Black Phosphorus Field-Effect Transistors withWork Function Tunable ContactsYuqiang Ma,†,# Chenfei Shen,†,# Anyi Zhang,† Liang Chen,† Yihang Liu,† Jihan Chen,† Qingzhou Liu,†

Zhen Li,† Moh. R. Amer,‡,§ Tom Nilges,∥ Ahmad N. Abbas,*,‡,⊥,¶ and Chongwu Zhou*,†

†Department of Electrical Engineering, University of Southern California, 3710 McClintock Avenue, Los Angeles, California90089-0271, United States‡Center of Excellence for Green Nanotechnologies, Joint Centers of Excellence Program, King Abdulaziz City for Science andTechnology, P.O Box 6086, Riyadh 11442, Saudi Arabia§Department of Electrical Engineering, University of California, Los Angeles, 420 Westwood Plaza, 5412 Boelter Hall, Los Angeles,California 90095, United States∥Department für Chemie, Technische Universitaẗ München, Lichtenbergstrasse 4, 85747 Garching b. München, Germany⊥Department of Electrical and Computer Engineering, University of Jeddah, Dhahban 23881, Saudi Arabia¶Department of Electrical and Computer Engineering, King Abdulaziz University, Abdullah Sulayman Street, Jeddah 22254, SaudiArabia

*S Supporting Information

ABSTRACT: Black phosphorus (BP) has been recentlyrediscovered as an elemental two-dimensional (2D) ma-terial that shows promising results for next generationelectronics and optoelectronics because of its intrinsicallysuperior carrier mobility and small direct band gap. Invarious 2D field-effect transistors (FETs), the choice ofmetal contacts is vital to the device performance, and it is amajor challenge to reach ultralow contact resistances forhighly scaled 2D FETs. Here, we experimentally show theeffect of a work function tunable metal contact on the device performance of BP FETs. Using palladium (Pd) as the contactmaterial, we employed the reaction between Pd and H2 to form a Pd−H alloy that effectively increased the work function ofPd and reduced the Schottky barrier height (ΦB) in a BP FET. When the Pd-contacted BP FET was exposed to 5%hydrogen concentrated Ar, the contact resistance (Rc) improved between the Pd electrodes and BP from ∼7.10 to ∼1.05 Ω·mm, surpassing all previously reported contact resistances in the literature for BP FETs. Additionally, with exposure to 5%hydrogen, the transconductance of the Pd-contacted BP FET was doubled. The results shown in this study illustrate thesignificance of choosing the right contact material for high-performance BP FETs in order to realize the real prospect of BPin electronic applications.

KEYWORDS: layered material, two-dimensional materials, field-effect transistor, black phosphorus, phosphorene, contact resistance

Black phosphorus (BP) has been recently unveiled as apromising anisotropic direct band gap semiconductingmaterial, with interesting optical and electrical proper-ties.1−6 Several controlled synthesis methods of BP have beenreported recently. Nilges et al. reported an ampule synthesismethod to produce bulk single-crystalline BP; however, thismethod requires mechanical exfoliation of BP, which limits thesize of BP flakes.7,8 Lau et al. reported pulsed laser deposition ofultrathin BP onto graphene/copper and Si/SiO2 substrates, butthe obtained BP is amorphous, which limits its electricalperformance.9 Ji et al. reported direct chemical vapor depositionof single-crystalline BP films onto Si/SiO2 substrates. Never-theless, large size and small thickness of the BP flakes cannot beachieved simultaneously.10 Consequently, methods that can

provide large-scale and highly crystalline BP are needed forhigh-performance electronics. Field-effect transistors (FETs) offew-layer BP have been reported to show high charge carriermobility, anisotropic transport behavior, high on/off ratios, andhigh operating frequencies.11−13 Although BP has exhibited thepotential to become a promising candidate for futureelectronics, FETs made with single-layer or few-layer BPusually possess small Schottky barriers. The Schottky barriercauses increased contact resistances (Rc), which significantlylimit the performance and scalability of BP FETs.14

Received: April 25, 2017Accepted: June 19, 2017

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Recent studies demonstrated several methods to overcomethe contact problems in BP FETs. A common method toreduce the Shottky barrier height and the contact resistance isby choosing the metal with the appropriate work function,depending on the 2D semiconductor carrier type. Ye et al.reported using high work function metals, such as Pd and Ni,for BP FETs and reported contact resistances of 1.75 and 3.15Ω·mm, respectively.15 Other studies have employed othermethods to mitigate the contact resistance problem. Forexample, by using TiO2 and cobalt contacts, a reduced Schottkybarrier of

significantly improve the BP FET performance (e.g., trans-conductance and contact resistance). Answering this question isimportant for the future development of high-performance BPdevices, and it will influence future studies to use nontraditionalcontact solutions for high-performance BP FETs.To change the work function of the contact metal, several

methods can be adopted, which include using gas absorp-tion,25,26 adding nanoparticles,27 or simply changing thetemperature.28 As a well-known metal catalyst, Pd was reportedto have high hydrogen solubility and can be used in hydrogen-sensing applications.29−31 Interestingly, it was reported that,according to density functional theory (DFT) calculations,atomically adsorbed hydrogen increases the work function ofPd up to 250 meV,32,33 whereas experimental results suggestthat one monolayer of atomic hydrogen increases the workfunction of Pd by 120 meV.34 These studies show that Pd, as acatalytically active noble metal, has a tunable work functionwhen exposed to hydrogen. This will further allow tuning of theSchottky barrier formed between Pd contacts and the channelmaterial (i.e., BP), thus improving the efficiency of chargeinjection and lowering the contact resistance.

In this study, we report that BP FETs contacted with Pdcould achieve a significantly low contact resistance, down to∼1.05 Ω·mm, when exposed to 5% hydrogen environment.Comparatively, previous studies have reported contactresistance around 1.75 ± 0.06 Ω·mm for Pd contacts, around1.14 ± 0.05 Ω·mm for Ti/Au, and around 3.15 ± 0.15 Ω·mmfor Ni.15,17,35 We also show that using absorption of hydrogen,the BP-based FET with Pd contacts can be switched fromSchottky behavior to Ohmic behavior. The transconductance ofthe device almost doubled from 0.499 to 0.937 μS/μm whenexposed to 5% hydrogen. We also find that BP-based FETswith Pd contacts can be used as hydrogen gas sensors, whereasBP-based FETs with Au contacts have almost no response toH2. Additionally, BP FETs with Pd contacts are sensitive to H2concentrations as low as 500 ppm, whereas the current ratio(e.g., current after H2 treatment divided by that beforetreatment) can reach up to 16×. These results shed light onthe importance of employing various methods, other than workfunction engineering, to lower the contact resistance andpotentially achieve high-performance scaled BP FETs.

Figure 2. Electronic properties of few-layer BP FET under different H2 concentrations. (a) Ids−Vds curves of few-layer BP FET under differentVg before exposure to H2. (b) Ids−Vds curves of few-layer BP FET under different Vg after exposure to 5% H2. (c) Ids−Vds curves of few-layerBP FET under different H2 concentrations. (d) Ids−Vg curves of few-layer BP FET under different H2 concentrations. (e) Ids−Vg curves of few-layer BP FET shown in log scale. Inset is the threshold voltage versus H2 concentration. (f) Plot of on-state drain current andtransconductance under different H2 concentrations.

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RESULTS AND DISCUSSION

Figure 1a shows a schematic of the device structure we used inour experiments. The image shows the reaction between H2and Pd electrodes of a BP FET. As reported by literature, atroom temperature and atmospheric pressure (standard ambienttemperature and pressure), palladium can absorb up to 900times its own volume of hydrogen, thus forming PdHx withdifferent x values.36,37 Hydrogen molecules dissociate sponta-neously at the Pd surface and produce H atoms which arepreserved in the Pd lattice.38 As a result, the work function ofPd can be tuned, which may be associated with either thepresence of highly protonized hydrogen species on the surfaceor the drastic shape distortion of thin Pd film in the course ofhydride formation.39 Figure 1b shows a schematic diagram ofhow different concentrations of H2 would alter the workfunction of Pd and the corresponding hole barrier change. Thisfeature facilitates a tunable Schottky barrier between thecontact material and the channel material, which can result in alower contact resistance in few-layer BP FET with Pdelectrodes. An optical image of a few-layer BP FET with aTLM structure is shown in Figure 1c. BP flakes were firstexfoliated using Scotch tape and then transferred onto a Si/SiO2 substrate, followed by electron-beam lithography andmetal evaporation using Pd as the source and drain electrodes.More experimental details on the device fabrication can befound in the Methods. The inset of Figure 1c shows an atomicforce microscopy (AFM) height profile of the few-layer BPFET, and it reveals a thickness of ∼31 nm. According to recentresearch,1,15 single-layer BP possesses a thickness of 0.5 nm.Hence, our sample should consist of ∼60 layers. Thecorresponding AFM image of this FET can be found in FigureS1 (Supporting Information). Channel lengths of the TLMstructure are measured by AFM to be 0.523, 1.517, and 2.294μm. The Ids−Vds curve of this BP FET before H2 treatment isshown as the black curve in Figure 1d. Even though this deviceshows a linear behavior at small Vds values, a small Schottkybarrier can be observed from the nonlinear behavior at higherVds values. On the contrary, the linear behavior and thesaturated drain current of BP FET after exposure to 5% H2 inAr suggest that the device contact is Ohmic even with such alow concentration of H2.We have systematically studied the device performance with

different concentrations of H2. First, the device was flushedwith pure Ar to clear the system from any residual gas species.The device subsequently was exposed to a specific H2concentration for 2 min. More details can be found in theMethods. As shown in Figure 2a, a Schottky contact can be

clearly observed from the nonlinear Ids−Vds curves of the as-made few-layer BP FET before exposure to H2. In contrast, theperformance of the few-layer BP FET after exposure to H2 isshown in Figure 2b, with the on-state drain current almostbeing doubled for Vg = −100 V, reaching 127 μA/μm aftertreatment as opposed to 77 μA/μm before treatment. Inaddition, the current saturation behavior of this device afterexposure to H2 is observed. The improved device performanceafter exposure to H2 shows that H2 treatment can transformSchottky contacts between BP and Pd into Ohmic contacts,giving rise to current saturation. Moreover, Ids−Vds curvesunder different H2 concentrations, plotted in Figure 2c,illustrate the gradual transition from Schottky to Ohmiccontact behavior of this device. The linearity of Ids−Vds curvesalso indicates the effectiveness of absorption of H2 on Pdelectrodes. Further characterization of the electronic propertiesof few-layer BP FET was done by measuring Ids−Vg curvesunder exposure to different H2 concentrations (Figure 2d). Theon-state current was also observed to be larger with higher H2concentrations. Further, we have treated this sample under pureH2 flow and found that the on-state current can reach up to 475μA/μm (Figure S2, Supporting Information). Additionally, anupshift of the threshold voltage in the Ids−Vg curved withincreasing H2 concentrations is shown in Figure 2e. Thisthreshold voltage shift is due to the reduced barrier for holecarrier injection after H2 exposure. A high on/off current ratioof ∼106 is observed based on Figure 2e. One possible reasonfor this high on/off ratio with the BP sample of 31 nm inthickness is that the top layers of the BP flake may havepartially oxidized during the fabrication process, as reported inprevious literature,40,41 so that the gate can now control all theremaining layers of BP. Figure 2f plots out a trend of increasingthe device transconductance when higher H2 content issupplied. The transconductance is improved from 0.499 μS/μm (no H2) to 0.937 μS/μm (5% H2). The results reveal thatthe increase in the on-state current under different H2concentration is due to not only a threshold voltage shift butalso an enhanced transconductance because of the reducedcontact resistance. It is also observed that a 5-fold increase ofthe on-state current could be achieved after 5% H2 exposure.To further study the changes on the device performance after

H2 exposure, we have performed TLM measurements tomeasure Rc under different H2 concentrations. Figure 3a showsthe fitted total device resistance versus channel length curves ofthe few-layer BP FET measured under different H2concentrations. Based on the equation Rtotal = 2Rc + Rchannel,where Rchannel is the channel resistance, the y-intercept of the

Figure 3. Contact resistance of Pd-contacted few-layer BP FET under different H2 concentrations. (a) Total device resistance versus channellength under different H2 concentrations. (b) Contact resistance versus H2 concentration.

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fitted lines in Figure 3a represents the value of 2Rc. Rc values forthe BP FET under different H2 concentrations are extractedand summarized in Figure 3b. It is found that with the increaseof H2 concentration from 0 to 5%, the contact resistancedecreases from ∼5 kΩ to ∼740 Ω. With normalization by thechannel width, our contact resistance under 5% H2concentration is calculated to be ∼1.05 Ω·mm. Compared toother recent studies reporting the contact resistance of BP-based FETs,15,29 our result has the lowest Rc reported to date,according to our knowledge. The increase of H2 concentrationfacilitates the molecular H2 adsorption, which changes the workfunction of Pd and therefore reduces the Schottky barrierformed between the Pd contact and the BP channel. Thereduced Schottky barrier would thus facilitate the hole injectionfrom the metal into the valence band of BP, leading to a lowercontact resistance. Our work indicates that by using metal witheven higher work function, one may be able to get even bettercontacts for opto/nanoelectronics.Due to the substantial change of drain current induced by the

formation of Pd−H contact and the reversibility of Pd to Pd−Htransformation process, BP FETs with Pd contacts can also beused for H2 gas sensing. Figure 4 shows the H2-sensingperformance of a Pd-contacted BP FET. The H2-sensingexperiment starts by loading the device in a gas chamber whileflushing the system with Ar for 5 min. Afterward, H2 is dilutedwith Ar to produce various concentrations, and the BP device isthen exposed to the desired concentration. After exposure to acertain concentration, the system is flushed with Ar again torecover the device to the original current level. The blue arrowsin Figure 4a indicate the points after Ar flushing. Based on thecurrent at each point, we can find that the device can recoveralmost to the original current level after Ar flushing after eachround of H2 exposure. More experimental details can be foundin the Methods. The recovery of current after Ar flushing showsthat the barrier tuning due to Pd contacts under H2 exposure isnot permanent and needs the presence of H2. To achieve betterperformance of BP devices under exposure to H2, metalcontacts with higher work function can be used. Figure 4ashows that the current changes to H2 concentrations as low as500 ppm. Another BP FET with Pd contact has shown similarperformance (Figure S3, Supporting Information), whichdemonstrates the reproducibility of this gas-sensing behavior.To achieve high current ratios, the gate voltage was set to bethe threshold voltage, which is −5 V for this device. Figure 4bshows the current ratios I/I0 calculated at different concen-trations of H2 for the same device shown in Figure 4a, where Istands for the average current value when H2 is on, and I0

represents the lowest current reached before the particular H2concentration is introduced. For instance, the I0 before 2000ppm of H2 was turned on is 5.6 μA/μm, and the averagecurrent is 91 μA/μm after 2000 ppm of H2 being purged insidethe chamber. Hence, the current ratio is calculated to be around16. As we are adopting average values, error bars are introducedto further clarify the range of current ratios. Our results showcomparable performance to previously reported Pd nanotubesand Pd nanoparticle-decorated ZnO nanorod arrays.42,43 Thesensitivity achieved by BP FETs with Pd contact indicates theirgreat potential to be effective H2 sensors.In order to confirm that the device performance changed,

and gas-sensing results observed are indeed originating fromthe Pd−H alloying, we have performed the same experimentswith a few-layer BP FET with Au contacts. The thickness of theBP flake used here is about 30−60 nm. Ids−Vds and Ids−Vgcurves of few-layer BP FET with Au contacts under different H2concentrations are shown in Figure 5. The device wasfabricated with the same process of Pd-contacted BP FET.Compared to Pd-contacted BP FET, the Ids changes of BP FETwith Au contacts obtained at different H2 concentrations arenegligible and not monotonic, indicating that H2 has a minimaleffect on Schottky barrier modulation with Au electrodes.Figure 5b shows a zoomed-in image of the Ids−Vds curvesshown in Figure 5a, and it is found that increasing the H2concentration from 0 to 100% can only lead to less than 1%change of Ids. The current change observed is much smallerthan that in Pd-contacted devices. Figure 5c shows Ids−Vgcurves of few-layer BP FET with Au contact under different H2concentrations. It is observed that the variation of Ids is ∼6%with a Vg of −100 V. Comparatively, the hydrogen-treated BPFET with Pd contacts reveals current ratios as high as 16×. Theinset in Figure 5c shows Ids−Vg curves of the device underdifferent H2 concentrations in log scale, from which we can findthat the on/off ratio of Au-contacted device is ∼102. We notethat whereas Figure 5 shows that H2 does not affect the currentof the Au-contacted devices, Figure 3a shows that for Pd-contacted devices, the H2 exposure affects both the contactresistance (as indicated in the change of intercepts) and thechannel resistance (as indicated in the change of slopes of thelines). We believe as the H2 exposure affects the Pd−BPSchottky barrier height, the depletion width due to theSchottky junctions at the contacts also is affected, thus leadingto changes of the channel resistance, as well. H2 exposure doesnot affect the Au−BP junction at all, so no change in currentwas observed in Figure 5. This control experiment using Aucontact illustrates that the increase of drain current in Pd-

Figure 4. H2-sensing performance of a few-layer BP FET. (a) Current versus time (in seconds) of the device used for H2 sensing underdifferent H2 concentrations. (b) Current ratio versus H2 concentration of this device.

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contacted FETs is mainly associated with Pd−hydrogenreaction. Therefore, all the effects observed thus far are causedby Pd contacts with hydrogen flow.

CONCLUSIONSIn summary, we have fabricated Pd-contacted BP FETs andshown that by exposing Pd to different concentrations of H2 wecan change the work function of the contacts and significantlyalter the device performance (e.g., transconductance, contactresistance, threshold voltage). The transconductance of thedevice changed from 0.499 to 0.937 μS/μm when the devicewas exposed to 5% hydrogen. Additionally, the contactresistance was significantly reduced from ∼7.10 to ∼1.05 Ω·mm by exposing the device to 5% hydrogen, with the thresholdvoltage shifting to more positive values. Moreover, the currentratio can reach as high as 16× under hydrogen exposure. Theresults shown in this study shed light on the importance ofusing contact engineering techniques in order to enhance BPFET performance and to realize full utilization of BP in futureelectronics and optoelectronics.

METHODSBlack Phosphorus Synthesis. The black phosphorus was

synthesized from red phosphorus (Chempur, 99.999t %) and tin/tin(IV) iodide (Sn/SnI4 = 10/5 mg per 250 mg batch). The reactantswere sealed in an evacuated (p < 10−3 mbar) silica ampule accordingto the procedure reported in literature.7,8 The ampule was heated to650 °C in 8 h, and the temperature was held for 5 h. After that, theampule was cooled to 550 °C with a ramp of 14 K/h and was kept atthe temperature for another 6 h. Finally, the mixture was cooled toroom temperature.

Black Phosphorus Device Fabrication and Measurements.The black phosphorus material was first exfoliated to nanometer scalethickness, using vacuum tapes (Kapton Tapes), as reportedpreviously.44 After exfoliation, the sample was dry-transferred ontoSi/SiO2 substrates with alignment markers (the thickness of SiO2 is300 nm). A bilayer poly(methyl methacrylate) (PMMA) consisting ofPMMA A6 and PMMA A2 was then spin-coated onto the Si/SiO2surface. After spin-coating, electron-beam lithography was conductedto pattern the source/drain electrodes, followed by development,metal deposition, and lift-off processes. Both Pd and Au electrodes (40nm) were deposited at the pressure of 9 × 10−7 Torr using an e-beamevaporator. The transfer curves and Ids−Vds curves were measuredusing Agilent 4156B with protective gas flow (Ar or Ar/H2).

Gas-Sensing Setup. Gas sensing was carried out by exposing theBP FET device to H2 gas diluted in Ar in a closed chamber. The FETdevice was bonded to a chip shell with aluminum or indium wires.Concentrations of H2 were adjusted by changing the flow rates of bothH2 and Ar while keeping the total flow rate constant. For each curve,the device was exposed to the desired concentration for 120 s and thenflushed with Ar for 300 s. A similar procedure was followed to measureIds−Vds and Ids−Vg for BP devices.

AFM Characterization. AFM (DI 3100 Digital Instruments)scanning was performed on BP-based FETs to determine both theheight of the material and the channel lengths between electrodes. Thescanned images have a resolution of 512 × 512. The scanning wasdone in an ambient condition.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.7b02858.

AFM image of Pd-contacted BP FET, Ids−Vds curves ofof Pd-contacted BP FET exposed to high H2concentrations, and H2-sensing measurement of anotherPd-contacted BP FET (PDF)

AUTHOR INFORMATIONCorresponding Authors*E-mail: ahmad.nabil.abbas@gmail.com.*E-mail: chongwuz@usc.edu.ORCIDChenfei Shen: 0000-0001-8635-3429Yihang Liu: 0000-0002-2491-9439Chongwu Zhou: 0000-0001-8448-8450Author Contributions#Y.M. and C.S. contributed equally to this work.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSThis work is financially supported by King Abdulaziz City forScience and Technology (KACST). We acknowledge StephenCronin at University of Southern California for access to thewire-bonding facility.

Figure 5. Electronic properties of few-layer BP FET with Aucontact under different H2 concentrations. (a) Ids−Vds curves offew-layer BP FET with Au contact under different H2 concen-trations. Inset is the optical image of the device. (b) Enlarged Ids−Vds curves in (a) to illustrate the small current change. (c) Ids−Vgcurves of few-layer BP FET with Au contact under different H2concentrations. Inset is the Ids−Vg curves of few-layer BP FET withAu contact shown in log scale.

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