Growth and oxidation of boron-doped diamond films
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Curtis E. Johnson Naval Air Warfare Center, Weapons Center, China Lake, California 93555 (Received 31 May 1994; accepted 7 March 1995)
Boron-doped diamond films have been grown by the hot filament chemical vapor deposition process. The feed gas was a mixture of argon, bubbled through a solution of B 2 O 3 in ethanol, and hydrogen. The highest growth rate was 0.7 /um/h. The boron concentration in the films depended on the concentration of B 2 O 3 in the ethanol. The highest boron doping level, as measured by secondary ion mass spectroscopy, was 6300 atomic ppm. Raman spectroscopy and x-ray diffraction both confirmed the presence of crystalline diamond in the films. The frequency of the diamond Raman line decreased with increasing boron concentration. This shift may arise from an interaction of the charged carriers (holes) produced by the boron doping and the Raman-active optic phonon. The oxidation rates of doped and undoped films were measured by thermogravimetric analysis at 700 °C in flowing high purity oxygen. Films with a boron concentration of 6300 ppm oxidized at one-tenth the rate of undoped diamond. A layer of B 2 O 3 , detected on the surface of an oxidized B-doped film, is believed to act as a protective barrier that decreases the oxidation rate.
I. INTRODUCTION Diamond has a combination of unique physical properties: it is the hardest known material, it has the highest thermal conductivity of any material at room temperature, it is chemically inert, it has low electrical conductivity, and it is optically transparent over large regions of the electromagnetic spectrum.' The physical properties of polycrystalline chemical vapor deposited (CVD) diamond films may depend on the film texture, the film microstructure, and amount of non-diamond carbon phases incorporated in the film. An undesirable characteristic of diamond is its susceptibility to oxidation when heated. When natural diamond is heated in the presence of oxygen, the onset of oxidation can occur at temperatures as low as 900 K.1 Polycrystalline diamond films also show poor oxidation resistance. If a method can be found to make diamond more oxidation-resistant, the use of diamond could be significantly extended. Simons and Cannon2 have reported that boron-doped (B-doped) diamond produced by the high-pressure hightemperature (HPHT) process oxidized at 1/70 the rate of undoped HPHT diamond and at 1/100 the rate of a natural diamond. The oxidation behavior of undoped CVD diamond3"6 and of CVD diamond doped with boron to an unspecified level7 have been reported. The boron doping appears to reduce the oxidation rate of CVD diamond as well. The present study was undertaken to determine, quantitatively, how the level of B-doping affects the 1448 http://journals.cambridge.org
oxidation rate of CVD diamond. B-doped diamond films were grown by the hot filament CVD (HFCVD) process, and the oxidation rate of the films was measured by thermogravimetric analysis (TGA). Boron concentrations in the films were determined by secondary ion mass spectrometry (SIMS).
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