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