Nucleation Mechanism of Cubic Boron Nitride in Vapor Phase Deposition

Cubic boron nitride (cBN) thin films were deposited on ultrathin Si flake substrates by inductively coupled plasma-enhanced chemical vapor deposition. The prepared films were sufficiently thin for direct HRTEM observation without inducing damage in the cBN films. Isolated cBN nuclei as small as 1 nm layer were observed on the textured tBN. The surface of cBN films was pure cubic phase without sp2-bonded BN, contrary to the so-called subsurface growth mechanisms. The chemical species responsible for cBN growth were analyzed by quadrupole mass analysis with ion energy analyzer (QMA-EA). QMA-IE analyses revealed that the boron source, B2H6, was totally ionized under the deposition conditions, while N2 was only partly ionized. The existence of neutral species such as N2 molecules inhibited cBN nucleation and growth. The introduction of Ar into the deposition system increased the N2 ionization yield markedly, which reduced the sp2-bonded BN formation from neutral N2 molecules on the growth surface and promoted cBN growth. In summary, both damage-free HRTEM observations and QMA-IE analyses suggest that cBN nucleates at a top surface rather than in subsurface region in vapor phase deposition.