UC Riverside

This dissertation describes the study of acoustic phonons in ultra-wide-band gap semiconductor materials using the Brillouin – Mandelstam and Raman light scattering spectroscopies. Acoustic phonons are the main heat carriers in this type of semiconductor materials and the knowledge of the phonon frequencies and group velocities is important for fundamental science and future practical applications. The Brillouin – Mandelstam spectroscopy allows the probing of both bulk and surface acoustic phonons. In the first part of the dissertation, I describe the investigation of bulk and surface acoustic phonons in the undoped and boron-doped diamond films. It was found that the frequency and the group velocity of acoustic phonons decrease non-monotonically with the increasing boron doping concentration, revealing pronounced phonon softening. The change in the velocity of the shear horizontal and the high-frequency pseudo-longitudinal acoustic phonons in the degenerately doped diamond, as compared to the undoped diamond, was as large as ~15% and ~12%, respectively. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. In the second part of the dissertation, I will describe the results of our 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. 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. These findings provide valuable insights into the phonon characteristics of the c-BN and diamond interfaces.