Research ArticleAPPLIED PHYSICS

Asymmetric hot-carrier thermalization and broadband photoresponse in graphene-2D semiconductor lateral heterojunctions

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Science Advances  14 Jun 2019:
Vol. 5, no. 6, eaav1493
DOI: 10.1126/sciadv.aav1493
  • Fig. 1 Graphene-MoS2 lateral heterojunction photodetector.

    (A) Schematic of the device. (B) Microscopic image of the as-fabricated device on a 285-nm SiO2/Si substrate. Multiple long and short electrodes (Ni/Au) were placed around the lateral junction to probe different regions of the device electrically. The inset diagram indicates the cathode and the anode of the device for the electrical and photocurrent measurements; the axis indicates the direction of the x axis for the X-Vg mappings and the simulation results. (C) Transfer characteristics of the graphene-MoS2 junction (orange, current applied across electrodes B and C and voltage probed across the short electrodes), MoS2 (green, across electrodes C and D), and graphene (red, across electrodes A and B), respectively. The channel voltage Vch = 0.1 V. (D) Spatial photoresponsivity (Ri = Iph/Pin, with Iph and Pin denoting the photocurrent and the incident light power, respectively) of the device under a 633-nm laser excitation with Vch = 0 V and Vg = 0 V. The dotted lines indicate the channel (in white) and electrodes (in orange) of the device. “G” and “M” are short for graphene and MoS2, respectively. (E) Schematics of the dominating hot-electron cooling processes (top) and the spatial hot-electron thermalization pathways (bottom) of the Dirac semimetallic graphene (left) and the parabolic semiconducting MoS2 (right). In the bottom panels, the arrows indicate possible heat transfer paths, and the diameters of the purple cylinders connecting the electron, the lattice, and the substrates indicate the strength of thermal couplings.

  • Fig. 2 Gate-dependent SPCM measurements of the device.

    (A) Gate voltage (Vg)–linecut (X) mapping of the photocurrent (Iph) under an 850-nm laser excitation. The three dotted lines indicate (from left to right) the junctions of metal-graphene, graphene-MoS2, and MoS2-metal. The magnitude of Iph was flipped to make the photoresponse at the graphene-MoS2 junction positive. (B) Stacked linecuts along the x axis of Iph with different Vg. The arrows indicate the peak positions. (C) Peak photocurrent Iph (left column), peak photovoltage Vph (middle column), and peak offsets ΔXpk (right column) as a function of the gate voltage with respect to the charge neutrality point of graphene (VDirac), extracted from the Vg-X mappings with laser excitations of 550 nm (first row), 650 nm (second row), 750 nm (third row), and 850-nm (fourth row). Vph is estimated by Iph·Rdark = Iph·Voffset/Idark, in which the dark resistance Rdark is inversely proportional to the dark current Idark averaged throughout the points whenever the laser spot is off the device in the Vg-X mappings, assuming that a constant voltage offset Voffset is supplied by the measurement setup. a.u., arbitrary units.

  • Fig. 3 Theoretical analysis of the thermalization pathways.

    (A to C) Simulated distributions of electron temperature increase (ΔTel) as the laser spot is on the graphene side (A), the junction (B), and the MoS2 side (C). The circles indicate the center positions of the incident laser. (D) Linecuts of ΔTel along the x axis normalized to the maximum ΔTel as in (A) to (C). (E) Schematics of the heat dissipations of photoinduced hot electrons when graphene is lightly doped (with the graphene Fermi level EF = 0.05 eV; top) and heavily doped (EF = 0.5 eV; bottom). (F) Normalized electron temperature at the graphene-MoS2 junction with different EF. The dashed line indicates the geometric junction. (G) Measured magnitudes of peak position offsets (|ΔXpk|) with 750-nm (filled circles) and 850-nm (open circles) laser excitations, as well as the simulated |ΔXpk| and calculated electron-lattice cooling length on the graphene side (ξ), as a function of EF.

  • Fig. 4 Spectral photocurrent response of the devices.

    Left axis: Photocurrent responsivity (Ri) as a function of the wavelength of incident light (λ) of three different devices. Right axis: Calculated absorbance (Abs.) of MoS2 (red dashed line) and graphene (red dotted line) on a 285-nm SiO2/Si substrate based on the complex refractive indices from (42, 43). The inset plots the spectral responsivity in log scale.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaav1493/DC1

    Section S1. Additional results on the material characterizations

    Section S2. Additional results on the electrical and photocurrent measurements

    Section S3. Band diagram of the graphene-MoS2 lateral heterojunction

    Section S4. Electron-phonon coupling strength in graphene and MoS2

    Section S5. Theoretical analysis of the electron temperature distributions

    Fig. S1. Atomic force microscope (AFM) image of the graphene-MoS2 lateral heterojunction.

    Fig. S2. HRTEM images of the graphene-MoS2 lateral heterostructure.

    Fig. S3. Raman and PL of the graphene-MoS2 lateral heterojunction.

    Fig. S4. Output characteristics.

    Fig. S5. I-V characteristics with 633-nm light illumination and various gate voltages.

    Fig. S6. Gate-dependent photocurrent response at the graphene-metal and MoS2-metal junctions.

    Fig. S7. Power-dependent and temperature-dependent photocurrent response.

    Fig. S8. Photocurrent peak position shift extracted from the Vg-X mappings with 850-nm laser excitation and at various temperatures.

    Fig. S9. SPCM mappings of the graphene-MoS2 junction with different excitation wavelengths.

    Fig. S10. Band diagram of the graphene-MoS2 lateral heterojunction.

    Fig. S11. Schematics showing two different hot-electron cooling pathways.

    Fig. S12. Extraction of graphene properties.

    Fig. S13. Calculated physical parameters for graphene with different Δ values.

    Fig. S14. Calculated physical parameters for graphene with different σmin values.

    Fig. S15. Simulated electron temperature distributions with different Fermi level of graphene.

    References (44, 45)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Additional results on the material characterizations
    • Section S2. Additional results on the electrical and photocurrent measurements
    • Section S3. Band diagram of the graphene-MoS2 lateral heterojunction
    • Section S4. Electron-phonon coupling strength in graphene and MoS2
    • Section S5. Theoretical analysis of the electron temperature distributions
    • Fig. S1. Atomic force microscope (AFM) image of the graphene-MoS2 lateral heterojunction.
    • Fig. S2. HRTEM images of the graphene-MoS2 lateral heterostructure.
    • Fig. S3. Raman and PL of the graphene-MoS2 lateral heterojunction.
    • Fig. S4. Output characteristics.
    • Fig. S5. I-V characteristics with 633-nm light illumination and various gate voltages.
    • Fig. S6. Gate-dependent photocurrent response at the graphene-metal and MoS2-metal junctions.
    • Fig. S7. Power-dependent and temperature-dependent photocurrent response.
    • Fig. S8. Photocurrent peak position shift extracted from the Vg-X mappings with 850-nm laser excitation and at various temperatures.
    • Fig. S9. SPCM mappings of the graphene-MoS2 junction with different excitation wavelengths.
    • Fig. S10. Band diagram of the graphene-MoS2 lateral heterojunction.
    • Fig. S11. Schematics showing two different hot-electron cooling pathways.
    • Fig. S12. Extraction of graphene properties.
    • Fig. S13. Calculated physical parameters for graphene with different Δ values.
    • Fig. S14. Calculated physical parameters for graphene with different σmin values.
    • Fig. S15. Simulated electron temperature distributions with different Fermi level of graphene.
    • References (44, 45)

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