  Boron-doped polycrystalline silicon-germanium (SiGe) thin films are grown by
low-pressure chemical vapor deposition (LPCVD) and their thermoelectric
properties are characterized from 120 K to 300 K for the potential applications
in integrated microscale cooling. The naturally formed grain boundaries are
found to play a crucial role in determining both the charge and thermal
transport properties of the films. Particularly, the unique columnar grain
structures result in remarkable thermal conductivity anisotropy with the
in-plane thermal conductivities of SiGe films about 50% lower than the
cross-plane values. By optimizing the growth conditions and doping level, a
high figure of merit (ZT) of 0.2 for SiGe films is achieved at 300 K, which is
about 100% higher than the previous record for p-type SiGe alloys, mainly due
to the significant reduction in the in-plane thermal conductivity caused by
nanograin boundaries. The low cost and excellent scalability of LPCVD render
these high-performance SiGe films ideal candidates for thin-film thermoelectric
applications.
