Morphology and electron emission properties of nanocrystalline CVD diamond thin films

Nanocrystalline diamond thin films have been produced by microwave plasma-enhanced chemical vapor deposition (MPECVD) using C₆₀/Ar/H₂ or CH₄/Ar/H₂ plasmas. Films grown with H₂ concentration ≤20% are nanocrystalline, with atomically abrupt grain boundaries and without observable graphitic or amorphous carbon phases. The growth and morphology of these films are controlled via a high nucleation rate resulting from low hydrogen concentration in the plasma. Initial growth is in the form of diamond, which is the thermodynamic equilibrium phase for grains ≤5 nm in diameter. Once formed, the diamond phase persists for grains up to at least 15-20 nm in diameter. The renucleation rate in the near-absence of atomic hydrogen is very high (approximately 10¹⁰ cm^-2 sec^-1), limiting the average grain size to a nearly constant value as the film thickness increases, although the average grain size increases as hydrogen is added to the plasma. For hydrogen concentrations less than approximately 20%, the growth species is believed to be the carbon dimer, C₂, rather than the CH₃* growth species associated with diamond film growth at higher hydrogen concentrations. For very thin films grown from the C₆₀ precursor, the threshold field (2 to approximately 60 volts/micron) for cold cathode electron emission depends on the electrical conductivity and on the surface topography, which in turn depends on the hydrogen concentration in the plasma. A model of electron emission, based on quantum well effects at the grain boundaries is presented. This model predicts promotion of the electrons at the grain boundary to the conduction band of diamond for a grain boundary width approximately 3-4 angstroms, a value within the range observed by TEM.