Contact Engineering of Two-Dimensional Layered Semiconductors beyond Graphene

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Contact Engineering of Two-Dimensional Layered Semiconductors beyond Graphene Zhixian Zhou Department of Physics and Astronomy Wayne State University Detroit, Michigan

Outline Introduction Ionic liquid gated few-layer MoS 2 FETs WSe 2 FETs with highly doped graphene contacts 2D metal contacted WSe 2 FETs 2D/2D semiconductor homo- and hetero-junctions as a new contact paradigm 9/26/2016

Motivation Ultrathin semiconductors with atomically smooth surfaces are highly desirable for multifunctional electronics (scaling, flexible electronics, sensing ) Graphene as a 2D material + one atomic layer thick + flexible + mechanically strong + chemically inert + thermally stable + high mobility - no bandgap Can we find a 2D material that is atomically thin, flexible, mechanically strong, thermally stable (like graphene), but with a reasonably large bandgap? 9/26/2016

Motivation MoS 2 TMD Transition Metal Dichalcogenides Bandgap 1-2 e Untra-thin and uniform channel Surface smoothness Mechanically flexible and strong Thermally stable Reasonablly good mobility MX 2 Metal M = Mo, W, Ti Chalcogenide ( X = S, Se, Te) MoS 2, MoSe 2, WSe 2

Early works of TMD FETs 4-terminal FE mobility of WSe 2 ~ 500 cm 2-1 s -1 at RT; 2-T FE mobility ~ 100 cm 2-1 s -1 MoS 2 FE mobility ~ 1 cm 2-1 s -1 MoS 2 FE mobility 10-50 cm 2-1 s -1 on/off ratio ~ 10 5

Monolayer MoS 2 High on/off ratio:10 8 S = 74 m/dec High mobility

Impact of contacts in early MoS 2 FET devices Actual μ = 2-7cm 2-1 s -1 Fuhrer and Hone, 2013 Nature Nanotech, 8, 146 (2013) Bare device With top HfO 2 and floating top-gate Kis et al. 2011 Nature Nanotech In crease of nominal μ by ~X1000 coupling between the back-gate and floating top-gate dielectric screening Contact resistance reduction (Schottky barrier reduction by n-doping by HfO 2 ) Threshold voltage negative shift (possible n-doping by HfO 2 )

Schottky Barrier at Metal/Semiconductor interface Simple Schottky-Mott Model Fermi level pinning

Electrical contacts to 2D semiconductors S D Metal TMDs Lateral depletion region (Schottky barrier ) Tunnel barrier Good contact materials: High conductivity, chemical and thermal stability High density of delocalized states across the interface at the Fermi level Low Schottky barrier Strong bonding and d-orbital hybridization narrow tunnel barrier Popov, Seifert, and Tomanek, PRL, 108, 156802 (2012) Allain, Kang, Banerjee and Kis, Nature Materials, 14, 1195 (2015) 9/26/2016 9

Strategies to make low resistance contacts 1. Lower the Schottky barrier height 2. Reduce the Schottky barrier width 3. Reduce the tunnel barrier Select metals with proper work function and reduce Fermi level pinning (reduce SB height) Doping to reduce the Schottky barrier width Hybridization of d orbitals

Making good contacts Approach 1 Thinning the lateral Schottky barrier thickness using ionic liquid gating at metal/mos 2 contact 9/26/2016

How do ionic-liquid-gated MoS 2 FETs work? positive gate voltage 1) negative ions near gate electrode 2) positive ions near device channel. electric double layers form at the interfaces between the ionic liquid and solid surfaces. area of the gate electrode >> the total area of the transport channel M.M. Perera et al. ACS Nano, 7, 4449, (2013) 9/26/2016 12

Transfer characteristics of two IL-gated MoS 2 FETs Ambipolar Behavior Bilayer Yes Trilayer Yes Holes On/Off ratio 10 6 10 4 Electron On/Off ratio > 10 7 > 10 7 M.M. Perera et al. ACS Nano, 7, 4449, (2013) 9/26/2016 13

Output characteristics of a trilayer MoS 2 device with IL-gate and back-gate without IL ( a) IL Gate The dielectric layer produced by IL, reduce the thickness of Schottky barrier by band bending near the contacts. ( b) Back gate without IL: Strongly nonlinear (upward turning) curve suggesting significant Schottky barrier M.M. Perera et al. ACS Nano, 7, 4449, (2013) 9/26/2016 14

Back-gate transfer characteristics with frozen IL I ds - bg between 77 and 180K, after the device had been quickly cooled from 250 K to 77 K at a fixed ILg 77 K< T< 180 K IL is frozen M.M. Perera et al. ACS Nano, 7, 4449, (2013) 9/26/2016 15

Field-effect mobility as a function of temperature W and W/O ionic liquid 7 X 10 12 < n < 9 X 10 12 M.M. Perera et al. ACS Nano, 7, 4449, (2013) 9/26/2016 16

Making good contacts to WSe 2 Approach 2 graphene as a work function tunable electrode material Nano Lett. 9, 3430 (2009) extremely large capacitance ionic liquid gate to tune graphene work function at the graphene/wse 2 contacts low resistance Ohmic contacts for both electrons and holes WSe 2 is protected by hbn 9/26/2016

Device Fabrication Au/Ti electrode CD graphene ILg h-bn Oxygen Plasma Etching WSe 2 S D SiO 2 Si

Highly doping graphene contacts by IL-gating 14 ILg = 6 WSe 2 Ti 14 ILg = - 6 12 ILg = 2 SiO 2 Si 12 10 Graphene 10 Graphene 8 6 Drain Au ds =10m Higher ILg 4-60 -40-20 0 20 40 60 () bg Graphene = 0 = 0 ILg 8 ILg h-bn Au + + Ionic Liquid 4 Ionic Liquid Mobile ions freeze below 180 K + + + + + + + + + + + + + + + + + + + + + + 6 ds = -10m -60-40 -20 0 20 40 60 () bg Graphene Higher ILg Source Au Si Back Gate H-J Chung et al. Nano Lett. 2014

Device image WSe2 h-bn Graphene Drain Graphene Source

Under positive ionic-liquid gate voltages 1 0.8 T = 293K = NA ILg Without Ionic liquid bg = 60 Ionic Liquid Gating On Graphene 3 40 2.5 T=170K T = 77K = ds = 100 6m ILg = = ILg 60 6 bg 0.6 30 2 Electron side 0.4 1.5 20 1 4 0.2 10 0.5 0 0 0 0.2 0.4 0.6 0.8 1 ds () -400-20 0.2 00.4 200.6 400.8 60 1 () bg ds E F E F Graphene Graphene h BN

Under negative ionic-liquid gate voltages 0-0.2-0.4 Iionic Liquid Gating On Graphene 0.40 0.35-5 0.3-10 0.25 ILg = - 7 T=180K = -10 m ds -0.6-0.8 Without Ionic -1 ILg liquid = NA = -60 bg T = 293K -1.2-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1 0 () ds -15 0.2-6 0.15 Hole side -20 0.1-25 = -7 0.05 ILg = -60 T = 77K 0 bg -30-80 -1-60-0.8-40 -20-0.6 0-0.4 20 40-0.260 80 0 () bg () ds E F E F Graphene Graphene h BN

Temperature dependent transfer characteristics 0.4 0.3 0.2 T=77 K 120 K 160 K 600 500 400 300 L di ds 1 W d C Hole side bg bg ds = -10 m ILg = - 7 ds Graphene Where C bg is determined to be 1.2 10-8 F cm -2 for 290nm SiO 2 based on the parallel capacitor model (C bg = 3.9ε 0 / 290 nm) 0.6 0.4 ILg = 6 ILg = 0 ds = 10 m Electron side No ILg 77 K 180 K 0.1 200 0.2 0 100 T= 170 K ds = 0.1-100 -50 0 50 () bg 0-60 Dimensions -40of Samples -20 : 0 20 40 60 Sample I : d= 6.0 nm L=6.8 µm W=4.8 () µm bg 0-80 -40 0 40 80 bg () H-J Chung et al. Nano Lett. 2014

2-terminal electron and hole FE mobilities 400 300 ILg = 6 ILg = -7 200 100 0 80 120 160 200 T(K) Dimensions of Sample : d= 6.0 nm L=4.8 µm W=4.8 µm H-J Chung et al. Nano Lett. 2014

WSe 2 diode with asymmetric graphene contacts 4 3.5 3 2.5 2 1.5 1 0.5 0 10 1 T=170K = 30 bg 10 0 Ideality factor~1.3 20 10-0.8-0.4 0 0.4 0.8 () ds 10-1 10-2 10-3 10-4 10-5 10-6 10-7 = 30 bg 20 10 T=170K -1-0.5 0 0.5 1 () ds E F E F Graphene Graphene h BN

Surface Charge Transfer doping Strong Electron Donor 20 B for n doping 18 Carrier density (cm -2 ) -5 10 12 0 5 10 12 ()= 10m ds T=295K 16 14 12 Strong Electron acceptor 10 F4-TCNQ for p doping 8 6 B Non doped F4-TCNQ 4-15 -10-5 0 5 10 15 bg () Drain Graphene h-bn Graphene Source Si Back Gate

WSe 2 with B and F4-TCNQ doped graphene contacts 10 2 ds ()= -1 2 probe F4-TCNQ doping 10 2 2 probe B doping ds ()= 1 10 0-0.1 T=294K 10 0 T=294K 0.1 10-2 0-12 10-2 8 bg =25 10-4 -3 10-4 6-6 4 10-6 10-8 -9 bg =-22-12 -1.2-0.9-0.6-0.3 0 ds () -25-20 -15-10 -5 0 5 10 bg () 10-6 10-8 2 0 0 0.3 0.6 0.9 1.2 ds () -10-5 0 5 10 15 20 25 bg () 7 Device Performance : On/Off ratio > 10 7 Linear I characteristics near Ohmic contacts

Electron and hole FE mobility in WSe 2 Drain 2 3 Source RT hole FE mobility: 258 cm 2 /s RT electron FE mobility: 46.5cm 2 /s 1000 800 600 400 4 probe measurement Mobility T ~ 1.5 γ Mobility(cm 2 /s) 10 3 10 2 10 1 200 300 T(K) m h ~ 0.3 +/- 0.2 m 0 m e ~ 0.9 m 0 Klein, A., et al. Solar materials and solar cells, 1997energy F4-TCNQ 4 Probe 200 ~ 2 B 0 150 200 250 300 350 T(K)

Partial List of Different Contact strategies by 2015 Low/high work function metals Muiltilayer-MoS 2 /Scandium (Appenzeller et al. Nano Lett. 2012 ) WSe 2 /In, Ag (also d-orbital hybridization) (Jene, Banerjee et al. Nano Lett. 2013 ) Pt under WSe 2 (reduced FLP) (Sanjay Banerjee et al.) Doping Surface doping of WSe 2 and MoS 2 using NO 2, K, B, and TiO 2-x (Javey et al. Nano Lett. 2012, Nano Lett. 2013, JACS 2014; S. Banerjee et al. Nano Lett. 2015 ) Body doping of MoS 2 and WS 2 with Cl and Nb (Ye et al. Nano Lett. 2014; Wu et al. Nano Lett. 2014) Graphene contacts MoS 2 /graphene ( Duan et al. Nano Lett. 2015; Kim and Hone et al. Nature Nano 2015) MoS 2 /Ni-graphene (T.L. Thong et al. ACS Nano 2014) WSe 2 /graphene (Das et al. Nano Lett. 2014) Phase engineered contacts 1T/2H MoS2 (Chhowalla et al. Nature Nano 2014) 1T /2H MoTe2 (Kim, Lee and Yang et al. Science 2015)

What next?

WSe 2 with NbSe 2 metallic 2D contacts Perspective view + suppressed interface states Au/Ti WSe 2 h-bn + reduced Fermi level pinning h-bn NbSe 2 Graphite SiO 2 Side view

WSe 2 with NbSe 2 metallic 2D contacts

WSe 2 with NbSe 2 metallic 2D contacts

Continued Search for New Contact Approaches For realistic device applications and fundamental physics air-stable thermally stable true ohmic contacts (ohmic even at low-t) contact-resistance at the order 100 Ω.µm 9/26/2016

Silicon-Based electronics MOSFET Working Principle S Source Ion implantation G Gate D Drain Ion implantation ++++++++ SiO 2 -------- ++++++++ N+ N+ P type substrate

Degenerately p-doped WSe 2 (Nb 0.005 W 0.995 Se 2 ) with Ti/Au metal contacts Low R c T-independent 18 nm thick Large drive current Stable

Degenerately p-doped WSe 2 (Nb 0.005 W 0.995 Se 2 ) with Ti/Au metal contacts Not suitable as channel materials Low mobility Low gate tunability (high off current and low on/off)

MOSFETs Contact S Source G Gate D Drain ++++++++ SiO 2 -------- ++++++++ N+ N+ P type substrate metal n+ Si n-si Low Resistance Contacts Were enabled by Ion implantation at contact regions only (not the channel)

Silicon-Based electronics However Ultrathin body of monolayer and few- layer TMDs prohibits effective (Local) doping by ion implantation What if we fabricate the channel and highly doped drain/source contacts seperately, and assemble them together?

New Contact Strategy Substitutional doping WSe 2 Nb 0.005 W 0.995 Se 2 Nb 0.005 W 0.995 Se 2 (Highly Doped WSe2 as Contact) + WSe 2 (undoped WSe2 as channel)

Layered Materials: van der Waal Assembling Geim and Grigorieva, Nature, 499, 419 (2013)

TMD FET with 2D/2D contacts 2D/2D Contacts SiO 2 Si Wafer Au/Ti h-bn TMDs h-bn Au/Ti

Optical image of a WSe 2 FET with 2D/2D contacts 2D/2D Contacts How do we do it? Dry transfer method!!!

Dry Transfer method Micro-manipulator Optical Microscope Micro-manipulator Optical Microscope Glass slide PDMS TMD Target (hbn on SiO2/Si wafer) Target (hbn on SiO2/Si wafer) Doped TMD hbn TMD hbn Doped TMD

Contact Mechanism 10um H.-J. Chuang, et. al.,nano Lett. 2016

Transfer and output characteristics of WSe 2 FETs with 2D/2D contacts P type WSe 2 transistors with degeneratley p doped WSe 2 as contacts 10um 3.5 nm thick WSe 2 channel H.-J. Chuang, et. al.,nano Lett. 2016

Contact Resistance: Transfer Length Method Metal (Au/Ti) to NbWSe2 to WSe2 High drive current > 300uA/um H.-J. Chuang, et. al.,nano Lett. 2016

Conductivity and FE hole mobility of WSe 2 down to 5 K WSe 2 channle with digenerately p-doped WSe 2 contacts 2-terminal conductivity 2-termainl FE mobility H.-J. Chuang, et. al.,nano Lett. 2016 Low-resistnace 2D/2D contacts enable the investigation of channel properties

WSe 2 Hall Bar with Nb-WSe 2 contacts Nb-WSe 2 Nb-WSe 2 WSe 2 7.9 nm T3L T4R

Improvement of 2-terminal FE mobility with improved Channle material quality WSe 2 channle with digenerately p-doped WSe 2 contacts 2500 2000 10K 2-terminal conductivity ds ()= -100m 10 4 2-termainl FE mobility ~6600 cm 2 /s 1500 1000 20K 10 3 500 50K 150K 300K 0-90 -80-70 -60-50 -40-30 bg () 10 2 10 100 T(K) ~200 cm 2 /s Improved channel material quality imporved FE mobility

Bilayer WSe 2 with 2D/2D contacts

n-type WSe 2 FET enabled by 2D/2D contact Also enables the n-type WSe 2 FET n-doped-wse 2 to WSe 2 2 terminal FET H.-J. Chuang, et. al.,nano Lett. 2016

Device stability H.-J. Chuang, et. al.,nano Lett. 2016

Conductivity and FE hole mobility of MoS 2 down to 5 K P-doped-MoS 2 to MoS 2 2 terminal FET 2-terminal conductivity 2-termainl FE mobility H.-J. Chuang, et. al.,nano Lett. 2016

Hetero-contacts H.-J. Chuang, et. al.,nano Lett. 2016

Hetero-contacts H.-J. Chuang, et. al.,nano Lett. 2016

Hetero-contacts H.-J. Chuang, et. al.,nano Lett. 2016

Top-gate WSe 2 FETs with Nb-MoS 2 hetero-contacts 10 2 10 0 ds = -1-100 m - 10 m SS = 100 m/dec 1 0.5 bg = - 10 10-2 10-4 0-2 10-6 293 K -0.5 293 K 10-8 -12-10 -8-6 -4-2 0 tg () -1-0.1 0 0.1 () ds +Low threshold voltage + Ohmic behavior +Small subthreshold swing + High off ratio

Top gate vs back gate: WSe 2 FETs with Nb-MoS 2 hetero-contacts 140 120 100 80 BG GT-grounded TG BG-grounded RT ds = - 100 m Nb-MoS 2 WSe 2 60 40 20 0 FE = 120 cm 2-1 s -1 293 K -20-1.5-1 -0.5 0 g C g ( C cm -2 )

Summary Ionic liquid gating Highly doped graphene contacts 2D/2D contacts as a universal approach to high performance TMD transistors Low Contact resistance ~ 0.3kΩ µm High On/off ratio > 10 9 High drive current > 320 µa/µm High 2-terminal FE mobility > 6000 cm 2 /s at low T Outlook: 2D/2D hetero-contacts to other 2D semiconductors (band alignment consideration) Sequential growth of channel and contacts TFET

Acknowledgements Current and former students (Wayne) Hsun-Jen (Ben) Chuang Bhim Chamlagain Meeghagee Perera Ming-Wei Lin Xuebin Tan Senior collaborators most directly related to the work presented Mark Ming-Cheng Cheng, Wayne David Mandrus and his group, UTK and ORNL David Tomanek MSU

Effect Ionic Liquid without gate voltage 10 0 10-1 10-2 10-3 10-4 10-5 10-6 10-7 T= 293K ds = 0.1-80 -60-40 -20 0 20 40 60 80 bg () 10 0 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 T= 170 K ILg = floating ds = 0.1-80 -60-40 -20 0 20 40 60 80 () bg H-J Chung et al. Nano Lett. 2014

5 4 T= 77K Possible Metal Insulator transition Metalic ds = -10 m ILg = -7 sample I L~4.8 ; W=4.8 Electron side 3 h-bn on WSe2_12-02-13_No1-3-2_15 2 160K 1 ~1.1 ~-33 ~1.1 1.2 1 ds = -10 m ILg = -7 0-100 -50 0 50 100 () bg 0.8 0.6 0.4 Insulating phase T= 160K 0.2 77K 0-50 -40-30 -20-10 0 () bg

MIT sample I L~4.8 W=4.8 Hole side 7 6 ds = 10 m ILg = 6 T=77 K 5 4 3 160K 1 0.8 ds = 10 m ILg = 6 2 1 0.6 0.4 160K 0-100 -50 0 50 100 () bg 0.2 T=77 K 0-10 -5 0 5 10 15 20 () bg

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