Peter K. Bachmann

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.

Diamond deposition technologies

Abstract Over the last 10 years a wide variety of techniques for growing diamond at low pressure and low temperature has emerged from laboratories all over the world. In this article, the state-of-the-art of the major techniques is summarized. The individual methods are briefly described, references to the original articles are given and major advantages and drawbacks are listed. The article also includes the comments, remarks and discussions elucidated by the participants of the European Community Workshop European Diamond Thin Film Technologies for the Nineties throughout the Deposition Technologies session.

Optical characterization of diamond

Abstract The state-of-the-art of major optical techniques to characterize diamond and related carbon-based materials is reviewed. The focus is on Raman spectroscopy, transmission spectroscopy, luminescence spectroscopy and ellipsometry.

Stress in optical waveguides. 2: Fibers

The stress properties of GeO(2)- and F-doped optical fibers drawn in different conditions have been investigated. The results are in excellent agreement with calculated data based on a generalized theoretical model. For constant drawing forces the influence on the stress profiles was found to be independent of drawing speed and temperature. The total observed stress is the sum of preform stress and drawing-induced stress.

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.

CVD diamond: a novel high γ-coating for plasma display panels?

Minimization of the firing voltage of plasma display panels requires electrode coatings with high ion-induced secondary electron emission coefficients (γ-coefficients). This paper discusses the methodology of Paschen-curve-based γ-measurements and compares measured γ-coefficients of MgO, the standard material for this application, with data measured for CVD diamond of various thicknesses, a-C:H and ta-C films grown on glass. Ne, Ar and Xe discharges are investigated. The negative electron affinity (NEA) of H-terminated CVD diamond is shown to lead to high γ-values, stable operation and low plasma firing voltages that rival or even surpass those of the best quality MgO(111) single crystals with the advantage of diamond being less sensitive to panel processing conditions. Replacing the H-termination of diamond by positive electron affinity (PEA) O-termination results in a dramatic increase of the firing voltage, thus demonstrating the importance of the electron affinity for ion-induced secondary electron emission. Diamond is found to be particularly interesting for high Xe panel fillings. The γ-coefficients of DLC- and ta-C layers are significantly lower and show considerable instability and degradation over time. The advantages and drawbacks of diamond as a PDP cell coating are discussed.

High emission current density microwave-plasma-grown carbon nanotube arrays by postdepositional radio-frequency oxygen plasma treatment

Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.

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.

Stress in optical waveguides. 1: Preforms.

The intrinsic stress properties of GeO2- and F-doped optical fiber preforms have been investigated in detail. The materials were prepared by low-pressure plasma-induced chemical vapor deposition (PCVD), and the dopant concentrations cover the range normally used in optical fiber manufacture (+1% > Δ > −1%). Homogeneously doped preform regions exhibit a constant stress level. This level is exclusively dependent on dopant concentrations. In GeO2-doped silica the stress increases linearly with the dopant concentration. For F-doped silica, however, this dependency is strongly nonlinear. A negative stress difference between undoped PCVD material and the substrate tube material can be explained by the reduced thermal expansion coefficient of PCVD-SiO2 caused by chlorine incorporated during the deposition step. The experiments agree excellently with theoretical predictions based on thermal expansion data.

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.