EVIDENCE FOR RHOMBOHEDRAL BORON-NITRIDE IN CUBIC BORON-NITRIDE FILMS GROWN BY ION-ASSISTED DEPOSITION

We present high-resolution transmission electron-microscopic observations of the sp(2)-bonded material that remains with the sp(3)-bonded cubic boron nitride (cBN) in films grown by ion-assisted deposition. These observations show regions of 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.