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Showcasing research from the Nano Energy Lab led by Jr-Hau He

at King Abdullah University of Science and Technology

High-efficiency omnidirectional photoresponses based on

monolayer lateral p–n heterojunctions

Lateral junctions enable new possibilities to control the width

of the depletion region, offering a more flexible strategy to

design complex circuit layouts. With the success in growing

p–n junction-based lateral heterostructures, the intriguing

optical properties have been explored. It is discovered that

lateral junctions function as diodes with gate-tuning behavior.

They generate a significant photocurrent with high responsivity,

outstanding omnidirectionality, fast response, and high

temperature operation capability. These unique characteristics

pave the way for the future development of sub-nano

semiconductor device applications.

rsc.li/nanoscale-horizons

As featured in:

See Lih-Juann Chen, Lain-Jong Li, Jr-Hau He et al.,Nanoscale Horiz., 2017, 2, 37.

This journal is©The Royal Society of Chemistry 2017 Nanoscale Horiz., 2017, 2, 37--42 | 37

Cite this:NanoscaleHoriz., 2017,2, 37

High-efficiency omnidirectional photoresponsesbased on monolayer lateral p–n heterojunctions†

Meng-Lin Tsai,ab Ming-Yang Li,cd Yumeng Shi,c Lih-Juann Chen,*b Lain-Jong Li*c

and Jr-Hau He*a

Electrical and optical properties of lateral monolayer WSe2–MoS2

p–n heterojunctions were characterized to demonstrate a high

responsivity of 0.26 A W�1 with an excellent omnidirectional photo-

detection capability. The heterojunction functioning as a diode

exhibits a prominent gate-tuning behavior with an ideality factor

of 1.25. In addition, ultrafast photoresponse, low-light detectability,

and high-temperature operation have been achieved. These unique

characteristics pave a way for the future development of sub-nano

semiconductor devices.

Introduction

2D materials including metals (graphene), semiconductors(transition metal dichalcogenides such as MoS2 and WSe2), andinsulators (BN) have emerged as exciting materials in variousresearch fields.1–5 As semiconductor p–n junctions are essentialbuilding blocks for modern electronics, it is crucial to constructatomically thin p–n heterojunctions and multiple transfer proce-dures have been adoped to form vertically stacked transition-metaldichalcogenides.6–8 But this would increase the complexity of theprocess and thus hinder the mass production for the hetero-structures. In the past few years, the direct epitaxial growthof lateral monolayer heterojunctions by the subsequent growthof 2 types of 2D materials has been proposed.9,10 However,the heterostructures are limited to either identical metals orchalcogens, resulting in the existence of alloy structures at theinterface and inhibiting the formation of ideal p–n junctions.

Very recently, the controlled epitaxial growth of WSe2–MoS2 lateralheterojunctions was successfully developed, which sets a corner-stone for the construction of future circuits based on monolayers.11

In contrast to vertically stacked 2D monolayers, lateral junctionsenable a new possibility to control the width of the depletionregion, which offers a more flexible strategy to design complexcircuit layouts. Accordingly, revolutionary optical and electronicintegrated circuits can possibly be realized for use in both funda-mental research and next-generation semiconductor industries.

To design highly efficient optical devices, omnidirectionallight-havesting capability is a crucial requirement. For example,the angle of incidence (AOI) of the sun changes during the day.Furthermore, when sunlight reaches the earth after passingthrough the atmosphere, it contains a large amount of scattered(diffused) light. On cloudy days, over 90% of the light is diffused.However, for polished Si surfaces, nearly 40% of light is reflectedwhen averaged over all AOIs and the solar spectrum.12 Additionally,as expensive sun-tracking and concentrator systems maketheir way into high-efficiency solar applications, most of thesetracking and concentrator systems are specified for directradiation only, which means that these tracking systems workwell on bright clear days but poorly on hazy days. Obviously,conventional solar devices show insufficient light harvesting dueto their angle-dependent absorption capabilities. Accordingly, anew optical requirement, the omnidirectional light-harvesting

a Computer, Electrical and Mathematical Sciences and Engineering,

King Abdullah University of Science and Technology, Thuwal, 23955-6900,

Kingdom of Saudi Arabia. E-mail: jrhau.he@kaust.edu.sab Department of Materials Science and Engineering, National Tsing Hua University,

Hsinchu, 30013, Taiwan, Republic of China. E-mail: ljchen@mx.nthu.edu.twc Physical Sciences and Engineering Division, King Abdullah University of Science

and Technology, Thuwal, 23955-6900, Kingdom of Saudi Arabia.

E-mail: lance.li@kaust.edu.sad Research Center for Applied Sciences, Academia Sinica, Taipei, 10617, Taiwan,

Republic of China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nh00075d

Received 26th April 2016,Accepted 12th October 2016

DOI: 10.1039/c6nh00075d

rsc.li/nanoscale-horizons

Conceptual insightsvan der Waals heterostructures of 2D materials have been investigated forthe fabrication of various electronic and optoelectronic devices in the pastseveral years. However, the stacking of heterostructures requires multipletransfer procedures and alignment techniques. It was not until recentlythat 2D lateral heterostructures had been developed to realize the junc-tion of multiple 2D materials in a single atomic layer. In this study, thegate-tunable electrical and optical properties of WSe2–MoS2 lateral mono-layer heterojunctions have been studied systematically and thoroughly forthe first time. The results and characteristics demonstrated here providethe possibilities for the future development of ultrathin, transfer-free,high-quality, and high-performance monolayer 2D devices.

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performance, emerges to reduce wasted photons caused by theangular effect. To date, photon management techniques thatenable omnidirectional light detectability such as hierarchicalsurface engineering, fabrication of graded refractive index coat-ings, employment of colloidal nanospheres, and utilization ofperiodic nanopillar arrays have been applied in various opticaldevices.13–16 These omnidirectional light-harvesting techniquesare often achieved with complex fabrication processes, whichcome with unwanted defects, recombination sites, and contam-inations deteriorating the device performance for the pursuitof omnidirectionality and thus requiring additional treatmentprocedures. Accordingly, a more straightforward and revolutionarydevice design is needed for achieving a highly efficient omni-directional optical performance.

Recently, van der Waals heterojunctions based on mono-layer 2D materials have been extensively studied for the designof ultrathin photodetecting and photovoltaic devices. Stackedvan der Waals heterostructures such as WS2–MoS2 and MoS2–graphene have been reported as the thinnest solar devices withultrahigh power densities per kilogram.17,18 The WSe2–MoS2

van der Waals heterojunction has been demonstrated to exhibita responsivity of 11 mA W�1.7 Despite the great success, therequired multiple transfer processes along with the alignmentprocedure largely increase the complexity of device fabrication.In addition, defects, lattice mismatch, additional functionalgroups, contaminants, and absorbed gas molecules at the junctionof these devices can act as quenching centers, resulting in severecharge carrier recombination and inhibiting the overall perfor-mance of the device.

With the success in growing p–n junctions based on lateralWSe2–MoS2 heterostructures, many properties are left to beexplored. First, the WSe2–MoS2 lateral junction functions as adiode with gate-tuning behavior with an ideality factor of1.25, demonstrating an excellent p–n heterojunction deviceperformance. Under 532 nm laser illumination, a significantphotocurrent has been generated in the lateral junction devicewith the responsivity as high as 0.26 A W�1, which is 20 timeshigher than that of the stacked monolayer heterojunction(0.011 A W�1) reported previously.7 Most importantly, the junctionpresents an outstanding omnidirectional detection capabilityfor low-intensity light, which can be further used for indoorlight-harvesting, since photons from all AOIs can directly get tothe active region of the lateral junction. The 2D lateral juctionalso demonstrates excellent photoresponsivity at an elevatedtemperature up to 120 1C.

ExperimentalChemical vapor deposition of lateral WSe2–MoS2

heterostructures

WSe2 single crystal monolayers were first grown by placing WO3

powders in a quartz boat at the heating zone center of thefurnace. The sapphire substrate was placed at the downstreamside next to the quartz boat. Se powders were placed in aseparate quartz boat at the upstream side of the furnace and

the temperature was maintined at 260 1C during the reaction.The Ar/H2 flow was controlled at Ar = 90 sccm and H2 = 6 sccmwith a chamber pressure of 20 Torr. The heating zone washeated to 925 1C, kept for 15 min, and cooled down to roomtemperature. Then the sample was moved to a separate furnace.MoS2 synthesis was carried out by switching the sources toMoO3 and S powders. The Ar flow was controlled at 70 sccmwith a chamber pressure of 40 Torr. The center zone and Ssource were heated to 725 1C and 190 1C, respectively, and keptfor 15 min. Finally, the sample was naturally cooled down toroom temperature.

Characterization of heterostructures

The surface morphology and surface potential of the hetero-structures were characterized using a commercial atomic forcemicroscope (Cypher ES – Asylum Research Oxford Instruments).Raman spectra were collected in a Witec alpha 300 confocalRaman microscopy system including a RayShield coupler withan exciting laser wavelength of 532 nm. The laser spot-size isaround 0.5 mm. The Raman signal was collected using a 100�objective lens (N.A. = 0.9) from Carl Zeiss Microscopy GmbHand dispersed using a 1800 lines per mm grating for Ramanmeasurements and a 300 lines per mm grating for PL measure-ments. PL measurements were also performed in the Witecalpha 300 confocal system.

Fabrication of devices

The WSe2–MoS2 heterojunction was first transferred onto aSiO2 (260 nm)/Si substrate via a PMMA (950 A4, MicroChem)assisted method. The WSe2–MoS2 heterojunction device wasfabricated via electron-beam lithography in a SEM system (JEOLJSM-7001F). Ti/Au (10/20 nm) and Au (20 nm) were depositedusing electron beam deposition.

Characterization of devices

The measurement was carried out in a probe-station and measuredusing a Keithley 4200 device equipped with a 532 nm laser. Theresponsivity of the device was measured using a monochromator(Spectral Products CM110) connected to a xenon light source(Spectral Products ASB-XE-175).

Results and discussion

The 2D lateral WSe2–MoS2 heterojunction shown in Fig. 1a wasepitaxially grown on sapphire substrates via a two-step chemicalvapor deposition of WSe2 at 925 1C and MoS2 at 725 1C.11 Afterthe growth, atomic force microscopy, photoluminescence (PL),and Raman measurements were carried out to identifythe morphology and optical properties of the lateral hetero-structures. Fig. 1b shows the optical microscopy image of themonolayer WSe2–MoS2 lateral heterostructures. To confirm thechemical composition of the WSe2 and MoS2 regions, PLcharacterization was performed at both the inner and outerregions of the triangular heterostructures, as shown in Fig. 1c.The PL peaks at 672 nm and 778 nm correspond to the direct

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band gaps of monolayer MoS2 (B1.8 eV) and WSe2 (B1.6 eV),respectively.19,20 Fig. 1d displays the PL peak position mappingof the same region shown in Fig. 1b, which verifies thattriangular WSe2 is surrounded by MoS2. In Fig. 1e, the Ramanpeak at 250 cm�1 measured from the inner region is the A1g

characteristic peak of WSe2, and the Raman peaks at 405 cm�1

and 385 cm�1 from the outer region are the A1g and E2g

characteristic peaks of MoS2.21,22 Fig. 1f shows the absorptionspectra of the WSe2 and MoS2 monolayers; the cutoff wave-lengths for WSe2 (B780 nm) and MoS2 (B680 nm) are alsoconsistent with the PL peaks shown in Fig. 1c.

To fabricate the 2D lateral p–n heterojunction devices,WSe2–MoS2 heterostructures were transferred onto the SiO2

(260 nm)/Si substrate for e-beam lithography processes. Fig. 2ashows the contact patterns of the electrodes. Ti/Au (10/20 nm)and Au (20 nm) were deposited using electron beam depositionto form low-resistance Ohmic contacts with MoS2 and WSe2,respectively. To avoid direct electrical contact between the outerregion and the inner electrode, we performed plasma etching inthe selective region for device fabrication.9,23 In addition, a backgate voltage was applied on the SiO2/Si substrate to investigatethe gate-controlled behavior, as shown in Fig. 2b. By applying agate voltage of �60 V, an ideality factor of 1.25 can be achieved,showing excellent diode characteristics. It has been proposedthat the Fermi level of 2D materials can be efficiently tunedby applying a gate voltage without extrinsic doping.7,20 Fig. 2cand d show the Ids–Vds characteristics under different back gate

voltages and Ids–Vds characteristics under 532 nm illumination(power = 2 � 104 W m�2), respectively. The improved rectifyingcurve when applying a high gate voltage (�60 V) for both darkand light measurements shows the gate modulating feature ofthe MoS2/WSe2 diode. The Ids–Vg characteristics at Vds = 2 V and�2 V are shown in Fig. S1a and b of the ESI,† respectively.When operated under a reversed bias (�2 V), the MoS2/WSe2

diode shows a higher photo-to-dark-current ratio at Vg rangingfrom �60 V to 60 V.

The gate tuning characteristics of the lateral heterojunctiondevice can possibly originate from the electrostatic dopingbehavior of WSe2.7,19 The CVD-grown WSe2 has been reportedas a lightly p-doped semiconductor.24 In contrast to WSe2, MoS2

is normally grown as highly n-doped unintentionally, whichmakes its Fermi level more resistant to additional electrostaticgating procedures. Therefore, by applying a positive bias, electrondoping in WSe2 occurs and WSe2 becomes more intrinsic, and therectifying behavior becomes less pronounced. When applying anegative bias, hole doping takes place in WSe2, leading to theformation of a heavily p-doped WSe2 region and thus achieving amore ideal p–n junction.

Fig. 3a and b show photoresponse and photovoltaic proper-ties of the lateral heterojunction device. Under 532 nm laserillumination with 2 � 104 W m�2, the device exhibits a 36-foldincrease in current with an applied voltage of �2 V, showingthe extrordinary photodetection ability of the lateral hetero-structure. In this case, the voltage is applied to p-type WSe2

while the n-type MoS2 is grounded. Meanwhile, the open-circuitvoltage of 0.3 V and the short-circuit current of 0.51 nA reveal aphotovoltaic effect of the device. Fig. 3c shows the reproduci-bility of the device performance using the lateral monolayerWSe2–MoS2 p–n heterojunction. Fig. 3d and e show the short-time-period photoresponses. The rising time is defined as thetime taken to increase from 20% to 80% of the maximumphotocurrent, and the falling time is defined as the time taken

Fig. 1 Characterization of WSe2–MoS2 monolayer lateral heterostructures.(a) Schematic of WSe2–MoS2 monolayer lateral heterostructures. (b) Opticalmicroscopy image of WSe2–MoS2 monolayer lateral heterostructures. (c) PLspectra of the MoS2 and WSe2 regions. (d) PL peak wavelength mappingimages. (e) Raman spectra of the MoS2 and WSe2 regions. (f) Absorptionspectra of the MoS2 and WSe2 monolayers.

Fig. 2 Characterization of the WSe2–MoS2 monolayer device. (a) Opticalmicroscopy images of the WSe2–MoS2 monolayer device. (b) Ids–Vds

characteristics of the device under a Vg of 0 V and �60 V. (c and d)Gate-tunable output characteristics of a lateral WSe2–MoS2 heterojunc-tion p–n diode (c) in the dark and (d) under 532 nm laser illumination.

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to decrease from 80% to 20% of the maximum photocurrent,respectively.25 The rising and falling response times of 20 and30 ms demonstrate its potential for high speed photosensorapplications. The response time could be limited by the diffu-sion time of the laterally connected WSe2 and MoS2 regions,which can be further improved with a better geometry designsuch as the reduction in device channel length. It is noted thatthe laser beam diameter is B1 mm, which is significantly largerthan the active area of the device. In this case, the contactbetween the metal electrodes and 2D materials would alsocontribute to the generation of photocurrent. We have previouslyexplored that the Ohmic contact between metal electrodes and2D materials would lead to a large photoresponse but a longerresponse time compared with the Schottky-contact devices.26

Therefore, the overall time response may also be limited by theresponse time at the Ti/MoS2 and Au/WSe2 interfaces. Thedynamics of the photogenerated carriers were further evaluatedusing a mechanical chopper to modulate the frequency of thelight source. As shown in Fig. 3f, the lateral heterojunctiondevice can be operated up to 100 Hz modulation frequency.Note that even with the low-resistance contacts used in thisstudy to optimize the photoresponsivity, the p–n junctiondevice has already demonstrated a fast response time potential

for fast detection. It is also noted that the gate voltage appliedfor Fig. 3 is 60 V. We have also measured the photoresponse ofthe device under the gate voltage of �60 V, as shown in Fig. S2of the ESI.† The responsivity of the device under a gate voltageof �60 V is very low. This can be attributed to the fact thatunder a gate voltage of �60 V, the carrier concentration in MoS2

is largely reduced due to the large negative gate bias, leading toa low responsivity. This phenomenon has also been observed inother 2D lateral structures.9

To further characterize the photosensing capability ofthe device, we measured the responsivity of the lateral hetero-junction at various light intensities, AOIs, wavelengths, andoperation temperatures. The responsivity is calculated by dividingthe measured output current by the illuminating optical power.The actual device area of B32 mm2 is estimated from the opticalmicroscopy image shown in Fig. 2a. The actual device area isidentified in Fig. S3, and the calculation details are included inthe ESI.† As shown in Fig. 4a and b, the lateral heterojunctionexhibits excellent omnidirectional light-harvesting propertiesand its responsivity shows a maximum of 0.26 A W�1 at a lightintensity of 500 W m�2, which is 420 times higher than thepreviously reported results obtained from the vertical hetero-junction based on stacked MoS2/WSe2 monolayers.7 In stacked2D layers, there are additional functional groups, contaminants,or adsorbed gas molecules at the junction interface, which act asthe carrier trapping centers and cause severe carrier recombina-tion of the charge carriers in the region. The figure of merit ofthe lateral heterojunction without any defects in the junction isthat the photoexcited carriers could be effectively collected,leading to an excellent device performance, as comparedto the vertical heterojunction. The excellent omnidirectionallight-harvesting behavior can be attributed to the ultrathin

Fig. 3 Photoresponse of the WSe2–MoS2 monolayer device. (a) I–Vcharacteristics of the device in the dark and under 532 nm laser illumination,under �2 V to 2 V biased voltage. (b) Photovoltaic characteristics under532 nm laser illumination. (c) Time response of the device under 532 nmlaser illumination with a bias voltage of�2 V. (d and e) Enlarged (d) rising and(e) falling time response of the device under 532 nm laser illumination with abias voltage of �2 V. (f) Frequency-dependent photogain of the deviceunder 532 nm laser illumination with a bias voltage of �2 V.

Fig. 4 Intensity, angular, wavelength, and temperature dependent photo-response. (a) Light intensity dependent responsivity of the device under532 nm laser illumination with a bias voltage of �2 V. (b) AOI-dependentresponsivity of the device under 532 nm laser illumination with a bias voltageof �2 V. (c) Responsivity spectrum of the device. (d) Temperature-dependentphoto-to-dark current ratio (PDCR) and responsivity of the device under532 nm laser illumination with a bias voltage of �2 V in a vacuum.

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nature and high absorptivity properties of the 2D materials. Forconventional photodiodes based on vertical junctions, theresponsivity is significantly decreased due to insufficientabsorption in the junction area at a certain depth caused byserious light scattering or reflection of the top surface at highAOIs. For example, planar Si-based optical devices have shownmore than 90% efficiency reduction when operated under highAOIs (751) as compared with light being incident normally(01).13 In contrast, for lateral heterojunction diodes, light canbe absorbed from any direction and can directly reach the activearea of the device. The maximum responsivity at 500 W m�2

can be attributed to the saturation of photocarriers. For lightintensity below 500 W m�2, there are abundant photocarriersavailable for photogeneration. However, at an intensity above500 W m�2, most of the charge carriers have been induced in thephotogeneration process, resulting in the saturation of thephotocarriers and the reduction of responsivity. For most photo-detectors, the responsivity is higher at lower light intensities. Inour case, the dark current and photocurrent are in the range of0.1 nA to a few nA. Therefore, at ultralow light intensity, thesignal to noise ratio becomes lower and the responsivity calcula-tion may fluctuate depending on the noise level. The decrease ofresponsivity at a low light power has also been reported in other2D material photodetectors.26–28 It is worth noting that even atan intensity as low as 50 W m�2, the responsivity of the deviceremains at 0.12 A W�1, demonstrating extraordinarily high lightdetectability. The outstanding omnidirectional low-light charac-teristics enable new indoor applications such as smart homesand intelligent buildings.

Fig. 4c shows the responsivity spectrum, showing that below750 nm, WSe2 starts to absorb photons and generate a photo-current. For a wavelength below 700 nm, more photons can beabsorbed both in the WSe2 and MoS2 regions, resulting in asignificant increase in responsivity. The results are consistentwith the absorption results shown in Fig. 1f. Compared to pureWSe2 and MoS2 films, the improvement of responsivity can beattributed to the following reasons. (1) The lower bandgap ofWSe2. Due to the slightly lower bandgap (B1.35 eV) of WSe2

as compared with MoS2 (B1.8 eV), more photons above thewavelength of 700 nm can be absorbed in the WSe2 region ascompared with the MoS2 films. (2) Higher absorption of MoS2.In Fig. 1f, MoS2 shows a higher absorption below 700 nm.Therefore, the heterojunction can absorb more light at wave-lengths below 700 nm to exhibit improved responsivity com-pared to pure WSe2 films. (3) Defect-free junction quality. Forpure 2D-material photodetectors, 2D material films are usuallycontacted with metal electrodes to form metal–semiconductor–metal Schottky devices. However, the interaction between 2Dmaterials and metals can lead to surface states at the junctioninterface, giving rise to the enhancement of leakage current. Inour device, the lateral heterojunction is almost defect-free andideal at the p–n interface. Therefore, the dark current underreversed bias can be minimized. Accordingly, the responsivitycan be improved due to the combined optical and electricalenhancement. Traditional photodetector devices require highbandgap materials at the top to absorb light with short

wavelengths and low bandgap materials at the bottom forlong-wavelength light, which increases the cost, complexity,and challenges of device fabrication. By designing lateralheterojunction devices, light with different wavelengths can besuccessfully absorbed within a single layer of material, whichreduces the complexity for device design. It is expected thatphotosensing devices for a broad range of the wavelength spec-trum from the ultraviolet (such as BN) to infrared region (such asgraphene) with a variety of combinations can be achieved. Fig. 4dshows that the responsivity of the device remains as high as0.16 A W�1 at 120 1C under a vacuum, illustrating the capabilityfor high-temperature operation. The device has been kept undera vacuum for high temperature measurements to prevent theoxidation of MoS2 and WSe2. Therefore, the noticeable deteriora-tion in responsivity may be due to the generation of thermalcarriers, leading to the increase of dark current, which cannot beeliminated at high temperatures.

Conclusions

In summary, by applying the novel design concept of mono-layer lateral p–n heterojunction WSe2–MoS2, we have fabricatedan atomically thin photodetector with a high responsivity of0.26 A W�1, which is 20 times higher than the device based onvan der Waals heterojunctions. Since the lateral p–n junction isformed via direct epitaxial growth, the junction is clean andwithout photon quenching sites as in stacked van der Waalsjunctions. An outstanding omnidirectional light harvestingbehavior has also been achieved and characterized to showthe ability to retain high responsivity even at large AOIs, whichis unachievable using the traditional top-down structuraldesign. In addition, low-light detectability with a maximumresponsivity at 500 W m�2 and a high-temperature operation upto 120 1C have been achieved. These unique and intriguingproperties pave a way for the future development of sub-nanosemiconductor devices and applications.

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