  We report an investigation of the bulk optical, bulk acoustic, and surface
acoustic phonons in thin films of turbostratic boron nitride (t-BN) and cubic
boron nitride (c-BN) grown on B-doped polycrystalline and single-crystalline
diamond (001) and (111) substrates. The characteristics of different types of
phonons were determined using Raman and Brillouin-Mandelstam light scattering
spectroscopies. The atomic structure of the films was determined using
high-resolution transmission electron microscopy (HRTEM) and correlated with
the Raman and Brillouin-Mandelstam spectroscopy data. The HRTEM analysis
revealed that the cubic boron nitride thin films consisted of a mixture of c-BN
and t-BN phases, with c-BN being the dominant phase. It was found that while
visible Raman spectroscopy provided information for characterizing the t-BN
phase, it faced challenges in differentiating the c-BN phase either due to the
presence of high-density defects or the overlapping of the Raman features with
those from the B-doped diamond substrates. In contrast, Brillouin-Mandelstam
spectroscopy clearly distinguishes the bulk longitudinal and surface acoustic
phonons of the c-BN thin films grown on diamond substrates. Additionally, the
angle-dependent surface Brillouin-Mandelstam scattering data show the peaks
associated with the Rayleigh surface acoustic waves, which have higher phase
velocities in c-BN films on diamond (111) substrates. These findings provide
valuable insights into the phonon characteristics of the c-BN and diamond
interfaces and have important implications for the thermal management of
electronic devices based on ultra-wide-band-gap materials.
