the generation of high velocity beams of fullerene particles, bombardment of the substrate surface by these beams, successive dissipation of kinetic energy at the surface and drastic increase of pressure and temperature in the interaction region and finally, formation of diamond crystal structure from deposited fullerenes. A possible setup for the film deposition is proposed. It is shown that that such a system could possibly achieve diamond film growth rates in excess of 1.4 mm/s.     

Thermal diffusivity of heteroepitaxial diamond films: Experimental setup and measurements

The thermal diffusivity of heteroepitaxial CVD diamond films grown on iridium buffer layers has been measured using a combined laser flash and converging thermal wave setup. Absolute values and anisotropy for a fiber-textured reference sample were in the range of former reports in the literature. The in-plane thermal conductivity for three heteroepitaxial samples grown on Ir/YSZ/Si(001) as deduced from the diffusivity measurements was around 20 W/cm K, similar to high purity large grain polycrystalline films. Laser flash measurements of the perpendicular diffusivity suggest that the defect rich first microns of the heteroepitaxial films represent a thermal series resistance which limits the perpendicular heat transport especially for thin films. For the parallel component of the diffusivity the contribution of this shunt resistance is negligible. The absolute values for the parallel component in the heteroepitaxial films with in-plane angular spread of the crystal lattice below 0.5° were discussed in the framework of the model proposed by Klemens for phonon scattering by grain boundaries. The present data indicate that the remaining defects in heteroepitaxial diamond films with low mosaic spread are significantly less detrimental for the heat transport than large angle grain boundaries. In addition we speculate that the exclusive deposition on the {100} growth sector may also reduce the influence of nitrogen in the gas phase on the heat transport properties.     

Control of surface and bulk crystalline quality in single crystal diamond grown by chemical vapour deposition

In order to improve the performance of existing technologies based on single crystal diamond grown by chemical vapour deposition (CVD), and to open up new technologies in fields such as quantum computing or solid state and semiconductor disc lasers, control over surface and bulk crystalline quality is of great importance. Inductively coupled plasma (ICP) etching using an Ar/Cl gas mixture is demonstrated to remove sub-surface damage of mechanically processed surfaces, whilst maintaining macroscopic planarity and low roughness on a microscopic scale. Dislocations in high quality single crystal CVD diamond are shown to be reduced by using substrates with a combination of low surface damage and low densities of extended defects. Substrates engineered such that only a minority of defects intersect the epitaxial surface are also shown to lead to a reduction in dislocation density. Anisotropy in the birefringence of single crystal CVD diamond due to the preferential direction of dislocation propagation is reported. Ultra low birefringence plates (< 10^? 5) are now available for intra-cavity heat spreaders in solid state disc lasers, and the application is no longer limited by depolarisation losses. Birefringence of less than 5 × 10^? 7 along a direction perpendicular to the CVD growth direction has been demonstrated in exceptionally high quality samples.     

Influence of substrate nature on the d.c.-glow discharge induced nucleation of diamond

The nature of the nucleation centers, formed during the so called bias enhanced nucleation (BEN) of chemical vapor deposition (CVD) diamond is still an open question. We address this question by investigating the chemical composition and structure of the material deposited during the “nucleation” stage on various substrates by near edge X-ray absorption fine structure technique (NEXAFS) and Raman spectroscopy.The key step of the BEN of diamond in hot filament CVD systems is the generation of a stable d.c.-glow discharge between the grounded substrate and a positively biased electrode. This process results in the deposition of a carbon based film which contains the diamond nucleation and growth centers. Different materials, such as Si(100), CVD diamond films, and Si(100) onto which thin films of Ni were evaporated were used as substrates.It was found that the structure of the material deposited during the d.c.-glow discharge process is affected by the nature of the substrate. The d.c.-glow discharge process applied to the Si substrate resulted in the formation of a graphite-like film in the earlier stages (5 min), which after prolonged treatment time (30 min) was predominantly composed of nanosized diamond. The CVD diamond film, used as a substrate, promoted the formation of nanosized diamond particles even after 5 min of the d.c.-glow discharge process. However, C-13 labeling experiments have shown that microcrystalline diamond does not grow on the pre-existing CVD diamond substrate under the d.c.-glow discharge conditions. In the case of the Ni modified Si, the deposited film was graphitic in nature both after short and prolonged d.c.-glow discharge treatment times.     

Diamond detectors for hadron physics research

The application of diamond for the detection of charged particles in atomic, nuclear and high-energy physics experiments is described. We compare the properties of three undoped diamond types, all of them produced by Chemical Vapor Deposition (CVD), in particular homoepitaxial single-crystal CVD Diamond (scCVDD), polycrystalline CVD Diamond (pcCVDD) grown on silicon, and CVD Diamond on Iridium (DoI) grown on the multi-layer substrate Ir/YSZ/Si001. The characteristics of the transient current (TC) signals generated from ²⁴¹Am-α-particles in the samples are exploited to evaluate the potential of the diamond crystals for particle timing and spectroscopy applications. The TC technique (TCT) results are correlated to the dark conductivity and the structural defects of the bulk materials as well as to the morphology and roughness of the diamond surfaces. The deterioration of the sensors performance after heavy irradiations with 26 MeV protons, 20 MeV neutrons, and 10 MeV electrons is discussed by means of charge-collection efficiency results, TC technique, and optical absorption spectroscopy (OAS). The important role of the diamond signal processing is underlined, which influences both the quality of the CVDD characterization data as well as the in-beam performance of the diamond sensors.     

Diamond coating on hard metals by traveling methods in a plasma-assisted chemical vapor deposition method above the liquid

In this work, two approaches were developed to extend the coating area of diamond by continuous deposition in a plasma-assisted chemical vapor deposition (CVD) method above the liquid. The techniques were based on the methods previously developed by our research group and the characteristic was to use dc (direct current) plasma generated between the liquid surface and the metal electrode. In the first approach, a tungsten rod was rotated in a chamber at reduced pressure so that a diamond film was formed as a ‘belt’ in 6 mm width around the side of the rod. The deposited diamond was polycrystalline with a grain size of 1–3 μm. The film thickness increased almost linearly with deposition time, whereas the grain size was almost constant against the deposition time. The second approach was for a plate substrate. A tungsten plate was hung with an iron wire and the plasma was horizontally generated between the liquid and plate surfaces. When the W plate was vertically slid down slowly, a diamond film was continuously deposited on the surface. The deposited film was covered with a soot-like carbon layer on the top and the post-treatment with H₂O/N₂ gas at 600 °C was effective in removing it. The continuous deposition successfully demonstrated the expansion of the deposition area with the novel plasma CVD method above the liquid.     

Diamond synthesis by plasma chemical vapor deposition in liquid and gas

The characteristics of diamond synthesis by 2.45 GHz microwave plasma chemical vapor deposition (CVD) under pressures greater than atmospheric pressure were investigated. The deposits on Si substrates were identified by scanning electron microscopy and Raman spectroscopy. The growth rate of diamond was found to be 250 μm/h at 300 kPa, which is ten times greater than that of the conventional low-pressure CVD method. In order to make high-speed deposition of diamond effective, the diamond growth rates for gas-phase microwave plasma CVD were compared to those from the in-liquid plasma CVD method. The growth rate was found to increase as system pressure increased, displaying the same tendency of that in-liquid plasma CVD. The amounts of input microwave energy per unit volume of diamond in the gas-phase and in-liquid plasma CVD methods were also compared. The amount of input microwave energy per unit volume of diamond was found to be 0.6 to 1 kWh/mm³.     

Kinetic Monte Carlo simulations of CVD diamond growth—Interlay among growth,etching, and migration

Diamond films are of interest to many practical applications but the technology of producing high-quality, low-cost diamond is still lacking. To reach this goal, it is necessary to understand the mechanism underlying diamond deposition. Most reaction models advanced thus far do not consider surface diffusion, but recent theoretical results, founded on quantum-mechanical calculations and localized kinetic analysis, highlight the critical role that surface migration may play in growth of diamond films. This study reports three-dimensional time-dependent Monte Carlo simulations of diamond growth that considers adsorption, desorption, lattice incorporation, surface migration, as well as filling atom-size voids. The reaction probabilities are founded on the results of quantum-chemical and transition-state-theory calculations. The analysis includes film growth rate, surface roughness, reaction frequencies, and the evolving film morphology upon varying intrinsic model parameters, like domain size, and growth environment variables, like surface temperature and gaseous precursor concentrations. The kinetic Monte Carlo simulations show that starting with an ideal [{100} − (2 × 1) : H] reconstructed diamond surface the model is able to produce continuous film growth, with the simulated behavior mimicking experiment.     

Synthesis of large single crystal diamond plates by high rate homoepitaxial growth using microwave plasma CVD and lift-off process

A process of making a large, thick single crystal CVD diamond plates has been developed. This process consists of high rate homoepitaxial growth of CVD diamond and subsequent lift-off process using ion implantation. By using this process, single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm³ have been successfully fabricated. The crystallinity of the CVD diamond plates has been evaluated by X-ray topography, polarized light microscopy and high resolution X-ray diffraction. The results indicate the pretreatment of the seed substrate has strong effect on the crystallinity of the CVD diamond plates.     

Diamond CVD film formation onto WC–Co substrates using a thermally nitrided Cr diffusion-barrier

Diamond film deposition onto WC-Co substrates exhibits several limitations regarding the final diamond quality in the film and its adhesion due to the chemical interaction between the Co in the substrate and the diamond CVD environment. In the present study, the use of a ~ 1.5 μm thermally nitrided Cr interlayer was examined as an effective diffusion barrier throughout the CVD process. Nitridation of the Cr PVD layer in NH₃ environment resulted in the formation of a graded CrN/Cr₂N layer comprised mainly of the CrN phase, accompanied with the formation of a porous ‘net-like’ microstructure at the surface. During both thermal nitridation and exposure to the CVD environment up to 360 min, the diffusion of C and Co from the substrate into the interlayer was limited to the region adjacent to the Cr–N interlayer/WC–Co substrate interface, which contained the Cr₂N phase. In this region, the Co interacted with the Cr lattice to form a CoCr phase, which was suggested to enhance the chemical binding between the interlayer and the substrate. The region containing the CrN phase was suggested to act as an effective diffusion barrier due to its fully occupied interstitial sites and relatively high crystalline density compared to the underlying Cr₂N phase. It was evident that the deleterious effects of Co during the CVD process were successfully suppressed using the Cr–N interlayer and the deposited diamond film exhibited improved adhesion and higher diamond quality.The formation of phases within the interlayer during nitridation and the diamond CVD process, and diamond quality evaluation in the deposited films were investigated by complementary techniques: SEM, XRD, XPS, SIMS and Raman spectroscopy.     

Hot filament chemical vapour deposition and wear resistance of diamond films on WC-Co substrates coated using PVD-arc deposition technique

Different Cr- and Ti-base films were deposited using PVD-arc deposition onto WC-Co substrates, and multilayered coatings were obtained from the superimposition of diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The behaviour of PVD-arc deposited CrN and CrC interlayers between diamond and WC-Co substrates was studied and compared to TiN, TiC, and Ti(C,N) interlayers. Tribological tests with alternative sliding motion were carried out to check the multilayer (PVD + diamond) film adhesion on WC-Co substrate. Multilayer films obtained using PVD arc, characterised by large surface droplets, demonstrated good wear resistance, while diamond deposited on smooth PVD TiN films was not adherent. Multilayered Ti(C,N) + diamond film samples generally showed poor wear resistance.Diamond adhesion on Cr-based PVD coatings deposited on WC-Co substrate was good. In particular, CrN interlayers improved diamond film properties and 6 μm-thick diamond films deposited on CrN showed excellent wear behaviour characterised by the absence of measurable wear volume after sling tests. Good diamond adhesion on Cr-based PVD films has been attributed to chromium carbide formation on PVD film surfaces during the CVD process.     

785 nm Raman spectroscopy of CVD diamond films

Raman spectroscopy is a powerful technique often used to study CVD diamond films, however, very little work has been reported for the Raman study of CVD diamond films using near-infrared (785 nm) excitation. Here, we report that when using 785 nm excitation with 1 µm spot size, the Raman spectra from thin polycrystalline diamond films exhibit a multitude of peaks (over 30) ranging from 400–3000 cm^− 1. These features are too sharp to be photoluminescence, and are a function of film thickness. For films > 30 µm thick, freestanding films, and for films grown in diamond substrates the Raman peaks disappear. This suggests that the laser is probing the vibrations of molecular units at the grain boundaries of the disordered crystallites present at the interface between the diamond and substrate.     

Effect of the boron content on the steam activation of boron-doped diamond electrodes

In order to investigate the initial stages of the steam-activation process of boron-doped diamond (BDD) electrodes, polycrystalline BDD electrodes with different levels of boron doping (800, 2500 and 5000 ppm) and crystal orientation were treated with water vapor at 800 °C. A higher degree of etching was observed for BDD electrodes with higher boron content. Based on scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, it is suggested that the {1 1 1} planes are preferentially etched. Thus, high-boron containing BDD electrodes which have a higher abundance of the {1 1 1} planes are heavily etched, while low-boron containing BDD electrodes with a mixed surface of {1 0 0} and {1 1 1} planes are less corroded. The steam activation of BDD electrodes have a higher electrochemically active surface area and wider potential window compared to pristine BDD electrodes.