contact engineering of two-dimensional layered...

Zhixian Zhou Department of Physics and Astronomy

Wayne State UniversityDetroit, Michigan

Contact Engineering of Two-Dimensional Layered Semiconductors beyond Graphene

9/26/2016

Outline

Introduction

Ionic liquid gated few-layer MoS2 FETs

WSe2 FETs with highly doped graphene contacts

2D metal contacted WSe2 FETs

2D/2D semiconductor homo- and hetero-junctions as a new contact paradigm

9/26/2016

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?

Motivation

Bandgap 1-2 eVUntra-thin and uniform channelSurface smoothnessMechanically flexible and strongThermally stableReasonablly good mobility

TMDTransition Metal Dichalcogenides

MX2

Metal M = Mo, W, Ti Chalcogenide ( X = S, Se, Te) MoS2, MoSe2, WSe2

MoS2

Motivation

4-terminal FE mobility of WSe2 ~ 500 cm2V-1s-1 at RT; 2-T FE mobility ~ 100 cm2V-1s-1

Early works of TMD FETs

MoS2 FE mobility ~ 1 cm2V-1s-1

MoS2 FE mobility 10-50 cm2V-1s-1

on/off ratio ~ 105

Monolayer MoS2 High on/off ratio:108

S = 74 mV/dec High mobility

Kis et al. 2011 Nature Nanotech

Bare device

With top HfO2 and floating top-gate

• In crease of nominal μ by ~X1000coupling between the back-gate and floating top-gate dielectric screening Contact resistance reduction (Schottky barrier

reduction by n-doping by HfO2)• Threshold voltage negative shift (possible n-doping by

HfO2)

Actual μ = 2- 7cm2V-1s-1

Fuhrer and Hone,2013 Nature Nanotech,8, 146 (2013)

Impact of contacts in early MoS2 FET devices

Schottky Barrier at Metal/Semiconductor interface

Simple Schottky-Mott Model

Fermi level pinning

Electrical contacts to 2D semiconductors

9/26/2016 9

Metal

S D

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)

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/MoS2 contact

9/26/2016

How do ionic-liquid-gated MoS2 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

9/26/2016 12M.M. Perera et al. ACS Nano, 7, 4449, (2013)

Transfer characteristics of two IL-gated MoS2 FETs

9/26/2016 13

Bilayer Trilayer

AmbipolarBehavior

Yes Yes

Holes On/Offratio

106 104

Electron On/Off ratio

> 107 > 107

M.M. Perera et al. ACS Nano, 7, 4449, (2013)

Output characteristics of a trilayer MoS2 device with IL-gate and back-gate without IL

9/26/2016 14

( 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


Back-gate transfer characteristics with frozen IL

9/26/2016 15

77 K< T< 180 K IL is frozen

•Ids-Vbg between 77 and 180K, after the device had been quickly cooled from 250 K to 77 K at a fixed VILg


Field-effect mobility as a function of temperature W and W/O ionic liquid

9/26/2016 16M.M. Perera et al. ACS Nano, 7, 4449, (2013)

7 X 1012 < n < 9 X 1012

Making good contacts to WSe2• Approach 2• graphene as a work‐function‐tunable electrode material

• extremely‐large‐capacitance ionic liquid gate to tune graphene work function at the graphene/WSe2 contacts

• low resistance Ohmic contacts for both electrons and holes

• WSe2 is protected by hBN

9/26/2016

Nano Lett. 9, 3430 (2009)

SiO2Si

WSe2

h-BN

CVD graphene

Au/Ti electrode

Oxygen Plasma Etching

DS

ILg

Device Fabrication

Ionic Liquid

Au Graphene Au

DrainSource

h-BN

Si Back Gate

Graphene

WSe2 Ti

SiO2 Si

Au

4

6

8

10

12

14

-60 -40 -20 0 20 40 60

Vds

=10mV

Vbg

(V)

VILg

= 0V

VILg

= 2V

VILg

= 6V

Ionic Liquid

+ +

‐ ‐‐

‐ ‐ ‐ ‐ ‐ ‐‐ ‐

‐‐

‐‐‐

‐‐ ‐

‐

‐

+++ + + +++ ++ + + ++ ++ + ++

++++ +

Higher ILg

4

6

8

10

12

14

-60 -40 -20 0 20 40 60

VILg

= - 6V

VILg

= 0V

Vds

= -10mV

Vbg

(V)

Higher ILg

Mobile ions freeze below 180 K

Graphene Graphene

H-J Chung et al. Nano Lett. 2014

Highly doping graphene contacts by IL-gating

Graphene

Graphene

WSe2h-BN

Drain

Source

Device image

Graphene Graphene

h‐BN

EF EF

Ionic Liquid Gating On Graphene

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1V

ds(V)

Vbg

= 60VT = 293KV

ILg= NA

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1

Vbg

= 60V

Vds

(V)

T = 77KV

ILg= 6V

0

0.5

1

1.5

2

2.5

3

-40 -20 0 20 40 60V

bg(V)

Vds

= 100 mV

VILg

= 6 V

4 V

0V

T=170K

Electron side

Without Ionic liquid

Under positive ionic-liquid gate voltages

Graphene Graphene

h‐BN

EF EF

Iionic Liquid Gating On Graphene

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0

T = 293KVbg

= -60V

Vds

(V)

VILg

= NA

-30

-25

-20

-15

-10

-5

0

-1 -0.8 -0.6 -0.4 -0.2 0V

ds(V)

Vbg

= -60V T = 77K

VILg

= -7V

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-80 -60 -40 -20 0 20 40 60 80

Vds

= -10 mVT=180K

Vbg

(V)

VILg

= - 7 V

-6 V

Hole side

Without Ionic liquid

Under negative ionic-liquid gate voltages

dsbgbg

ds

VCdVdI

WL 1

0

100

200

300

400

500

600

-60 -40 -20 0 20 40 60

T= 170 K

VILg

= 0V

No ILg

Vds

= 0.1V

Vbg

(V)

Graphene

0

0.2

0.4

0.6

-80 -40 0 40 80V

bg(V)

77 K

180 K

VILg

= 6 V

Vds

= 10 mV

0

0.1

0.2

0.3

0.4

-100 -50 0 50V

bg(V)

Vds

= -10 mV

VILg

= - 7 V

T=77 K

160 K

120 K

Hole side Electron side


Where Cbg is determined to be 1.2 ×10-8 F cm-2

for 290nm SiO2 based on the parallel capacitormodel (Cbg = 3.9ε0 / 290 nm)

Dimensions of Samples :Sample I : d= 6.0 nm L=6.8 µm W=4.8 µm

Temperature dependent transfer characteristics

0

100

200

300

400

80 120 160 200

VILg

= 6 VV

ILg = -7 V

T(K)Dimensions of Sample :d= 6.0 nm L=4.8 µm W=4.8 µm


2-terminal electron and hole FE mobilities

Graphene Graphene

h‐BN

EF EF

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

-1 -0.5 0 0.5 1

10 V

Vbg

= 30V

Vds

(V)

T=170K

20V

Ideality factor~1.3

0

0.5

1

1.5

2

2.5

3

3.5

4

-0.8 -0.4 0 0.4 0.8

20 V

Vds

(V)

Vbg

= 30VT=170K

10 V

WSe2 diode with asymmetric graphene contacts

GrapheneDrain Source

h-BN

Si Back Gate

Graphene

BV for n doping

Surface Charge Transfer doping

4

6

8

10

12

14

16

18

20

-15 -10 -5 0 5 10 15

-5 1012 0 5 1012Carrier density (cm-2)

Vbg

(V)

Vds

(V)= 10mVT=295K

F4-TCNQ

BV

Non doped

F4-TCNQ for p doping

Strong Electron Donor

Strong Electron acceptor

• Device Performance : On/Off ratio > 107

10-8

10-6

10-4

10-2

100

102

-25 -20 -15 -10 -5 0 5 10

2 probe F4-TCNQ doping

T=294K

Vbg

(V)

- 0.1V

Vds

(V)= -1V

10-8

10-6

10-4

10-2

100

102

-10 -5 0 5 10 15 20 25

2 probe BV doping

T=294K

0.1V

Vds

(V)= 1V

Vbg

(V)

-12

-9

-6

-3

0

-1.2 -0.9 -0.6 -0.3 0

-12V

Vbg

=-22V

Vds

(V)0

2

4

6

8

0 0.3 0.6 0.9 1.2V

ds(V)

Vbg

=25V

7V

• Linear IV characteristics near Ohmic contacts

WSe2 with BV and F4-TCNQ doped graphene contacts

RT hole FE mobility: 258 cm2/VsRT electron FE mobility: 46.5cm2/Vs

• mh ~ 0.3 +/- 0.2 m0• me ~ 0.9 m0Klein, A., et al. Solar materials and solar cells, 1997energy

0

200

400

600

800

1000

150 200 250 300 350T(K)

F4-TCNQ

BV

4 probe measurement

101

102

103

200 300T(K)

Mob

ility

(cm

2 /Vs)γTMobility

Drain

SourceV2V3

5.1~

2~

4 Pr

obe Electron and hole FE mobility in WSe2

Partial List of Different Contact strategies by 2015

• Low/high work function metals Muiltilayer-MoS2/Scandium (Appenzeller et al. Nano Lett. 2012 ) WSe2/In, Ag (also d-orbital hybridization) (Jene, Banerjee et al. Nano Lett. 2013 ) Pt under WSe2 (reduced FLP) (Sanjay Banerjee et al.)

• Doping Surface doping of WSe2 and MoS2 using NO2, K, BV, and TiO2-x

(Javey et al. Nano Lett. 2012, Nano Lett. 2013, JACS 2014; S. Banerjee et al. Nano Lett. 2015 )

Body doping of MoS2 and WS2 with Cl and Nb(Ye et al. Nano Lett. 2014; Wu et al. Nano Lett. 2014)

• Graphene contacts MoS2/graphene ( Duan et al. Nano Lett. 2015; Kim and Hone et al. Nature Nano 2015) MoS2/Ni-graphene (T.L. Thong et al. ACS Nano 2014) WSe2/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?

WSe2 with NbSe2 metallic 2D contacts

Perspective view

Side view

Au/Ti

Graphite

SiO2

NbSe2h-BN

WSe2 h-BN

+ suppressed interface states

+ reduced Fermi level pinning

WSe2 with NbSe2 metallic 2D contacts

9/26/2016

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

Continued Search for New Contact Approaches

MOSFET Working Principle

P‐type substrate

N+ N+SiO2

GateSource Drain

++++++++- - - - - - - -++++++++

VG

VS VD

Ion implantationIon implantation

Silicon-Based electronics

Degenerately p-doped WSe2 (Nb0.005W0.995Se2) with Ti/Au metal contacts

18 nm thick

• Low Rc

• T-independent

• Large drive current

• Stable

Not suitable as channel materials• Low mobility• Low gate tunability (high off current and low on/off)

Degenerately p-doped WSe2 (Nb0.005W0.995Se2) with Ti/Au metal contacts

n-Si

n+ Simetal

MOSFETs Contact

P‐type substrate

N+ N+

Source

- - - - - - - -

VSDrain

SiO2++++++++

++++++++

GateVG

VD

Low Resistance ContactsWere enabled by Ion implantation

at contact regions only( not the channel)

• Ultrathin body of monolayer and few- layer TMDs prohibits effective (Local) doping by ion implantation

However

Silicon-Based electronics

What if we fabricate the channel and highly doped drain/source contacts seperately, and assemble them together?

New Contact Strategy

WSe2 Nb0.005W0.995Se2

Nb0.005W0.995Se2

+WSe2

Substitutional doping

(Highly Doped WSe2 as Contact)

(undoped WSe2 as channel)

Layered Materials: van der Waal Assembling

Geim and Grigorieva, Nature, 499, 419 (2013)

SiO2

Si Wafer

h-BN

h-BNTMDs

Au/Ti Au/Ti

TMD FET with 2D/2D contacts

2D/2DContacts

2D/2D Contacts

Optical image of a WSe2 FET with 2D/2D contacts

How do we do it ?

Dry transfer method !!!

Target (hBN on SiO2/Si wafer)

Optical MicroscopeMicro-manipulator Optical

Microscope

Target (hBN on SiO2/Si wafer)

Micro-manipulator

TMD hBNDoped TMD

hBN

Doped TMD

Glass slidePDMSTMD

Dry Transfer method

10um

Contact Mechanism

H.-J. Chuang, et. al.,Nano Lett. 2016

10um


P‐type WSe2 transistorswith degeneratley p‐doped WSe2 as contacts

Transfer and output characteristics of WSe2 FETs with 2D/2D contacts

3.5 nm thick WSe2 channel

Contact Resistance: Transfer Length Method

High drive current > 300uA/umMetal (Au/Ti) to NbWSe2 to WSe2


WSe2 channle with digenerately p-doped WSe2 contacts

Low-resistnace 2D/2D contacts enable the investigation of channel properties

2-terminal conductivity 2-termainl FE mobility

Conductivity and FE hole mobility of WSe2down to 5 K


WSe2 Hall Bar with Nb-WSe2 contacts

T4R

T3L

7.9 nmWSe2

Nb-WSe2

Nb-WSe2

102

103

104

10 100T(K)

~6600 cm2 /Vs

Improvement of 2-terminal FE mobility with improved Channle material quality

Improved channel material quality imporved FE mobility


WSe2 channle with digenerately p-doped WSe2 contacts

0

500

1000

1500

2000

2500

-90 -80 -70 -60 -50 -40 -30

Vbg

(V)

300K150K

50K

20K

10KV

ds(V)= -100mV

~200 cm2 /Vs

Bilayer WSe2 with 2D/2D contacts

n-type WSe2 FET enabled by 2D/2D contact

Also enables the n-type WSe2 FET

n-doped-WSe2 to WSe2 2 terminal FET


Device stability


Conductivity and FE hole mobility of MoS2 down to 5 K


P-doped-MoS2 to MoS2 2 terminal FET


Hetero-contacts


Top-gate WSe2 FETs with Nb-MoS2 hetero-contacts

10-8

10-6

10-4

10-2

100

102

-12 -10 -8 -6 -4 -2 0Vtg(V)

SS = 100 mV/decVds = -1 V

- 100 mV

- 10 mV

293 K

-1

-0.5

0

0.5

1

-0.1 0 0.1Vds(V)

Vbg = - 10 V

- 2 V

293 K

+Low threshold voltage + Ohmic behavior +Small subthreshold swing + High off ratio

Top gate vs back gate: WSe2 FETs with Nb-MoS2hetero-contacts

-20

0

20

40

60

80

100

120

140

-1.5 -1 -0.5 0

BG GT-groundedTG BG-grounded

VgCg(C cm-2)

RT Vds = - 100 mV

293 K

FE = 120 cm2V-1s-1

Nb-MoS2 WSe2

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 > 109

High drive current > 320 µA/µm High 2-terminal FE mobility > 6000 cm2/Vs at low T

Outlook:• 2D/2D hetero-contacts to other 2D semiconductors

(band alignment consideration)• Sequential growth of channel and contacts• TFET

AcknowledgementsCurrent and former students (Wayne)

Hsun-Jen (Ben)Chuang Xuebin TanMing-Wei Lin

MeeghageePerera

Bhim Chamlagain

Senior collaborators most directly related to the work presented

Mark Ming-Cheng Cheng, Wayne

David Mandrusand his group, UTKand ORNL

David TomanekMSU

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

-80 -60 -40 -20 0 20 40 60 80V

bg(V)

T= 293K

Vds

= 0.1V

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

-80 -60 -40 -20 0 20 40 60 80V

bg(V)

Vds

= 0.1V

VILg

= floating

T= 170 K


Effect Ionic Liquid without gate voltage

0

1

2

3

4

5

-100 -50 0 50 100

160K

T= 77K

~1.1~-33V

Vds

= -10 mV

VILg

= -7V

Vbg

(V)

Metalic

~1.1

0

0.2

0.4

0.6

0.8

1

1.2

-50 -40 -30 -20 -10 0

T= 160K

77K

Vds

= -10 mV

VILg

= -7V

Vbg

(V)

Insulating phase

sample I L~4.8 ; W=4.8Electron side

h-BN on WSe2_12-02-13_No1-3-2_15

Possible Metal Insulator transition

0

0.2

0.4

0.6

0.8

1

-10 -5 0 5 10 15 20

Vds

= 10 mV

VILg

= 6V

T=77 K

Vbg

(V)Vbg

(V)

160K

0

1

2

3

4

5

6

7

-100 -50 0 50 100

Vds

= 10 mV

VILg

= 6V

160K

T=77 K

Vbg

(V)

MIT sample IL~4.8W=4.8Hole side

contact engineering of two-dimensional layered...

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