  We investigate the energy relaxation of hot carriers produced by
photoexcitation of graphene through coupling to both intrinsic and remote
(substrate) surface polar phonons using the Boltzmann equation approach. We
find that the energy relaxation of hot photocarriers in graphene on commonly
used polar substrates, under most conditions, is dominated by remote surface
polar phonons. We also calculate key characteristics of the energy relaxation
process, such as the transient cooling time and steady state carrier
temperatures and photocarriers densities, which determine the thermoelectric
and photovoltaic photoresponse, respectively. Substrate engineering can be a
promising route to efficient optoelectronic devices driven by hot carrier
dynamics.
