In situ plasma diagnostics of the chemistry behind sulfur doping of CVD diamond films

Abstract

Microwave plasma enhanced chemical vapour deposition (CVD) has been used to grow sulfur doped diamond films using a 1% CH4/H2 gas mixture with various levels of H2S addition (100–5000 ppm), upon undoped Si substrates. X-Ray photoelectron spectroscopy has shown that S is incorporated into the diamond at number densities (≤0.2%) that are directly proportional to the H2S concentration in the gas phase. Four-point probe measurements showed the resistivity of these S-doped films to be a factor of three lower than undoped diamond grown under similar conditions. Sulfur containing diamond film was also obtained using a 0.5% CS2/H2 gas mixture, although the high resistivity of the sample indicated that the sulfur had been incorporated into the diamond lattice in a different manner compared with the H2S grown samples. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of H2S doping levels (1000–10000 ppm in the gas phase). CS and CS2 have been detected in significant concentrations in the plasma region as a result of gas phase reactions. Additional measurements from a 1% CS2/H2 plasma gave similar species mole fractions except that no CS was detected. These results suggest that CS may be the first step toward CS bond formation in the film and thereby a pathway allowing S incorporation into diamond. Optical emission spectroscopy has shown the presence of S2 in both gas mixtures, consistent with the observed deposition of sulfur on the cool chamber walls.