Dielectric properties of conducting boron-doped diamond

We extract the infrared dielectric function from Fourier transform infrared reflectance spectra 650-4000cm^-1 for conducting single crystal and polycrystalline boron-doped (3-6×10²⁰cm^-3) diamond (BDD) by Kramers-Kronig (K-K) analysis, validating our method on commercial SiC substrates. This method highlights the importance of using integrable functions in K-K, solving issues with convergence near the end-points of the measurement spectral range. Polycrystalline BDD shows an elevated optical index ∼3-3.5 below 2000cm^-1 compared to the ∼2.5-2.7 in single-crystal BDD, and the ∼2.4 for ideal diamond, accompanied by increased dielectric loss at ∼1250cm^-1, termed the L2-mode. Raman mapping of this IR-active ω_L2 revealed elevated levels of graphitic sp² inclusions, particularly at grain boundaries. These optical measurements are consistent with polycrystalline BDD showing lower electrical resistivity 0.05Ω-cm, one order lower than that of the single crystal 0.6Ω-cm, highlighting the strong role of sp² inclusions in grain boundary conductivity as a source of dielectric loss. These results show that extrinsic factors beyond B-doping alone should be considered in the interpretation of the optoelectronic properties of BDD and inform its use for thermal management in codesigned microelectronics for power and RF systems, plasma-assisted epitaxy of cubic boron nitride and electrochemistry.