Atomically thin p–n junctions with van der Waals heterointerfaces

Journal name:
Nature Nanotechnology
Volume:
9,
Pages:
676–681
Year published:
DOI:
doi:10.1038/nnano.2014.150
Received
Accepted
Published online

Semiconductor p–n junctions are essential building blocks for electronic and optoelectronic devices1, 2. In conventional p–n junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a p–n junction at the ultimate thickness limit3, 4, 5, 6, 7, 8, 9, 10. Van der Waals junctions composed of p- and n-type semiconductors—each just one unit cell thick—are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions10, 11, 12. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin p–n heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the p–n interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p–n junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals p–n heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.

At a glance

Figures

  1. Charge transport in an atomically thin p–n heterojunction.
    Figure 1: Charge transport in an atomically thin p–n heterojunction.

    a, Bottom left: Schematic diagram of a van der Waals-stacked MoS2/WSe2 heterojunction device with lateral metal contacts. Top: enlarged crystal structure, with purple, red, yellow and green spheres representing Mo, S, W and Se atoms, respectively. Bottom right: Optical image of the fabricated device, where D1 and D2 (S1 and S2) indicate the metal contacts for WSe2 (MoS2). Scale bar, 3 µm. b, Current–voltage curves at various gate voltages measured across the junction (between contacts D1 (+) and S1 (–) in the optical image in a). Inset: Gate-dependent transport characteristics at Vds = 0.5 V for individual monolayers of MoS2 (blue curve, measured between S1 and S2) and WSe2 (red curve, measured between D1 and D2). c,d, Band profiles in the lateral (c) and vertical (d) directions, obtained from electrostatic simulations. Under forward bias (Vds = 0.6 V), electrons (blue circles with ‘e’) in the conduction band (CB) of MoS2 and holes (red circles with ‘h+’) in the valence band (VB) of WSe2 undergo interlayer recombination (black arrows in c) via Shockley–Read–Hall (red dashed arrows in d) or Langevin (blue arrow in d) mechanisms, contributing the dark current. τ and B are, respectively, the tunnelling-assisted recombination lifetime and the Langevin recombination constant. Note that there is no significant band bending in the lateral transport direction. Band offsets for electrons (ΔEc) and holes (ΔEv) across the junction are indicated in c.

  2. Gate-tunable photovoltaic response.
    Figure 2: Gate-tunable photovoltaic response.

    a, Photoresponse characteristics at various gate voltages under white-light illumination. Inset: Colour plot of photocurrent as a function of voltages Vds (x axis) and Vg (y axis). The dashed line represents the profile of short-circuit current density Jsc at Vds = 0 V. b, Photocurrent map of the device presented in Fig. 1a for Vds = 0 V and 532 nm laser excitation. The junction area and metal electrodes are indicated by dashed and solid lines, respectively. Scale bar, 3 µm. c, Photoluminescence spectra measured from the isolated monolayers (blue curve for MoS2; red curve for WSe2) and the stacked junction region (brown curve). d, Photoluminescence spatial maps for emission at 1.66 eV (top) and 1.88 eV (bottom), corresponding to direct gap transitions of monolayer WSe2 and MoS2, respectively. The junction area is indicated by dashed lines. Scale bars, 3 µm. e,f, Schematic illustrations of exciton dissociation (e) and interlayer recombination (f) processes. In e, horizontal and vertical arrows represent charge transfer and intralayer recombination processes, respectively. In f, red and blue arrows indicate Shockley–Read–Hall (SRH) and Langevin recombination processes, respectively. g, Simulations of the gate-voltage-dependent majority (red curve for holes in WSe2; blue curve for electrons in MoS2) and minority (red dashed curve for holes in MoS2; blue dashed curve for electrons in WSe2) carrier densities in each layer (top), spatially averaged 〈nMpW1.2〉 (middle) and 〈nMpW/(nM + pW)〉 (bottom). h, Measured (circles and dashed curve) and simulated (green curve for two-dimensional Langevin process and purple curve for SRH mechanism) photocurrent at Vds = 0 V as a function of gate voltages. For the fit, B = 4.0 × 10−13 m2 s−1 and τ = 1 µs are used for the two-dimensional Langevin (s = 1.2) and SRH mechanisms, respectively.

  3. Graphene-sandwiched van der Waals p–n heterojunctions.
    Figure 3: Graphene-sandwiched van der Waals p–n heterojunctions.

    a, Schematics of MoS2/WSe2 junction sandwiched between top and bottom graphene electrodes. b, Optical image and corresponding photocurrent map (for Vds = 0 V) of the graphene-sandwiched monolayer p–n junction device (1L–1L). Photocurrent is uniformly observed only in the junction area indicated by the dashed lines, separated from the metal electrodes (indicated by solid lines). Scale bars, 3 µm. c, Current–voltage curves of the device in b, measured in the dark (black) and under 532 nm laser excitation (red). Inset: Schematic of the bandstructure, with exciton dissociation and charge-collection processes indicated by horizontal arrows. The dashed line represents the Fermi level. Note that the bottom graphene (GR) is slightly p-doped from the SiO2 substrate. d, Photoresponse characteristics of graphene-sandwiched p–n junctions with different thicknesses. For comparison, the applied voltages (horizontal axis) are normalized with respect to the junction thicknesses. e, EQE plots as a function of excitation energy (wavelength) for the devices in d. The laser powers were in the range 3–7 µW, corresponding to 380–890 W cm−2. Results in d and e are shown for devices composed of monolayer–monolayer (1L–1L), bilayer–bilayer (2L–2L) and multilayer–multilayer (ML–ML (10/9 nm)) junctions.

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Author information

Affiliations

  1. Department of Physics, Columbia University, New York, New York 10027, USA

    • Chul-Ho Lee,
    • Yilei Li,
    • Tony F. Heinz &
    • Philip Kim
  2. Department of Chemistry, Columbia University, New York, New York 10027, USA

    • Chul-Ho Lee &
    • Colin Nuckolls
  3. KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea

    • Chul-Ho Lee
  4. Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea

    • Gwan-Hyoung Lee
  5. Energy Frontier Research Center (EFRC), 1001 Schapiro Center (CEPSR), Columbia University, New York, New York 10027, USA

    • Arend M. van der Zande
  6. Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida 32611-6200, USA

    • Wenchao Chen &
    • Jing Guo
  7. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA

    • Minyong Han
  8. Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA

    • Xu Cui,
    • Ghidewon Arefe &
    • James Hone
  9. Department of Electrical Engineering, Columbia University, New York, New York 10027, USA

    • Tony F. Heinz

Contributions

C-H.L., G-H.L., M.H., X.C. and G.A. fabricated and characterized the van der Waals p–n heterostructures. C-H.L. and G-H.L. performed device fabrication and transport measurements. A.M.v.d.Z. and C-H.L. performed photocurrent measurements. W.C. and J.G. provided theoretical support. Y.L. performed optical characterization. P.K., J.H., T.F.H., J.G. and C.N. advised on experiments. C-H.L. and P.K. wrote the manuscript in consultation with G-H.L., A.M.v.d.Z., W.C., C.N., T.F.H., J.G. and J.H.

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The authors declare no competing financial interests.

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