Howard Wilson

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.

Secondary electron emission measurements on synthetic diamond films

Abstract During the electron irradiation of synthetic diamond films, three successive regimes are encountered as a function of the electron dose: (1) a reduction of the downward band bending of energy levels at the sample surface because an excess of secondary electrons leaves the sample; (2) the creation of an internal electric field in which secondary electrons drift to the surface, leading to an appreciable increase in the secondary emission and to a linear relation between the primary electron energy and the secondary electron yield; and (3) the desorption of hydrogen terminating the carbon surface bonds. The secondary emission thus decreases to very low values. The rate of decrease of secondary emission is similar for C:H- and C:H:Ba-terminated diamond surfaces.