The Effects of Boron Doping on the Bulk and Surface Acoustic Phonons in Single-Crystal Diamond

TL;DR

This study investigates how boron doping affects the acoustic and optical phonons in single-crystal diamond, revealing phonon softening and velocity reductions that impact thermal transport properties relevant for electronic applications.

Contribution

It provides experimental and theoretical evidence of phonon softening due to boron doping in diamond, highlighting its effects on phonon velocities and optical phonon frequencies.

Findings

Phonon velocities decrease by up to 15% with doping.

Optical phonon frequencies decrease significantly at high doping levels.

Theoretical calculations confirm phonon softening due to boron doping.

Abstract

We report the results of the investigation of bulk and surface acoustic phonons in the undoped and boron-doped single-crystal diamond films using the Brillouin-Mandelstam light scattering spectroscopy. The evolution of the optical phonons in the same set of samples was monitored with Raman spectroscopy. 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. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. The density-functional-theory calculations of the phonon band structure for the pristine and highly-doped sample confirm the phonon softening as a result of boron doping in diamond. The obtained results have important implications for thermal transport in heavily doped diamond, which is a promising material for ultra-wide-band-gap electronics.