Chemical composition,thermal stability and hydrogen plasma treatment of laser-cut single-crystal diamond surface studied by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

X-ray photoelectron spectroscopy examination shows that after laser cutting under ambient condition, the upper surface of diamond consists of a heavy oxidized layer consisting of a variety of carbon–oxygen chemical states comprising –C═O, –C–O–C– and –C–O–H species. The thickness of the oxide layer was estimated to be ～22 ?. Upon vacuum annealing to 700 °C the thickness of the oxide layer decreases to ～10 A and the upper surface layer becomes more diamond-like through desorption of C–O species. Exposure of the laser cut diamond surface to a microwave hydrogen (MW-H) plasma results in removal of the oxide layer and exposure of the diamond phase. This is evidenced by the appearance of characteristic diamond surface and bulk plasmons which accompanied the C (1s) X-ray photoelectron peak. Our studies show that the surface chemical composition and thermal stability of the laser cut and polished surfaces both after MW-H exposure are nearly similar. The morphology of the laser cut surface shows an ill-defined laminar structure without any characteristic features which is not significantly affected by MW-H plasma exposure. This is in contrast to the polished surfaces for which exposure to the MW-H may result in its planarization.     

In-situ oxidation and annealing of hydrogenated diamond (100) surfaces studied by high resolution electron energy loss spectroscopy

Homoepitaxially grown single crystal diamond (100) surface on polished natural type 2a diamond was carefully prepared and examined for morphology and chemical composition. Atomic force microscopy (AFM) analysis showed smooth polishing scratches consisting of broad and shallow patterns also seen by electron microscope examination in the secondary electron emission detection mode. High resolution electron energy loss spectroscopy (HREELS) was used to determinate different chemical bonding configurations present on the as grown surface and their thermal stability induced by vacuum annealing and exposure to activated hydrogen. In-situ hydrogen terminated surfaces were oxidized by exposure to thermally activated oxygen. Gradual annealing leads to the removal of oxygen containing species and to regeneration of the hydrogenated diamond surface. Annealing results in desorption of peroxide and ether groups and recovers the diamond optical phonon overtones in the HREELS. There was no indication of hydroxyl groups after oxygen exposure. Possible chemical processes involving activated hydrogen and oxygen during the exposures and subsequent annealing are suggested. Activated hydrogen and oxygen possess sufficient energy to attack absorbed species in order to form new chemical bonds absorbed onto the diamond surface.     

A near edge X-ray absorption fine structure study of oxidized single crystal and polycrystalline diamond surfaces

The influence of oxidizing environments on single crystal diamond and polycrystalline chemical vapor deposited CVD diamond films was studied using the near edge X-ray absorption fine structure (NEXAFS) pre-edge region in both bulk and surface sensitive modes. The NEXAFS of (100) oriented single crystal diamond was measured following (i) exposure to a microwave (MW) hydrogen plasma, (ii) annealing to 1000 °C, (iii) exposure of the as annealed surface to H₂O, and (iv) exposure of the as annealed surface to O₂. From these measurements particular surface bonding configurations have been assigned to features in the pre-edge structure. The NEXAFS of microcrystalline CVD diamond films was studied following different oxidative treatments using (i) a thermal atomic oxygen (AO) environment, (ii) a hyperthermal (5 eV) AO source, and (iii) an RF oxygen plasma exposure. The nature of the surface layer was found to be different for differently oxidized surfaces. These treatments were carried out as part of a study of CVD diamond durability in the low Earth orbit space environment.     

Electronic properties of plasma hydrogenated diamond surfaces: A microscopic study

The quality of hydrogen termination of undoped diamond surfaces is investigated on a microscopic level by applying atomic force microscopy, Kelvin force microscopy, and scanning electron microscopy. The results show that high temperature hydrogenation in a microwave plasma can lead to microscopic inhomogeneities at the surface, both in morphology and electron emission properties. These inhomogeneities are due to a very thin (1 nm) layer of non-diamond material on the surface, which appears to be inherently generated by the hydrogen plasma process. By scanning in contact AFM an underlying diamond surface can be exposed, which retains properties typical for hydrogenated diamond and exhibits significantly improved electron emission properties.     

Multiple twinning and nitrogen defect center in chemical vapor deposited homoepitaxial diamond

Homoepitaxial diamond films were grown on polished {100} faces of single crystal type IIa diamond substrates using microwave plasma assisted chemical vapor deposition system. 14 homoepitaxial diamond films were grown under a variety of substrate temperatures (1000–2000°C), methane concentration (1–6% in hydrogen gas) and processing pressure (60–200 Torr). Electron paramagnetic resonance (EPR) studies demonstrate that nitrogen is incorporated as a singly substitutional impurity (P1-defect center) and the nitrogen concentration is in the range 10–100 parts per million (ppm). The substitutional nitrogen concentration in homoepitaxial diamond was observed to decrease with increasing substrate temperature. Multitwin percentages of all grown diamonds derived from EPR spectra are correlated with the growth parameter α, which is simply the growth velocity along the 〈100〉 direction divided by the growth velocity along the 〈111〉 direction. With the aid of multitwin morphology and multitwin percentages derived from EPR, we describe conditions under which a twin-free and low defect single crystal diamond can be grown from the vapor phase on the {100} oriented substrates.     

Plasma amination of ultrananocrystalline diamond/amorphous carbon composite films for the attachment of biomolecules

Ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) have been deposited by microwave plasma chemical vapour deposition at 600 °C from 17% CH₄/N₂ mixtures. The as-grown films turned out to be hydrogen terminated and very stable. Photochemical amination of H-terminated diamond is a well-established route to attach functional groups to such surfaces for applications in biosensors. Here we report on experiments to aminate UNCD surfaces directly by exposure to ammonia plasmas. Thereafter the surfaces were reacted with the heterobifunctional crosslinker molecule SSMCC bearing a N-hydroxysuccinimide (NHS) ester group which should react with the surface NH₂ groups. By means of X-ray photoelectron spectroscopy (XPS), contact angle measurements and fluorescence microscopy it is shown that both steps, plasma amination and SSMCC attachment lead to the desired aims. On the other hand, experiments to attach a thiol-bearing fluorescein molecule directly to H-terminated UNCD films turned out to be partially successful although according to literature such a reaction should be very unlikely.     

Friction force microscopy study of diamond films modified by a glow discharge treatment

A glow discharge treatment technique has been developed which enables control of the surface roughness and morphology of diamond films for applications in optical and electrical components. A conventional hot filament chemical vapour deposition (CVD) system was used to deposit the diamond films onto silicon substrates via a three-step sequential process: (i) deposition under normal conditions; (ii) exposure to either a pure hydrogen plasma or 3% methane in an excess of hydrogen using DC-bias; and (iii) diamond deposition for a further 2 h under standard conditions. The frictional characteristics and roughness of the film surfaces were investigated by atomic force microscopy (AFM) and the morphology and the growth rates determined from scanning electron microscope images. Lateral force microscopy (LFM) has revealed significant differences in frictional behaviour between the high quality diamond films and those modified by a glow discharge treatment. Friction forces on the diamond films were very low, with coefficients ∼0.01 against silicon nitride probe tips in air. However, friction forces and coefficients were significantly greater on the DC-biased films indicating the presence of a mechanically weaker material such as an amorphous carbon layer. A combination of growth rate and frictional data indicated that the exposure to the H₂ plasma etched the diamond surface whereas exposure to CH₄/H₂ plasma resulted in film growth. Re-Nucleation of diamond was possible (stage iii) after exposure to either plasma treatment. The resultant friction forces on these films were as low as on the standard diamond film.     

Planarizing CVD diamond films by using hydrogen plasma etching enhanced carbon diffusion process

Polycrystalline diamond films, deposited by microwave plasma chemical vapor deposition (MPCVD), were planarized in hydrogen plasma under the graphitization of iron film obtained by reduction of iron chloride under hydrogen plasma ambient. For this process, the free-standing diamond films were dipped in a saturated iron chloride solution and dried horizontally in atmospheric ambient. Then the diamond samples were heated by hydrogen plasma in the same MPCVD reactor. Under the effect of hydrogen reduction, iron thin film was formed on the surface of diamond films. Under ca. 800 °C, the carbon diffusion process was carried out under the graphitization effect of iron thin film. Since the iron film used in this process is very thin, the diffused carbon will diffuse from the diamond side to the hydrogen plasma side and then etched away by the plasma. Therefore, the etching rate of diamond film can be kept consistent. After etching the growth surface of a free-standing diamond film, we investigated the surface morphologies and the carbon phases on the etched surfaces of diamond films. Finally, compared with the result of mechanical lapping experiments, we suggest that the hydrogen plasma etching enhanced carbon diffusion process can serve as a new planarization method for rough diamond film surface. A mechanism for this enhanced etching effect is also presented and discussed.     

高功率微波等离子体环境下甲烷浓度对金刚石膜的影响

在实验室自制的10 kW微波等离子体化学气相沉积装置中,分析了高功率微波等离子体环境中甲烷浓度对金刚石膜生长的影响。利用等离子体发射光谱诊断分析高功率微波等离子体放电环境的特征,同时利用SEM及Raman光谱对不同沉积条件下获得的金刚石膜的形貌及质量进行表征,以确定高功率微波等离子体环境下金刚石膜生长的最优甲烷浓度范围。实验表明在保持微波功率为5000 W,CH₄/H₂≤1%时,金刚石膜中二次形核现象明显,晶粒尺寸较小;CH₄/H₂≥2.5%时,金刚石膜可获得较大的晶粒,但易于产生孪晶体;CH₄/H₂=1.5%～2%时,可获得晶粒完整且质量较高的金刚石膜。     

A wear simulation study of nanostructured CVD diamond-on-diamond articulation involving concave/convex mating surfaces

Using microwave plasma chemical vapor deposition, a 3-µm-thick nanostructured diamond layer was deposited onto polished, convex, and concave components that were machined from Ti–6Al–4V alloy. These components had the same radius of curvature, 25.4 mm. Wear testing of the surfaces was performed by rotating articulation of the diamond-deposited surfaces (diamond-on-diamond) with a load of 225 N for a total of 5 million cycles in bovine serum resulting in polishing of the diamond surface and formation of very shallow, linear wear grooves of less than 50 nm depth. The two diamond surfaces remained adhered to the components and polished each other to an average surface roughness that was reduced by as much as a factor of 80 for the most polished region located at the center of the condyle. Imaging of the surfaces showed that the initial wearing-in phase of the articulating surfaces by the end of the 5 million cycles. Atomic force microscopy, scanning electron microscopy, Raman spectroscopy, and surface profilometry were used to characterize the surfaces and verify that the diamond remained intact and uniform over the surface, thereby protecting the underlying metal. These wear simulation results show that diamond deposition on Ti alloy has potential application for joint replacement devices with improved longevity over existing devices made of cobalt chrome and ultra-high molecular weight polyethylene.     

Predominant physical quantity dominating macroscopic surface shape of diamond synthesized by microwave plasma CVD

To specify the most important factor which dominates macroscopic surface shape of single crystal diamond synthesized by microwave plasma chemical vapor deposition, we have carried out numerical simulations and corresponding experiments. Comparison between the numerical and experimental results has shown that temperature distribution on the top surface of the substrate predominantly controls the surface shape.     

Diamond deposition on Ni3Ge single- and polycrystalline substrates

In order to find new materials for heteroepitaxial diamond growth Ni₃Ge single- and polycrystalline wafers were produced and used as substrates for diamond deposition in a microwave plasma system.The cubic phase Ni₃Ge substrate revealed to be an interesting and potential material for heteroepitaxial diamond chemical vapour deposition due to its: (1) lattice parameter matching within <1% the lattice parameter of diamond; and (2) coexistence with carbon up to its (congruent) melting point. Thus centimetre-size crystal boules were pulled from the melt using the Czochralski crystal growth method. These boules were sectioned into wafers and polished.Low-pressure diamond was grown on the Ni₃Ge wafers under various deposition conditions. The orientation of isolated diamond single crystals grown on the Ni₃Ge substrate surface show that heteroepitaxial nucleation occurred. Diamond nucleation was low, as seeding methods to enhance nucleation were not used.     

Hydrogen incorporation control in high quality homoepitaxial diamond (111) growth

The effect of growth conditions (temperature, microwave power, and pressure) on the growth rate and hydrogen concentration in the (111) homoepitaxial diamond grown by temperature-controlled microwave-assisted plasma chemical vapor deposition is systematically studied by secondary ion mass spectrometry. The most effective parameter in the hydrogen incorporation is the substrate temperature. The growth rate and hydrogen concentration are found to increase with elevated substrate temperature. The increase in microwave power enhances the growth rate and suppresses hydrogen incorporation. The effect of pressure rise below 50 Torr is similar to that of microwave power. The effect of microwave power and pressure on the decrease of hydrogen incorporation can be explained as a result of the hydrogen abstraction from the growing surface by atomic hydrogen in the gas phase.     

Preparation of low index single crystal diamond surfaces for surface science studies

In this review I summarise the procedures that have been developed to prepare well ordered, low index single crystalline diamond surfaces for surface science studies. Particular emphasis is placed on methods to smooth as polished surfaces with the aim to obtain well developed terraces with atomic order by different atomic hydrogen etching techniques.     

Surface properties of differently prepared ultrananocrystalline diamond surfaces

Ultrananocrystalline diamond/amorphous carbon composite films have been deposited by microwave plasma chemical vapour deposition from 17% CH₄/N₂ mixtures at 600 °C. Thereafter the films were subjected to various treatments (plasma processes, UV/O₃ exposure) to obtain hydrogen, oxygen, and fluorine terminated surfaces, which then have been characterized with respect to their composition, roughness, wettability, and other properties. Among others, it will be shown that H- and F-terminated surfaces are very stable even if exposed for long time to air, while O-terminated ones are prone to contaminations. H- and O-termination can be patterned by applying the UV/O₃ treatment through a mask. Finally, it will be shown that a non-fouling poly(ethylene glycol) layer can be grafted directly on oxygen terminated surfaces by an atom transfer radical polymerization process using α-bromoisobutyryl bromide as an initiator.     

Observations of nanotube and ‘celery’ structures following diamond CVD on single crystal diamond substrates

Diamond films have been grown onto polished single crystal high-pressure high-temperature diamond substrates using microwave plasma chemical vapour deposition. After deposition, unusual whiskers were observed on the surface of the film originating from growth step edges. Some of these whiskers curled up lengthwise to resemble ‘celery’-like structures of length ∼20 μm and diameter <1 μm. Electron microscope analyses of these structures revealed the whiskers to be largely amorphous, although some smaller whiskers contained crystalline material embedded within them. A suggestion for their mechanism of formation involving metal-catalysed growth at step edges is presented.     

Surface modification of nanocrystalline diamond/amorphous carbon composite films

The surfaces of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films deposited from a 17% CH₄/N₂ mixture have been subjected to a variety of plasma and chemical treatments, namely H₂ and O₂ microwave plasmas, a CHF₃ 13.56 MHz plasma, and a chemical treatment with aqua regia (HCl:HNO₃ 3:1). The resulting surfaces have been studied with respect to their chemical nature by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS), concerning their morphology with atomic force microscopy, and by contact angle measurements to study their hydrophobicity and their stability. As-grown surfaces are hydrogen terminated, but the number of C–H bonds can slightly be increased by a H₂ microwave plasma, while treatment with aqua regia considerably lowers the number of C–H bonds at the surface. O₂ and CHF₃ plasmas, on the other hand, lead to a replacement of the terminating C–H bonds by C–O or C–OH and C–F_x groups, respectively. Finally, by contact angle measurements over a period of 150 days it could be shown that the H-terminated surface is very stable whereas the contact angle of the O-treated surface changed considerably with time, probably due to the adsorption of contaminants.     

Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

Atomic force microscopy observations of a single crystal diamond surface lifted-off via ion implantation

The microscopic surface morphology of “lifted-off” surfaces, produced via ion implantation was observed by atomic force microscopy. A polished single-crystal diamond substrate with an average surface roughness of less than 0.1 nm was used for precise observations. After the lift-off process, the lifted-off surface became rough with pits appearing. Hydrogen plasma treatment close to the chemical vapor deposition conditions for diamond (1150 °C, 160 Torr) completely removed these pits and the surface was subsequently covered by a strip-like structure consisting of atomic steps. The surface roughness, however, was not further influenced by the plasma treatment. The observed morphological evolution reflects the graphite/diamond interface formed by the lift-off.     

Investigation of nanocrystalline diamond films grown on silicon and glass at substrate temperature below 400 °C

We present investigation of nanocrystalline diamond films deposited in a wide temperature range. The nanocrystalline diamond films were grown on silicon and glass substrates from hydrogen based gas mixture (methane and hydrogen) by microwave plasma CVD process. Film composition, nano-grain size and surface morphology were investigated by Raman spectroscopy and scanning electron microscopy. All samples showed diamond characteristic line centred at 1332 cm^− 1 in the Raman spectrum. Nanocrystalline diamond layers revealed high surface flatness (under 10 nm) with crystal size below 60 nm. Surface morphology of grown films was well homogeneous over glass substrates due to used mechanical seeding procedure. Very thin films (40 nm) were successfully grown on glass slides (i.e. standard size 1 × 3″). An increase in delay time was observed when the substrate temperature was decreased. A possible origin for this behaviour was discussed.