Abstract-Microscopic origins of the terahertz carrier relaxation and cooling dynamics in graphene

• Momchil T. Mihnev,
• Faris Kadi,
• Charles J. Divin,
• Torben Winzer,
• Seunghyun Lee,
• Che-Hung Liu,
• Zhaohui Zhong,
• Claire Berger,
• Walt A. de Heer,
• Ermin Malic,
• Andreas Knorr
• & Theodore B. Norris

http://www.nature.com/ncomms/2016/160525/ncomms11617/abs/ncomms11617.html

The ultrafast dynamics of hot carriers in graphene are key to both understanding of fundamental carrier–carrier interactions and carrier–phonon relaxation processes in two-dimensional materials, and understanding of the physics underlying novel high-speed electronic and optoelectronic devices. Many recent experiments on hot carriers using terahertz spectroscopy and related techniques have interpreted the variety of observed signals within phenomenological frameworks, and sometimes invoke extrinsic effects such as disorder. Here, we present an integrated experimental and theoretical programme, using ultrafast time-resolved terahertz spectroscopy combined with microscopic modelling, to systematically investigate the hot-carrier dynamics in a wide array of graphene samples having varying amounts of disorder and with either high or low doping levels. The theory reproduces the observed dynamics quantitatively without the need to invoke any fitting parameters, phenomenological models or extrinsic effects such as disorder. We demonstrate that the dynamics are dominated by the combined effect of efficient carrier–carrier scattering, which maintains a thermalized carrier distribution, and carrier–optical–phonon scattering, which removes energy from the carrier liquid.

Abstract-Evidence for bandgap opening in buckled epitaxial graphene from ultrafast time-resolved terahertz spectroscopy

Momchil T. Mihnev^1,2, Feng Wang³, Gang Liu⁴, Sara Rothwell⁵, Philip I. Cohen⁵, Leonard C. Feldman⁴, Edward H. Conrad³ and Theodore B. Norris^1,2,a)

1 Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, USA
2 Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109, USA
3 School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
4 Institute for Advanced Materials Devices and Nanotechnology, Rutgers University, Piscataway, New Jersey 08854, USA
5 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, USA
a) Author to whom correspondence should be addressed. Electronic mail: tnorris@umich.edu
Appl. Phys. Lett. 107, 173107 (2015); http://dx.doi.org/10.1063/1.4934781

http://scitation.aip.org/content/aip/journal/apl/107/17/10.1063/1.4934781?TRACK=RSS

We utilize ultrafast time-resolved terahertz (THz) spectroscopy as a direct, sensitive, and non-contact all-optical probe to investigate the hot-carrier relaxation and cooling dynamics of buckled epitaxial graphene. This special form of graphene is grown epitaxially on nitrogen-seeded single-crystal silicon carbide (SiC( 0001¯ )) substrates by thermal decomposition of Si atoms. The pre-deposited interfacial nitrogen atoms pin the first graphene layer to the SiC substrate, and cause it and subsequent graphene layers to buckle into nanoscale folds, which opens an energy gap of up to ∼0.7 eV. We observe a remarkable increase of up to two orders of magnitude in the relaxation rate of the THz carrier dynamics of this semiconducting form ofepitaxial graphene relative to pristine epitaxial graphene, which we attribute to a large enhancement of the optical-phonon-mediated carrier cooling and recombination over a wide range of electron temperatures due to the finite bandgap. Our results suggest that the introduced bandgap is spatially non-homogenous, with local values close to the optical phononenergy of ∼200 meV, which allows the conduction and the valence band to be bridged by opticalphonon emission. We also demonstrate that carrier relaxation times can be modified by orders of magnitude by careful bandgap engineering, which could find application in novel graphene-based devices that incorporate both metallic and semiconducting forms of graphene.