n-MoS2 van der

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Vertical and In-plane Current Devices Using NbS/n-MoS van der Waals Schottky Junction and Graphene Contact Hyung Gon Shin, Hyong Seo Yoon, Jin Sung Kim, Minju Kim, June Yeong Lim, Sanghyuck Yu, Ji Hoon Park, Yeonjin Yi, Taekyeong Kim, Seong Chan Jun, and Seongil Im Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05338 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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Vertical and In-plane Current Devices Using NbS2/n-MoS2 van der Waals Schottky Junction and Graphene Contact Hyung Gon Shin†,†, Hyong Seo Yoon⊥,†, Jin Sung Kim†,†, Minju Kim†, June Yeong Lim†, Sanghyuck Yu†, Ji Hoon Park†, Yeonjin Yi†, Taekyeong Kim‡, Seong Chan Jun⊥ and Seongil Im†*

†

Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-gu,

Seoul 03722, Korea ⊥

School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul

03722, South Korea ‡

Department of Physics, Hankuk University of Foreign Studies, 81 Oedae-ro, Chein-gu,

Yongin-si, 17035, South Korea *Corresponding author. Tel: 82-2-2123-2842, Email: [email protected] †

These authors contributed equally to this work.

Keywords: MoS2, NbS2, graphene, Schottky Diode, MESFET, van der Waals interface

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Table of Contents

Abstract Van der Waals (vdW) Schottky junction between two dimensional (2D) transition metal dichalcogenides (TMDs) is introduced here for both vertical and in-plane current devices: Schottky diodes and metal semiconductor field effect transistors (MESFETs). The Schottky barrier between conducting NbS2 and semiconducting n-MoS2 appeared as large as ~0.5 eV due to their work function difference. While Schottky diode shows an ideality factor of 1.8~4.0 with ON/OFF current ratio of 103~105, Schottky effect MESFET displays little gate hysteresis and ideal subthreshold swing of 60~80 mV/dec due to low density traps at the vdW interface. All MESFETs operate with a low threshold gate voltage of -0.5 ~ -1 V exhibiting easy saturation. It was also found that the device mobility is significantly dependent on the condition of source/drain (S/D) contact for n-channel MoS2. The highest 2 ACS Paragon Plus Environment

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room temperature mobility in MESFET reaches to ~more than 800 cm2/V s with graphene S/D contact. The NbS2/n-MoS2 MESFET with graphene was successfully integrated into an organic piezoelectric touch sensor circuit with green OLED indicator, exploiting its predictable small threshold voltage, while NbS2/n-MoS2 Schottky diodes with graphene were applied to extract doping concentrations in MoS2 channel.


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Two dimensional (2D) transition metal dichalcogenide (TMD) materials have been extensively studied for the last decade, since those materials are found to be semiconducting, conducting, and sometimes superconducting depending on the combination of transition metal and chalcogene elements.

1-11

Main focus has been on 2D TMD semiconductors,

because semiconductors could always be the most important for existing or future device technologies. 6-14 Many of metal insulator semiconductor field effect transistors (MISFETs) using 2D TMD channels have thus been reported

13-20

, along with their use for

complementary metal-oxide-semiconductor transistor (CMOS) inverters.

21-25

But stability

issues such as gate bias-induced hysteresis and unpredictable threshold voltage still remain unresolved in 2D MISFETs, due to defect traps at the dielectric/2D channel interface.

19,26,27

Heterojunction 2D TMD PN diodes with van der Waals (vdW) interface have also received much attention from researchers. 28-33 These vdW PN junction interfaces basically experience out-of-plane/or vertical current across the junction during device operation. However, any in– plane current device using hetero 2D-2D vdW interface seems not reported yet, although vdW interface may be more favorable for in-plane current than for vertical one. Some inplane current devices were suggested in conducting thin film oxide/MoS2 or black phosphorous/ZnO nanowire junction systems, but they were not 2D-2D vdW junctions at all. 34,35

In the present work, we have fabricated vdW Schottky junction between two 2D TMDs aiming at hysteresis-reduced bifunctional devices for both vertical and in-plane current: Schottky diode and metal semiconductor field effect transistor (MESFET) with predictable small threshold voltages. Those two devices could simultaneously be formed with a common vdW junction. Those 2D-2D junctions are based on high work function NbS2 conductor and semiconducting MoS2, formed by dry-transfer method. 4 ACS Paragon Plus Environment

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TMD NbS2 is

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known to be a superconducting material at very low temperature, but behaves as a conductor with a large work function at room temperature. 38 While NbS2/n-MoS2 Schottky diode shows an ideality factor of 1.8~4.0 with ON/OFF current ratio of 103~105, Schottky effect MESFET displays little gate hysteresis of 0.05~0.1 V and ideal subthreshold swing of 60~80 mV/dec due to low trap density at the vdW interface. Operational threshold voltage of all MESFETs appears as low as -0.5 ~ -1 V and it was always predictable. Easy saturation behavior was observed as their own advantages, too. The highest mobility of MESFET reaches to ~more than 800 cm2/V s at room temperature when graphene source/drain/gate contact was used. This value is comparable or approaches to previous results from conducting oxide thin film NiOx/MoS2 MESFET whose high mobility was once doubted. 34,39 In the end, we put our MESFET with graphene contact to an extended device application such as current driver of piezoelectric touch sensor circuit, where the advantage of 2D-2D junction MESFET, predictable low threshold voltage was nicely exploited for piezoelectric switching of ±1 V. On the one hand, NbS2/n-MoS2 Schottky diodes with graphene were applied to extract unintentionally doped electron concentration in MoS2 channel, which supports the results from Hall measurement. For device patterning, we used e-beam lithography for a MESFET on SiO2/p-Si but also employed photo-lithography techniques for other MESFETS on glass substrate. Figure 1a shows an optical microscope image of our first prototype NbS2/MoS2 MESFET which works as Schottky diode as well when fabricated on SiO2/p-Si by dry transfer

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of TMD flakes and e-beam lithography. Each TMD material’s identity was

confirmed by Raman spectroscopy results in Figure 1b, where the spectrum of NbS2 is overlaid on that of MoS2 and displays vertical (A1) and in-plane (E1, E2) vibration peaks of NbS2 crystal structure. 40,41 Thicknesses of those two TMDs were measured by atomic force 5 ACS Paragon Plus Environment

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microscopy (AFM) scan, to be 7 and 12 nm for MoS2 and NbS2. (Figure 1c) The AFM scan also clearly shows the 3D contour for overlaid TMD area in Figure 1d. Our first device is again visualized in a 3D schematic view for atomic TMD layers in Figure 1e, where Au and Pt electrodes are shown as used for ohmic electrodes of MoS2 and NbS2, respectively [but for this prototype MESFET we did not carry out any contact improvement-anneal, to keep the 2D-2D Schottky junction thermal budget-free]. Although the two layers are overlaid, they would form a van der Waals gap in between. Basic conductance of NbS2 on MoS2 was estimated from the two terminal current – voltage (I-V) measurements of Figure 1f, to be as high as ~1 mS (~1 mA at 1 V). As well known, it is conducting. In contrast, the conductance of MoS2 appears incomparably small showing ~10 pA at 2 V. The small conductance in MoS2 has two reasons: 1. Small Schottky barrier between Au and n-type MoS2 still exists causing some contact resistance, and 2. Schottky effects between NbS2 and n-type MoS2 would cause carrier depletion in MoS2 underneath NbS2. Electrons undergo contact resistance during their injection from Au terminal but then should meet with more resistance in MoS2 channel which could be somewhat depleted due to work function difference between NbS2 and MoS2. Figure 2a and b respectively show schematic device cross section of NbS2/n-MoS2 Schottky junction diode and the rectifying I-V characteristics for vertical current from four possible diode combinations (see the inset circuits that come from the inset photo of Figure 1e). According to Figure 2b, ON/OFF current ratio appears ~105 from all the diodes, however the ideality factors are initially as large as ~25 and the forward bias-induced current turns on not at 0 V but at ~1 V. Moreover, lowest current is not obtained at 0 V but at -1.8 V. It seems that besides the NbS2/MoS2 Schottky barrier, another barrier may exist in this diode system, probably at the Au/MoS2 contact which is not annealed for this prototype device. On the one hand, the NbS2/n-MoS2 Schottky junction introduces in-plane carrier transport for MESFET 6 ACS Paragon Plus Environment

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transistor action quite well, due to vdW interface between NbS2 and MoS2. The two schematic device cross sections of Figure 2c illustrate ON and OFF behavior of MESFET with NbS2/n-MoS2 junction. A forward bias (+) on NbS2 gate opens n-MoS2 channel (for ONstate) but the forward bias should be limited not to cause gate leakage current (vertical charge/carrier transport). In such a gate voltage range, carrier transport can be controlled by drain bias (VDS). Positive gate voltage always means a forward bias for source terminal (usually grounded) during ON-state, however it is not necessarily a forward bias to drain terminal. When VDS becomes larger than VGS, reverse bias becomes dominant between gate and drain terminals forming a partial depletion of channel as illustrated. Upper figure of Figure 2c shows such partial depletion in the channel length (L), where in particular, a pinchoff state as a starting point voltage (VDSAT) of drain current (ID) saturation is depicted with a depletion triangle. The output characteristics (ID-VDS) of Figure 2d display the VDSAT (ID saturation point) which gradually increases with (+) VGS. It is seen that ID increase stops at the point to become flat-saturated. If the gate bias (VGS) on NbS2 becomes strongly negative as reverse bias to both source and drain, the MoS2 channel is charge-depleted with scarce number of carriers that cause OFF-state (see the lower figure of Figure 2c). In such depletion or OFF state, light illumination can cause photo-current as illustrated in the lower figure. The output curves in Figure 2d do not show quite good linear curves at small VDS, indicating that our prototype MoS2 channel MESFET on SiO2/p-Si substrate experiences some contact resistance problem which can also cause somewhat low ID.

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Despite the contact resistance,

the transfer characteristics (ID-VGS) of the MESFET (inset device symbol) in Figure 2e display decent device performances: high ON/OFF ratio of 106, decent saturation mobilities (µ) of 15~50 cm2/Vs (inset plots), much small SS of 67 mV/dec, small hysteresis less than 0.05 V, and a small threshold voltage less than -1 V. Mobility measurements were conducted in saturation regime using the following equation 42-44 for MESFET. (According to Hall effect 7 ACS Paragon Plus Environment

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measurement results in Supporting Information 1, Figure S1, sheet electron concentration, ns (# cm-2) in MoS2 was determined; ns increases with channel TMD thickness.)

ߤ=

௚ಾೌೣ ௅

(1)

௤௡ೞ ௐ

where gMax is a maximum transconductance (g=

డூವ డ௏ಸೄ

) as a function of VGS, ns is

1.75x1010 cm-2 for 7 nm-thin MoS2 (and this number is later confirmed in Supporting Information 2, Figure S2 with capacitance –voltage (C-V) measurements on NbS2/MoS2 Schottky diodes with graphene terminal of Figure 5b), 45-48 q is electronic charge, W and L are the width and length of the channel, respectively. In particular, the SS value seems to be almost the smallest among reported 2D FETs with thermionic source-to-channel injection barrier. 1,45,49 Next point in Figure 2e is that the mobility increases with the VDS. This mobility increase is attributed to the contact resistance lowering by VDS or in-plane direction electric (E) field; Au/n-MoS2 contact junction must have had a Schottky barrier and the barrier is lowered by VDS in principle. The transfer curves of Figure 2f show that the prototype MESFET displays good photo-response under static and dynamic illumination using 630 nm wave length red light emitting diode (LED). For the dynamic photo-response at 1 Hz (light switching frequency), OFF-state was used as discussed in Figure 2c (lower figure). Second device with vdW Schottky junction was fabricated on glass substrate as patterned by photolithography. For this second MESFET we conducted post-anneal at 250 oC for S/D contact improvement unlike the case of the first prototype device.

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Optical

microscopy image of Figure 3a displays the MESFET with metal (Au/Au/Pt for source/drain/gate [S/D/G]) while Figure 3b shows a 3D AFM image of NbS2 overlaid on MoS2 in the Schottky junction. AFM scanned thickness of the two TMDs was 18 and 7 nm for NbS2 and MoS2, respectively. The surface potential or surface work function (qΦ) of the 8 ACS Paragon Plus Environment

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two TMDs was measured by scanning Kelvin probe (SKPM). As imaged by outlined potentials in Figure 3c, the work function difference between conducting NbS2 (qΦNbS2 =4.81 eV) and n-MoS2 (qΦMoS2 =4.53 eV) was apparent. Experimental qΦNbS2 value (4.81 eV) is different from theoretical ones (5~6 eV)

36,37

probably due to surface oxidation of NbS2 or

mismatch between theoretical models and experiments, but in fact no experimental value has been reported yet except ours. Despite of this discrepancy, the device characteristics and SKPM results appear compatible in our study. Two terminal I-V curves of Figure 3d simply indicates the effects of work function difference, showing that initial current in MoS2 channel becomes an order of magnitude lowered by overlaid NbS2. Here, with the SKPM work function values and Schottky barrier height (SBH, qΦB), we could propose an energy band diagram for NbS2/MoS2 Schottky junction as depicted in Figure 3f. For the diagram, Schottky barrier height for electrons was also measured, to be ~0.5 eV based on Richardson’s equation 50

(inset of Figure 3e) and temperature-dependent I-V characteristics (see Figure 3e and

Supporting Information 3, Figure S3 for details). The built-in potential energy (qΦi) or work function difference between NbS2 and MoS2 is 0.28 eV. We regard that this energy band diagram must be a useful information for Schottky diode and MESFET devices. (We also added another energy band diagrams in Supporting Information 4, Figure S4, providing more detailed information for electron carrier transport in MoS2 channel.) Overall device performances of our second MESFET on glass appear more improved than those of the first prototype one on SiO2/p-Si as seen in output and transfer characteristics of Figure 3g and h. Compared to the first device, ON current of the second one is basically a few times higher while its SS value is kept as low as ~80 mV/dec. According to the inset mobility plot of Figure 3h, the saturation mobility appears to show a maximum of 170 cm2/Vs at 5 V of VDS. Only negative point may be that OFF ID current also increases 9 ACS Paragon Plus Environment

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lowering the ON/OFF ratio of MESFET which appears now to be 104~105. Such increase in both ON and OFF ID is attributed to an improved Au source/drain contact on MoS2 channel. However, the mobility plot reveals that some contact resistance or barrier at the Au/MoS2 interface still remains even after contact annealing, showing that the peak mobility varies from 70 to 170 cm2/Vs depending upon in-plane E-field or VDS (mobility minimum ~70 cm2/Vs. at 1 V of VDS); Au/n-MoS2 contact is still not ideal although the contact barrier is lowered by annealing, and it could be further lowered with higher VDS. Such contact resistance could be effectively removed by utilizing graphene S/D/G contacts. Figure 4a displays an optical image of our graphene-contact MESFET fabricated on glass. (Shadow area is metal contact on graphene.) A 3D device scheme is also presented along with its AFM image for TMD thickness in Figure 4b. Figure 4c and its inset respectively present the transfer and output characteristics of the graphene-contact device, in which ~ a few µA has been measured as ON state ID and it is one or two orders of magnitude higher than those of other MESFET devices with metal S/D/G electrode. Inset output curves again display easy saturation behavior at less than 1 V of VDS, which are shown in previous MESFETs. While the thickness of MoS2 channel is quite similar to previous ones (~7 nm) in MESFETs with metal contacts, the saturation mobility of our graphene-contact MESFET in Figure 4d appears to be as high as ~800 cm2/Vs, which is in fact comparable to the room temperature record from thin film NiOx-gated MoS2 MESFET. 34 Most interesting feature in the figure is that the mobility value seldom changes with the increase of VDS. It must be attributed to ideal graphene S/D contact for MoS2 channel. Although the contact improvement by graphene has been reported 20 in 2D or other FETs using gate insulator (MISFETs), our experimental confirmation on contact resistance or barrier removal is regarded still effective and important. 10 ACS Paragon Plus Environment

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The effects of contact resistance reduction by graphene have been actually observed from Schottky diodes as well as MESFETs. Figure 5a summarizes I-V characteristics for three Schottky diodes, that we have fabricated to simultaneously use for MESFETs. According to their rectification I-V curves, NbS2/n-MoS2 Schottky diodes with Au/n-MoS2 contact always display delayed turn-on behavior in forward bias current, also showing minimum current at reverse bias (non zero) voltage (delayed minimum current behavior). These simply mean that more forward and reverse bias voltages are required for good diode behavior. In contrast to the case with metal/n-MoS2, device with graphene/n-MoS2 contact displays turn-on at a forward bias near zero volt (linear plot) and minimum current (logarithmic plot) at zero volt as well, along with a good ideality factor of ~1.8. For better explanation on the contact improvement, schematic band diagrams are proposed in Figure 5b, to describe electron injection from source to MoS2 channel in two different cases: Au/MoS2 and graphene/MoS2 contacts. Au-induced Fermi pinning is expected at the Au/MoS2 junction which has a small Schottky barrier and this barrier can be lowered (q∆Φ) by in-plane E-field (or by VDS) increase. However, graphene does rarely induce any Schottky barrier or Fermi pinning on MoS2 due to the vdW interaction between graphene and MoS2; graphene rather tunes its own Fermi level to that of MoS2 without making any barrier. So, employing the graphene contact for our NbS2/n-MoS2 MESFET and Schottky diode with graphene contacts must be effective. Using graphene/n-MoS2 contact in NbS2/n-MoS2 Schottky diodes which belong to MESFET structure, we could work out electron carrier concentration, Nd of 7 nmthin MoS2 channel, as described in Supporting Information 2. The C-V (or 1/C2-V) measurements on the two diodes were conducted in consideration that the Schottky diode would show an abrupt capacitance change right at the moment when the MoS2 layer thickness under NbS2 gets to the total depletion state by a gate voltage 44 As a result, the slope of 1/C2V plot is extracted out to show Nd value, which turns out ~2.5x1016 cm-3. The number is 11 ACS Paragon Plus Environment

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almost the same as one achieved from Hall measurements (ns / 7 nm = 1.75x1010 cm-2/ 7x10-7 cm). As a final substantial application of our 2D MESFET to demonstrate the benefit from its low threshold and operating voltages, the NbS2/n-MoS2 MESFET with graphene was integrated into an organic piezoelectric touch sensor circuit, where a green organic light emitting diode (OLED) was also connected as an indicator. Figure 6a shows input voltage signals generated by P(VDF-TrFE) polymer piezoelectric capacitor, which generates ~1 and 1 V respectively when pressed and released. Its inset presents sensor circuit working at a supplied voltage (VDD) of 3.5 V, as integrated with MESFET, P(VDF-TrFE), and OLED. Supporting Information 5 and 6 (with Figure S5 and S6) show all the details of touch sensor and OLED properties, respectively. Our MESFET devices appear to well operate, switching on and off the OLED indicator according to the external input of only ~±1 V according to Supporting Information 7, short video file: Video S1.avi. Successful low voltage switching operation is attributed to the predictable small threshold voltage of our 2D MESFET, and it should be regarded as a great advantage over 2D MISFETs which still request many process conditions for such predictable small threshold. Figure 6b exhibits dynamic output current (IOUT) and voltage signals (VOUT) as obtained by repeated touching and releasing behavior. ON/OFF behavior of OLED and MESFET is clearly demonstrated by input touching. In summary, we have simultaneously fabricated both vdW Schottky junction diodes and MESFETs using high work function conducting NbS2 TMD gate on MoS2 TMD channel. While NbS2/n-MoS2 Schottky diode shows SBH of ~0.5 eV as estimated with SKPM and temperature-dependent I-V measurements, it also displays an ideality factor of 1.8~4.0 with ON/OFF current ratio of 103~105. Schottky effect MESFET displays little gate hysteresis of 0.05~0.1 V and ideal subthreshold swing of 60~80 mV/dec due to low density traps at the 12 ACS Paragon Plus Environment

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vdW interface. Operational threshold voltage of all MESFETs appears as low as -0.5 ~ -1 V and easy saturation behavior was observed as their own advantages. It was also found that the device mobility is significantly dependent on the condition of source/drain (S/D) contact for n-channel MoS2. The highest mobility in MESFET reaches to ~more than 800 cm2/V s with graphene S/D contact, although the lowest was ~15 cm2/Vs as obtained from Au/MoS2 contact when the contact was not annealed at all. The NbS2/n-MoS2 MESFET with graphene was successfully integrated into an organic piezoelectric touch sensor circuit with green OLED indicator, exploiting its predictable small threshold voltage, while NbS2/n-MoS2 Schottky diodes were applied to extract unintentionally doped Nd value in MoS2 channel. We conclude that our bifunctional 2D-2D Schottky device using NbS2/n-MoS2 vdW junction is promising and useful in view of future 2D device electronics being able to provide both inplane and vertical current.

Experimental Section Device Fabrication: In order to fabricate Schottky devices (diodes and MESFETs), 285 nmthick SiO2/p+-Si wafer and glass substrate (Eagle 2000) were chosen. The substrates were cleaned in acetone and ethyl alcohol using an ultrasonicator. For the MESFET devices in figures, a MoS2 nanoflake was initially dry-transferred onto the substrates, to be used for active channel. 27 Then NbS2 flake was also dry-transferred onto the MoS2, to form a gate (G) conductor. For the source (S) and drain (D) contact electrodes Au (30 nm) were sequentially deposited by DC magnetron sputtering system and then patterned at room temperature by using lithography (photolithography for the device on the glass, and e-beam lithography for 13 ACS Paragon Plus Environment

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the device on the wafer) and lift-off processes. For the gate contact pad of Pt (30 nm), same deposition and patterning processes were carried out. However, for the last device (in Figure 4), multilayer graphene was used for S, D, and G contact by dry transfer, before final 30 nmthick Pt pads were patterned as lead on the graphene by photolithography. Our devices were preserved in low vacuum (1 Torr) for several hours to improve the layer’s adhesion by van der Waals forces, right after device fabrication.

Characterization: The thickness of nanoflakes were characterized by Raman spectroscopy and AFM (Nanowizard I, JPK Instrument). SKPM measurements were performed by using Park Systems XE7 with non-contact mode. In SKPM imaging, we applied an AC bias voltage of 1.5 V with a frequency of 17 kHz to Au coated tip (PPP-NCSTAu, nanosensors). The Au tip was calibrated by highly oriented pyrolytic graphite (HOPG, work function=4.6 eV), and the work function of tip was calculated to be about 4.90 eV. All electrical measurements of device characteristics were operated mostly in the dark, ambient air (relative humidity, RH ~40%), and room temperature conditions by using a semiconductor parameter analyzer (HP4155C, Agilent Technologies) and function generator (AFG310, Tektronix). Since our MESFET devices should properly operate as Schottky diodes as well, their diode behavior was always checked before MESFET characterizations. C-V measurements on Schottky diodes with graphene were conducted with LCR meter (4284A, Agilent Technologies) at 10 kHz.


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ASSOCIATED CONTENT Supporting Information Available: Hall measurements. C-V measurements for Nd extraction. Schottky barrier height measurements. Energy band diagrams. P(VDF-TrFE) polymer piezoelectric capacitor performance. Circuitry of piezoelectric sensor and green OLED.

AUTHOR INFORMATION Corresponding Author *Seongil Im, E-mail: [email protected]. Phone: 82-2-2123-2842. Fax: 82-2-392-1592. Address: Electron Device Laboratory, Science Building, Room 240, Yonsei University, Seoul, 03722, Korea.

ACKNOWLEDGMENT H.G.S., H.S.Y. and J.S.K. contributed equally to this work. The authors acknowledge the financial support from NRF (NRL program: Grant No. 2017R1A2A1A05001278, SRC program vdWMRC: Grant No.2017R1A5A1014862, center) and the Korean Government (MSIP) (No. 2015R1A5A1037668).


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REFERENCES (1)

Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nat Nanotechnol 2011, 6, 147-150.

(2)

Efetov, D. K.; Wang, L.; Handschin, C.; Efetov, K. B.; Shuang, J.; Cava, R.; Taniguchi, T.; Watanabe, K.; Hone, J.; Dean, C. R.; Kim, P. Nat Phys 2016, 12, 328-332.

(3)

Yabuki, N.; Moriya, R.; Arai, M.; Sata, Y.; Morikawa, S.; Masubuchi, S.; Machida, T. Nat Commun 2016, 7, 10616.

(4)

Staley, N. E.; Wu, J.; Eklund, P.; Liu, Y.; Li, L. J.; Xu, Z. Phys Rev B 2009, 80, 184505.

(5)

Zhou, Y. G.; Wang, Z. G.; Yang, P.; Zu, X. T.; Yang, L.; Sun, X.; Gao, F. Acs Nano 2012, 6, 9727-9736.

(6)

Chen, X. L.; Wu, Z. F.; Xu, S. G.; Wang, L.; Huang, R.; Han, Y.; Ye, W. G.; Xiong, W.; Han, T. Y.; Long, G.; Wang, Y.; He, Y. H.; Cai, Y.; Sheng, P.; Wang, N. Nat Commun 2015, 6, 6088.

(7)

Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H. Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Science 2011, 331, 568-571.

(8)

Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. Nat Chem 2013, 5, 263-275.

(9)

Sarkar, D.; Xie, X. J.; Liu, W.; Cao, W.; Kang, J. H.; Gong, Y. J.; Kraemer, S.; Ajayan, P. M.; Banerjee, K. Nature 2015, 526, 91-95.

(10)

Radisavljevic, B.; Whitwick, M. B.; Kis, A. Acs Nano 2011, 5, 9934-9938.

(11)

Kang, K.; Xie, S. E.; Huang, L. J.; Han, Y. M.; Huang, P. Y.; Mak, K. F.; Kim, C. J.; Muller, D.; Park, J. Nature 2015, 520, 656-660.

(12)

Walia, S.; Balendhran, S.; Wang, Y. C.; Ab Kadir, R.; Zoolfakar, A. S.; Atkin, P.; Ou, J. Z.; Sriram, S.; Kalantar-zadeh, K.; Bhaskaran, M. Appl Phys Lett 2013, 103, 232105.

(13)

Shokouh, S. H. H.; Jeon, P. J.; Pezeshki, A.; Choi, K.; Lee, H. S.; Kim, J. S.; Park, E. Y.; Im, S. Adv Funct Mater 2015, 25, 7208-7214.

(14)

Desai, S. B.; Madhvapathy, S. R.; Sachid, A. B.; Llinas, J. P.; Wang, Q. X.; Ahn, G. H.; Pitner, G.; Kim, M. J.; Bokor, J.; Hu, C. M.; Wong, H. S. P.; Javey, A. Science 2016, 354, 99-102.

(15)

Cheng, R.; Jiang, S.; Chen, Y.; Liu, Y.; Weiss, N.; Cheng, H. C.; Wu, H.; Huang, Y.; Duan, X. F. Nat Commun 2014, 5, 5143.

(16)

Choi, K.; Lee, Y. T.; Kim, J. S.; Min, S. W.; Cho, Y.; Pezeshki, A.; Hwang, D. K.; Im, S. Adv Funct Mater 2016, 26, 3146-3153.

(17)

Wang, H.; Yu, L. L.; Lee, Y. H.; Shi, Y. M.; Hsu, A.; Chin, M. L.; Li, L. J.; Dubey, M.; Kong, J.; Palacios, T. Nano Lett 2012, 12, 4674-4680.

(18)

Liu, H.; Ye, P. D. D. Ieee Electr Device L 2012, 33, 546-548.

(19)

Choi, K.; Raza, S. R. A.; Lee, H. S.; Jeon, P. J.; Pezeshki, A.; Min, S. W.; Kim, J. S.; Yoon, W.; Ju, S. Y.; Leec, K.; Im, S. Nanoscale 2015, 7, 5617-5623.

(20)

Lee, Y. T.; Choi, K.; Lee, H. S.; Min, S. W.; Jeon, P. J.; Hwang, D. K.; Choi, H. J.; Im, S. Small 2014, 10, 2356-2361.

(21)

Shokouh, S. H. H.; Pezeshki, A.; Raza, S. R. A.; Lee, H. S.; Min, S. W.; Jeon, P. J.; Shin, J. M.; Im, S. Adv Mater 2015, 27, 150-156.


Page 16 of 31

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(22)

Lee, H. S.; Shin, J. M.; Jeon, P. J.; Lee, J.; Kim, J. S.; Hwang, H. C.; Park, E.; Yoon, W.; Ju, S. Y.; Im, S. Small 2015, 11, 2132-2138.

(23)

Jeon, P. J.; Kim, J. S.; Lim, J. Y.; Cho, Y.; Pezeshki, A.; Lee, H. S.; Yu, S.; Min, S. W.; Im, S. ACS Appl Mater Interfaces 2015, 7, 22333-22340.

(24)

Pezeshki, A.; Shokouh, S. H. H.; Jeon, P. J.; Shackery, I.; Kim, J. S.; Oh, I. K.; Jun, S. C.; Kim, H.; Im, S. Acs Nano 2016, 10, 1118-1125.

(25)

Lim, J. Y.; Pezeshki, A.; Oh, S.; Kim, J. S.; Lee, Y. T.; Yu, S.; Hwang, D. K.; Lee, G. H.; Choi, H. J.; Im, S. Adv Mater 2017, 29, 1701798.

(26)

Kwon, H.; Jeon, P. J.; Kim, J.S.; Kim, T. Y.; Yun, H.; Lee, S. W.; Lee, T.; Im, S. 2d Mater 2016, 3, 044001.

(27)

Choi, K.; Lee, Y. T.; Min, S. W.; Lee, H. S.; Nam, T.; Kim, H.; Im, S. J Mater Chem C 2013, 1, 78037807.

(28)

Jeon, P. J.; Min, S. W.; Kim, J. S.; Raza, S. R. A.; Choi, K.; Lee, H. S.; Lee, Y. T.; Hwang, D. K.; Choi, H. J.; Im, S. J Mater Chem C 2015, 3, 2751-2758.

(29)

Pezeshki, A.; Hossein, S.; Shokouh, H.; Nazari, T.; Oh, K.; Im, S. Adv Mater 2016, 28, 3216-3222.

(30)

Ahn, J.; Jeon, P. J.; Raza, S. R. A.; Pezeshki, A.; Min, S. W.; Hwang, D. K.; Im, S. 2d Mater 2016, 3, 045011.

(31)

Nourbakhsh, A.; Zubair, A.; Dresselhaus, M. S.; Palacios, T. Nano Lett 2016, 16, 1359-1366.

(32)

Yan, R. S.; Fathipour, S.; Han, Y. M.; Song, B.; Xiao, S. D.; Li, M. D.; Ma, N.; Protasenko, V.; Muller, D. A.; Jena, D.; Xing, H. G. Nano Lett 2015, 15, 5791-5798.

(33)

Roy, T.; Tosun, M.; Cao, X.; Fang, H.; Lien, D. H.; Zhao, P. D.; Chen, Y. Z.; Chueh, Y. L.; Guo, J.; Javey, A. Acs Nano 2015, 9, 2071-2079.

(34)

Lee, H. S.; Baik, S. S.; Lee, K.; Min, S. W.; Jeon, P. J.; Kim, J. S.; Choi, K.; Choi, H. J.; Kim, J. H.; Im, S. Acs Nano 2015, 9, 8312-8320.

(35)

Jeon, P. J.; Lee, Y. T.; Lim, J. Y.; Kim, J. S.; Hwang, D. K.; Im, S. Nano Lett 2016, 16, 1293-1298.

(36)

Liu, Y. Y.; Stradins, P.; Wei, S. H. Sci Adv 2016, 2, 1600069.

(37)

Yang, Z. X.; Pan, J. L.; Liu, Q.; Wu, N. N.; Hu, M. L.; Ouyang, F. P. Phys Chem Chem Phys 2017, 19, 1303-1310.

(38)

Guillamon, I.; Suderow, H.; Vieira, S.; Cario, L.; Diener, P.; Rodiere, P. Phys Rev Lett 2008, 101, 166407.

(39)

Yang, Z.; Wang, J.; Liao, L. Acs Nano 2016, 10, 1714-1715.

(40)

Li, S. L.; Tsukagoshi, K.; Orgiu, E.; Samori, P. Chem Soc Rev 2016, 45, 118-151.

(41)

Mcmullan, W. G.; Irwin, J. C. Solid State Commun 1983, 45, 557-560.

(42)

Frenzel, H.; Lajn, A.; von Wenckstern, H.; Lorenz, M.; Schein, F.; Zhang, Z. P.; Grundmann, M. Adv Mater 2010, 22, 5332-5349.

(43)

Sze, S. M., Physics of semiconductor devices. 2nd ed.; Wiley: New York, 1981; chap. 7.

(44)

Muller, R. S.; Kamins, T. I.; Chan, M., Device electronics for integrated circuits. 3rd ed.; John Wiley & Sons: New York, 2002; chap. 3.

(45)

Kim, S.; Konar, A.; Hwang, W. S.; Lee, J. H.; Lee, J.; Yang, J.; Jung, C.; Kim, H.; Yoo, J. B.; Choi, J. Y.; Jin, Y. W.; Lee, S. Y.; Jena, D.; Choi, W.; Kim, K. Nat Commun 2012, 3, 1011.


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(46)

El-Mahalawy, S. H.; Evans, B. L. physica status solidi (b) 1977, 79, 713-722.

(47)

Fivaz, R.; Mooser, E. Physical Review 1967, 163, 743-755.

(48)

Grant, A. J.; Griffiths, T. M.; Pitt, G. D.; Yoffe, A. D. Journal of Physics C: Solid State Physics 1975, 8, L17-L23.

(49)

Das, S.; Chen, H. Y.; Penumatcha, A. V.; Appenzeller, J. Nano Lett 2013, 13, 100-105.

(50)

Yang, H.; Heo, J.; Park, S.; Song, H. J.; Seo, D. H.; Byun, K.-E.; Kim, P.; Yoo, I.; Chung, H.-J.; Kim, K. Science 2012, 336, 1140-1143.


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Figure

Figure 1. Schottky diode and MESFET schemes. (a) Optical microscope image of prototype NbS2/MoS2 MESFET fabricated on 285 nm-thick SiO2/p+-Si substrate. (b) Raman spectra obtained from NbS2 and MoS2 flakes which were exfoliated on SiO2/p+-Si. (c) Thickness profiles and (d) 3D AFM images for top NbS2 and bottom MoS2 flakes. (e) Threedimensional schematic view of atomic layer NbS2/MoS2 MESFET. (f) Two terminal current – voltage (I-V) measurements of NbS2 and MoS2 of the device.


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Device physics and I-V results in prototype diode and MESFET. (a) Schematic NbS2/n-MoS2 Schottky diode in forward bias operation. (b) I-V characteristics of four Schottky diodes from possible combination according to Figure 1e. (c) ON (upper figure) and OFF (lower figure) behavior of MESFET with conducting NbS2/n-MoS2 junction. (d) Drain current−drain voltage (ID−VDS) output curves of our MESFET on SiO2/p+-Si substrate. (e) 20 ACS Paragon Plus Environment

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ID−VGS transfer and IG-VGS gate leakage curves of the MESFET, as measured with VDS increase. SS was as small as only ~67 mV/Dec. Inset curves show the saturation mobility plots. It is noted that the peak mobility increases with VDS increase. (f) Photo-induced transfer curves and photo-dynamic switching behavior of the MESFET with 12 nm-thin NbS2 gate electrode, as obtained under red LED and OFF bias conditions; the 12 nm-thin NbS2 is regarded light-transparent.


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Band diagram of NbS2/n-MoS2 vdW junction and second MESFET performance. (a) Bottom illumination optical microscope (inset: top illumination OM image) and (b) 3D AFM images of our second MESFET fabricated on glass substrate. (c) SKPM image of the MESFET to measure the surface potential or surface work function (qΦ) of the two TMDs. (d) Two terminal I-V curves of MoS2 channel: the I-V curve before NbS2 transfer shows much higher current level than after transfer. (e) Richardson plot of ln(I/T2) versus q/(kBT). [T: Kelvin temperature, kB: Boltzmann constant] The Schottky barrier qΦB, ∼0.5 eV, is obtained from the slope of the linear fit. (f) Energy band diagram of MoS2/nNbS2 heterojunction with vdW gap as constructed on our measurement results. (g) ID−VDS output curves of the MESFET. (h) ID−VGS transfer and IG-VGS gate leakage curves of the MESFET, as measured with VDS increase. Inset mobility plots clearly show that the peak mobility increases with VDS increase.


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Figure 4. NbS2/n-MoS2 MESFET with graphene S/D contact. (a) Optical microscope image (bottom illumination) of NbS2/MoS2 MESFET with graphene source/drain/gate contacts and Pt leads (shadow image) as fabricated on glass. (b) 3D schematic and 3D AFM views of the MESFET. (c) ID−VGS transfer and ID−VDS output (inset) curves of the MESFET.

(d) Saturation mobility plots of the MESFET: unlike the other devices, the similar mobility is maintained regardless of VDS increase.


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Contact electrode effects on NbS2/n-MoS2 Schottky diode performance. (a) I-V curves of three Schottky diodes of NbS2/n-MoS2; with the smaller barrier between contact and MoS2 the I-V behavior becomes more ideal. (b) Energy diagrams to explain electron injection at the Au/n-MoS2 and graphene/n-MoS2 systems.


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Figure 6. Piezoelectric touch sensor application of NbS2/n-MoS2 MESFET. (a) Generated input voltage by touching P(VDF-TrFE) polymer capacitor, and inset piezoelectric sensor circuit composed of MESFET, piezo sensor, and OLED indicator. Supplied voltage (VDD) was 3.5 V. Press and release the capacitor for turning ON and OFF the n-MoS2 channel MESFET (and OLED), respectively. (b) Arbitrary dynamic touching on the P(VDF-TrFE) capacitor is sensed to result in output signals in the circuit.


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schottky diode and MESFET schemes. (a) Optical microscope image of prototype NbS2/MoS2 MESFET fabricated on 285 nm-thick SiO2/p+-Si substrate. (b) Raman spectra obtained from NbS2 and MoS2 flakes which were exfoliated on SiO2/p+-Si. (c) Thickness profiles and (d) 3D AFM images for top NbS2 and bottom MoS2 flakes. (e) Three-dimensional schematic view of atomic layer NbS2/MoS2 MESFET. (f) Two terminal current – voltage (I-V) measurements of NbS2 and MoS2 of the device. 313x139mm (150 x 150 DPI)

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Figure 2. Device physics and I-V results in prototype diode and MESFET. (a) Schematic NbS2/n-MoS2 Schottky diode in forward bias operation. (b) I-V characteristics of four Schottky diodes from possible combination according to Figure 1e. (c) ON (upper figure) and OFF (lower figure) behavior of MESFET with conducting NbS2/n-MoS2 junction. (d) Drain current−drain voltage (ID−VDS) output curves of our MESFET on SiO2/p+-Si substrate. (e) ID−VGS transfer and IG-VGS gate leakage curves of the MESFET, as measured with VDS increase. SS was as small as only ~67 mV/Dec. Inset curves show the saturation mobility plots. It is noted that the peak mobility increases with VDS increase. (f) Photo-induced transfer curves and photodynamic switching behavior of the MESFET with 12 nm-thin NbS2 gate electrode, as obtained under red LED and OFF bias conditions; the 12 nm-thin NbS2 is regarded light-transparent. 162x187mm (150 x 150 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Band diagram of NbS2/n-MoS2 vdW junction and second MESFET performance. (a) Bottom illumination optical microscope (inset: top illumination OM image) and (b) 3D AFM images of our second MESFET fabricated on glass substrate. (c) SKPM image of the MESFET to measure the surface potential or surface work function (qΦ) of the two TMDs. (d) Two terminal I-V curves of MoS2 channel: the I-V curve before NbS2 transfer shows much higher current level than after transfer. (e) Richardson plot of ln(I/T2) versus q/(kBT). [T: Kelvin temperature, kB: Boltzmann constant] The Schottky barrier qΦB, ∼0.5 eV, is obtained from the slope of the linear fit. (f) Energy band diagram of MoS2/n-NbS2 heterojunction with vdW gap as constructed on our measurement results. (g) ID−VDS output curves of the MESFET. (h) ID−VGS transfer and IG-VGS gate leakage curves of the MESFET, as measured with VDS increase. Inset mobility plots clearly show that the peak mobility increases with VDS increase. 299x138mm (150 x 150 DPI)


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Figure 4. NbS2/n-MoS2 MESFET with graphene S/D contact. (a) Optical microscope image (bottom illumination) of NbS2/MoS2 MESFET with graphene source/drain/gate contacts and Pt leads (shadow image) as fabricated on glass. (b) 3D schematic and 3D AFM views of the MESFET. (c) ID−VGS transfer and ID−VDS output (inset) curves of the MESFET. (d) Saturation mobility plots of the MESFET: unlike the other devices, the similar mobility is maintained regardless of VDS increase. 214x149mm (150 x 150 DPI)


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Contact electrode effects on NbS2/n-MoS2 Schottky diode performance. (a) I-V curves of three Schottky diodes of NbS2/n-MoS2; with the smaller barrier between contact and MoS2 the I-V behavior becomes more ideal. (b) Energy diagrams to explain electron injection at the Au/n-MoS2 and graphene/nMoS2 systems. 174x157mm (150 x 150 DPI)


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Figure 6. Piezoelectric touch sensor application of NbS2/n-MoS2 MESFET. (a) Generated input voltage by touching P(VDF-TrFE) polymer capacitor, and inset piezoelectric sensor circuit composed of MESFET, piezo sensor, and OLED indicator. Supplied voltage (VDD) was 3.5 V. Press and release the capacitor for turning ON and OFF the n-MoS2 channel MESFET (and OLED), respectively. (b) Arbitrary dynamic touching on the P(VDF-TrFE) capacitor is sensed to result in output signals in the circuit. 179x116mm (150 x 150 DPI)


n-MoS2 van der

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