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High intrinsic mobility and ultrafast carrier dynamics in multilayer metal-dichalcogenide MoS2

Jared H. Strait, Parinita Nene, and Farhan Rana
Phys. Rev. B 90, 245402 – Published 1 December 2014
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  • INTRODUCTION
  • CONDUCTIVITY SPECTRA AND MOMENTUM…
  • CARRIER MOBILITY
  • CARRIER DYNAMICS ON SHORT TIME SCALES
  • CARRIER DYNAMICS ON LONG TIME SCALES
  • DISCUSSION
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    The ultimate limitations on carrier mobilities in metal dichalcogenides, and the dynamics associated with carrier relaxation, are unclear. We present measurements of the frequency-dependent conductivity of multilayer dichalcogenide MoS2 by optical-pump terahertz-probe spectroscopy. We find mobilities in this material approaching 4200 cm2 V−1 s−1 at low temperatures. The temperature dependence of scattering indicates that the mobility, an order of magnitude larger than previously reported for MoS2, is intrinsically limited by acoustic phonon scattering at THz frequencies. Our measurements of carrier relaxation reveal picosecond cooling times followed by recombination lasting tens of nanoseconds and dominated by Auger scattering into defects. Our results provide a useful context in which to understand and evaluate the performance of MoS2-based electronic and optoelectronic devices.

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    • Received 25 September 2014
    • Revised 7 November 2014

    DOI:https://doi.org/10.1103/PhysRevB.90.245402

    ©2014 American Physical Society

    Authors & Affiliations

    Jared H. Strait*, Parinita Nene, and Farhan Rana

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

    • *jhs295@cornell.edu

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    Issue

    Vol. 90, Iss. 24 — 15 December 2014

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    • Figure 1
      Figure 1

      Optical-pump THz-probe spectroscopy. (a) A schematic of optical-pump and THz-probe measurement scheme. (b) In-plane band structure of multilayer MoS2 [27] depicting carrier photoexcitation and recombination. (c) (Solid line) Measured ΔE(t,u) in units of detector current for a fixed pump-probe delay u=5 ps. ΔE(t,u=−5 ps) is subtracted for clarity. (Dashed line) The scaled reference THz pulse E0(t).

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    • Figure 2
      Figure 2

      THz conductivity. The real (circles) and imaginary (triangles) parts of the measured THz conductivity Δσ(ω,u) plotted versus frequency for various temperatures. In each case, the probe delay is u=5 ps, and the pump fluence is ∼1.2 μJ cm−2. (Dashed lines) Drude model fits to the measured conductivity spectra using the corresponding values of τD.

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    • Figure 3
      Figure 3

      Mobility and scattering rate. (a) Electron mobility versus temperature, determined from the measured momentum scattering rate. (b) Measured momentum scattering rate versus temperature with a linear fit for T≤200 K. The 1/τD∝T dependence at low temperatures suggests acoustic phonon scattering as the dominant scattering mechanism. Measured mobility reaches ∼4200 cm2 V−1 s−1 at 30 K.

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    • Figure 4
      Figure 4

      Intraband relaxation. The change in dc conductivity Δσdc(u) versus pump-probe delay on short time scales (−2≤u≤5 ps). We show data for two representative temperatures [(a) 300 K and (b) 45 K], each with a pump fluence of ∼1.2 μJ cm−2. Immediately after the pump pulse, the conductivity increases, reaching its maximum value within 5 ps at all temperatures. The rise time is slower at 45 K than at 300 K.

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    • Figure 5
      Figure 5

      Density-dependent recombination. (a) (Solid line) The measured change in the carrier density ΔN(u) plotted on a log scale as a function of probe delay. Shown are transients for four pump fluences (0.2, 0.4, 0.8, and 1.2 μJ cm−2) at 45 K. (Blue dot-dashed line) Initial exponential relaxation corresponding to the estimated τ1. (Red dashed line) Full transient simulations using Eqs. (6). (b) Measured ΔN+=ΔN(u=0+) (circles) and ΔN−=ΔN(u=0−) (diamonds) versus pump fluence at 45 K. (Solid line) Curves obtained from simulations of Eqs. (6). (c) The measured inverse decay time 1/τ1 at 45 K versus ΔN+. The slope of the line estimates the electron capture rate And.

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    • Figure 6
      Figure 6

      The real and imaginary parts of the transmission are shown for the case of the exact expression and the approximate expression for n(ω)=3 and d=4 μm.

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    • Figure 7
      Figure 7

      The real and imaginary parts of the ratio ΔẼ(ω,u)/E0(ω) for u=5 ps and maximum pump fluence at 45 K. For all other temperatures, fluences, and pump-probe delays, ΔẼ(ω,u)/E0(ω) is either similar or smaller.

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    • Figure 8
      Figure 8

      The four basic Auger processes for the capture of electrons and holes at defect states are illustrated. The energies of the conduction band bottom, valence band top, and the defect levels are Ec,Ev, and Ed, respectively. In each case, the approximate carrier density dependence of the capture rates are indicated [48].

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