Abstract
The combination of its high electron mobility1,2,3,4,5, broadband absorption6 and ultrafast luminescence7,8,9,10 make graphene attractive for optoelectronic and photonic applications11,12,13, including transparent electrodes14, mode-locked lasers15 and high-speed optical modulators16. Photo-excited carriers that have not cooled to the temperature of the graphene lattice are known as hot carriers, and may limit device speed and energy efficiency. However, their roles in charge and energy transport are not fully understood17,18,19,20. Here, we use time-resolved scanning photocurrent microscopy to demonstrate that hot carriers, rather than phonons, dominate energy transport across a tunable graphene p–n junction excited by ultrafast laser pulses. The photocurrent response time varies from 1.5 ps at room temperature to 4 ps at 20 K, implying a fundamental bandwidth of ∼500 GHz (refs 12, 13, 21). Gate-dependent pump–probe measurements demonstrate that both thermoelectric and built-in electric field effects contribute to the photocurrent, with the contribution from each depending on the junction configuration. The photocurrent produced by a single pulsed laser also displays multiple polarity reversals as a function of carrier density, which is a possible signature of impact ionization19,22,23.
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Acknowledgements
The authors thank Z. Zhong for helpful suggestions on device fabrication and T. Norris for useful discussions. X. Xu acknowledges support from DARPA YFA. The research was supported in part by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (DE-SC0002197). D. Sun acknowledges partial support from IBM (4910031201). A. Jones was supported by the NSF Graduate Research Fellowship (DGE-0718124). Device fabrication was performed at the University of Washington Nanotechnology User Facility funded by the NSF. X. Xu thanks B. Heckel, B. Blinov and L. Sorensen for help with the laboratory set-up.
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X.X. conceived the experiments. G.A. fabricated the devices, assisted by A.J., J.R. and D.S. Measurements were performed by D.S., with X.X., G.A. and D.H.C. providing assistance. W.Y. contributed to the theoretical explanation. All authors discussed the results and contributed to writing the manuscript.
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Sun, D., Aivazian, G., Jones, A. et al. Ultrafast hot-carrier-dominated photocurrent in graphene. Nature Nanotech 7, 114–118 (2012). https://doi.org/10.1038/nnano.2011.243
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DOI: https://doi.org/10.1038/nnano.2011.243
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