Diamond films grown from fullerene precursors

Abstract

Fullerene precursors have been shown to result in the growth of diamond films from argon microwave plasmas. In contradistinction to most diamond films grown using conventional methane-hydrogen mixtures, the fullerene-generated films are nanocrystalline and smooth on the nanometer scale. They have recently been shown to have friction coefficients approaching the values of natural diamond. It is clearly important to understand the development of surface morphology during film growth from fullerene precursors and to elucidate the factors leading to surface roughness when hydrogen is present in the chemical vapor deposition (CVD) gas mixtures. To achieve these goals, we are measuring surface reflectivity of diamond films growing on silicon substrates over a wide range of plasma processing conditions. A model for the interpretation of the laser interferometric data has been developed, which allows one to determine film growth rate, root mean square (rms) surface roughness, and bulk losses due to scattering and absorption. The rms roughness values determined by reflectivity are in good agreement with atomic force microscope (AFM) measurements. A number of techniques, including high-resolution transmission electron microscopy (HRTEM) and near-edge x-ray absorption fine structure (NEXAFS) measurements, have been used to characterize the films. A mechanism for diamond-film growth involving the C2 molecule as a growth species is presented. The mechanism is based on (1 the observation that the optical emission spectra of the fullerene-containing plasmas are dominated by the Swan bands of C₂ and (2) the ability of C₂ to insert directly into C-H and C-C bonds with low activation energy barriers, as shown by recent theoretical calculations of reactions of C₂ with carbon clusters.