contact engineering of two-dimensional layered...
TRANSCRIPT
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
M.M. Perera et al. ACS Nano, 7, 4449, (2013)
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
M.M. Perera et al. ACS Nano, 7, 4449, (2013)
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
H-J Chung et al. Nano Lett. 2014
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
H-J Chung et al. Nano Lett. 2014
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
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
H.-J. Chuang, et. al.,Nano Lett. 2016
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
H.-J. Chuang, et. al.,Nano Lett. 2016
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
H.-J. Chuang, et. al.,Nano Lett. 2016
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
2-terminal conductivity 2-termainl 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
H.-J. Chuang, et. al.,Nano Lett. 2016
Device stability
H.-J. Chuang, et. al.,Nano Lett. 2016
Conductivity and FE hole mobility of MoS2 down to 5 K
2-terminal conductivity 2-termainl FE mobility
P-doped-MoS2 to MoS2 2 terminal FET
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 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
H-J Chung et al. Nano Lett. 2014
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