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Ultrafast hot-carrier-dominated photocurrent in graphene

Nature Nanotechnology volume 7, pages 114–118 (2012)Cite this article

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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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Figure 1: Standard photocurrent microscopy of a graphene device.
Figure 2: Power dependence of the pump–probe measurements.
Figure 3: Temperature dependence of photocurrent amplitude and dynamics.
Figure 4: Gate dependence of pump–probe measurements implying two contributions.

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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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Authors and Affiliations

  1. Department of Physics, University of Washington, Seattle, 98195, Washington, USA

    Dong Sun, Grant Aivazian, Aaron M. Jones, David Cobden & Xiaodong Xu

  2. Department of Materials Science and Engineering, University of Washington, Seattle, 98195, Washington, USA

    Jason S. Ross & Xiaodong Xu

  3. Department of Physics and Center of Theoretical and Computational Physics, The University of Hong Kong, Hong Kong, China

    Wang Yao

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  1. Dong Sun
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Contributions

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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Correspondence to Xiaodong Xu.

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The authors declare no competing financial interests.

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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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