M.N.R. Ashfold

Use of different excitation wavelengths for the analysis of CVD diamond by Laser Raman Spectroscopy

Abstract Raman spectroscopy has been shown to be an accurate technique for the qualitative characterisation of chemical vapour deposited (CVD) diamond films. The intensities of the diamond and non-diamond components in the spectrum vary with the wavelength of the laser excitation. This shows that laser Raman at different wavelengths can be used as a selective probe for the different constituents of the deposited film. In the present work, this selectivity has been used to examine the effect of methane concentration during growth on the Raman spectra of CVD diamond. Diamond films were deposited on single crystal Si(100) wafer substrates by microwave plasma enhanced, and hot filament assisted CVD. Methane concentrations of 0.36–2.16% in hydrogen were used as the feedstock. Laser wavelengths ranging from the ultraviolet (244 nm) to the infra-red (780 nm) were used to perform Raman spectroscopy on the deposited diamond films. Scanning electron microscopy (SEM) was used to determine the morphology of the films and related to the Raman spectra.

Studies of phosphorus doped diamond-like carbon films

Phosphorus doped diamond-like carbon (DLC ) films have been deposited on Si substrates using CH 4 -based RF plasmas with addition of 0‐21% PH 3 into the source gas mixture. Scanning Auger studies reveal that the films contain P:C ratios as high as 0.89, and that the degree of P incorporation is roughly proportional to the PH 3 concentration in the gas phase. Reduction in intensity and finally loss of the laser Raman G-band with increasing P content in the film shows that excessive P incorporation causes amorphisation of the film. The electronic properties of the films, such as field emission threshold and optical band gap, are a complicated function of film composition. Minimum field emission thresholds occur at P:C ratios of around 0.02, and voltage bias values ~30% lower than that for the undoped film. Annealing in vacuum at 150°C can improve the field emission threshold of the low P content films by a factor of four.

CVD diamond wires and tubes

Abstract Diamond has been uniformly deposited onto the surface of thin metal wires using hot filament CVD. The diamond-coated wires are stronger and stiffer than the uncoated wires. Subsequent etching of the metal core in a suitable chemical reagent allows free-standing diamond tubes to be made, the typical dimensions being 1 cm long with an internal diameter of 10–150 μm. The formation of a thick, chemical-resistant carbide layer at the metal-diamond interface when using Ti and W wires is investigated.

In situ plasma diagnostics of the chemistry behind sulfur doping of CVD diamond films

Microwave plasma enhanced chemical vapour deposition (CVD) has been used to grow sulfur doped diamond films using a 1% CH yH gas mixture with various levels of H S addition (100–5000 ppm), upon undoped Si substrates.X-Ray photoelectron 42 2 spectroscopy has shown that S is incorporated into the diamond at number densities (F0.2%) that are directly proportional to the H S concentration in the gas phase.Four-point probe measurements showed the resistivity of these S-doped films to be a 2 factor of three lower than undoped diamond grown under similar conditions.Sulfur containing diamond film was also obtained using a 0.5% CS yH gas mixture, although the high resistivity of the sample indicated that the sulfur had been incorporated into 22 the diamond lattice in a different manner compared with the H S grown samples.Molecular beam mass spectrometry has been 2 2 plasma region as a result of gas phase reactions.Additional measurements from a 1% CS yH plasma gave similar species mole 22 fractions except that no CS was detected.These results suggest that CS may be the first step toward C–S bond formation in the film and thereby a pathway allowing S incorporation into diamond.Optical emission spectroscopy has shown the presence of S 2 in both gas mixtures, consistent with the observed deposition of sulfur on the cool chamber walls. 2002 Elsevier Science B.V. All rights reserved.

Diamond deposition in a DC-arc Jet CVD system: investigations of the effects of nitrogen addition

Abstract Studies of the chemical vapour deposition of diamond films at growth rates >100 μm h −1 with a 10-kW DC-arc jet system are described. Additions of small amounts of N 2 to the standard CH 4 /H 2 /Ar feedstock gas results in strong CN(B→X) emission, and quenches C 2 (d→a) and H α emissions from the plasma. Species selective, spatially resolved optical emission measurements have enabled derivation of the longitudinal and lateral variation of emitting C 2 , CN radicals and H ( n =3) atoms within the plasma jet. Scanning electron microscopy and laser Raman analyses indicate that N 2 additions also degrade both the growth rate and quality of the deposited diamond film; the latter technique also provides some evidence for nitrogen inclusion within the films.

Gas-phase concentration measurements and diamond film composition from chlorine assisted CVD

Abstract Molecular beam mass spectrometry has been used to obtain quantitative measurements of the composition of the gas-phase species prevailing during diamond chemical vapour deposition (CVD) using a variety of chlorine containing source gases. Gas mixtures used were 1% of a chlorinated methane (CH4 −n,Cln,n = 1 − 4) in H2 and 1% CH4 in H2 with added chlorine varying from 1%–4%. At filament temperatures optimum for diamond growth (≈2300 °C) the relative concentrations of the various hydrocarbon species (CH4, C2H2, C2H4) in the gas mixture are remarkably similar to those measured when the carbon precursor species is CH4. At these filament temperatures almost all the chlorine is reduced to HCl, its concentration being proportional to the chlorine fraction in the source gas, regardless of the form of the chlorine in the input mixture. Auger electron spectroscopy analysis of the as-grown diamond films indicated that no chlorine was present in the bulk of the films, though trace amounts of chlorine were detected on the film surface. These observations are consistent with the supposition that chlorine atoms are involved in the gas-surface reactions which produce active growth sites on the diamond surface.

Young's modulus of diamond-coated fibres and wires

Abstract Diamond-coated fibres and wires were produced by hot filament chemical vapour deposition (HFCVD) of diamond on a variety of core materials including tungsten(W) and silicon carbide (SiC). Fibres with a diamond volume fraction exceeding 95% have been produced. Three different methods of measuring the fibre Youngs modulus(a resonance method, a bend test and a tensile test) are presented, together with recent results. Possible applications for such fibres include reinforcements in metal matrix composites (MMCs).

Simulation of H-C-S containing gas mixtures relevant to diamond chemical vapour deposition

Mole fractions of 24 species present within x%H Sy1%CH yH (xs0–1%) and 1%CS yH gas mixtures have been calculated 24 2 2 2 using the CHEMKIN computer package in conjunction with a mechanism based on the composite conversion: CH q 4 2H S|CS q4H .Arrhenius parameters for each elementary reaction involving S-containing species are presented, along with 22 2 associated thermodynamic properties for each species.Molecular beam mass spectrometric measurements of species mole fractions in microwave activated x%H Sy1%CH yH (xs0–1%) mixtures agree well with the model calculations, if we assume a 24 2 (reasonable) gas temperature of 1630 K.The agreement between similar measurements of both 0. 5%H S y1%CH yH and 24 2 1%CS yH hot filament activated gas mixtures is less good, but the calculations succeed in reproducing many of the observed 22 trends in species mole fraction with change in filament temperature.

Phosphorus carbides: theory and experiment

The recent finding that radio frequency plasma activation of CH(4)/PH(3) gas mixtures can yield films with P : C ratios < or = 3 has served to trigger further research into new phosphorus carbide materials. Theoretical and experimental results relating to periodic and amorphous materials, respectively, are presented here: (i) The electronic structure and stability of different crystalline phosphorus carbide P(x)C(y) phases have been studied using first-principles density-functional theory. Calculations have been carried out for P(4)C(3+8 n) (n= 0-4), PC, and PC(3) and the most likely periodic structures examined in detail. Particular attention is paid to the composition PC(3), for which there are several possibilities of similar energy. (ii) Recent experimental efforts have involved use of pulsed laser ablation methods to produce hydrogen-free phosphorus carbide thin films. Mechanically hard, electrically conducting diamond like carbon films containing 0- approximately 26 at.% P have been deposited on both Si and quartz substrates by 193 nm PLA of graphite/phosphorus targets (containing varying percentages of phosphorus), at a range of substrate temperatures (T(sub)= 298-700 K), in vacuum, and analysed via laser Raman and X-ray photoelectron spectroscopy.

Molecular beam mass spectrometry investigations of low temperature diamond growth using CO2/CH4 plasmas

Diamond films have been successfully deposited at substrate temperatures as low as 435C using CO CH gas mixtures in a 24 Ž. microwave plasma chemical vapour deposition CVD reactor. In order to understand why it is possible to grow diamond at these low temperatures using these gases, we have performed the first in situ molecular beam mass spectrometry studies to measure, simultaneously, the concentrations of the dominant gas phase species present during growth over a wide range of plasma gas Ž. mixtures 080% CH , balance CO . Optical emission spectroscopy has also been used to investigate gas phase species present 42 in the microwave plasma. These experimental measurements give further evidence that CH radicals may be the key growth 3 species and suggest that CO may be of greater importance to the plasma chemistry of CO CH gas mixtures than previously 24 thought. 2001 Elsevier Science B.V. All rights reserved.