atomically thin p-n junctions with van der waals...

Atomically thin p–n junctions with van der WaalsheterointerfacesChul-Ho Lee1,2,3, Gwan-Hyoung Lee4, Arend M. van der Zande5, Wenchao Chen6, Yilei Li1,Minyong Han7, Xu Cui8, Ghidewon Arefe8, Colin Nuckolls2, Tony F. Heinz1,9, Jing Guo6,James Hone8 and Philip Kim1*

Semiconductor p–n junctions are essential building blocks forelectronic and optoelectronic devices1,2. In conventional p–njunctions, regions depleted of free charge carriers form oneither side of the junction, generating built-in potentials associ-ated with uncompensated dopant atoms. Carrier transportacross the junction occurs by diffusion and drift processesinfluenced by the spatial extent of this depletion region. Withthe advent of atomically thin van der Waals materials andtheir heterostructures, it is now possible to realize a p–n junc-tion at the ultimate thickness limit3–10. Van der Waals junctionscomposed of p- and n-type semiconductors—each just one unitcell thick—are predicted to exhibit completely different chargetransport characteristics than bulk heterojunctions10–12. Here,we report the characterization of the electronic and optoelec-tronic properties of atomically thin p–n heterojunctions fabri-cated using van der Waals assembly of transition-metaldichalcogenides. We observe gate-tunable diode-like currentrectification and a photovoltaic response across the p–n inter-face. We find that the tunnelling-assisted interlayer recombina-tion of the majority carriers is responsible for the tunability ofthe electronic and optoelectronic processes. Sandwiching anatomic p–n junction between graphene layers enhances thecollection of the photoexcited carriers. The atomically scaledvan der Waals p–n heterostructures presented here constitutethe ultimate functional unit for nanoscale electronic andoptoelectronic devices.

Heterostructures based on atomically thin van der Waalsmaterials are fundamentally different and more flexible than thosemade from conventional covalently bonded materials, in that thelack of dangling bonds on the surfaces of van der Waals materialsenables the creation of high-quality heterointerfaces without theconstraint of atomically precise commensurability9,13,14. The abilityto build artificial van der Waals heterostructures, combined withrecent rediscoveries of transition-metal dichalcogenides (TMDCs)as atomically thin semiconductors3–5, provides a route to a widevariety of semiconductor heterojunctions and superlattices. Thestrong light–matter interaction and distinctive optical propertiesof TMDCs render them promising candidates for optoelectronicdevices such as photodiodes, photovoltaic cells and light-emittingdevices3,8,15–20. The availability of TMDCs with different bandgapsand workfunctions allows for bandgap engineering of heterostruc-tures21. Furthermore, the ability to electrostatically tune the carrier

densities and band alignments of two-dimensional semiconductorsoffers an alternative way to design functional heterostructures22–26

and to understand the mechanisms underlying their operation.Here, we have realized the ultimate limit of p–n junction scalingin a heterostructure consisting of semiconducting TMDC mono-layers. We use individually contacted layers of p-type tungsten di-selenide (WSe2) and n-type molybdenum disulphide (MoS2) tocreate an atomically thin p–n junction. The difference in workfunc-tion and bandgap between the two monolayers creates an atomicallysharp heterointerface, predicted to be of type II band alignment21.

Figure 1a presents a schematic of the van der Waals heterostruc-ture of MoS2/WSe2. Each TMDC monolayer consists of a hexagon-ally packed plane of transition-metal atoms sandwiched betweentwo planes of chalcogen atoms6. This heterojunction is created byvan der Waals assembly of individual monolayers on a SiO2/Sisubstrate (Supplementary Section 1). To measure the electricalproperties of the junction, we fabricated metal contacts on eachlayer (optical image in Fig. 1a). Al and Pd were chosen to inject elec-trons and holes into the n-MoS2 and p-WSe2 layers, respectively27. Agate voltage Vg applied to the Si substrate adjusts the carrier densitiesin each semiconductor layer. As shown in the inset of Fig. 1b, theindividual MoS2 and WSe2 layers exhibit, respectively, n- and p-typechannel characteristics due to the unintentional doping present ineach crystal. From the measured threshold voltages (Vth), weestimate (using n (electron) or p (hole) = q−1Cg|Vth – Vg|, whereq = 1.6 × 1019 C and Cg = 1.23 × 10−8 F cm−2) carrier densities of1 × 1012 to 3 × 1012 cm−2 at Vg = 0 V for both materials.Accordingly, a p–n junction is formed between the two atomicallythin semiconductors. Figure 1b presents the I–V curves of the junc-tion at various gate voltages, as measured between metal electrodeson the WSe2 (D1) and MoS2 (S1). We observe gate-voltage-tunedrectification of current as electrostatic doping modulates thedensity of free electrons and holes in the junction.

Although the observed I–V characteristics are similar to those of aconventional p–n junction diode, the underlying physical mechanismof rectification is expected to differ in view of the lack of a depletionregion in the atomically thin junction. Figure 1c,d presents band pro-files in the lateral and vertical directions, respectively, at a forward biasof 0.6 V, based on simulations for the electrostatic configuration ofthe device (Supplementary Section 3). We find that most of thevoltage drop occurs across the vertical p–n junction, leaving noappreciable potential barriers in the lateral transport direction

1Department of Physics, Columbia University, New York, New York 10027, USA, 2Department of Chemistry, Columbia University, New York, New York10027, USA, 3KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea, 4Department of Materials Scienceand Engineering, Yonsei University, Seoul 120-749, Korea, 5Energy Frontier Research Center (EFRC), 1001 Schapiro Center (CEPSR), Columbia University,New York, New York 10027, USA, 6Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida 32611-6200, USA,7Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA, 8Department of MechanicalEngineering, Columbia University, New York, New York 10027, USA, 9Department of Electrical Engineering, Columbia University, New York, New York10027, USA. *e-mail: [email protected]

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within each semiconductor layer. In contrast, under reverse bias, sub-stantial potential barriers result from band bending in the lateraldirection (Supplementary Fig. 4). Consequently, under forwardbias, the current is governed by tunnelling-mediated interlayerrecombination between majority carriers at the bottom (top) of theconduction (valence) band of MoS2 (WSe2).

This unusual interlayer recombination can be described bytwo possible physical mechanisms or a combination of both: (1)Shockley–Read–Hall (SRH) recombination (R ≈ nM pW/τ (nM + pW))mediated by inelastic tunnelling of majority carriers into trapstates in the gap; and (2) Langevin recombination (R ≈ BnMpW

s)by Coulomb interaction. Here R is the recombination rate, nM andpW are, respectively, the electron and hole densities in MoS2 andWSe2, and τ and B are, respectively, the tunnelling-assistedrecombination lifetime and the Langevin recombination constant2.An exponent (1.2 < s < 1.5) is used in the modelling of thetwo-dimensional Langevin mechanism28, in contrast to the linear

dependence on p for the three-dimensional case (SupplementarySection 3). The rectifying I–V characteristics are understood withan increasing interlayer recombination rate at higher forwardbiases (Supplementary Fig. 5). In addition, the current underforward bias can be tuned by varying the carrier densitiesthrough electrostatic gating; this is maximized with nearly balancednM and pW at Vg = 0 (Fig. 1b), as predicted for both SRHand Langevin recombination. We estimate a lower bound for life-time τ > 1 µs from the measured current and the simulated carrierdensities (Supplementary Section 4). This lifetime is relativelylong because inelastic tunnelling processes occur between randomlystacked TMDC layers with lateral momentum mismatch13. This isbeneficial for reducing the interlayer recombination in photocurrentgeneration, as discussed below.

Based on this understanding of the charge transport mechan-ism, we next explore the optoelectronic response of the atomicallythin p–n junction. Figure 2a presents representative I–V curves

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Figure 1 | Charge transport in an atomically thin p–n heterojunction. a, Bottom left: Schematic diagram of a van der Waals-stacked MoS2/WSe2heterojunction device with lateral metal contacts. Top: enlarged crystal structure, with purple, red, yellow and green spheres representing Mo, S, W and Seatoms, respectively. Bottom right: Optical image of the fabricated device, where D1 and D2 (S1 and S2) indicate the metal contacts for WSe2 (MoS2). Scalebar, 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 (redcurve, measured between D1 and D2). c,d, Band profiles in the lateral (c) and vertical (d) directions, obtained from electrostatic simulations. Under forwardbias (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 WSe2undergo 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 thatthere 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.

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and a colour plot of the photocurrent (inset) in the gate range of–30 to 30 V under white-light illumination. We observe a gate-tunable photovoltaic response. In particular, the short-circuitcurrent density reaches its maximum at Vg = 0 V and decreaseson varying the gate voltage in either polarity (the line profileat source–drain voltage (Vds) = 0 V in the colour plot). Themaximum photoresponsivity is ∼2 mAW−1, as measured using afocused 532 nm laser with a power of 0.8 µW (corresponding to100 W cm−2). Note that we observed a similar gate dependencefor all light sources used, although the gate voltage was shiftedby extrinsic substrate- or photo-induced doping effects29

(Supplementary Fig. 7).To further elucidate the underlying physical processes, we

performed spatial mapping of the photoresponse by scanning thelight source in a confocal opticalmicroscope. Figure 2b displays the re-sulting photocurrent map taken at Vds = 0 V. The strongest

photoresponse is observed in the MoS2/WSe2 junction area, indi-cating spontaneous charge separation occurring at the junction.This charge separation process is also compatible with the photolumi-nescence characteristics of the MoS2/WSe2 junction. Figure 2c showsphotoluminescence spectra obtained from the separated monolayersand from the overlapping layers forming the p–n junction. We findthat emission from the direct gap transitions of both monolayerMoS2 (1.88 eV) and monolayer WSe2 (1.66 eV) is strongly quenchedonly at the MoS2/WSe2 junction area. The integrated intensitiesdecrease, respectively, by 81% and 98% compared to isolated MoS2and WSe2 monolayers. The spatially resolved photoluminescencemaps at the fixed bandgap energies also reveal luminescence quenchingin the p–n junction area (Fig. 2d). We attribute the observed strongdecrease in photoluminescence to the rapid separation of chargecarriers in the junction region. Although an energy transferprocess could explain the decrease in photoluminescence by the

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Figure 2 | Gate-tunable photovoltaic response. a, Photoresponse characteristics at various gate voltages under white-light illumination. Inset: Colour plot ofphotocurrent 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 bydashed and solid lines, respectively. Scale bar, 3 µm. c, Photoluminescence spectra measured from the isolated monolayers (blue curve for MoS2; red curvefor 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 chargetransfer and intralayer recombination processes, respectively. In f, red and blue arrows indicate Shockley–Read–Hall (SRH) and Langevin recombinationprocesses, 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 ⟨nM pW

1.2⟩ (middle)and ⟨nM pW/(nM+ pW)⟩ (bottom). h, Measured (circles and dashed curve) and simulated (green curve for two-dimensional Langevin process and purplecurve 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.

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material with the larger gap, the observed decrease of photolumine-scence of both materials suggests a charge transfer mechanism. Notethat we can exclude the photo-thermoelectric effect as the origin ofthe photocurrent; its contribution is minor compared to themeasured photoresponse as a consequence of the small Seebeckcoefficient and very slight temperature gradient across the verticalvan der Waals interface (Supplementary Section 6)30,31.

Spontaneous dissociation of a photogenerated exciton into freecarriers can be driven by large band offsets for electrons (ΔEc)and holes (ΔEv) across the atomically sharp interface, as shown inFig. 2e. This charge separation may also be understood in termsof highly asymmetric charge transfer rates for electrons and holesin a heterostructure with type II band alignment; one processinvolves allowed transitions into the band, while the other involvesforbidden transitions into the gap. These charge transfer and separ-ation processes are analogous to those considered in excitonicorganic solar cells11. In atomically thin p–n junctions, the(allowed) charge transfer processes are expected to be fast and

efficient because exciton (or minority carrier) diffusion is notrequired. Indeed, our observation of significant photoluminescencequenching and photocurrent generation in the p–n junction indi-cates that exciton dissociation at the junction is significantly fasterthan other non-radiative (or radiative) decay channels existingwithin the layers. For typical TMDC monolayers, such intralayerrelaxation processes are present on a timescale of ∼10 ps (refs 3,32).

For an atomically thin p–n heterojunction, even after excitondissociation, the majority carriers spatially confined in each layercan undergo recombination though inelastic tunnelling. Such inter-layer tunnelling-mediated recombination plays an important role indetermining the photocurrent. Figure 2f represents interlayer SRHand Langevin recombination processes. Because the gate voltagetunes the majority carrier densities, we can model the gate modu-lation of the photocurrent observed in our experiments. Figure 2gpresents plots of the simulated densities of majority and minoritycarriers in each layer as a function of gate voltage (top panel),under illumination. Here, the interlayer recombination rate will be

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Figure 3 | Graphene-sandwiched van der Waals p–n heterojunctions. a, Schematics of MoS2/WSe2 junction sandwiched between top and bottom grapheneelectrodes. 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 thebandstructure, with exciton dissociation and charge-collection processes indicated by horizontal arrows. The dashed line represents the Fermi level. Note thatthe bottom graphene (GR) is slightly p-doped from the SiO2 substrate. d, Photoresponse characteristics of graphene-sandwiched p–n junctions with differentthicknesses. For comparison, the applied voltages (horizontal axis) are normalized with respect to the junction thicknesses. e, EQE plots as a function ofexcitation energy (wavelength) for the devices in d. The laser powers were in the range 3–7 µW, corresponding to 380–890W cm−2. Results in d and e areshown for devices composed of monolayer–monolayer (1L–1L), bilayer–bilayer (2L–2L) and multilayer–multilayer (ML–ML (10/9 nm)) junctions.






proportional to nMpWs (middle panel in Fig. 2g) or nMpW/(nM + pW)

(bottom panel in Fig. 2g), depending on whether the dominantrecombination mechanism follows Langevin or SRH behaviour.These quantities, spatially averaged over the junction area, are mini-mized near Vg = 0 V. They increase with |Vg| due to accumulation ofone type of majority carrier. Consequently, as shown in Fig. 2h, thesharply peaked photoresponse observed experimentally is modelledwith both Langevin and SRH recombination because the photo-current is determined by the difference of the recombination rateand the gate-independent generation rate (Supplementary Section5 provides more details of the simulation). Note that althoughLangevin recombination may be a dominant mechanism due tothe enhanced Coulomb interaction between electrons and holesspatially confined in the two-dimensional layered system33, SRHrecombination could still be induced by imperfections at thejunction interface, as well as defects in the materials. Therefore,we could not achieve full quantitative agreement using only aspecific mechanism, despite reasonable qualitative agreement.

In our devices, the relatively low carrier mobility in the lateraltransport channel results in a diffusion time for the majority carriersto leave the junction region as long as ∼1 µs, a timescale comparableto the interlayer recombination lifetime. Because photocurrent col-lection competes with interlayer recombination, one strategy toreduce interlayer recombination losses is to increase the rate ofcollection of the photogenerated carriers. To realize this improve-ment, we used graphene electrodes directly on the top and bottomof the vertical p–n junction (Fig. 3a). This allows carrier collectionby direct vertical charge transfer, rather than through lateral diffu-sion. For comparison, we fabricated van der Waals stacks usingvarious thicknesses of MoS2/WSe2 layers between two grapheneelectrodes (Supplementary Sections 1 and 2)34. In this devicegeometry, graphene serves as a transparent van der Waalscontact that also minimizes recombination compared to typicalmetal contacts8.

Figure 3b shows that photocurrent (at Vds = 0 V) is only observedin the region where the two active TMDC monolayers overlap withthe top and bottom graphene electrodes (Supplementary Section 8).Figure 3c presents I–V curves of the vertical p–n junction consistingof both monolayers in the dark and under 532 nm laser excitation.The device produces linear I–V characteristics, apparently domi-nated by direct tunnelling between the two graphene electrodes(Supplementary Section 7). Nevertheless, we still observe ashort-circuit current of ∼70 nA at an excitation power of ∼7 µW(corresponding to 920 W cm−2), corresponding to a photoresponsiv-ity of ∼10 mAW−2. This value for vertical charge collection is largerthan that for a typical laterally contacted device by a factor of ∼5.

The p–n photodiode characteristics are improved significantly asthe number of atomic layers in the MoS2/WSe2 heterojunctionsincreases and direct tunnelling between the top and bottom gra-phene electrodes is suppressed. Figure 3d shows the photoresponsecharacteristics of three devices with different junction thicknesses.As the thickness increases, the overall transport curve changesfrom a linear to a rectifying diode characteristic because direct tun-nelling current decreases exponentially (Supplementary Section 7).In particular, the vertical p–n junction consisting of two multilayers(10/9 nm) shows a photovoltaic response with an open-circuitvoltage of ∼0.5 V (corresponding to ∼26 mV nm−1). Additionally,the photocurrent gradually increases with thickness up to∼120 mAW−1 because of increased light absorption. These photo-response characteristics could also be tuned by varying the gatevoltage (Supplementary Section 9).

We also investigated the external quantum efficiency (EQE) as afunction of excitation energy for graphene-sandwiched p–n junc-tions of different thicknesses (Fig. 3e). The measured EQEs at532 nm were 2.4%, 12% and 34% for monolayer, bilayer and multi-layer p–n junctions, respectively. The quantity does not scale

linearly with junction thickness because of the thickness-dependentcompetition between generation, charge collection and recombina-tion, permitting further optimization of the overall efficiency.Regardless of thickness, all three p–n junctions show similar spectralresponses, with two prominent peaks at 1.64 eV and 1.87 eV, corre-sponding to the excitonic absorption edges of WSe2 and MoS2,respectively. This spectral photoresponse matches well with theabsorption spectrum measured from the monolayer MoS2/WSe2stack (Supplementary Section 10), indicating that excitons generatedin both semiconductor layers contribute to the photocurrent.

In conclusion, we have fabricated atomically thin p–n junctionsfrom van der Waals-bonded semiconductor layers. Although thejunction exhibits both rectifying electrical characteristics and photo-voltaic response, the underlying microscopic processes differstrongly from those of conventional devices in which an extendeddepletion region play a crucial role. In particular, interlayer tunnel-ling recombination of the majority carriers across the van der Waalsinterface, which can be tuned by gating, is found to influence signifi-cantly both the electrical and optoelectronic properties of the junc-tion. Further optimization of the band alignment, number ofatomic layers and interface lattice mismatching will lead to uniquematerial platforms for novel, high-performance electronic andoptoelectronic devices.

MethodsThe MoS2/WSe2 stacks were fabricated on 280-nm-thick SiO2-coated Si substratesusing a co-lamination and mechanical transfer technique. The thickness of theTMDC layers was confirmed by atomic force microscopy, photoluminescenceand/or Raman spectroscopy (Supplementary Section 2). The vertical stacks ofgraphene/MoS2/WSe2/graphene were prepared using a polymer-free van der Waalsassembly technique with edge electrical contact to the graphene layers34. Furtherdetails of the fabrication processes are provided in Supplementary Section 1. Formodel simulation of the device, Poisson and drift-diffusion equations were solvedself-consistently using a two-dimensional finite difference method (SupplementarySection 3). The photoresponse characteristics were investigated under white-lightillumination and laser excitation. Quantitative analysis of the response was obtainedwith focused laser excitation of known power. Scanning photocurrent measurementswere performed using a confocal optical microscopy equipped with a two-axisscanning mirror or scanning mechanical stage. In these experiments, a 532 nmcontinuous-wave laser (Crystalaser) and a supercontinum laser source (NKT photonics)were used as excitation light sources. The laser radiation was focused to a 0.7–1 µmfull-width at half-maximum (FWHM) spot through a ×50 objective. The powersused for wavelength-dependent EQE measurements were in the range 3–7 µW,where the devices exhibit a linear response (Supplementary Section 11).

Received 14 March 2014; accepted 24 June 2014;published online 10 August 2014

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AcknowledgementsThis work was supported as part of the Center for Re-defining Photovoltaic EfficiencyThrough Molecule Scale Control, an Energy Frontier Research Center funded by the USDepartment of Energy, Office of Science, Office of Basic Energy Sciences (award no.DE-SC0001085), and in part by the National Science Foundation (DMR-1124894) and bythe FAME Center, one of six centres of STARnet, a Semiconductor Research Corporationprogramme sponsored by MARCO and DARPA. G.H.L. and M.H. acknowledge supportfrom the Basic Science Research Program (2014R1A1A1004632) (G.H.L.) and the NanoMaterial Technology Development Program (2012M3A7B4049966) (M.H.) through theNational Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT andFuture Planning.

Author contributionsC-H.L., G-H.L., M.H., X.C. and G.A. fabricated and characterized the van der Waals p–nheterostructures. C-H.L. and G-H.L. performed device fabrication and transportmeasurements. A.M.v.d.Z. and C-H.L. performed photocurrent measurements. W.C. andJ.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 inconsultation with G-H.L., A.M.v.d.Z., W.C., C.N., T.F.H., J.G. and J.H.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondence andrequests for materials should be addressed to P.K.

Competing financial interestsThe authors declare no competing financial interests.





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