Evidence for rhombohedral boron nitride in cubic boron nitride films grown by ion-assisted deposition.

Abstract

We present high-resolution transmission electron-microscopic observations of the ${\mathit{sp}}^{2}$-bonded material that remains with the ${\mathit{sp}}^{3}$-bonded cubic boron nitride (cBN) in films grown by ion-assisted deposition. These observations show regions of ${\mathit{sp}}^{2}$-bonded material that are in a three-layer stacking configuration rather than the two-layer configuration of hexagonal boron nitride. Measurement of the lattice fringe angles shows that the observed three-layer stacking is consistent with the metastable, rhombohedral structure (rBN). Significantly, rBN allows for a diffusionless pathway for cBN synthesis under high pressure, unlike the high-activation-energy route that is required to directly convert the hexagonal phase to cBN. This low-energy pathway is considered in relation to recent work in the literature indicating that ion-induced compressive stress plays a critical role in the synthesis of thin-film cBN.