Dieter Leers

Towards a general concept of diamond chemical vapour deposition

Abstract A C H O phase diagram is introduced providing a common scheme for all major diamond chemical vapour deposition (CVD) methods used to date. It reveals that low pressure diamond synthesis is only feasible within a well-defined field of the phase diagram, a diamond domain that allows general predictions of gas phase compositions and starting materials suitable for diamond synthesis. It gives an explanation for the quality variations of diamonds deposited from different gas mixtures. The concept originates from the thorough analysis of experiments by thermal CVD, hot filament CVD, various plasma deposition techniques and combustion flames. This analysis also shows that the large deposition rate differences between the various methods correlate well with the coirresponding gas temperatures, indicating that a hot spot in the gas phase fosters high rate diamond growth.

Raman and X-ray studies of polycrystalline CVD diamond films

Abstract Differently prepared microwave plasma-deposited diamond films with a broad spectrum of morphological and Raman spectroscopic features were investigated by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction (XRD). XRD indicates the presence of graphitic polytypes in most samples, independent of growth conditions or morphology. X-ray texture analysis reveals pronounced fibre textures not only for well-faceted deposits, but also for smooth, fine grain films. Information about internal strain, stacking faults and average crystallite sizes is obtained from diffraction peak shifts and widths. Crystallite sizes deduced from XRD profiles are found to be orders of magnitudes smaller than the grain sizes observed by SEM. An analysis of the diamond Raman peak position, width and shape suggests that Raman peak broadening is dominated by phonon lifetime reduction. Raman and X-ray diffraction data reveal a reciprocal relationship between the width of the diamond Raman line and the crystallite size.

Thermal properties of C/H-, C/H/O-, C/H/N- and C/H/X-grown polycrystalline CVD diamond

Abstract Over 60 CVD diamond films with thicknesses in the range 2–600 μm, grown from C/H, C/H/O, C/H/Cl and C/H/N gas mixtures by microwave plasma CVD, combustion flame synthesis and r.f. plasma torch CVD, were compared in terms of their thermal, morphological, Raman and luminescence data. Correlation diagrams reveal that the content of sp2-hybridized carbon is the main factor determining the thermal properties of the films. Other parameters, e.g. thickness, crystallinity and defects, only influence the thermal performance by changing the phase purity. The presence of oxygen and nitrogen in the CVD gas phase restricts the thermal conductivity of the films to values well below the 2200 ± 200 W m−1 K−1 achieved for polycrystalline films, 250 μm thick, grown from methane and hydrogen. Diamond films with thicknesses of less than 4 μm and thermal conductivities of more than 700 W m−1 K−1 were grown from C/H and C/H/O mixtures.

Post-depositional diamond etching

Abstract Differently prepared polycrystalline vapour deposited diamond films were exposed to pure oxygen for well defined periods of time at 700°C in order to monitor their time-dependent oxidation by scanning electron microscopy (SEM), photoluminescence measurements, and Raman spectroscopy. Raman and SEM data reveal that defective diamond or non-diamond carbon components located at the grain boundaries of the polycrystalline material are preferentially etched. The time-dependences of both Raman and luminescence intensities help to separate oxidation-resistant from oxidation-sensitive films and indicate that the presence of an interconnected network rather than the absolute concentration of non-diamond carbon is important for the oxidation behaviour. Nitrogen-induced defects in the vapour deposited films, manifested by a 1.83 eV luminescence line, seems to promote the formation of such a network and, above a threshold level, drastically enhance the oxidation sensitivity of diamond deposits.

Improved model for OH absorption in optical fibers

The OH absorption spectrum between 1200 and 1550 nm has been analyzed in plasma chemical vapor deposition (PCVD) optical fibers. Using these data, a model for the structure of the spectrum is developed so that the wings and tails can be taken into account properly. This model is able to correlate almost quantitatively the OH peak absorption intensity at approximately 1380 nm with the intensity of the absorption tails in the transmission windows around 1300 and 1550 nm. Three different OH species and two combination bands are involved in the model. An attempt is made to identify the spectral components, with special OH environments on a microscopic scale. >

Wear testing of CVD diamond films

Abstract A variety of diamond films with varying thicknesses from 40 to 1200 nm have been prepared by microwave plasma chemical vapour deposition (CVD). Their wear performance has been investigated as a function of morphology and layer thickness along with their surface roughness and Raman features. Accelerated wear testing was performed with a modified sphere-on-tape test using lapping tapes of differing abrasiveness. The wear data of the thin film deposits are compared with data obtained for bulk WC, AlN, Si, Al 2 O 3 and other materials. The CVD diamond films are shown to be much more wear resistant than other well-established low wear materials, almost independent of morphology and layer thickness. Layers with an RMS roughness above 20 nm are shown to damage the abrading tapes severely. Ultralow wear, smooth CVD diamond coatings, down to thicknesses of 40 nm, are shown to be feasible.