  We design a hybrid graphene/hexagonal boron nitride superlattice monolayer
and investigate its thermoelectric properties using density functional theory
and Boltzmann transport equations with the relaxation time accurately treated
by electron-phonon coupling calculations. Compared with that of pristine
graphene, the lattice thermal conductivity of the superlattice structure is
more than two orders of magnitude lower due to the enhanced three-phonon
scattering process originated from the mixed-bond characteristics. Besides, the
coexistence of light and heavy bands around the Fermi level leads to an
ultrahigh power factor along the zigzag direction, where the highest ZT value
of ~2.5 can be achieved for the n-type system at 1100 K. Moreover, it is noted
that the carrier transport near the valance band minimum is almost entirely
contributed by the graphene part of the superlattice. As a consequence, the
thermoelectric performance of p-type system can be enhanced to be comparable
with that of n-type one by appropriate substitution of nitrogen atom with
phosphorus, which can suppress the lattice thermal conductivity but nearly have
no influence on the hole transport.
