Showing papers on "Diamond published in 1990"

Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films

Abstract: An empirical many-body potential-energy expression is developed for hydrocarbons that can model intramolecular chemical bonding in a variety of small hydrocarbon molecules as well as graphite and diamond lattices. The potential function is based on Tersoff's covalent-bonding formalism with additional terms that correct for an inherent overbinding of radicals and that include nonlocal effects. Atomization energies for a wide range of hydrocarbon molecules predicted by the potential compare well to experimental values. The potential correctly predicts that the \ensuremath{\pi}-bonded chain reconstruction is the most stable reconstruction on the diamond {111} surface, and that hydrogen adsorption on a bulk-terminated surface is more stable than the reconstruction. Predicted energetics for the dimer reconstructed diamond {100} surface as well as hydrogen abstraction and chemisorption of small molecules on the diamond {111} surface are also given. The potential function is short ranged and quickly evaluated so it should be very useful for large-scale molecular-dynamics simulations of reacting hydrocarbon molecules.

Current Issues and Problems in the Chemical Vapor Deposition of Diamond

Abstract: Current issues and problems in the chemical vapor deposition (CVD) of diamond are those which relate to its characterization, its nucleation on foreign surfaces, the question of its formation in preference to the other phases of solid carbon (for example, graphite, chaoite, or lonsdaleite), why different morphologies and crystallographic orientations (textures) are seen in different experiments or with different parameters in the same experiment, and finally whether well-crystallized metastable phases can be obtained by CVD in other material systems or are only a peculiarity of carbon chemistry. Whether a given carbon coating is justly described as diamond has been such an issue, and coatings should clearly show evidence for diamond by x-ray diffraction and Raman spectroscopy before the claim of diamond is made. Experimental results have not been consistent in many cases, and much work remains to be done before an accurate assessment can be made of the technological impact of the development.

Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates

Abstract: Epitaxial liftoff is an alternative to lattice‐mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin‐film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self‐bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin‐film atomic lattices can be simultaneously imaged, showing only a thin (20–100 A) amorphous layer in between.

Thermal diffusivity of isotopically enriched 12C diamond

Abstract: Type-IIA diamond single crystals containing approximately 0.1%, 0.5%, and 1% $^{13}\mathrm{C}$ were synthesized and their thermal diffusivities were measured at room temperature by the thermal-wave-mirage technique. The measured value (18.5 ${\mathrm{cm}}^{2}$/s) of the 0.1% $^{13}\mathrm{C}$ crystal was 50% higher than the 1% $^{13}\mathrm{C}$ (natural isotope abundance). This is the highest room-temperature thermal diffusivity of any solid naturally occurring or previously synthesized. The laser damage threshold at 193 nm for the isotopically enriched crystal is more than an order of magnitude higher than that of natural diamond.

Mechanism for diamond growth from methyl radicals

Abstract: We use a 9‐carbon model compound to describe a proposed mechanism for homoepitaxial growth of diamond from methyl radicals on a hydrogenated, electrically neutral (100) surface. We estimate enthalpy and entropy changes for each step in the mechanism using group additivity methods, taking into account the types of bonding and steric repulsions found on the (100) surface. Rate constants are estimated based on analogous reactions for hydrocarbon molecules, while gas phase species concentrations are taken from our previous measurements. The rate equations are then integrated. The method, which contains no adjustable parameters or phenomenological constants, predicts a growth rate of between 0.06 and 0.6 μm/h, depending on the local details of the surface. Uncertainties related to the use of a model compound rather than diamond are discussed. The analysis demonstrates that the proposed mechanism is feasible.

Nanometre-sized diamonds are more stable than graphite

Abstract: DIAMONDS just 3–5 nm in diameter have recently been recovered from carbonaceous residues of detonations1. They have also been found in meteorites2, nucleate homogeneously in the gas phase3, and diamond-like films can be grown in low-pressure hydrogen4. Conditions are very different in these four cases, but the nearly equal sizes in both meteorites and detonations, and the necessity of nucleation centres for growth of synthetic diamonds, indicate the existence of a common underlying factor. Ultra-small diamonds, too small to be detected readily, may in fact be far more prevalent than presently realized. We arrive at this conclusion by comparing calculated heats of formation of small tetrahedral (diamond) and hexagonal (graphitic) clusters. In agreement with Nuth's discussions on diamonds in interstellar media5,6, we conclude that surface energies are an important aspect in the stabilization of microcrystalline diamonds. For surface bonds terminated with hydrogen atoms, we find that diamonds smaller than ∼3 nm in diameter are energetically favoured over polycyclic aromatics (the precursors to graphite), withou/equiring the high pressures or extreme kinetic conditions usually associated with diamonds.

Chemical Preparation and Shock Wave Compression of Carbon Nitride Precursors

Abstract: Two synthetic routes have been developed to produce high-molecular-weight organic precursors containing a high weight fraction of nitrogen. One of the precursors is a pyrolysis residue of melamine-formaldehyde resin. The second precursor is the byproduct of an unusual low-temperature combustion reaction of tetrazole and its sodium salt. These precursors have been shock compressed under typical conditions for diamond and wurtzite boron nitride synthesis in an attempt to recover a new ultrahard carbon nitride. The recovered material has been analyzed by X-ray diffraction, FTIR, and Raman microprobe analysis. Diamond is present in the recovered material. This diamond is extraordinarily well ordered relative to diamond shock synthesized from carbonaceous starting materials.

Structure and bonding studies of the C:N thin films produced by rf sputtering method

Abstract: Thin C:N films were prepared by rf diode sputtering of a graphite target in a mixed argon/nitrogen plasma. We have observed a systematic variation of the properties of these C:N films with an increase in the nitrogen partial pressure. XPS, AES, and TEM studies show that nitrogen will stabilize the diamond sp3 bonding. From XPS studies, we found that the density of our C:N films is increased from 1.37 × 1023 atoms/cm3 to 1.63 × 1023 atoms/cm3 using a 100% nitrogen plasma. The energy gap of our nitrogen carbon also shows an increase from 1.1 eV to 1.4 eV using a 100% nitrogen plasma. The mechanical properties also are shown to be enhanced for certain applications. By using the same method, we can also show that it can produce 100% amorphous C:N films which are more diamond-like as compared with other methods.

Micromechanical fracture strength of silicon

Abstract: In order to test the statistical influence of some process and micromachining parameters on the fracture strength of silicon microelements, arrays of identical microsized cantilever beams were bulk micromachined in single‐crystalline silicon wafers. The beams were exposed to various surface treatments (diamond polishing with different grades, oxidization, stripping of oxide) in different combinations. The influence on fracture strength was investigated by bending the beams to fracture in a micromanipulator mounted in situ in a scanning electron microscope while registering force‐versus‐deflection curves. Average fracture strengths, standard deviations, Weibull moduli, crack‐initiating flaw sizes, and in some cases elastic moduli were evaluated. Diamond polishing was found to decrease the fracture strength drastically, but polishing followed by oxidization not only restored the original strength, but actually increased it, due to crack healing. Polishing, oxidization, and subsequent stripping of oxide resu...

Fourier-transform and continuous-wave EPR studies of nickel in synthetic diamond: Site and spin multiplicity

Abstract: Pulsed, Fourier-transform, and continuous-wave electron paramagnetic resonance methods are used to study the g=2.0319 EPR signal in synthetic diamond crystals. This signal is from Ni which is found to be located at a substitutional site in the diamond lattice without detectable nearby charge compensation. The effective spin state of S=(3/2 is determined from Fourier-transform nutational EPR spectroscopy. It is proposed that the center is a ${\mathrm{Ni}}^{\mathrm{\ensuremath{-}}}$ ion with electronic configuration 3${\mathit{d}}^{7}$.

Numerical modeling of the filament-assisted diamond growth environment

Abstract: A numerical model of the filament-assisted diamond growth environment has been developed and used to calculate temperature, velocity, and species concentration profiles, accounting both for transport and detailed chemical kinetics. The computed hydrocarbon concentrations agree well with previously measured values, when allowance is made for 3D effects not included in our model. Upper-bound, diffusion-limited film growth rates for various assumed growth species have been computed, and it has been found no hydrocarbon species other than CH3, C2H2, or CH4 can account for measured diamond growth rates. The effect of thermal diffusion on H-atom profiles has been examined, and found to be only 10%. Although the environment is far from thermodynamic equilibrium, several reactions are close to partial equilibrium throughout the region from the filament to the substrate. It is also shown that homogeneous H-atom recombination is too slow to explain the experimentally observed decrease in the concentration of H with increasing initial methane concentration.

Epitaxial growth of diamond thin films on cubic boron nitride {111} surfaces by dc plasma chemical vapor deposition

Abstract: Diamond thin films have been grown epitaxially on high‐pressure synthesized cubic boron nitride (c‐BN) particles by using dc plasma chemical vapor deposition. At the early growth stage of the film on c‐BN{111} surfaces, the island structure is observed and the number density of islands is about 1011 cm−2. The growth and the coalescence of islands are also found by scanning electron microscopy observation. The continuous film is obtained at the thickness of about 2000 A and the surface of the film is rather smooth. The Raman peak of the epitaxial diamond film shows the shift toward the lower wave number due to the tensile stress involved in the film.

The effect of surface treatment on the electrical properties of metal contacts to boron-doped homoepitaxial diamond film

Abstract: Both doped and undoped homoepitaxial diamond films were fabricated using microwave plasma-enhanced chemical vapor deposition (CVD). The conductivity of the diamond film is strongly affected by the surface treatment. In particular, exposure of film surface to a hydrogen plasma results in the formation of a conductive layer which can be used to obtain linear (ohmic) I-V characteristics of the Au/diamond contacts, regardless of the doping level. It is shown how the proper chemical cleaning of the boron-doped homoepitaxial diamond surface allows the fabrication of Au-gate Schottky diodes with excellent rectifying characteristics at temperatures of at least 400 degrees C. >

Force microscopy with a bidirectional capacitance sensor

Abstract: A new method for sensing cantilever deflection in the atomic force microscope (AFM), based on capacitance measurement, is described. Parameters governing the design of such an instrument are considered in detail. Two different geometries are compared, wire on plate and an integrated flat plate sensor. The electronic circuitry, providing 6×10−19 F noise in a 0.01–1000 Hz bandwidth, is also described. Implementation of the design ideas into a working AFM in ultrahigh vacuum is demonstrated. This AFM allows simultaneous measurement of cantilever deflection in two orthogonal directions, necessary for our nanotribology studies. The theoretical sensitivity of 5×10−7 F/m is not achieved due to roughness. The bidirectional sensing and imaging capabilities are demonstrated for an Ir tip on cleaved graphite, and a diamond tip on diamond films. The capacitance detection technique is compared and contrasted with other AFM sensors.

Methyl radical and H-atom concentrations during diamond growth

Abstract: The gas‐phase composition at the surface of a growing diamond film was measured as a function of the initial methane (CH4) fraction and, for a 2% methane fraction, as a function of added oxygen (O2). The results were modeled with a one‐dimensional reactor flow code that includes diffusion and detailed chemical kinetics. We found that most of the ethylene (C2H4) and ethane (C2H6) that was detected was actually not present in the growth chamber but was instead formed in the probe by recombination of methyl radicals (CH3) that were present in the gas phase. Thus, C2H4 and C2H6 acted as surrogates for CH3 in our system, and measurement of those two stable species allowed us to estimate the mole fraction of the CH3 radical. We then took advantage of the fact that CH3, CH4, H2, and H were in partial equilibrium in the diamond growth chamber in order to estimate the concentration of H. A comparison between the mole fractions of CH3 and H, as determined from our experiments, and the mole fractions calculated from...

Amorphic diamond films produced by a laser plasma source

Abstract: Recently, attention has been focused upon laser plasma sources of thin‐film diamond. These depend upon laser‐ignited discharges in which intense pulsed currents flow through the small volume of carbon plasma ablated from graphite feedstock by a focused laser beam. The materials produced in this way generally resemble the hard amorphic films deposited by ion beams. This paper reports a detailed characterization of these films which we call amorphic diamond. The combination of an optical band gap of 1.0 eV with a grain size of 100–200 A places this material far outside the range of possibilities available to the model of graphitic islands. A structure of very fine grained diamond would more readily explain the hardness of 13 GPa determined in the absence of any measurable fraction of hydrogen. Such amorphic diamond films have been grown uniformly on 100‐cm2 areas at ambient room temperatures with no seeding or abrasion of the substrate.

Early formation of chemical vapor deposition diamond films

Abstract: Nanometer‐size diamond particles formed on a silicon substrate by the hot‐filament chemical vapor deposition method were examined by a high‐resolution electron microscope. The particles developed well‐faceted cuboctahedral habits. Examination of their morphologies and microstructures provides a wealth of information on their crystal growth mechanism. The effect of the pretreatment of the substrate by diamond powder, which has been known to enhance thin‐film growth, was found to be due to seeding by ‘‘diamond dust’’ on the substrate surface.

Diamond Film Preparation by Arc Discharge Plasma Jet Chemical Vapor Deposition in the Methane Atmosphere

Abstract: Diamond films are synthesized by arc discharge plasma jet chemical vapor deposition in the methane atmosphere. The apparatus developed generates a stabilized plasma jet continuously for more than 12h. The plasma jet consists of hydrogen and argon. is used as an atmospheric gas and is mixed into the plasma jet. The plasma jet is sprayed onto a cool‐down substrate, and a diamond film is deposited on the substrate surface. By means of this method, a diamond film is deposited on a molybdenum substrate at the growth rate of 930 μm/h. The crystallinity of the diamond film measures well by means of x‐ray diffraction and Raman spectroscopy. The weight of diamond obtained by this method in a unit time seems to be almost the same as that obtained by high‐temperature and high‐pressure synthesis method.

The effect of oxygen in diamond deposition by microwave plasma enhanced chemical vapor deposition

Abstract: High quality diamond thin films were deposited on different substrates at temperatures from 300 to 1000 °C by the microwave plasma enhanced chemical vapor deposition (MPCVD) system. The quality of deposited diamond films was improved by adding oxygen in the gas mixtures. Different ratios of methane to oxygen concentration in hydrogen at different temperatures have been studied. At high temperatures (800–1000 °C), the addition of oxygen will not only enhance the growth rate of deposited films but also extend the region of diamond formation. At low temperatures ( 900 °C) were either graphitic or diamond containing a large amount of graphitic or amorphous carbon and at low temperatures (<500 °C) were white, soot-like coatings which were easily scraped off. The quality of the deposited films was characterized by Raman spectroscopy and scanning electron microscopy.

Tight-binding molecular-dynamics study of phonon anharmonic effects in silicon and diamond

Abstract: The anharmonic effects on phonons in silicon and diamond have been studied by molecular-dynamics simulations using an empirical tight-binding Hamiltonian. One-phonon spectral intensities of the zone-center and zone-boundary ({ital X}) modes have been calculated through the Fourier transform of the velocity-velocity correlation functions. This scheme allows a quantitative and nonperturbative study of phonon frequency shift and phonon linewidth as a function of temperature. The results obtained are in good agreement with experimental data.

Ubiquitous interstellar diamond and SiC in primitive chondrites: abundances reflect metamorphism

Abstract: It is shown here that interstellar diamond and SiC were incorporated into all groups of chondrite meteorites. Abundances rapidly go to zero with increasing metamorphic grade, suggesting that metamorphic destruction is responsible for the apparent absence of these grains in most chondrites. In unmetamorphosed chondrites, abundances normalized to matrix content are similar for different classes. Diamond samples from chondrites of different classes have remarkably similar noble-gas constants and isotropic compositions, although constituent diamonds may have come from many sources. SiC seems to be more diverse, partly because grains are large enough to measure individually, but average characteristics seem to be similar from meteorite to meteorite. These observations suggest that various classes of chondritic meteorites sample the same solar system-wide reservoir of interstellar grains.

Properties of Boron-Doped Epitaxial Diamond Films

Abstract: Boron-doped homo-epitaxial films were deposited by microwave plasma CVD on synthesized single-crystal diamonds. Boron concentration in the films was determined by the boron concentration in the reactant gases. The crystallinity of boron- doped films was found to be affected by the boron concentration in the reactant gases. The conductivity of boron doped films was determined by the concentration of boron, and the activation energy was nearly the same as for natural IIb diamonds. The effect of the surface orientation of substrates was found in the conductivity of highly doped films. The result of the Hall effect measurement indicated that the mobility of boron-doped diamond epitaxial films was 70 cm2/Vs even at 500°C.

Methyl versus acetylene as diamond growth species

Abstract: We have modeled plasma-assisted diamond growth on substrates placed in a high velocity 1-dimensional flow. The gas consisted of methane or acetylene injected into a flow of partially dissociated hydrogen gas at 800 °C. Diamond is formed only near the injector. More diamond is formed when methane is the additive, and Raman spectra show that the quality of the diamond films is also higher when methane is the additive. The model, which includes detailed chemistry, convection, concentration diffusion, and thermal diffusion, shows that with this experimental arrangement only methane and methyl radicals are present in significant quantities when methane is added, while only acetylene is present when acetylene is added. We conclude that (1) Diamond films can be grown directly from methyl radicals (or, possibly, from methane) and from acetylene. This suggests that a variety of hydrocarbons could act as growth species. (2) An environment containing methane and methyl is much more effective for growing diamond films than one containing acetylene. (3) The quality of the diamond film depends on the identity of the growth species, with acetylene producing lower quality films than methyl (or methane). (4) The fall-off in diamond formation with distance from the injector is due to destruction of species crucial to diamond growth on the silicon substrates.

The effect of environment on the tribological properties of polycrystalline diamond films

Abstract: Essentially pure, ~15 µm thick, polycrystalline diamond films DC-PACVD deposited on polycrystalline α–SiC flats were friction and wear tested against similarly coated α–SiC pins The oscillatory sliding tests were performed with a Knudsen cell-type, wide temperature range tribometer built into a scanning electron microscope Experiments were completed in 133 × 10–3 Pa (1 × 10–5 Torr) and 133 Pa (1 × 10–11 Torr) Pair, at test flat temperatures cycled to 850 °C and back to room temperature The results are compared with similar tests previously completed with diamond versus bare a-SiC and diamond versus Si(100) sliding combinations In the absence of in situ surface analytical capability in the SEM tribometer, the findings are interpreted based on relevant information collected from the literature The data indicate that the friction of the respective, tangentially sheared interfaces depends on the generation and annihilation of dangling bonds Desorption of hydrogen during heating and sliding under the electron beam, in vacuum, create unoccupied orbitals on the rubbing surfaces These dangling bonds spin-pair with others donated from the counterface, leading to high friction Resorption of adsorbates such as hydrogen (eg, by cooling the tribosystem) lowers the interfacial adhesion and friction At high temperatures, in vacuum, the interaction energy may also be reduced by surface reconstruction and graphitization At high temperatures, in Pair, the coefficient of friction of diamond versus itself is substantially lower than that in vacuum This reduction is most probably caused by the generation of oxidation products combined with the temperature-shear-oxygen-induced phase transformation of diamond to graphite The wide temperature wear rates of pure, polycrystalline diamond, characteristic of our test procedure, varied from ~4 × 10–16 m3/N • m in 133 × 10–3 Pa vacuum to ~1 × 10–15 m3/N • m in 133 Pair

A spectroscopic study of optical centers in diamond grown by microwave-assisted chemical vapor deposition

Abstract: Absorption and cathodoluminescence spectra have been recorded for single crystals of diamond and polycrystalline films of diamond, grown by microwave-assisted chemical vapor deposition (CVD) using methane and hydrogen. The investigation has been carried out to see to what extent the properties of CVD diamond are similar to those of conventional diamond, and to what extent they are unique. Studies have been made of the as-grown material, which has not been intentionally doped, and also samples that have been subjected to radiation damage and thermal annealing. The single crystals grown using methane concentrations of 0.5 to 1.0% exhibit bright blue “band A” emission and also intense edge emission, similar to the cathodoluminescence spectra of some natural type IIa diamonds. This implies that the crystals are relatively free from structural and chemical defects, a conclusion which is reinforced by the absence of any zero-phonon lines in the absorption spectra of crystals which have been subjected to radiation damage and annealing at 800 °C. Before radiation damage the spectrum does, however, reveal an absorption which increases progressively to higher energies, and which may be associated with sp2-bonded carbon. The cathodoluminescence spectra after radiation damage indicate that the crystals contain some isolated nitrogen, and the detection of H3 luminescence, following thermal annealing at 800 °C, demonstrates for the first time that these samples contain small concentrations of nitrogen pairs. All of the polycrystalline films, grown using methane concentrations between 0.3 and 1.5%, have an absorption which increases progressively to higher energies, and which again is attributed to sp2-bonded carbon. This absorption is stronger in the films grown using higher methane concentrations. Films grown at a methane concentration of 0.3% also exhibit bright blue cathodoluminescence, although the edge emission is undetectably weak. The use of higher methane concentrations produces films with evidence in the cathodoluminescence spectra of nitrogen + vacancy and nitrogen + interstitial complexes, as well as optical centers unique to CVD diamond. One particular defect produces an emission and absorption line at 1.681 eV. By implanting conventional diamonds with 29Si ions it has been confirmed that this center involves silicon, and it has been shown that the 1.681 eV luminescence is relatively more intense in implanted diamonds which have a high concentration of isolated nitrogen.

A thermally activated solid state reaction process for fabricating ohmic contacts to semiconducting diamond

Abstract: Techniques have been developed to produce ohmic contacts to naturally occurring boron doped semiconducting diamond. Thin films of Mo, Mo/Au, and Mo/Ni/Au deposited on diamond produced adherent ohmic contacts after annealing at 950 °C. A thermally activated solid state reaction which produces a refractory carbide precipitate at the original diamond/metal interface is the principal factor in affecting the properties of the contacts. The interface reaction has been characterized using Auger electron spectroscopy, scanning electron microscopy, x‐ray diffraction, metallography, and I‐V measurements.

Drill bit having improved cutter configuration

Abstract: Drill bits may include cutting members which have cutting faces formed of segments of differing cutting materials. The faces of the cutting members may include two or more segments, with the segments formed from at least two different materials. For example, a first segment could be formed of a polycrystalline diamond compact surface while a second segment could be formed of a thermally stable diamond product material.