Diamond growth in a direct-current low-pressure supersonic plasmajet

Abstract

Abstract A direct-current, water-cooled hydrogen plasmajet was designed and built. The arc burning between a thoriated tungsten cathode and a Laval nozzle acting as anode was operated at a power level of 4 kW. The plasmajet expands supersonically into a vacuum vessel in which a pressure of a few millibars is sustained. Methane as carbon-containing species is mixed into the decaying plasma close to the exit of the Laval nozzle. Molybdenum plates were used as substrates for diamond deposition. The substrates were placed on a water-cooled movable substrate holder, which allowed depositions at distances ranging from 2 to 30 cm. The bulk temperature of the substrate was monitored by a thermocouple. The deposited coatings were analysed by Raman spectroscopy for diamond phase purity and by scanning electron microscopy for morphology. The film thickness was determined by a Talley Surf profilometer and by optical thickness analysis. At the conditions investigated, a maximum growth rate of about 6 μm h−1 was observed. The growth rate showed a maximum at a distance of 6–10 cm from the nozzle. With increasing distance from the nozzle the growth rate as well as the crystallite size decreases and the diamond phase purity as reflected in the Raman spectra deteriorates. An alteration of arcjet power had only little effect on growth rate and no effect on film quality.