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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 41026-41033
Transient Carrier Cooling Enhanced by Grain Boundaries in Graphene Monolayer Hee Jun Shin,†,∥,⊥ Jaesu Kim,‡,⊥ Seongho Kim,§,⊥ Hyeongmun Kim,† Van Luan Nguyen,§ Young Hee Lee,‡,§ Seong Chu Lim,*,‡,§ and Joo-Hiuk Son*,† †
Department of Physics, University of Seoul, Seoul 02504, Republic of Korea Department of Energy Science and §IBS Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science, Sungkyunkwan University, Suwon 16419, Republic of Korea ∥ Research Group of Food Safety, Korea Food Research Institute, Wanju 55365, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Using a high terahertz (THz) electric field (ETHz), 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 μm) and medium (70 μm) to large grains (500 μm), 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. KEYWORDS: high-field THz, graphene, grain size, carrier relaxation, optical phonons
1. INTRODUCTION A monolayer of graphene exhibits a unique band structure with a linear relation between the energy E and momentum p featuring the Dirac point, which enables carriers to move similar to massless particles through graphene carbon atoms.1 Therefore, the low-field carrier mobility of a graphene monolayer exceeding 2 × 105 cm2/V s2,3 is observed. Furthermore, such a transport property is important for realizing various applications,4−10 e.g., high-speed graphene electronics and optoelectronic devices demonstrating an operational frequency greater than 0.1 THz.4,11−13 In the low-field regime, long-range Coulomb scattering dominates the carrier transport in graphene. Therefore, to reduce the scattering by the charged impurities in the SiO2 substrate, a hexagonal boron nitride layer was used as an underlying gate oxide to suppress the Coulomb scattering.14 As the applied field increases, the kinetic energy of the carriers also increases. Because the scattering mechanism depends on the energy of the carriers,15,16 as the field increases, new dominant scattering sources, such as defects,17,18 surface optical phonons of the substrate,19−21 and optical phonons,22 are expected to emerge. In modern microdigital electronics, the operation © 2017 American Chemical Society
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