Contact Resistance of
Graphene/Single- Walled Carbon Nanotube
Thin Film Transistor
Contact Resistance of
Graphene/Single- Walled Carbon Nanotube
Thin Film Transistor
Master’s Thesis Defense
Dec 15 2015
Houngkyung Kim(Advisor : Seokwoo Jeon)
Department of Materials Science and EngineeringKAIST Institute for the NanoCentury
KAIST1
Contents
I. Introduction
II. Experimental
III. Results and Discussion
IV. Summary and Further Works
2
Outstanding Physical Properties§ 1D: Carbon Nanotube (CNT) § 2D: Graphene
CNT Graphene
Electricalproperty
Hole mobility :790,000 cm2V−1s−1 @
RT 1)
Electron mobility :100,000 cm2V−1s−1 @ RT 2)
230,000 cm2V−1s−1 @ Low T 3)
Young’s Modulus 0.27 TPa to 1.47 TPa 4) 1.0 TPa 5)
Tensile Strength 3.6~63 GPa 4) 130 GPa 5)
Thermal Conductivity 3.50×103 Wm−1K−1 6) 5.30×103 Wm−1K−1 7)
Optical Property 2 % Absorption 8)
1) T. Kurkop et al., Nano Lett. (2004), 2) A. S. Mayorov et al., Nano Lett. (2011), 3) K. I. Bolotin et al., Solid State Commun. (2008), 4) B. G. Demczyket al., Mater. Sci. Eng. (2002), 5) C. Lee et al., Science (2008), 6) A.A. Balandin et al., Nano Lett. (2008), 7) E. Pop et al., Nano Lett. (2006), 8) R. R. Nair et al., Science (2008) 3
CNT and Graphene Applications
Flexible Electronics
Transparent Electronics
Super CapacitorThermoelectric Generator
Composite
Solar Cell 4
CNT Channel and Graphene Electrodes§ Semiconducting SWCNT Channel
§ Graphene Electrodes
Sub-10 nm CNT transistorFranklin A. D. et al., Nano Lett. (2012)
Integrated CMOS logic circuitsChen, Z., Science. (2006)
CNT computerShulaker M. M. et al., Nature (2013)
Graphene/PET-based touchscreenSukang Bae et al., Nat. Nanotech. (2010)
Flexible and Transparent organic thin-film transistorsLee W H et al., Adv. Mat. (2011) 5
All-Carbon Transistors§ All-CNT transistors
§ All-graphene transistors
§ CNT-graphene transistors
Limited Output Current
Limited ON/OFF Ratio
Q. Cao et al., Adv Mat. (2006) V. K. Sangwan et al., Micro. Eng. (2011) J. Lee et al., MRS Commun. (2012)
S. Lee et al., Nano Lett. (2011) S. Lee et al., Nano Lett. (2012) B. J. Kim et al., ACS Nano (2012)
S. Jang et al., Nanotech. (2010) W. J. Yu et al., Nano Lett. (2012)S. Hong et al., Adv. Mat. (2011) 6
Importance of Contact Resistance
AD Franklin et al., ACS Nano. (2014)
§ Total resistance is a series combination of channel resistance and the contact resistance.
§ Contact resistance is the major limiting factor of device performance
= +
7
Research Objective
Graphene
Semiconducting SWCNT Metallic SWCNT
Graphene
- Characterization of Contact Resistance
between Single Walled Carbon Nanotube and Graphene
- Gate Voltage and Contact Length effect on contact resistance
- Despite their importance, contact resistance of graphene/SWCNT has not been reported!
8
Schematic of Device Fabrication
Growth and TransferGraphene onto SiO2/Si
Growth and Transferof SWCNTs
Deposit Cr/Au pads9Anneal at 400 for 4 h PR mask and O2 plasma etching
Pattern graphene and O2 plasma etching
Growth and Transfer of Graphene
Xuesong Li et al., Science (2009)
v Chemical Vapor Deposition on Cu foil
§ OM image
Etch Cu and Transfer
Cu foil
Graphene/CuGraphene/SiO2/Si
10
Schematic of Device Fabrication
Growth and Transferof SWCNTs
Deposit Cr/Au pads11Anneal at 400 for 4 h
Growth and TransferGraphene onto SiO2/Si
PR mask and O2 plasma etching
Pattern graphene and O2 plasma etching
Growth of Aligned SWCNTsv Chemical Vapor
Deposition
SJ Kang et al., Nat. Nanotech. (2007)
Fe catalysts line
ST cut quartz wafer
Fe catalysts line
Aligned SWNTs array
§ SEM image
2μm300μm12metallic:semiconducting = 1:2
Schematic of Device Fabrication
Growth and Transferof SWCNTs
Deposit Cr/Au padsPR mask and O2 plasma etching 13Anneal at 400 for 4 h
Growth and TransferGraphene onto SiO2/Si
Pattern graphene and O2 plasma etching
Graphene/SWSNT Thin Film Transistors
10 μm
Gra
phen
e
SWN
Ts
Au
§ AFM
§ OM § SEM
- Density ~0.8 ± 0.2 SWCNTs/μm
- Avg. Dia. ~ 0.87 nm
14
S
D
Electrical Characteristics
- Ambipolar
- Schottky barrier, p-n like junction
- Monotonic decrease of mobility
- non-negligible role of contact resistance
- Linear response
- Diffusive transport
§ Output Curves § Transfer Curves
15
Characterization of Contact Resistance
, ,,
, ,,
DrainSource
§ Transfer Length Method (TLM)
§ Simple Equivalent Circuit Model
Xinning Ho et al, Nano Lett., 2010
- The slope : specific resistivity
- The y-axis intercept : contact resistance (2Rc)
- The x-axis intercept: transfer length (LT)
16
Contact Resistance of Graphene/SWCNT§ Metallic SWCNT § Semiconducting SWCNT
17
Material Graphene Palladium1) Gold1)
m-SWCNTResistivity (kΩ/µm) 34 24 30
2Rc (kΩ) 512 20 50LT (µm) 6.861 0.417 0.83
s-SWCNTResistivity (kΩ/µm) 56~92 25~45 60~95
2Rc (kΩ) 664~961 48~58 180~240LT (µm) 5.93 0.89 1.5
1) Xinning Ho et al, Nano Lett., 2010
18
Schottky Barrier of Graphene/SWCNT
§ hole branch
§ Electrostatic doping
h+ h+h+
v Effect of gate voltage on 2Rc
§ electron branch
Contact Resistance of Graphene/SWCNT§ Metallic SWCNT § Semiconducting SWCNT
19
Material Graphene Palladium1) Gold1)
m-SWCNTResistivity (kΩ/µm) 34 24 30
2Rc (kΩ) 512 20 50LT (µm) 6.861 0.417 0.83
s-SWCNTResistivity (kΩ/µm) 56~92 25~45 60~95
2Rc (kΩ) 664~961 48~58 180~240LT (µm) 5.93 0.89 1.5
1) Xinning Ho et al, Nano Lett., 2010
Effects of Contact Length Scaling
- On-state current decrease as contact length decrease
- Indicating increase of contact resistance
- Contact length should be carefully designed when we fabricate
graphene/SWCNTs transistors20
21
Contact Resistance Effect of Scaled Device§ Contour Plot for the ratio 2Rc/RT
0 4 8 12 16 200
50100150200250300
Lcontact (µm)
L cha
nnel
(µm
)
0.1
0.30.5
- The ratio 2Rc/RT dependence on the two scaled lengths, Lcontact and Lchannel
- According to the plot the portion of the contact resistance can be predicted.
- For high performance device the target must be carefully targeted
Target Area
Summary & Further Works
i) Contact resistance of graphene/SWCNTs have been investigated.
ii) The Schottky barrier between graphene and SWCNT leads to a p-n
like junction behavior.
iii) Contact resistance of graphene/s-SWCNT can be modulated by
electrostatic doping.
iv) The portion of contact resistance can estimated by knowing their
contact length and channel length.
§ Summary
§ Further worksContact engineering such as work function control and surface
engineering can be employed to reduce the contact resistance
22
감사합니다
23
24
34
68
1.0E+02
1.4E+02
1.7E+02
2.0E+02
2.4E+02
2.7E+02
3.1E+02
0 2 4 6 8 10 12 14 16 18 200
50
100
150
200
250
300
Y Ax
is Ti
tle
X Axis Title0 4 8 12 16 200
50100150200250300
20101998
Carbon nanotube transistor timeline
The First carbon nanotube field-effect transistorTans, S. et al., DelftNatureMartel, R. et al., IBMAppl. Phys. Lett.
2003
The 15-nm CNT transistorFranklin, A. D. et al., IBMNat. Nanotechnol.
2012
Sub-10 nm CNT transistorFranklin A. D. et al., IBMNano Lett.
2006
Sub 20-nm CNT transistorSeidel, R. V., Infineon Technologies Nano Lett.
Ballistic p-type CNT FETJavey A. et al., Stanford, PurdueNature. 2004
Integrated CN logic circuitsChen, Z., IBMScience.
2013
CNT computerShulaker M. M. et al., StanfordNature
Simple Equivalent Circuit Model
The analysis assumes that the resistance of m-SWNTs are independent of
= () + () = ()+ () = 1() = ,() + ()= 1() + 1() = 1,() + () = 1 = , + = 1,() + ()
1 = = 1() = 1 = = 1 ()
= ()Xinning Ho et al, Nano Lett., 2010
Support 2. Simple Equivalent Circuit model
= () + () = ()+
The analysis assumes that the resistance of m-SWNTs are independent of
() = 1() = ,() + () = 1() + 1() == 1,() + () = 1 = , + = 1,() + ()
1 = = 1() = 1 = = 1 ()
= ()
Xinning Ho et al, Nano Lett., 2010
2um10 μm
Graphene
SWNTs Array
, ,,
, ,,
Drain – 10 mV
Source = ℎ2 + 2,
28
29
30
Contact Length Scaling
-. Contact length
31
32
-30 -15 0 15 300
1
2
3
4
5
6
-30 -15 0 15 30
2
4
6
8
10
Vg (V)
-I d( µ
A)
Vg (V)
R (k
Ω)
Vds = -0.01 V
Graphene Device
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-1
0
1
30 V 20 V 10 V 0 V -10 V
33
Modulation-doped growth of mosaic graphene with single-crystalline p–n junctions for efficient photocurrent generation•Kai Yan, et al. Nat. Comm. (2012)
34
Support 3. SEM image
10 μm
§ Chemical doping of CVD graphene with nitric acid (63 wt% HNO3)
Contact Resistance after Doping
0 2 4 6 8 10 12 140
20
40
60
R (kΩ)
(μm)§ 30~50 % of resistance reduction within the range of channel
length investigatedà Linear fit of R indicates negligible contact resistance
CNT FETs Performance
Gate
DS
m
m
ss
ss
I on
gm
SJ Kang. et al., Nat. Nanotech., 2007
On
CVD SWNTs metallic: semiconducting = 1:2 → I on/off ratio: 3~5
I off
37
CNT FETs Performance
Gate
DS
m
m
ss
ss
I off
I on
gm
Off
SJ Kang. et al., Nat. Nanotech., 2007
CVD SWNTs, metallic: semiconducting = 1:2 → I on/off ratio: 3~5
I on/off ratio: 10~10 > 99.9% s-SWNTRemoval of m-SWNT is remained key issue 38
Electrical breakdown in air
§ High-power operation (1 ~ 90 )
§ Only remove the m-SWNT in isolated, narrow regions and uncontrolled regions
P.G. Collins et al., Science, 2001SJ Kang et al., Nat. Nanotech., 2007E Pop et al., Nanotech., 2007
39
G-SWNTs FET Characteristics
= = + 12 [ 2 ]High mobility ~ 1800 High on/off switching ratios of ~ .
-4 -2 0 2 4
10-12
10-10
10-8
10-6
10-4
(V)
− (A)
After electrical breakdown process
(L : Channel length, W : Channel width, D : Density of nanotubes,CQ : Quantum capacitance of nanotubes, : Dielectric constant)
= −
41-. Graphene is gapless semi-metal.ACH Neto et al., Rev. of Mod. Phys. (2009)
Chiralities of SWNTs
l Chiral vector: (m, n)
l m-n = 3k, metallic
l m-n != 3k, semiconducting
l As grown SWNTs (CVD)metallic: semiconducting = 1:2
l Chirality control of SWNT is key issue
42
Graphene Doping effect of H2O/O2 in Air
Redox potential of the electronIn this reaction : - 5.3eV
Lies under the graphene Fermi level.(~ 4.6eV)
§ Redox system
43
Raman Analysis : Graphene Properties and Layers
§ ID/IG ratio: indicator of the defect concentration
§ I2D/IG ratio: > ~2 single layer graphene
A. C. Ferrari et. al., PRL 2006, 97, 187401
G peakD peak
※ D peak can not be observed in perfect lattice structure.
44
SWCNTs
A. Ismach, et al., Angew. Chem. Int. Ed. 2004
Alignment on Al2O3 substrate
C. Kocabas, et al., Small 2005
Alignment on Quartz substrate
C. Kocabas, et al., J. Phys. Chem. 200745
46
C. Kocabas, et al., J. Phys. Chem. 200747
Fermi Level Engineering
Ki Kang Kim et al, JACS, 2008
à
§ Fermi levels of both graphene and SWNTs are shifted down as the increased work function
à Reduces the Schottky barrier height, resulting in lower
Graphene-CNT Contact Geometry
SWNTs on Graphene Graphene on SWNTs
Larger Contact Area?
50
Contact Geometry
Graphene
CNT
Metal
The Role of Metal-CNT Contact in CNT FET
§ Different metal contacts → different on-current
§ Schottky barrier (SB) height determine the contact property
§ For optimum CNT FET performance electrodes that show low contact resistance is desired
Chen Z. et al., Nano Lett. 2005
0
20
40
60
80
100
# of
SW
NT (%
)
Semiconducting Metallic
10 μm
10μm
Graphene-SWNTs Transistor
a) b)
c)d)
§ Graphene and SWNTs were grown by CVD
§ 100 nm SiO2/p++Si BackgateFET
§ PMMA assisted transfer§ Conventional photo
lithography process§ SWNT avg. dia = 088 nm§ s-SWNT:m-SWNT=2:1
4.7 4.8 4.9 5 5.110
0
102
104
106
2Rc
(kO
hms)
Work function [ eV ]-0.1 0 0.1 0.2 0.3 0.4
10-10
10-8
10-6
10-4
1/2R
c
Schottky Barrier [ eV ]
Contact Resistance Model
§ Schottky barrier (SB) height determine the contact property
§ Different graphene work fucntion → Rc ?
§ Better gco for graphene ?
Transfer Characteristics of G/SWNTs TFT
-30 -20 -10 0 10 20 30100
200
300
400
500
Vg (V)
I d(n
A)
Vds = -0.01 V
§ Transfer Curves :- Ambipolar characteristics- 2 separated peaks: p-n junction b/w graphene and SWNTs
§ Field effect mobility: = , = [ ]- Monotonic decrease of indicate non-negligible effects of contact
Lchannel (µm)
(cm2 V
-1s-1
)
2 4 6 8 10 12 14
1000
2000
3000
4000
5000