Hartmut Lade

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.

Thermoluminescence spectroscopy of Eu2+ and Mn2+ doped BaMgAl10O17

Abstract Thermoluminescence (TL) studies of Eu2+ and Mn2+ doped BaMgAl10O17 (BAM) are reported and discussed. The TL spectra that are measured after irradiation with ultraviolet (120– 350 nm ) show a series of TL peaks between 100 and 300 K . The TL spectra are similar for BAM with the two dopants, which suggest that the shallow traps are typical for the BAM host lattice. Using the Hoogstraaten analysis trap depths between 0.1 and 0.2 eV are determined. A model is proposed based on thermally activated recombination in local TL centres (not via the conduction band). Further support for this model is obtained from the observation that the TL signal is strongest for excitation around the band edge of BAM (∼180 nm ) . Upon heating the samples in air all low temperature TL peaks decrease in intensity. In addition a new peak appears in the TL spectrum, which is connected with a deeper trap (0.3 eV ) and also a partial oxidation of Eu2+ to Eu3+ is observed. The luminescence efficiency is lower and the UV induced degradation is faster after annealing in air. These results indicate that the shallow traps are related to oxygen vacancies. The shallow traps do not have a negative influence on performance (efficiency and degradation) of BAM as a lighting phosphor. The luminescence efficiency and stability are strongly influenced by the formation of Eu3+ and a deeper trap during annealing in air. Subsequent annealing in a reducing atmosphere restores the original properties.

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.

Influence of surface modifications on the electronic properties of CVD diamond films

The effects of different surface treatments, including ex-situ H2 and H2+O2 plasma exposure, chromic acid treatment and in-situ vacuum annealing at elevated temperatures, on the electronic properties, particularly the electron affinity of microwave plasma-grown polycrystalline diamond films, were investigated using UV photo-electron spectroscopy (UPS) and X-ray photo-electron spectroscopy (XPS). H2 and H2 + O2 plasma exposure results in a negative electron affinity (NEA) for all diamond films, independent of morphology, thickness or phase purity. An additional peak in the region of low kinetic energies of the UPS spectra correlates with plasma-generated defects that are removed by in-situ vacuum annealing for several minutes at 700 °C. NEA is not affected by this annealing procedure. Oxidation of the diamond surface by hot chromic acid results in a positive electron affinity (PEA) that correlates with a pronounced increase in film surface resistivity and complete suppression of electron emission. NEA alone is not sufficient to ensure good electron emission properties.

The performance of depressed-cladding single-mode fibers with different b/a ratios

Depressed-cladding single-mode fibers with a first cladding-to-core ratio (b/a) ranging from seven to one and relative refractive-index differences of the core from 0.25 to 0.35% and of the cladding from 0 to -0.2% are investigated experimentally with respect to the LP/sub 11/ mode cutoff properties, the mode-field diameter, the dispersion, and the bending attenuation. Numerical calculations based on actual profile parameters of the bend loss and the mode-field diameter agree well with experiment. It is demonstrated that low attenuation and bend losses independent of b/a can be obtained if the cutoff wavelength is kept constant by adjusting the core radius, the core index or the index of the first cladding. >

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.