2 research outputs found
Unsaturated Drift Velocity of Monolayer Graphene
We
observe that carriers in graphene can be accelerated to the
Fermi velocity without heating the lattice. At large Fermi energy
|<i>E</i><sub>F</sub>| > 110 meV, electrons excited by
a
high-power terahertz pulse <i>E</i><sub>THz</sub> relax
by emitting optical phonons, resulting in heating of the graphene
lattice and optical-phonon generation. This is owing to enhanced electron–phonon
scattering at large Fermi energy, at which the large phase space is
available for hot electrons. The emitted optical phonons cause carrier
scattering, reducing the drift velocity or carrier mobility. However,
for |<i>E</i><sub>F</sub>| ≤ 110 meV, electron–phonon
scattering rate is suppressed owing to the diminishing density of
states near the Dirac point. Therefore, <i>E</i><sub>THz</sub> continues to accelerate carriers without them losing energy to optical
phonons, allowing the carriers to travel at the Fermi velocity. The
exotic carrier dynamics does not result from the massless nature,
but the electron–optical-phonon scattering rate depends on
Fermi level in the graphene. Our observations provide insight into
the application of graphene for high-speed electronics without degrading
carrier mobility
Transient Carrier Cooling Enhanced by Grain Boundaries in Graphene Monolayer
Using
a high terahertz
(THz) electric field (<i>E</i><sub>THz</sub>), 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 ÎĽm) and medium (70 ÎĽm)
to large grains (500 ÎĽm), the dominant carrier scattering source
in large- and small-grained graphene differs at high THz field, i.e.,
there is 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, the enhanced
optical phonon scattering in the 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 graphene electronics
by suppressing the heating of optical phonons