  At high phonon temperature, defect-mediated electron-phonon collisions
(supercollisions) in graphene allow for larger energy transfer and faster
cooling of hot electrons than the normal, momentum-conserving electron-phonon
collisions. Disorder also affects the heat flow between electrons and phonons
at very low phonon temperature, where the phonon wavelength exceeds the mean
free path. In both cases, the cooling rate is predicted to exhibit a
characteristic cubic power law dependence on the electron temperature, markedly
different from the T^4 dependence predicted for pristine graphene. The impact
of defect-induced cooling on the performance of optoelectronic devices is still
largely unexplored. Here we study the cooling mechanism of hot-electron
bolometers based on epitaxial graphene quantum dots where the defect density
can be controlled with the fabrication process. The devices with high defect
density exhibit the cubic power law. Defect-induced cooling yields a slower
increase of the thermal conductance with increasing temperature, thereby
greatly enhancing the device responsivity compared to devices with lower defect
density and operating with normal-collision cooling.
