Paul W May

Diamond thin films: a 21st-century material

Diamond has some of the most extreme physical properties of any material, yet its practical use in science or engineering has been limited due its scarcity and expense. With the recent development of techniques for depositing thin films of diamond on a variety of substrate materials, we now have the ability to exploit these superlative properties in many new and exciting applications. In this paper, we shall explain the basic science and technology underlying the chemical vapour deposition of diamond thin films, and show how this is leading to the development of diamond as a 21st century engineering material.

XPS and laser Raman analysis of hydrogenated amorphous carbon films

Abstract Hydrogenated amorphous carbon films were deposited in an RF parallel plate plasma reactor using various values of process pressure (10–50 mTorr) and DC self-bias (0–300 V). The films were then analysed by laser Raman spectroscopy (LRS) at 514.5 nm and X-ray photoelectron spectroscopy (XPS). Values for the ratio of sp 2 :sp 3 bonded carbon in the various films were obtained by suitable fitting of the XPS carbon 1s energy peaks, using a three-curve fitting procedure, which recognises a portion of the peak attributable to CO surface bonding. The sp 3 content was found to depend upon the DC self bias (and hence the ion impact energy) during deposition, peaking at a value of 81% at approximately 150 V. The softer films grown at lower DC bias values still had an sp 3 content of approximately 70%. Microcombustion analysis showed that films deposited with low DC bias contained 7 at.% H compared to less than 2 at.% for films deposited at biases greater than 100 V. This high sp 3 content can be explained by H-termination of dangling bonds, suggesting that sp 3 content alone is not a reliable indication of film properties. Curve-fittings of LRS spectra of the films showed that the Breit–Wigner–Fano lineshape is inappropriate for use with hydrogen containing films. Fitting using a Gaussian profile gave precise values for the FWHM, intensity, and Stokes’ shift of the G and D-peaks. A linear relationship between the intensity ratio of the D to G peaks and the width of the G peak was found for films deposited at high DC bias (with low H content), but not for films deposited at low DC bias. This is consistent with the increased H content of the films causing a change in the elastic constants and/or affecting the stress levels within the films.

Reevaluation of the mechanism for ultrananocrystalline diamond deposition from Ar∕CH4∕H2 gas mixtures

Various mechanisms for the growth and renucleation of ultrananocrystalline diamond (UNCD) films are discussed and evaluated in the light of experimental and theoretical evidences in recent publications. We propose that the most likely model for UNCD growth is that where most of the diamond is formed via a similar mechanism to that of microcrystalline diamond films, i.e., gas phase H atoms abstracting surface hydrogens, followed by a CHx, x=0–3, addition. Calculations of the gas composition close to the substrate surface in the microwave plasma reactor for both the microcrystalline diamond and the UNCD growth, at substrate temperatures of 1073 and 673K, suggest that CH3 and C atoms are the most likely precursors for the growth of UNCD. However, the deposition is interrupted by an event which prevents the smooth growth of a continuous layer, and instead creates a surface defect which changes the growth direction and acts as a renucleation site. The possible nature of this event is discussed in detail. Using...

Microcrystalline, nanocrystalline, and ultrananocrystalline diamond chemical vapor deposition: Experiment and modeling of the factors controlling growth rate, nucleation, and crystal size

Ar∕CH4∕H2 gas mixtures have been used to deposit microcrystalline diamond, nanocrystalline diamond, and ultrananocrystalline diamond films using hot filament chemical vapor deposition. A three-dimensional computer model was used to calculate the gas phase composition for the experimental conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology, growth rate, and across-sample uniformity can be rationalized using a model based on competition between H atoms, CH3 radicals, and other C1 radical species reacting with dangling bonds on the surface. Proposed formulas for growth rate and average crystal size are tested on both our own and published experimental data for Ar∕CH4∕H2 and conventional 1% CH4∕H2 mixtures, respectively.

CVD diamond: a new technology for the future?

Abstract Diamond exhibits a unique range of properties including extreme hardness, mechanical strength, high thermal conductivity, broad-spectrum optical transparency, electrical insulating properties, and chemical resistance. However, its high cost and problems in manipulation have hitherto made it difficult to realize this potential. This situation has dramatically changed with the advent of techniques to confer the properties of diamond on a variety of substrates by depositing very thin diamond films on to them.

The New Diamond Age

After the hype, what realistic applications might synthetic diamond films have in the near future?

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.

Field emission conduction mechanisms in chemical vapor deposited diamond and diamondlike carbon films

Field emission properties of undoped chemical vapor deposited diamond and diamondlike carbon films have been measured for a variety of different deposition conditions. The nature and appearance of the damage site after testing, together with the mathematical form of the observed current–voltage relations, are correlated with the conductivity of the film. This is consistent with a model for the overall current which is a combination of conduction mechanisms through the bulk of the film with Fowler–Nordheim tunneling.

Examination of the effects of nitrogen on the CVD diamond growth mechanism using in situ molecular beam mass spectrometry

Abstract Molecular beam mass spectrometry (MBMS) has been used to obtain quantitative measurements of the composition of the gas-phase species prevalent during diamond hot filament CVD using a variety of nitrogen-containing source gas mixtures. The ratio of C:N in the feedstock was maintained at 1:1, and the gas mixtures used were 0.5% each of CH 3 NH 2 and HCN in H 2 , and 0.5% CH 4 in H 2 with added NH 3 and N 2 at 0.5 and 0.25% respectively. The deposition rate and resulting film quality at optimum growth temperatures depend critically on the origin of carbon-containing species. At the relevant process temperatures, most of the gas-phase carbon exists in the form of unreactive HCN (∼70–90%) for all precursor gas mixtures (except CH 4 /N 2 ), with very little C 2 H 2 detected. As a result, poor quality diamond films were deposited at rates of less than 0.1 μm h −1 . For CH 4 /N 2 mixtures, however, equal amounts of HCN and C 2 H 2 were detected, and well-faceted diamond films were produced at higher deposition rates (∼0.45 μm h −1 ). These observations are explained in terms of the effects that nitrogen, and its resulting reaction products (NH 3 , HCN, CH 3 NH 2 , etc.), have on the gas-phase chemistry occurring during the CVD process. In particular, we suggest that N 2 can act as a catalyst for the destruction of H atoms, which in turn leads to significant changes in the gas-phase chemistry.

Field emission from chemical vapor deposited diamond and diamond-like carbon films: Investigations of surface damage and conduction mechanisms

Field emission properties of undoped chemical vapor deposited diamond and diamond-like carbon films have been measured for a variety of different deposition conditions. The nature and appearance of the damage site after testing has been investigated with scanning electron microscopy and laser Raman mapping. These observations, together with the mathematical form of the observed current–voltage relations, are correlated with the conductivity of the film. The results are consistent with a model for the overall emission current that combines conduction mechanisms through the bulk of the film with Fowler–Nordheim tunneling.