  The structural modifications of polycrystalline hexagonal boron nitride
implanted with He+ ion beams at energies between 200 keV and 1.2 MeV to
fluences of 1.0 \times 1017 ions \cdot cm-2 were investigated using micro-Raman
spectroscopy. The measured Raman spectra show evidence of implantation-induced
structural transformations from the hexagonal phase to nanocrystalline cubic
boron nitride, rhombohedral boron nitride and amorphous boron nitride phases.
The first-order Longitudinal-Optical cBN phonon was observed to be downshifted
and asymmetrically broadened and this was explained using the spatial
correlation model coupled with the high ion implantation-induced defect
density.
