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Approaching the intrinsic exciton physics limit in two-dimensional semiconductor diodes

Nature volume 599, pages 404–410 (2021)Cite this article

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Abstract

Two-dimensional (2D) semiconductors have attracted intense interest for their unique photophysical properties, including large exciton binding energies and strong gate tunability, which arise from their reduced dimensionality1,2,3,4,5. Despite considerable efforts, a disconnect persists between the fundamental photophysics in pristine 2D semiconductors and the practical device performances, which are often plagued by many extrinsic factors, including chemical disorder at the semiconductor–contact interface. Here, by using van der Waals contacts with minimal interfacial disorder, we suppress contact-induced Shockley–Read–Hall recombination and realize nearly intrinsic photophysics-dictated device performance in 2D semiconductor diodes. Using an electrostatic field in a split-gate geometry to independently modulate electron and hole doping in tungsten diselenide diodes, we discover an unusual peak in the short-circuit photocurrent at low charge densities. Time-resolved photoluminescence reveals a substantial decrease of the exciton lifetime from around 800 picoseconds in the charge-neutral regime to around 50 picoseconds at high doping densities owing to increased exciton–charge Auger recombination. Taken together, we show that an exciton-diffusion-limited model well explains the charge-density-dependent short-circuit photocurrent, a result further confirmed by scanning photocurrent microscopy. We thus demonstrate the fundamental role of exciton diffusion and two-body exciton–charge Auger recombination in 2D devices and highlight that the intrinsic photophysics of 2D semiconductors can be used to create more efficient optoelectronic devices.

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Fig. 1: Atomically thin WSe2 p–n diode with atomically clean vdW contacts.
Fig. 2: Doping-dependent optoelectronic performance of a 2D WSe2 p–n diode.
Fig. 3: Doping-dependent TRPL and exciton–charge Auger.
Fig. 4: Correlation between photocurrent and exciton lifetime.

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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

Xiangfeng Duan acknowledges the support from the Office of Naval Research through Award N00014-18-1-2707. J.R.C. acknowledges NSF Career grant number 1945572. Y.H. acknowledges the financial support from the Office of Naval Research through award N00014-18-1-2491. Y.P. acknowledges the support from Air Force Office of Scientific Research under AFOSR award no. FA9550-YR-1-XYZQ.

Author information

Author notes

  1. These authors contributed equally: Peng Chen, Timothy L. Atallah

Authors and Affiliations

  1. Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA, USA

    Peng Chen, Timothy L. Atallah, Zhaoyang Lin, Peiqi Wang, Justin R. Caram & Xiangfeng Duan

  2. Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA, USA

    Sung-Joon Lee, Zhihong Huang & Yu Huang

  3. Department of Chemistry and Biochemistry, University of California Santa Cruz, Santa Cruz, CA, USA

    Junqing Xu & Yuan Ping

  4. State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, China

    Xidong Duan

  5. California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA, USA

    Yu Huang, Justin R. Caram & Xiangfeng Duan

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Contributions

Xiangfeng Duan and P.C. conceived the research. P.C., T.L.A., J.R.C. and Xiangfeng Duan designed the experiment. P.C. fabricated the devices and performed optoelectrical measurements. Z.L., P.W., S.-J.L., Z.H., Xidong Duan and Y.H. contributed to materials, device fabrications, measurements and discussions. J.X. and Y.P. conducted band structure calculations. T.L.A. and P.C. conducted the time-resolved photoluminescence and photocurrent scanning measurements. P.C., T.L.A., J.R.C. and Xiangfeng Duan performed the data analysis. P.C., T.L.A., J.R.C. and Xiangfeng Duan co-wrote the manuscript. All authors discussed the results and commented on the manuscripts.

Corresponding authors

Correspondence to Justin R. Caram or Xiangfeng Duan.

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Peer review information Nature thanks Andrey Chaves, Lain-Jong Li and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Band diagram and photocurrent generation in diode.

a, The carriers generated by the Schottky barrier are blocked by the barrier at the p–n interface. b, The carriers generated by the p–n junction may tunnel through the Schottky junction and contribute to the photocurrent.

Extended Data Fig. 2 Fitting the IDS–VDS characteristic of p–n junction diode.

Black dot: experimental data; solid red line: fit of diode equation. a, The fit of p–n configuration; we extract the RS = 36 MΩ, RSH = 47 GΩ, IS = 4.6 × 10−22 A and η = 1.18; b, The fit of NP configuration; we extract the RS = 28 MΩ, RSH = 35 GΩ, IS = 8.1 ×10−21 A and η = 1.3.

Extended Data Fig. 3 Apparent external quantum efficiency (EQE) of 2D diodes by assuming the device area as the active area.

a, EQE dependence on charge density for the evap-diode (red dots) and the vdW-diode (black dots) at VG1 = −5 V. The line serves as a guide for the eyes. b, EQE dependence on charge density for the evap-diode (red dots) and the vdW-diode (black dots) at VG1 = 5 V. The EQE is calculated as EQE = ISCEph/(ePin), where ISC is the short circuit photocurrent, Eph is the energy per photon, e is the elementary charge and Pin is the input power. Pin = power density (Pd) × illuminated exciton-collection area (A). Note we estimated the apparent EQE by using the device area (the entire WSe2 area between the source and drain electrodes) as A for simplicity, which may lead to a considerably underestimated EQE value as the device area is usually larger than the active area. If we consider the exciton diffusion model with a total exciton collection length of ~1 μm, the maximum EQE is estimated ~21%.

Extended Data Fig. 4

Fitting lifetimes and doping dependence of relative PL intensity and lifetime for different components. a, An example of biexponential fit: (VG = −0.8 V, P = 244 nW). The top panel is the residuals of tri-exponential fitting. The middle panel is the residuals of bi-exponential fitting. The bi-exponential residual is identical to the tri-exponential implying the tri-exponential is an over-fit confirmed by the error in k3 being larger than the value of k3 (Extended Data Table 1); therefore, we used the bi-exponential fit. The bottom panel is the TRPL data and bi-exponential and tri-exponential fitting curve. b, An example of triexponential fit: (VG = −4 V, P = 244 nW). The top panel is the residuals of tri-exponential fitting. The middle panel is the residuals of bi-exponential fitting. The tri-exponential residual is better than the bi-exponential without fit errors larger than the fit values; there we used the tri-exponential fit. The bottom panel is the TRPL data and bi-exponential and tri-exponential fitting curve. c, Doping dependence of relative PL intensity for different components. There are three components, which are t1, t2 and t3. d, Doping dependence of the PL lifetime for different components. The inset shows the lifetime of t3.

Extended Data Fig. 5 A highly simplified band diagram showing the relevant states for band-edge carriers in WSe2.

EF,0 denotes the Fermi level of undoped system; EF,t denotes the Fermi level at turning point.

Extended Data Fig. 6 Deconvolution of the exciton diffusion from scanning photocurrent microscopy studies.

Specifically, we used VG1 = 4 V and VG2 = −4 V (black line) as our measure of laser spot size since the photocurrent collection is exclusively from the diode interface, which is much smaller than our laser spot size (instrument response function, IRF) and fit (red dashed line) it to a single Gaussian function. We fit (pink dashed line) VG1 = 4 V and VG2 = −0.4 V (blue line) with a function being convolution of the IRF Gaussian with an exponential centred at the middle of the interface (X = 0 μm) for the low-doping limit. The decay constant for the fit corresponds to exciton diffusion length Lexc = 0.72 ± 0.10 μm. The yellow square denotes the position of electrodes.

Extended Data Fig. 7 Gate dependent ISC in monolayer, bilayer and four-layer WSe2 vdW-diodes.

a, IDS–VDS curve of the monolayer WSe2 diode under illumination. b, IDS–VDS curve of the bilayer WSe2 diode under illumination. c, IDS–VDS curve of the four-layer WSe2 diode under illumination. d, Gate dependent ISC in monolayer diode. e, Gate dependent ISC in bilayer diode. f, Gate dependent ISC in four-layer diode.

Extended Data Fig. 8

Power dependent apparent EQE in a bilayer diode at VG1 = 5 V and different VG2.

Extended Data Table 1 Fitting parameters for Extended Data Fig. 4a, 4b
Full size table
Extended Data Table 2 Summary of the diode parameters in different studies
Full size table

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Chen, P., Atallah, T.L., Lin, Z. et al. Approaching the intrinsic exciton physics limit in two-dimensional semiconductor diodes. Nature 599, 404–410 (2021). https://doi.org/10.1038/s41586-021-03949-7

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