Anisotropic Thermal Conductivity in Boron Doped Diamond

Diamond has a number of outstanding properties that make it an appealing candidate for electronic devices that must withstand extremes, e.g. extreme temperature, voltage or power. These properties include high thermal conductivity, a large breakdown voltage and good electron mobility. The success of diamond in high powered electronics relies on developing effective strategies for doping that produce a high-quality electrically conductive diamond. In addition to effecting electrical conductivity, doping diamond effects its thermal conductivity. However, only a few experimental studies have explored the effect that p-type doping of diamond with boron has on diamonds thermal transport properties. We report time-domain thermoreflectance measurements as a function of temperature of the thermal properties of boron doped diamond (BDD). We perform comparative studies of the thermal conductivity of a degenerately BDD films with a concentration of ~10^20 cm-3 and a low concentration BDD ~10^16 cm-3. We find that, in the direction perpendicular to diamond film surface, the highly BDD has a thermal conductivity an order of magnitude lower than undoped or weakly doped diamond. The thermal conductivity of low concentration BDD is proportional to 1/T, while heavily BDD thermal depends weakly on T. Surprisingly, we find that the thermal conductivity of highly concentration BDD is strongly anisotropic. At room temperature, we observe an in-plane thermal conductivity of ~600 W m^-1 K^-1 and through-plane thermal conductivity of ~200 W m^-1 K^-1. This indicates strong boron doping of diamond causes planar defects parallel to the diamonds surface.