Boron-Doped Diamond and the Discovery of New Properties in an Old Material

Doped semiconductors can possess metallic-like properties such as superconductivity and localized surface plasmon resonances. Among semiconductors, diamond stands apart because of its extraordinary mechanical, thermal, electronic, and optical properties. While diamond has been studied for many decades now, new properties continue to be unearthed because of the following: 1) synthetic challenges related to the thermodynamic equilibrium which can only be overcome under near ambient conditions by a handful of techniques including plasma-based chemical vapor deposition; 2) a limited number of impurity dopants that can produce electronic effects; and 3) the lack of measurement tools that can probe the relevant energy transitions.<br/><br/>In this talk, I will present our recent discovery of a yet another property in boron-doped diamond. Specifically, we report on intervalence band plasmons, defined as the collective electronic excitations between the valence subbands. To probe these low-energy (&lt;0.5 eV) transitions, we applied two relatively advanced techniques, scanning transmission electron microscopy-valence electron energy loss spectroscopy and photoinduced force infrared spectroscopy. The measured loss and absorbance spectra, respectively, are found to exhibit an intense signal at an energy of ~0.15 eV for boron-doped diamond, which is completely absent in undoped diamond. We carried out first-principle calculations and were able to reproduce the measured spectra based on the contributions of intervalence band transitions to the dielectric function. The calculations are then used to reveal that the real part of the dielectric function contains a resonance and a zero-crossing that energetically increases with carrier density, which are characteristic of metal-like collective excitations of a plasmon. While plasmonic behavior in doped semiconductors is well-documented, it has typically been attributed to Drude excitation of free charge carriers (e.g., holes). Our study shows the possibility of other mechanisms for the measured plasmonic response. In addition, the introduction of plasmonic properties in diamond may lead to applications that take advantage of its ability to host emissive and spin-active defects.