Abstract: Recent advances in device miniaturization have led to highly nonuniform power distribution in microelectronic devices, producing ​“hot spots” with heat fluxes on the order of 1 kW cm^-2. Thermal management of concentrated heat sources is a critical bottleneck for a broad range of applications: microprocessors, smart phones, flexible electronics and telecommunication systems. Although strategies such as power optimization design and localized cooling have been developed, mitigating the temperature excursion of ​“hot spots” remains challenging.

Heat spreaders that can effectively conduct heat from a small high-power device to a larger heat exchanger, an important component of thermal management system, require materials with high thermal conductivity (Λ). Although processing the highest Λ in all bulk materials, heat spreaders constructed from single-crystal diamond are costly and can be produced in limited sizes. Meanwhile, the thermal conductivity of more cost-effective chemical vapor deposition (CVD) grown diamond films is often significantly compromised by microstructural defects. Novel high thermally conducting materials are needed to effectively dissipate heat and thereby enable enhanced performance and reliability of electronic devices.

In this seminar, I will first present the experimental discovery of unconventional high thermal conductivity in cubic BAs and isotopically modified BP single crystals that originate from phonon dispersion feature. Measurements of the thermal conductivity of these crystals with sub-millimeter dimensions are enabled by time-domain thermoreflectance (TDTR). At room temperature, Λ for BAs and BP are 1000 and 490 W m^-1 K^-1, respectively, surpassing the values of conventional high-Λ materials such as Ag, BeO, and SiC.

I will then present the anisotropic cross‐ and in‐plane thermal conductivities of 20- to 140-nm graphite thin film grown by CVD on Ni(111) measured by TDTR, which are found to be <50% of the that of highly oriented pyrolytic graphite. The reduced thermal conductivity is attributed to a combination of grain boundaries, structural disorder, and size effects. We define a figure‐of‐merit for flexible heat spreaders and find that CVD graphite has potential advantages over metal films for thermal management of flexible electronics.