Resonant Raman scattering in cubic and hexagonal boron nitride

We measured first- and second-order Raman scattering in cubic and hexagonal boron nitride using excitation energies in the visible and in the UV. The nonresonant first-order Raman susceptibilities for cubic and hexagonal BN are 1 and $10\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{2}$, respectively. Raman scattering is thus very powerful in detecting the hexagonal phase in mixed thin boron nitride films. In cubic BN the constant Raman sucseptibility in the visible and the UV is due to its indirect band gap. For hexagonal BN a Raman enhancement is found at $5.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is well explained by the energy dependence of the dielectric function of hexagonal BN. The second-order spectrum of cubic boron nitride is in excellent agreement with first-principles calculations of the phonon density of states. In hexagonal BN the overbending of the LO phonon is $\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, five times larger than in graphite.