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CXXXX American Chemical Society

High-Performance MolybdenumDisulfide Field-Effect Transistors withSpin Tunnel ContactsAndre Dankert,* Lennart Langouche, Mutta Venkata Kamalakar, and Saroj Prasad Dash*

Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296, Göteborg, Sweden

Two-dimensional (2D) materials arecurrently explored for their potentialrole in next-generation nanoelectro-

nic and spintronic devices.1�4 Graphene hasattracted the major attention for its remark-ably high electron mobility and long dis-tance spin transport at room tempera-ture.1,5 However, pristine graphene hasneither a band gap nor spin�orbit coupling,which are indispensable requirements for aswitching action in charge- or spin-basedtransistor applications. External doping toengineer a band gap or to induce a spin�orbit coupling in graphene has so far proveddetrimental to its intrinsic properties.6

Semiconducting molybdenum disulfide(MoS2) has emerged as a potential alterna-tive 2D crystal demonstrating solutions forseveral novel nanoelectronic and optoelec-tronic applications.7�10 In MoS2, the lack ofinversion symmetry, high atomic mass, andconfinement of electron motion in theplane leads to a very strong spin�orbitsplitting in the valence band up to 0.3 eV.9,10

These distinct characteristics lead to the cou-pling of the spin and valley degrees of free-dom, which can only be simultaneously

flipped to conserve energy, and thereforeprovide long spin and valley lifetimes.10

These spin, orbit, and valley properties arequite unique to MoS2 and may lead to yetunexplored applications.For the practical realization of spin-based

devices, the electrical injection, trans-port, manipulation, and detection of spin-polarized carriers in the MoS2/ferromagnet(FM) heterostructures are primary require-ments. However, in such contacts, a Schottkybarrier of 100�180 meV is associated withthe charge depletion region at the inter-faces.11,12 Therefore, only electrons that arethermally emitted over the Schottky barriercan flow across the contact, resulting in areduced spin polarization and a low drivecurrent.13 An even more important con-cern is the conductivity mismatch betweenthe FM and the MoS2, which can result ina negligible magnetoresistance (MR).14,15

Recently, it has been possible to over-come the conductivity mismatch issues forspin injection into semiconductors16�19 andgraphene20 by employing narrow Schottkybarriers or insulating tunnel barriers. For the2D semiconductor MoS2, similar approaches

* Address correspondence toandre.dankert@chalmers.se,saroj.dash@chalmers.se.

Received for review September 19, 2013and accepted December 30, 2013.

Published online10.1021/nn404961e

ABSTRACT Molybdenum disulfide has recently emerged as a promis-

ing two-dimensional semiconducting material for nanoelectronic, optoe-

lectronic, and spintronic applications. Here, we investigate the field-effect

transistor behavior of MoS2 with ferromagnetic contacts to explore its

potential for spintronics. In such devices, we elucidate that the presence of

a large Schottky barrier resistance at the MoS2/ferromagnet interface is a

major obstacle for the electrical spin injection and detection. We

circumvent this problem by a reduction in the Schottky barrier height

with the introduction of a thin TiO2 tunnel barrier between the ferro-

magnet and MoS2. This results in an enhancement of the transistor on-state current by 2 orders of magnitude and an increment in the field-effect mobility

by a factor of 6. Our magnetoresistance calculation reveals that such integration of ferromagnetic tunnel contacts opens up the possibilities for MoS2-based

spintronic devices.

KEYWORDS: MoS2 . field-effect transistor . ferromagnetic contacts . magnetoresistance . spintronics . Schottky barrier

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can be explored by using heterostructures with FMtunnel contacts. More recently, a reduction of theSchottky barrier resistance was demonstrated by in-troducing a MgO tunnel barrier between the FM andsingle-layer MoS2.

12 In other studies, it has been shownthat a high contact resistance has adverse effects onMoS2 field-effect transistor (FET) characteristics, redu-cing the effective mobility (μeff) and transistor on�offratio.11 Additionally, it has also been demonstratedthat a maximum mobility of the channel could beachieved by using multilayer (ML) MoS2 with an opti-mal thickness of about 10 nm.11 Such devices alsoprovide larger density of states and multichannelconduction.21 Therefore, achieving a suitable inter-face resistance and better transistor properties withFM tunnel contacts in single-layer and ML MoS2 FETsare the crucial requirements for further developmentsin spintronic devices.In this article, we address amajor issue related to the

electrical spin injection and detection in MoS2. Ourexperiments reveal that the presence of a Schottkybarrier at the Co/ML MoS2 interface seriously limitsthe device performance. We present an approach toengineer the contact resistance by introducing a thinTiO2 tunnel barrier reducing the Schottky barrier anddemonstrating a reduction of the contact resistanceand an improved MoS2 transistor performance. Fur-thermore, we correlate the contact resistances of ourdevices with MR calculations on the basis of the spindiffusion model.

RESULTS AND DISCUSSION

MoS2 flakes were mechanically exfoliated onto aSiO2/Si substrate. The FM Co electrodes with andwithout a TiO2 tunnel barrier were prepared by nano-fabrication techniques (see Methods and Figure 1a,b).We used ML MoS2 with a thickness of about 13 nm(Figure 1c), which promotes a larger density of statesand multichannel conduction, allowing for high drivecurrents.11,21

Initially, we studied the two-terminal source�draincurrent�voltage characteristics (Ids�Vds) of the MoS2/Codevice at zero gate bias (Vgs = 0) and room temperature(Figure 1d). The quasi-symmetric nonlinearity of Ids�Vdsis due to the back-to-back Schottky diode structuresof the devices with a contact resistance area productRcontA = 3 � 10�7 Ωm2 and a channel resistivity ofF = 3 � 10�3 Ωm. Such high Schottky barrier andchannel resistance of the MoS2 devices yield problemsin spin injection and detection.13,15 These problemscan be resolved by reducing the Schottky barrier andintroducing an optimal tunnel barrier resistance.Next we examine the role of such a TiO2 tunnel

barrier at the interface, which was found to be excel-lent for spin injection into graphene.5 The TiO2 isfound to grow homogeneously on MoS2, without anychange in roughness before and after the deposition.

The Ids�Vds characteristic of such MoS2/TiO2/Co con-tact also shows a quasi-symmetric nonlinear behavior(Figure 1d). However, the contact resistance is drama-tically reduced by 2 orders of magnitude (RcontA =2.4 � 10�9 Ωm2) when compared to the directCo/MoS2 contact. Such behavior has been reproducedin multiple devices prepared independently. This re-duction in contact resistance can be attributed to alowering of the Schottky barrier height. Similar resultswere reported with MgO as a tunnel barrier on single-layer MoS2

12 and in germanium.22

In a Co/TiO2/MoS2 contact, the Schottky barrierheight Φb is determined by various parameters, suchas the work function of Co (Φm), the electron affinity ofMoS2, the thickness and dielectric constant of TiO2, thecharge neutrality level, and the surface density of gapstates at the MoS2 interface.

22�25 Considering that theproperties of Co, TiO2, and MoS2 are unaffected, thecritical parameters which can influence the Schottkybarrier are the charge neutrality level and the surfacedensity of gap states at the MoS2 interface with thepreparation of the TiO2 barrier. The interface gap statescan alleviate the Fermi level pinning and result in alowering of the Schottky barrier height.11,12,22,25,26

To understand the modification of the Schottkybarrier height, we performed detailed temperature- and

Figure 1. MoS2 FET with FM contacts. (a) Schematic of aMoS2 FET with FM tunnel contacts as source and drain andthe measurement configuration. (b) Optical image of thefabricated MoS2 FET with contacts. (c) AFM image and linescan of a 13 nm thin MoS2 flake exfoliated on SiO2/n-Sisubstrate. (d) Current�voltage (Ids�Vds) characteristics ofthe MoS2 devices with ferromagnetic source and draincontacts at room temperature and zero gate voltage. Left:MoS2/Co device; the band diagram demonstrates theSchottky barrier at the interface (inset). Right: MoS2/TiO2/Codevice; the band diagram depicts a reduction of the Schottkybarrier (inset).

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gate-dependent drain�source IV characteristics ofdevices with and without the TiO2 tunnel barrier(Figure 2).12 The Ids�Vds characteristics (Figure 2a)show a weak temperature dependence for theMoS2/TiO2/Co device compared to the direct Co con-tact. Such a weak temperature dependence is a man-ifestation of a reduced Schottky barrier height due tothe introduction of the TiO2 layer. To be noted, toextract the true Schottky barrier values, the Ids�Vdsmeasurements were performed in the flat band con-dition by applying a gate voltage of Vgs = 3 V in theMoS2/Co and Vgs = �15 V in the MoS2/TiO2/Co device.These flat band voltages, Vfb, are obtained from theVgs dependence of the Schottky barrier.11

In order to extract the Schottky barrier height ata certain Vgs, we employed the three-dimensionalthermionic emission equation describing the electricaltransport through a Schottky barrier into the ML MoS2channel:11,12,27

Ids ¼ AA�T2exp � e

kBTΦb � Vds

n

� �" #(1)

where A is the contact area, A* is the Richardsonconstant, e is the electron charge, kB the Boltzmannconstant,Φb is the Schottky barrier height, and n is theideality factor. Figure 2b shows the Arrhenius plot(ln(IdsT

�2) versus T�1) for different bias voltagesVds. The slopes S(Vds) extracted from the Arrheniusplot follow a linear dependence with Vds (S(Vds) =�(e/kB)(Φb � (Vds/n))), as displayed in Figure 2c. TheSchottky barrier is evaluated from the extrapolatedvalue of the slope at zero drain�source voltage(S0 = �(eΦb/kB)). We obtained a drastic reduction ofthe Schottky barrier height Φb from 121 meV for thedirect Co contact to 27 meV in the MoS2/TiO2/Codevice calculated at their respective Vfb. These flatband voltages were extracted from the gate-voltage-dependent Schottky barrier height plots shownin Figure 2d. For Vgs < Vfb, the thermionic emissiondominates the measured current, resulting in a lineardependence of Φb on Vgs. However, for Vgs > Vfb, thetunnel current through the deformed Schottky barrierstarts to contribute increasingly, resulting in a devia-tion from the linear response.11 The true Schottkybarrier height is obtained from the point of onset ofthe deviation.To rule out the presence of metallic Ti in our TiO2

tunnel barrier and its influence on the reduction of theSchottky barrier, we performed a control experimentwith direct Ti/Au contacts (see Supporting Information).The MoS2/Ti/Au device shows a similar high contactresistance as the Co/MoS2 devices (Supporting Informa-tion Figure S1). The independence of the contact resis-tance with change in work function of themetals is alsoevidence of a Fermi level pinning close to the conduc-tionbandof theMoS2.

11 Therefore, this depinningof theFermi level and the reduction of the contact resistance

by introducing a TiO2 tunnel barrier is extremely pro-mising for electrical spin injection into MoS2.Apart from the requirement of an ideal contact

resistance for spin injection and detection, it is alsovery important to achieve higher channel mobilitiesand better transistor properties for spin transport andespecially spin manipulation in MoS2 devices. There-fore, we investigate the transistor behavior of Co/MoS2devices with and without TiO2 tunnel barriers. Thetransistor characterization of the devices was per-formed in a source�drain geometry with an appliedgate voltage. Different measurement configurations

Figure 2. Temperature-dependent measurements andSchottky barrier height. The data presented are for aMoS2/Co (left panel) and a MoS2/TiO2/Co (right panel)contact. (a) Ids�Vds characteristics for temperatures be-tween 194 and 267 K at the flat band voltages. (b) IdsT

�2

dependence on T�1 in an Arrhenius plot with linear fits. (c)Bias voltage dependence of the extracted slope S returnsthe zero bias slope S0 to calculate the Schottky barrierheight: 121 meV for the MoS2/Co and 27 meV for theMoS2/TiO2/Co contact. (d) Gate-voltage-dependent Schottkybarrier height; the deviation from the linear response at lowVgs (dotted line) defines the flat band voltage and the realSchottky barrier height.

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were used to determine output and transfer character-istics of the devices. Output characteristics were mea-sured by sweeping the drain�source voltage (Vds),while measuring the drain�source current (Ids) atseveral gate voltages (Vgs) (Figure 3a). The transfercharacteristics were recorded by sweeping the Vgswhile measuring the Ids at different Vds at room tem-perature (Figure 3b,c). The output characteristics ofboth devices show a linear Ids�Vds behavior at smallbias voltages (Figure 3a). However, such behavior doesnot necessarily indicate an Ohmic contact as perceivedpreviously.8 Thermally assisted tunneling in nanolayersof MoS2 at finite temperatures can cause a linear de-pendence despite the presence of a sizable Schottkybarrier at the MoS2/Co interface. As we discussedpreviously, the Schottky barrier and the carrier deple-tion at the interfaces give rise to higher contactresistances and nonlinear Ids�Vds characteristics athigh voltages. Introducing a TiO2 tunnel barrier be-tween the Co and MoS2 lowers the contact resistanceby 2�3 orders of magnitude (Figure 1d). The decreasein contact resistance with the TiO2/Co contact alsoresults in an improved transistor performance. Themeasured transfer characteristics are typical for n-typeFET devices, which is determined by the position ofthe Co Fermi level close to the conduction band of the

MoS2. The high contact resistance in the Co/MoS2device limits the drive current to about 4 μAμm�2

and results in the low transistor performance (Figure 3b).The introduction of TiO2 leads to a high saturation currentdensity up to 560 μAμm�2 with a carrier concentrationof 9 � 1012 cm�2, making the device characteristicseven more favorable for spin transport (Figure 3c).Furthermore, the ferromagnetic tunnel contact

MoS2/TiO2/Co also showed a significant enhancementin field-effect mobility and transistor on�off ratio. Theeffective field-effectmobility is estimated by extractingthe subthreshold slope dIds/dVgs from the transfercharacteristics (Figure 3a):

μeff ¼ dIdsdVgs

L

wCiVds(2)

where L is the length,w is thewidth of the channel, andCi is the gate capacitance.8 The effective field-effectmobility is found to be μeff = 12 cm2 V�1 s�1 for theMoS2/Co device (Figure 3b). This value is comparableto previously reported mobilities for ML MoS2 on SiO2

substrates without using any high-k dielectric cappinglayer.11,28 In contrast, the devices with TiO2 exhibitsa much higher mobility of μeff = 76 cm2 V�1 s�1

(Figure 3c). This demonstrates that the introductionof a TiO2 tunnel barrier reduces the contact resistance

Figure 3. Characterization of MoS2 FETs. (a) Output characteristics Ids�Vds measured at low bias range for differentgate voltages (Vgs) for both Co/MoS2 (left panel) and Co/TiO2/MoS2 devices (right panel). (b) Transfer characteristic Ids�Vdsfor theMoS2/Co FETwith different source�drain bias voltages (Vds) in a linear scaling. An effective field-effectmobility can beextracted fromthe subthreshold slope (μeff=12cm

2V�1 s�1). The logarithmical scaling forVds=0.5V (inset) reveals aon�off ratioof Ion/Ioff > 105. (c) Transfer characteristic for theMoS2/TiO2/Co FET. The effective field-effectmobility is μeff = 76 cm2 V�1 s�1, andIon/Ioff > 106 (inset).

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and increases the effective field-effect mobility by afactor of 6. Furthermore, the MoS2 devices with directCo contact show a current on�off ratio of Ion/Ioff > 105,whereas a device with TiO2/Co contact exhibits adrastic increase in Ion/Ioff > 106. These results clearlyindicate the requirement of lower contact resistance toextract intrinsic device parameters of MoS2.

29

In order to understand the viability of ferromagnetictunnel contacts on MoS2 for the observation of two-terminal MR, we put the contact resistances in our de-vices into perspective with the spin diffusion theory.14,30

We consider a FM/I/MoS2/I/FM structure, where I is theinterface contact. By taking experimentally measuredmobility and resistances into account, we estimate theoptimal contact resistance. To observe a high MR, theinterface resistance should lie within the range

FNL < RcontA <FNl2sfL

(3)

where FN is the resistivity of the MoS2 channel, Rcont isthe contact resistance, A is the contact area, lsf is thespin diffusion length, and L is the channel length in theMoS2.

14 Figure 4a shows the calculated MR as a func-tion of contact resistance area product (RcontA) for thechannel length of L = 100 nm and different expectedspin lifetimes in MoS2. The latter is a relevant factorto consider because the experimentally observed life-time could be well below the theoretical prediction ofτsf = 1 ns.10 This calculation shows the possibility ofobservation of a significant MR in a narrow contactresistance range. The low MR for small contact resis-tances is due to the conductivity mismatch, preventinga spin injection from the Co contact into MoS2. Thedecay of the MR for larger resistances is caused by therelaxation of the spins during the time spent in theMoS2 channel.

14

For our MoS2/Co device, the contact resistanceis very high (RcontA = 4 � 10�9 Ωm2 in FET on-statewith Vgs = 60 V) and lies in the higher end of the

resistance window corresponding to the low MR re-gion (Figure 4a). In contrast, by introducing a thin TiO2

tunnel barrier, we dramatically reduce the contactresistance (RcontA = 4 � 10�10 Ωm2) to the optimumrange for the observation of a large MR. Furthermore,we show the calculated MR as a function of gate voltageVgs and contact resistance RcontA in Figure 4b. Theobserved shift in MR is because of the tuning of thecarrier density in theMoS2 channel by the gate voltage.In addition, the change in contact resistance of ourdevices (dashed lines in Figure 4a) is due to deforma-tion of the Schottky barrier. Notably, the reducedSchottky barrier height for the MoS2/TiO2/Co contactresults in a weaker Vgs dependence compared to thedirect Co contact. Our results demonstrate the tun-ability of the channel, contact, and MR by the gatevoltage. In addition to the improved transistor perfor-mance, the TiO2/Co spin tunnel contacts are in theoptimum range for spin injection and detection inMoS2 over the full gate voltage range. Further en-hancements in the MR can be achieved by improvingthe mobility of the MoS2 channel. Such improvementsin mobility can be realized by recently demonstratedtechniques, such as encapsulating MoS2 structures inhigh-k dielectrics21,28 or atomically flat hexagonalboron nitride.31,32 This allows not only to explorespin-transistor-based devices but also to detect spincurrents in MoS2 created by spin Hall effect and otherspin-related phenomena.

CONCLUSION

In conclusion, we presented an improved field-effecttransistor behavior of MoS2 with ferromagnetic tunnelcontacts. The higher contact resistance and limitedtransistor performance of direct Co contacts on MoS2could be circumvented by introducing a TiO2 tunnelbarrier. We demonstrated a drastic reduction of thecontact resistance, which resulted in an increase in thetransistor on-state current by 2 orders of magnitude

Figure 4. Contact resistance for observation of MR. Calculated MR of FM/I/MoS2/I/FM spin FET structure as a function of thecontact resistance area product (RcontA) for a channel length of 100 nmat room temperature. The values are normalized to thepeak value at a spin lifetime τsf = 1 ns. (a) Calculation for different spin lifetimes τsf at a gate voltage Vgs = 60 V (on-state). Thevertical lines represent the RcontA value for the TiO2/Co tunnel and the direct Co contact onMoS2 in our experiments. (b)MR asa function of gate voltage and contact resistance. The dashed lines represent the variation of contact RA of our devices withgate voltage.

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reaching 560 μAμm�2 and the effective field-effectchannel mobility by a factor of 6. Our calculations forthe observation of two-terminal MR indicate thatthe implementation of TiO2 has many advantages todevelop efficient spintronic devices based on MoS2.The integration of such ferromagnetic tunnel contacts

on MoS2 bypasses the conductivity mismatch problemand improves the transistor performance. Further-more, the channel conductance, contact resistance,and expected MR can be tuned by an applied gatevoltage. This offers a multitude of possibilities to com-bine spintronic effects with regular electronics.

METHODSTheMoS2flakeswereexfoliated fromabulk crystal (SPI Supplies),

using the conventional cleavage technique, onto a clean SiO2

(285 nm)/highly doped n-type Si substrate. The flakes wereidentified using a combination of optical (Figure 1b) and atomicforce microscopy (Figure 1c). We used multilayer MoS2 with athickness of about 13 nmandwidths of 1�2 μm. The SiO2/n-Si isused as a gate to control the carrier concentration in the MoS2.Contacts of Co (65 nm)/Au (20 nm) were prepared on the MoS2samples by electron beam lithography, electron beam evapora-tion, and lift-off technique. Electrodes with widths of 0.5�1 μmand channel lengths of 0.5�1 μm are used. The 1 nm TiO2

tunnel barrier was deposited by sputtering technique. Usinga dc argon/oxygen plasma with a Ti source results in a TiO2

deposited on the target. Afterward, the sample transferred intoan electron beam evaporation system to deposit the Co(65 nm)/Au (20 nm) contacts.

Conflict of Interest: The authors declare no competingfinancial interest.

Acknowledgment. The authors acknowledge the support ofcolleagues at the Quantum Device Physics Laboratory andNanofabrication Laboratory at Chalmers University of Technol-ogy. The authors would also like to acknowledge the financialsupported from the Nano Area of the Advance program atChalmers University of Technology.

Supporting Information Available: MoS2 FET characteristicswith Ti/Au contacts without TiO2 tunnel barrier. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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