Statistical Study on the Schottky Barrier Reduction of TunnelingContacts to CVD Synthesized MoS2Seunghyun Lee,*,† Alvin Tang,† Shaul Aloni,‡ and H.-S. Philip Wong†

†Department of Electrical Engineering and Stanford SystemX Alliance, Stanford University, Stanford, California 94305, United States‡The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

*S Supporting Information

ABSTRACT: Creating high-quality, low-resistance contacts isessential for the development of electronic applications usingtwo-dimensional (2D) layered materials. Many previouslyreported methods for lowering the contact resistance rely onvolatile chemistry that either oxidize or degrade in ambient air.Nearly all reported efforts have been conducted on only a fewdevices with mechanically exfoliated flakes which is notamenable to large scale manufacturing. In this work, Schottkybarrier heights of metal-MoS2 contacts to devices fabricatedfrom CVD synthesized MoS2 films were reduced by inserting athin tunneling Ta2O5 layer between MoS2 and metal contacts. Schottky barrier height reductions directly correlate withexponential reductions in contact resistance. Over two hundred devices were tested and contact resistances extracted for largescale statistical analysis. As compared to metal-MoS2 Schottky contacts without an insulator layer, the specific contact resistivityhas been lowered by up to 3 orders of magnitude and current values increased by 2 orders of magnitude over large area (>4 cm2)films.

KEYWORDS: MoS2, tunneling insulator, low-resistance contact, CVD synthesis

Two-dimensional (2D) layered materials ranging from zerobandgap graphene to wide bandgap insulators such asboron nitride offer new opportunities in the field ofnanoelectronics with its rich variation of physical properties.Although graphene has been the most widely studied 2Dmaterial, its lack of a bandgap and the resulting leakage currentof graphene transistors severely limits its use. 2D layeredmaterials such as transition metal dichalcogenides (TMDs)provide a solution to this dilemma as most TMDs have a finitebandgap while retaining the ideal qualities of ultrathin 2Dmaterials. Layered TMDs represent the ideal channel materialfor device scalability as their few-atom thick layers devoid ofsurface dangling bonds will be robust against short-channeleffects down to very short gate lengths for ultrathin shortchannel devices.1,2

Although TMDs exhibit excellent intrinsic properties fordevice scaling, creating high-quality, low-resistance contacts isan open challenge in the development of 2D layered materialsfor electronics applications. While metal contacts to semimetalssuch as graphene have proven to be less problematic,3 metalcontacts to semiconducting TMDs have proven to be morechallenging due to Schottky barrier formation.4,5 Conventionalsubstitutional doping method for decreasing the contactresistance of bulk semiconductors is not suitable for low-dimensional materials as doping impurities introduce largestrain to the lattice as well as additional defect sites that lowercarrier mobility.6 Previous studies show that even smallSchottky barriers have significant impact on the device current

drive.4 In fact, the high resistance of metal-2D semiconductorcontacts have largely been attributed to sizable Schottkybarriers4,5 and Fermi level pinning4,7,8 upon band alignment.While earlier reports have reported that gold contacts to MoS2are ohmic,9,10 the linear I−V relation is limited to the lowvoltage regime. Many subsequent reports confirm the existenceof sizable Schottky barriers at the contacts.4,5 Experimentalevidence of Fermi level pinning also has been presented forvarious metal-2D semiconductor contacts and the pinning isbelieved to be caused by gap state formation.4,7,8

Various methods have previously been used to reduce thecontact resistance of transistors with TMD channels. Chemicaladsorption doping techniques such as the use of NO2 gasambient11 or potassium ions12 to dope contact regions ofWSe2-based devices have been shown to reduce contactresistance. Low work function metals have been used ascontact metals to form improved contacts with thin MoS2flakes, resulting in higher carrier injection.4 However, airstability is a major issue with these techniques because adsorbedchemical species may desorb from the surface and the chemicalspecies may react with oxygen and water molecules uponprolonged exposure to ambient air.13,14 Similarly, low workfunction metals are susceptible to oxidation.15

Received: September 15, 2015Revised: December 15, 2015Published: December 23, 2015

Letter

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A robust, air-stable technique to reduce the contact resistanceis to use metal−insulator−semiconductor (MIS) contacts,which are formed by inserting a thin insulator layer betweenmetal and semiconductor. Such a contact structure haspreviously been shown to reduce the effective Schottky barrierheight of metal contacts to Si,16 Ge,17 and III−V material.18,19The barrier height reduction in these contact structures hasbeen attributed to the attenuation of metal induced gap states(MIGS)20,21 in the insulator and/or electronic dipoleformation22 at the insulator-semiconductor interface.In this work, a systematic study of the application of MIS

contacts has been conducted on a large batch of MoS2 devicesfabricated using CVD grown large area cm2 scale four-layerMoS2. True Schottky barrier heights were extracted at flat bandgate bias conditions for various MIS contact insulatorthicknesses. Over 200 two terminal transfer length measure-ment (TLM) structures were fabricated and measured topresent the statistical variation of specific contact resistivity.Ta2O5, which has a low conduction band offset

23−25 to MoS2,has been used as the MIS contact insulator (SupportingInformation, Figure S1). Large area (>4 cm2), uniform four-layer MoS2 has been synthesized by CVD method, transferred,and patterned. (Supporting Information, Figure S2 and S3).Although large-scale syntheses of both single-layer andmultilayer MoS2 are possible, multilayer (four-layer) MoS2was used in this work because multilayer MoS2 was found tohave lower contact resistance and higher on-current. This is inagreement with several reports claiming that multilayer TMDshave lower contact resistance26 and higher mobility4,27 due tosuppression of both quantum confinement effect and interfacialCoulomb impurities scattering.The MIS technique has also been previously applied to

metal−TMD contacts by using MgO and TiO2 as the thininsulators.28,29 Although these works also showed reduction ineffective Schottky barrier height, no quantitative study on thereduction of specific contact resistivity has been done.Moreover, all of these previous studies used mechanicallyexfoliated samples with flake sizes on the order of μm2. Withsuch small flake areas, a proper statistical study on thecorrelation of insulator thickness to specific contact resistivityand effective Schottky barrier height was not feasible due to thelimited number of devices. Additionally, no report exists on theapplication of MIS contacts on larger area cm2 scale TMDsgrown by synthesis techniques such as chemical vapordeposition (CVD) that are amenable to large scale manufactur-ing. Lastly, and most importantly, some of the early works29,30

did not taken into account the influence of the gate field in theircalculation of Schottky barrier height. An important goal of thiswork was to address the limitations of previous works, and indoing so, provide deeper insights for the application of MIScontacts.An optical image of the multilayer MoS2 synthesized on 285

nm thick SiO2 substrate is shown in Figure 1a. The film showsuniform contrast over large area in the microscope image(Figure 1b). Raman spectra taken at 10 random spots on theMoS2 film indicate that the peak frequency difference betweenthe MoS2 characteristic A1g peak (407 cm

−1) and the E12g peak(383 cm−1) was 24 cm−1, indicative of 4−5 layer MoS2according to literature.31 However, it is known that Ramansignature becomes less sensitive to layer thickness as thenumber of MoS2 layers increase,

31 and another verificationmethod is necessary to determine the thickness of CVD grownMoS2. The MoS2 film is characterized by atomically smooth

edges and high crystallinity. From the high-resolution images atthe edges (Figure 1c), we can define the thickness of the film tobe four layers, a value that is in close agreement with the Ramanmeasurement. A selective area diffraction pattern taken from∼1 μm in diameter area (Figure 1c inset) shows high-qualitycrystalline MoS2 with two domains rotated by about 15°,suggesting that typical domain size is about 1 μm. Thethickness of the film is further confirmed by atomic forcemicroscopy (AFM) measurement at the edge of the film(Figure 1d). These films can be transferred to an arbitrarysubstrate and patterned by dry etching as shown in Figure 1e.The side view schematic of the fabricated structure is shown

in Figure 2a. MoS2 films were transferred onto a 40 nm thickHfO2 substrate, and various thicknesses of Ta2O5, ranging from0 to 5 nm, were deposited using atomic layer deposition (ALD)technique (details provided in the Supporting Information).Metal contacts (Ti/Au: 3 nm/30 nm) were then deposited andpatterned to form the TLM structures (Figure 2b). Theseparation between contacts of the TLM structure were varied,and the resistance across the contacts were extracted from theI−V curve and plotted as a function of the contact separation. Arepresentative plot of TLM total resistance vs contactseparation is shown in Figure 2c. From such a plot, the specificcontact resistivity can be extracted. Devices made from pristine,exfoliated MoS2 flakes are reported to have specific contactresistivities ρc ranging from ∼3 × 10−4 Ω·cm2 to 3 × 10−2 Ω·

Figure 1. (a) An optical image of the multilayer MoS2 layersynthesized on a SiO2 substrate. (b) A microscopic image of thecontinuous MoS2 layer with uniform contrast and its Raman spectrum(scale bar: 100 μm). The Raman peak frequency difference betweenthe MoS2 characteristic A1g peak (407 cm

−1) and the E12g peak (383cm−1) was found to be 24 cm−1, indicative of 4−5 layer MoS2. (c) Ahigh-resolution TEM image of the side edge profile of the MoS2 film.The film is polycrystalline as shown from the selected area electrondiffraction pattern (inset). (d) Atomic force microscopy (AFM) imageof MoS2 layer on SiO2 substrate. (e) MoS2 layer transferred to HfO2substrate and patterned with lithography.

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cm2 at zero gate bias using gold as the metal contact.26 Fromour work, we found the ρc of our CVD grown MoS2 withoutany modification to be between 3.2 × 10−3 Ω·cm2 and 5.9 ×10−2 Ω·cm2 with Ti/Au contact as shown in Figure 2f. Suchrelatively high contact resistivity is due to the large barrierheight and Fermi level pinning at the metal-MoS2 contact

interface as investigated in many reports.4,5,7,8 To confirm theexistence of Fermi level pinning, we measured the Schottkybarrier heights for another metal (Pt) contacted to MoS2 andcompared it with Ti contacts to extract the pinning factor(Supporting Information S4). From the temperature-depend-ent measurement, the pinning factor, S, was extracted to be S =

Figure 2. (a) Side view schematic of the fabricated transfer length measurement (TLM) structure. MoS2 films were transferred onto a 40 nm thickHfO2 substrate. Various thicknesses of Ta2O5, ranging from 0 to 5 nm, were deposited using ALD process. (b) Optical microscope image of theTLM structures. The device width was fixed; the separation between contacts was varied, and the resistance across the contacts was measured. (c)Typical plot of the resistance vs contact separation distance. Specific contact resistivity was extracted from the x- and y-axis intercepts. (d) Schematicband diagram of a pinned MoS2 Fermi level. The metal and MoS2 are in direct contact, allowing the tail of the metal electron wave function to decayinto the semiconductor bandgap. (e) Schematic band diagram with Ta2O5 inserted between metal and MoS2. The metal wave function is attenuatedin the gap states. (f) Schematic band diagram with Ta2O5 inserted between metal and MoS2. Dipole formation at the interface may also lower theeffective Schottky barrier. (g) Measured specific contact resistivity ρc as a function of Ta2O5 dielectric thickness.

Figure 3. (a) Energy band diagrams corresponding to the applied gate biases in three distinct regions: below flat band, at the flat band, and above flatband. The contribution from the tunneling current becomes negligible only when VGS is at or below VFB. (b, d) The Arrhenius plot for various gatebiases when the thickness of Ta2O5 is 0 nm (b) and 1.5 nm (d), respectively. The slopes of these lines represent the effective Schottky barrier height,for the corresponding gate bias. (c, e) The effective Schottky barrier height as a function of gate bias when the thickness of Ta2O5 is 0 nm (c) and 1.5nm (e), respectively.

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0.107 which was slightly smaller than the S = 0.113 valueextracted from the trend line shown in another article.4

According to the MIGS theory,20,21 the metal electron wavefunction decays into the semiconductor bandgap at the metal−semiconductor junction and charges the interface states of thesemiconductor (Figure 2d). This pulls the intrinsic Fermi level(EF,int) at the interface toward the charge neutrality level (ECNL)of the gap states. A thin insulator inserted between metal andsemiconductor attenuates the metal electron wave functionprior to penetrating the semiconductor (Figure 2e). Thisresults in fewer charges available to move EF,int toward ECNL, asshown in Figure 2e. Another explanation for how the MIScontact structure reduces the effective Schottky barrier height isthrough dipole formation at the insulator−semiconductorinterface. As shown in Figure 2f, interfacial dipoles of oppositepolarity can effectively neutralize charges that cause the intrinsicFermi level (EF,int) to move toward the charge neutrality level(ECNL), reducing the effective Schottky barrier height.From the collective plots of total resistance vs contact

separation (e.g., Figure 2c), the specific contact resistivity (ρc)for contacts with varying Ta2O5 thicknesses was extracted, andthe data are shown in Figure 2g. The insertion of a 1.5 nm thickTa2O5 layer resulted in 2−3 orders of magnitude reduction inρc. However, as the insulator thickness is more than 1.5 nm, ρcincreased again. Such an effect is shown in Figure 2g. Tounderstand this trend, the contact resistance of the metal/insulator/MoS2 MIS contact can be modeled as two resistancesin series: the resistance due to the Schottky barrier (RSB) andthe resistance due to tunneling through the insulator (RT).Without any insulator, the large Schottky barrier causes RSB todominate contact resistance. By inserting a thin insulator toreduce the effective Schottky barrier height RSB is greatlylowered while a small RT component is added, effectivelyreducing the overall contact resistance. Once the insulator isincreased beyond an optimal thickness, RT begins to dominate

as current becomes tunneling limited, resulting in higher overallcontact resistance. This is the first time such trade-offs betweenRT and RSB have been observed for metal/insulator/2D materialMIS contacts, and the existence of an optimal insulatorthickness for MIS contacts is confirmed.To investigate the physical origin of lowering of RSB,

temperature-dependent carrier transport measurements wereconducted, and the effective Schottky barrier heights ΦSB wereextracted for different insulator thicknesses. We note that theeffective Schottky barrier heights ΦSB of this work are notequivalent to the Schottky barrier heights of conventionalmetal/semiconductor junctions. Because the insulator is notaccounted for in the expression for current−voltage character-istics (1) employed to determine the barrier height, theeffective Schottky barrier heights extracted here are arepresentation of overall electrical behavior. As illustrated inFigure 3a, the band diagrams depict the contributions ofthermionic emission current and thermally assisted tunnelingcurrent under various gate bias conditions. When the gate biasis above the flat band voltage, VFB, both components contributeto the current flow during temperature-dependent measure-ments.4 The contribution from the tunneling current becomesnegligible only when the gate voltage is at or below VFB. Toextract the accurate ΦSB based on diode eq 1 and thermionicemission theory, ΦSB must be extracted at the flat band voltagecondition (VGS = VFB).

4

= −Φ

− −⎛⎝⎜

⎞⎠⎟⎡⎣⎢⎢

⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥I AT

ek T

eVk T

exp exp 1d2 SB

B

ds

B (1)

In this equation 1, Id is the current through the device, A is theRichardson’s constant, T is the absolute temperature, kB is theBoltzmann constant, q is the electronic charge, and Vds is thedrain to source bias. The values of ln(Id/T

2) at a drain bias ofVds = 1 V for various gate biases are plotted in an Arrhenius plot(Figure 3b). The slopes of these lines directly provide the

Figure 4. Effective Schottky barrier height as a function of gate bias when the thickness of Ta2O5 is 0.5 nm (a), 2.5 nm (b), and 5 nm (c),respectively. (d) The Schottky barrier height at flat band condition as a function of Ta2O5 thickness. (e) The diode current flow across the devices inlogarithmic scale for various Ta2O5 thicknesses.

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effective Schottky barrier height (units in eV), for thecorresponding gate bias, VGS. This extraction is repeated forall measured gate voltages, and the slopes (i.e., effective ΦSB) asa function of the gate voltage is plotted in Figure 3c. For VGSless than or equal to VFB, the effective ΦSB extracted fromFigure 3b linearly responds to VGS as shown in Figure 3c.However, as VGS increases above VFB, the tunneling currentcomponent becomes relevant, and the plot deviates from itslinear relation. Hence the accurate ΦSB extracted at flat bandvoltage conditions for devices without any Ta2O5 layer wasfound to be 95 meV (Figure 3c). Compared to this 95 meVbarrier height, the ΦSB of devices with the lowest contactresistance was extracted to be 29 meV for a 1.5 nm thick Ta2O5insulator layer as shown in Figure 3d,e.Such analysis was repeated for other fabricated devices with

various thicknesses of Ta2O5 (Figure 4a−c). ΦSB as a functionof Ta2O5 insulator thickness is plotted in Figure 4d. Suchchange in ΦSB had a dramatic effect on the current flow acrossthe devices because of the significant reduction in the specificcontact resistivity. In Figure 4e, the current level increased by 2orders of magnitude after the optimized thickness (1.5 nm) ofTa2O5 was inserted between the metal-MoS2 contact. A linearplot of the transfer curve for devices both with and without theTa2O5 layer is also shown in Supporting Figure S5. To gainmore insight into the effect of the Ta2O5 layer, we extracted thethreshold voltages for transistors both with and without a 1.5nm Ta2O5 layer (Supporting Figure S6). From the gateresponse of the MoS2 bottom gate transistor, we found a slightdecrease in the threshold voltage, VTh, before and after Ta2O5layer insertion. However, the drive current increased dramat-ically by several orders of magnitude after Ta2O5 layer insertion.In this work, we demonstrate how a low temperature (200

°C) ALD process can be used to reduce contact resistance ofdevices fabricated from CVD synthesized large area 2Dmaterials. With ever-growing interest in layered two-dimen-sional (2D) materials, controlling material interfaces has been acritical challenge in utilizing 2D materials for their uniqueproperties. This has been especially important for recentlyemerging TMDs because, unlike graphene, these materials havefinite bandgaps. The tunneling insulator thickness of this MIScontact technique is highly controllable using conventionalALD tools and does not rely on low-work function metal orvolatile chemistry techniques that are unstable in ambient air.This large area contact resistance study on CVD grown filmswill enable more facile integration of 2D material technologywith today’s semiconductor technology.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.5b03727.

Band alignment of MoS2 and Ta2O5; synthesis of largearea MoS2; device fabrication process (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: seansl@stanford.edu.Author ContributionsS.L. and A.T. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported in part by the member companies ofStanford Initiative for Nanoscale Materials and Processes(INMP) affiliate program, and Function Accelerated nanoMa-terial Engineering (FAME) Center, one of six centers ofSemiconductor Technology Advanced Research Network(STARnet), a Semiconductor Research Corporation (SRC)program sponsored by Microelectronics Advanced ResearchCorporation (MARCO) and Defense Advanced ResearchProjects Agency (DARPA). Work at the Molecular Foundrywas supported by the Office of Science, Office of Basic EnergySciences, of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231.

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