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

We present high-resolution transmission electron-microscopic observati ons of the sp(2)-bonded material that remains with the sp(3)-bonded cu bic boron nitride (cBN) in films grown by ion-assisted deposition. The se observations show regions of sp(2)-bonded material that are in a th ree-layer stacking configuration rather than the two-layer configurati on of hexagonal boron nitride. Measurement of the lattice fringe angle s shows: that the observed three-layer stacking is consistent with the metastable, rhombohedral structure (rBN). Significantly, rBN allows f or a diffusionless pathway for cBN synthesis under high pressure, unli ke the high-activation-energy route that is required to directly conve rt the hexagonal phase to cBN. This low-energy pathway is considered i n relation to recent work in the literature indicating that ion-induce d compressive stress plays a critical role in the synthesis of thin-fi lm cBN.