Transient carrier cooling enhanced by grain boundaries in graphene monolayer.

Using a high terahertz electric field (E_THz), the carrier scattering in graphene was studied with an electric field of up to 282 kV/cm. When the grain size of graphene monolayers varies from small (5 um) and medium (70 um) to large grains (500 um), the dominant carrier scattering source in large- and small-grained graphene differs, i.e., optical phonon scattering for large grains and defect scattering for small grains. Although the electron-optical phonon coupling strength is the same for all grain sizes in our study, however, the enhanced optical phonon scattering in high THz field from the large-grained graphene is caused by a higher optical phonon temperature, originating from the slow relaxation of accelerated electrons. Unlike the large-grained graphene, lower electron and optical phonon temperatures are found in the small-grained graphene monolayer, resulting from the effective carrier cooling through the defects, called supercollisions. Our results indicate that the carrier mobility in the high crystalline graphene is easily vulnerable to scattering by the optical phonons. Thus, controlling the population of defect sites, as a means for carrier cooling, can enhance the carrier mobility at high electric fields in the graphene electronics by suppressing the heating of optical phonons.