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Applied Physics Letters / Volume 92 / Issue 4 / ELECTRONIC TRANSPORT AND SEMICONDUCTORS
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Appl. Phys. Lett. 92, 042116 (2008); http://dx.doi.org/10.1063/1.2837539 (3 pages)

Measurement of ultrafast carrier dynamics in epitaxial graphene

Jahan M. Dawlaty, Shriram Shivaraman, Mvs Chandrashekhar, Farhan Rana, and Michael G. Spencer

School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA

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(Received 29 November 2007; accepted 28 December 2007; published online 30 January 2008)

Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70–120 fs range is followed by a slower relaxation process in the 0.4–1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.

© 2008 American Institute of Physics

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KEYWORDS and PACS

Keywords

carbon, carrier relaxation time, epitaxial layers, high-speed optical techniques, Raman spectra

PACS

  • 78.66.Tr

    Fullerenes and related materials

  • 72.20.Jv

    Charge carriers: generation, recombination, lifetime, and trapping

  • 78.30.Na

    Fullerenes and related materials

  • 63.22.Dc

    Free films

  • 78.47.D-

    Time resolved spectroscopy (>1 psec)

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.2837539

PUBLICATION DATA

ISSN

0003-6951 (print)  
1077-3118 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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Figures (click on thumbnails to view enlargements)

FIG.1
Measured transmittivity transients for (a) sample A, (b) sample B, (c) sample B with different pump powers, and (d) sample C. The dark solid lines with markers are the experimental data and the light solid lines without markers are analytical fits to the data using exponentials with time constants τ1 and τ2. The transients in (b) and (c) show that the slower time constant τ2 does not change much as the pump energy is varied.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
(a) Band structure of graphene showing an intrinsic population of electrons and holes near the Dirac point. Optical excitation is indicated by the arrow. (b) The nonequilibrium distribution of photoexcited carriers account for the initial rise in transmittivity. (c) The carriers equilibrate among themselves through carrier-carrier scattering on a time scale given by τ1 resulting in a hot carrier distribution. (d) Subsequent cooling and decay of the hot distribution through carrier-phonon scattering (and possibly electron-hole recombination) occurs on a time scale given by τ2.

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
The slower relaxation time τ2 is plotted vs the ratio of the intensity of the Raman G and D peaks for samples A, B, and C. This ratio is a measure of the crystal coherence length. Larger crystal disorder (smaller coherence length) results in shorter relaxation times.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.4
Peak transmittivity after photoexcitation is plotted for various pump energies for sample B. Linear relation between the transmittivity peak and the the pump pulse energy agrees with the linear absorption and band filling model.

FIG.4 Download High Resolution Image (.zip file) | Export Figure to PowerPoint



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