  Understanding the energy transport by charge carriers and phonons in
two-dimensional (2D) van der Waals heterostructures is essential for the
development of future energy-efficient 2D nanoelectronics. Here, we performed
in situ spatially resolved Raman thermometry on an electrically biased graphene
channel and its hBN substrate to study the energy dissipation mechanism in
graphene/hBN heterostructures. By comparing the temperature profile along the
biased graphene channel with that along the hBN substrate, we found that the
thermal boundary resistance between the graphene and hBN was in the range of
(1-2) x 10^(-7) m^(2) KW^(-1) from ~100 C to the onset of graphene break-down
at ~600 C in air. Consideration of an electro-thermal transport model together
with the Raman thermometry conducted in air showed that local doping occurred
under a strong electric field played a crucial role in the energy dissipation
of the graphene/hBN device up to T ~ 600 C.
