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.     

Diamond nucleation on Ni3Al substrate using bias enhanced nucleation method

Diamond deposition with positive and negative bias enhanced nucleation (BEN) pretreatments on mirror-polished polycrystalline Ni₃Al substrates has been investigated, respectively. It was found that diamond deposition on the substrates under both biasing exhibited significant variations among grains of different orientations. The substrate surface was found to be rough in the case of negative biasing, whereas it was smooth in the case of positive biasing. Thus, the correlation of the crystallographic orientation of grains on the samples with the diamond nucleation behavior was systematically characterized for the case of positive biasing by electron backscattered diffraction method with scanning electron microscopy. Diamond deposition on Ni₃Al grains near (111) orientation results in higher nucleation densities, while the densities are low on (110) and (100) oriented grains. Also, the interfacial microstructure between diamond deposited and Ni₃Al was characterized by cross-sectional transmission electron microscopy.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Effects of cobalt and cobalt oxide buffer layers on nucleation and growth of hot filament chemical vapor deposition diamond films on silicon (100)

An initial study on the nucleation and growth of diamond, using hot filament chemical vapor deposition (HFCVD) technique, was carried out on Co and CoO thin buffer layers on non-carbon substrates (Si (100)), and the results were compared with conventional scratching method. The substrate temperature during the growth was maintained at 750±50 °C. A mixture of CH₄ and H₂ (1: 100 volume %) was used for deposition. The total pressure during the two hour deposition was 30±2 Torr. X-ray photoelectron spectroscopy (XPS) study showed the diamond nucleation at different time periods on the Co and CoO seed layers. It is observed that Co helps in nucleation of diamond even though it is known to degrade the quality of diamond film on W-C substrate. The reason for improvement in our study is attributed to (i) the low content of Co (~0.01%) compared to W-C substrate (~5–6%), (ii) formation of CoSi₂ phase at elevated temperature, which might work as nucleation sites for diamond. SEM analysis reveals a change in the morphology of diamond film grown on cobalt oxide and a significant reduction in the size of densely packed crystallites. Raman spectroscopic analysis further suggests an improvement in the quality of the film grown on CoO buffer layer.     

Nucleation stability of diamond nanowires inside carbon nanotubes: A thermodynamic approach

Aiming at synthesis diamond nanowires, a simple thermodynamic approach was performed with respect to the effect of nanosize-induced additional pressure on the Gibbs free energy of critical nuclei to elucidate diamond nucleation inside carbon nanotubes upon chemical vapor deposition, based on the carbon thermodynamic equilibrium phase diagram. Notably, these analysis showed that the diamond nucleation would be preferable inside a carbon nanotube due to the effect of surface tension induced by the nanosize curvature of the carbon nanotube and diamond critical nuclei, compared with diamond nucleation on the flat surface of a silicon substrate. Meanwhile, the metastable phase region of diamond nucleation would be driven into a new stable phase region in the carbon thermodynamic equilibrium phase diagram by the effect of nanosize-induced additional pressure. Eventually, we predicted that carbon nanotubes would be an effective path to grow diamond nanowires by chemical vapor deposition.     

Epitaxial Ir layers on SrTiO3 as substrates for diamond nucleation: deposition of the films and modification in the CVD environment

Iridium films on SrTiO₃(001) have recently proven to be a superior substrate material for the heteroepitaxy of diamond thin films by chemical vapour deposition in the effort towards the realization of single crystal diamond films. In this paper we report on the growth and structural properties of iridium (Ir) films deposited by electron-beam evaporation on SrTiO₃(001) surfaces varying the deposition temperature between 280 and 950°C. The films were studied by scanning electron microscopy, atomic force microscopy and X-ray diffraction. At the highest temperature film growth proceeds via three-dimensional nucleation, coalescence and subsequent layer-by-layer growth. The resulting films show a cube-on-cube orientation relationship with the substrate and a minimum mosaic spread of 0.15°. Towards lower deposition temperatures the orientation spread increases only slightly down to ∼500°C while the surface roughness, after passing through a maximum at ∼860°C, decreases significantly. For the lowest temperatures (below 500°C) the mosaic spread rises accompanied by the occurrence of twins until the epitaxial order is lost. Plasma treatment in the diamond deposition reactor at high temperature (920°C) yields low nucleation densities and modifies the Ir surface. At the same time {111} facets show a significantly higher structural stability as compared with {001} facets. Nucleation at 700°C results in highly aligned diamond grains with low mosaic spread and a vanishing fraction of randomly oriented grains, proving the superior properties of Ir films on SrTiO₃ for diamond nucleation as compared with pure silicon substrates.     

Studies of heteroepitaxial growth of diamond

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substrate system. Oxide substrates, notably MgO and SrTiO₃, on which thin epitaxial films of iridium serve as a nucleation layer for diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond by low-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for the process. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films, typically 150–300 nm thick, with Ir (1 0 0) parallel to the surface of all substrates as confirmed by X-ray and electron backscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhanced nucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 10¹² cm⁻² were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating the nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless, (0 0 1) diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38 μm and lateral dimensions near 4 mm. The crystals were transparent in visible light and cleaved on (1 1 1) planes along 〈1 1 0〉 directions, similar to natural diamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfection results in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO₃. However, Al₂O₃ possesses superior interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond. Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth of heteroepitaxial diamond should be feasible in the near future.     

Influence of CF4 addition for HFCVD diamond growth on silicon nitride substrates

This work presents a study of CVD diamond growth on silicon nitride-based ceramics with the addition of carbon tetrafluoride (CF₄) in a hot filament-assisted reactor (HFCVD). Silicon nitride substrates were hot pressed under a nitrogen atmosphere for 90 min at 1750°C, giving specimens of very high density and good mechanical properties. The CF₄ addition is known to bring several advantages to diamond growth and, in particular, in this work, an important interaction of the CF₄-containing gas phase with the silicon nitride (Si₃N₄) substrates has been proven to be very beneficial for nucleation, growth and adherence of the diamond films. A basic gas mixture of H₂/1.5 vol.% CH₄/0.5 vol.% CF₄ was used in the growth experiments. The nucleation study reveals a strong interaction of the halogen-containing gas phase with the vitreous phase on the substrate surface. A strong erosion of the surface has been observed, which induced a high nucleation density (N_d) of the order of 10⁸ particles cm⁻², without any surface pre-treatment. Silicon nitride surface analysis was performed with Raman and infrared specular reflectance spectroscopy. Results suggest the erosion of the vitreous phase, mainly the silica (SiO₂) component, and the formation of silicon carbide, prior to diamond growth. Raman spectra and scanning electron microscopy (SEM) show better quality film grown with CF₄ addition. Indentation tests with a Rockwell C tip, at variable charge, show a better film adherence if grown with CF₄ addition.     

Highly oriented diamond growth on SixGe1−x (100) thin films

Highly oriented (100) diamond films have been successfully grown on Si_xGe_1−x (100) thin films by bias enhanced nucleation (BEN) in microwave plasma chemical vapor deposition (MPCVD) system. Raman spectra show the 1332 cm⁻¹ peak which proves the formation of diamond. Diamond nucleation density on Si_xGe_1−x substrate estimated by scanning electron microscopy is higher than 10⁹ cm⁻². The interface between diamond and Si_xGe_1−x substrate was characterized by transmission electron microscopy (TEM). About 20 nm decrease in thickness of the Si_xGe_1−x film was observed after bias enhanced nucleation step. TEM shows the existence of silicon carbide and heteroepitaxial diamond grains grown on Si_xGe_1−x substrate. Characterization from high-resolution TEM on the specimen of short time deposition reveals that a number of epitaxial diamond grains were directly nucleated on Si_xGe_1−x with {111} interplanar spacing ratio of diamond and Si_xGe_1−x of 2:3. The diamond nucleation is found to be preferred on the ridge position of the rough substrate surface. Diamond {100} facets were quickly developed in the early stage of growth.     

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.     

Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp³ and sp² network and the quantitative analysis were done by Fourier transform infrared spectroscopy.     

Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

Influence of H2S addition during diamond deposition on hardmetal substrates

Diamond deposition on hardmetal substrates is an industrial process to increase the wear resistance of tools during machining operations, but till now an increase in diamond layer adhesion is desirable. The main problem during diamond deposition on hardmetal substrates is the Co content in the binder phase.H₂S was used for immobilizing the cobalt on the substrate surface. The H₂S should react with the metallic Co covering its surface with CoS. Because of this the diamond nucleation occurs easier and the Co vapour pressure is also reduced. Similar mechanisms were observed using silicon and boron vapour during substrate pre-treatment.Positive effects of H₂S addition were achieved if the H₂S is added only during the diamond nucleation period. The experiments with continuous H₂S addition were not successful.For comparison diamond deposition on Murakami/Carrot pre-treated substrates were carried out.     

Substrate pre-treatment by ultrasonication with diamond powder mixtures for nucleation enhancement in diamond film growth

Pre-treatment of silicon substrates by ultrasonic abrasion for nucleation enhancement in diamond film formation by hot-filament chemical vapour deposition is discussed. Scanning electron microscopy, atomic force microscopy and visible Raman spectroscopy were employed as analysis techniques. Ultrasonication was applied by suspensions of isopropanol with micro-or nanosized diamond powders, micro-sized metal and alumina particles and mixtures thereof. The root mean square roughness of the ultrasonically pre-treated samples varied from 0.2 to 12.0 nm depending on the applied powder mixture. All samples that were ultrasonically pre-treated had a larger diamond nucleation density than the untreated silicon wafer. As expected, for an effective increment of the diamond nucleation density by several orders of magnitude the application of diamond powder is necessary, since the generation of surface roughness alone is not sufficient to enhance the diamond nucleation kinetics satisfactorily. The simultaneous action of diamond powders and large alumina or titanium particles leads to an increase in diamond nucleation density up to a factor of 10⁶. When nano-diamond powder is used, the embedment of diamond fragments is best and in combination with titanium grains (50–75 µm) a diamond nucleation density of 8 × 10⁹ cm^− 2 is obtained. After 8 h of film growth, the diamond surface grains are significantly smaller for the samples that demonstrated higher nucleation densities, whereas the quality of the diamond layers is equal.     

Nanometer rough,sub-micrometer-thick and continuous diamond chemical vapor deposition film promoted by a synergetic ultrasonic effect

In this paper we report on a surface treatment to seed substrates for the promotion of diamond nucleation. This surface treatment consists of an ultrasonic abrasion process using poly-disperse slurry composed of a mixture of small diamond particles (<0.25 μm) and larger particles (>3 μm) which may consist of diamond, alumina, titanium, etc. Whereas ultrasonic abrasion with a mono-disperse diamond slurry results in a diamond nucleation density of ∼2–3×10⁸ particles/cm², treatment with poly-disperse slurries results in diamond nucleation density of values up to ∼5×10¹⁰ particles/cm². This effect was found to display a similar effectiveness on a variety of substrates such as silicon, sapphire, quartz, etc. The enhancement in diamond nucleation is interpreted by a ‘hammering’ effect whereby the larger particles insert very small diamond debris onto the treated surface, thus increasing the density of nuclei onto which diamond growth takes place during the chemical vapor deposition process. By increasing the nucleation density to values of ∼5×10¹⁰ particles/cm², continuous diamond films of thickness of less than ∼100 nm were grown after only 5 min of deposition. The roughness of continuous diamond films grown on substrates treated at optimum conditions obtains values of 15–20 nm. The effect of ultrasonic treatment on silicon substrates and the deposited films was investigated by atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.     

Micro-Raman analysis of the cross-section of a diamond film prepared by hot-filament chemical vapor deposition

Raman micro-spectroscopy using exciting light with an approximately 2-μm diameter exciting spot was undertaken to investigate the micro-structure of cross-section of a 100-μm thick diamond film prepared by hot-filament chemical vapor deposition (HFCVD). The Raman spectra exhibited different features with changing position, suggesting that the composition of crystalline diamond, amorphous carbon and graphitic phases varied in the HFCVD process. The presence of a broad band and high background intensity in the spectra near the substrate surface was attributed to the amorphous carbon synthesized in the nucleation process of the film. A decreasing proportion of sp²-bond structure in the amorphous carbon phase with increasing film thickness was believed to account for the decline in the intensity of the 1200–1600-cm⁻¹ band. The different composition of the diamond grains and of the grain boundaries in the film was shown in the Raman spectra obtained from different positions at the same cross-sectional thickness by scanning the exciting light parallel to the surface of the film.     

Pre-nucleation techniques for enhancing nucleation density and adhesion of low temperature deposited ultra-nanocrystalline diamond

Effect of pre-nucleation techniques on enhancing nucleation density and the adhesion of ultra-nanocrystalline diamond (UNCD) deposited on the Si substrates at low temperature were investigated. Four different pre-nucleation techniques were used for depositing UNCD films: (i) bias-enhanced nucleation (BEN); (ii) pre-carburized and then ultrasonicated with diamond powder solution (PC-U); (iii) ultrasonicated with diamond and Ti mixed powder solution (U-m); (iv) ultrasonicated with diamond powder solution (U). The nucleation density is lowest for UNCD/U-substrate films ( 10⁸ grains/cm²), which results in roughest surface and poorest film-to-substrate adhesion. The UNCD/PC-U-substrate films show largest nucleation density ( 1 × 10¹¹ grains/cm²) and most smooth surface (8.81 nm-rms), whereas the UNCD/BEN-substrate films exhibit the strongest adhesion to the Si substrates (critical loads =  67 mN). Such a phenomenon can be ascribed to the high kinetic energy of the carbon species, which easily form covalent bonding, Si–C, and bond strongly to both the Si and diamond.     

The effect of ion bombardment on the nucleation of CVD diamond

The nucleation effect of CVD diamond by ion bombardment was studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. It was found that after the ion bombardment, an amorphous layer embedded with nano-crystalline diamond particles formed on the Si substrate. These nano-crystalline diamond particles were proposed to serve as the nucleation centers for the growth in the second step. The nucleation density depended strongly on the ion dosage and a nucleation density of up to 2×10⁹ cm⁻² could be achieved under optimized conditions.     

Improvements in the Formation of Boron-Doped Diamond Coatings on Platinum Wires Using the Novel Nucleation Process (NNP)

In order to increase the initial nucleation density for the growth of boron-doped diamond on platinum wires, we employed the novel nucleation process (NNP) originally developed by Rotter et al. [1]. This pretreatment method involves (i) the initial formation of a thin carbon layer over the substrate followed by (ii) ultrasonic seeding of this “soft” carbon layer with nanoscale particles of diamond. This two-step pretreatment is followed by the deposition of boron-doped diamond by microwave plasma-assisted CVD. Both the diamond seed particles and sites on the carbon layer itself function as the initial nucleation zones for diamond growth from an H₂-rich source gas mixture. We report herein on the characterization of the pre-growth carbon layer formed on Pt as well as boron-doped films grown for 2, 4 and 6 h post NNP pretreatment. Results from scanning electron microscopy, Raman spectroscopy and electrochemical studies are reported. The NNP method increases the initial nucleation density on Pt and leads to the formation of a continuous diamond film in a shorter deposition time than is typical for wires pretreated by conventional ultrasonic seeding. The results indicate that the pre-growth layer itself consists of nanoscopic domains of diamond and functions well to enhance the initial nucleation of diamond without any diamond powder seeding.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.