  The minimization of electronics makes heat dissipation of related devices an
increasing challenge. When the size of materials is smaller than the phonon
mean free paths, phonons transport without internal scatterings and laws of
diffusive thermal conduction fail, resulting in significant reduction in the
effective thermal conductivity. This work reports, for the first time, the
temperature dependent thermal conductivity of doped epitaxial 6H-SiC and
monocrystalline porous 6H-SiC below room temperature probed by time-domain
thermoreflectance. Strong quasi-ballistic thermal transport was observed in
these samples, especially at low temperatures. Doping and structural boundaries
were applied to tune the quasi-ballistic thermal transport since dopants
selectively scatter high-frequency phonons while boundaries scatter phonons
with long mean free paths. Exceptionally strong phonon scattering by boron
dopants are observed, compared to nitrogen dopants. Furthermore, orders of
magnitude reduction in the measured thermal conductivity was observed at low
temperatures for the porous 6H-SiC compared to the epitaxial 6H-SiC. Finally,
first principles calculations and a simple Callaway model are built to
understand the measured thermal conductivities. Our work sheds light on the
fundamental understanding of thermal conduction in technologically-important
wide bandgap semiconductors such as 6H-SiC and will impact applications such as
thermal management of 6H-SiC-related electronics and devices.
