Flexible SWNTs ThinFilm Transistors

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Flexible SWNTs Thin-Film Transistors
Seung-Hyun Hur, John A. Rogers
Mar 31, 2005
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Table of Content

• 1. Introduction
• - Motivation
• - Single-Walled Carbon Nanotubes
• - Soft Lithography
• 2. Fabrication of SWNTs Device by nano Transfer
  Printing
• - Electrodes fabrication
• - P/N/Ambipolar SWNTs TFT
• 3. Transfer of CVD SWNTs on plastic substrates
• - Transfer Results
• - Bending Folding Test
• 4. Conclusions

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1. Introduction
4
Flexible Electronics
PNAS 98(9), 4835 (2001) Science 291, 1502 (2001)
Outdoor/Indoor Advertising
Radio Frequency Identification Tag (RFID)
Electronic Papers
Smart Gloves
1. Motivation
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Electronic Devices on Mechanically Flexible
Materials

• Substrate Materials
• Transparency, stability, Flexibility, etc.
• PET, PEN

• Active Materials
• Mechanically Flexible Equivalent
  or better performances than Conventional
  materials(i.e. a-Si)
• Organic semiconductor, µs-Si
• SWNTs

• Processing
• Low Temperature processable,
  Curved Surface Patternable Low
  Cost, Large Area applicable Non-destructive to
  active materials
• Soft Lithography

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Organic Semiconducting Materials
Amorphous-Si 1cm2/Vs
Organic semiconductors 20 cm2/Vs Measured
from single crystal
Poly-Si 100 cm2/Vs gt expensive, Uniformity
gt Cheap, Flexible High mobility Sc
Materials !!!!
Brotherton et al., Semicon. Sci. Tech. 10,
721(1995)
3. SWNTs TFT
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Single Wall Carbon Nanotubes (SWNTs)

• SWNT Rollup from a single graphene sheet
• Discovered in 1991 by Iijima
• S. Iijima, Nature 354, 56 (1991)
• High mobility 10,000 100,000 cm2/Vs
• Fuhrer et al., Nano Letters 4(1), 35
  (2004)
• Flexibility 30 break elongation
• 510 intra-tube electronic
  scattering
• Bozovic et al., Phys. Rew. B 67,
  33407(2003)
• Metallic or semiconducting (12)
• Band Gap 0.9 eV / D (nm)

(n-m)/2
Integer
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Random Network SWNTs TFT
TFT Schemes
Fully aligned tube device
Random network device

• Ideal
• Device mobility close to single tube mobility

• Tube Density
• Film Uniformity

TFT by carbon nanotube network 270 cm2/Vs
(p-type, high OFF current for higher
mobilities)) Snow et al., APL 82 (13), 2145
(2003)
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SWNTs Growth

• CNT Growth
• CVD (Methane, CO, Alcohol)
• HiPCO
• Laser ablation

CVD Growth Catalyst Fe nanoparticles (Ferritin
precursor) Gas Methane Temperature 900
?C Wafer Doped Si wafer with 100 nm dry oxide
1?
10 ?
Tube length 5-10 ?m Tube height 1-3 nm Typical
density 1-30 tubes/?m2
SEM / AFM Image of SWNTs on SiO2 wafer
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Device Mobility and On/off ratio as Tube Density
W 250?, L 100? SiO2 100nm
Metallic(1)
Semiconducting(2)
High Density
Low Density
20x 1hr 28 tubes / ?m2
40x 10 min 6 tubes / ?m2
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Optimization of Growing Conditions
Temperature
Feeding Gas
Temp. 900 ? CH4 500 sccm H2 75 sccm
W 250 ?, L 25? 20X 10min growth
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Soft Lithography
Soft Lithography
Nano Transfer Printing(nTP)
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Comparison
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2. Fabrication of SWNTs Device by nano Transfer
Printing
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Contact Printing of Electrode for Flexibe
Electronics
Cold welding C.Kim et al. Science(2000) Princeton
Cathode transfer J. Rhee et al. APL(2002) SNU
nTP Y. Loo et al. JACS(2002) Bell/UIUC
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Fabrication of SWNTs Device by nano Transfer
Printing
PDMS
S. H. Hur et al., Appl. Phys. Lett. 85,
5730(2004) S. H. Hur et al., J. of Appl. Phys.
submitted(2005)
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Device Performance Comparison with conventional
Methods
(a)
(b)
W 250?, L 100? SiO2 100nm
PR Shipley 1805 S/D Ti(2 nm) / Au(20 nm)
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Contact Resistance and Threshold Voltage Shift
Vgs -16V
-14V
-12V
-6V
Rc
Rch
Intrinsic ?device 22 cm2/Vs
Rc,on 20 40 ?cm
3. SWNTs TFT
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Threshold Voltage Shift and Mobility change
Sum of total current From three paths
Mobility is same Vt is different
Short channel ScSgs 21
Simplified long channel All paths act like Sc
Long channel ScSgs 21
3. SWNTs TFT
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P/N SWNTs TFT Oxygen Effect
V. Derycke et al., Nano Lett., 1, 453 (2001).
Vac-1 1 day annealing at 10-5 torr, room
temperature. Vac-2 1 day annealing at 10-5
torr, 150 C Air Open to ambient
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P/n/ambipolar with polymer coating
(a)
(b)
PEI coating (Polyethyleneimine)
(d)
(c)
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CMOS Inverter
(a)
(b)
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3. Transfer of CVD SWNTs on plastic substrates
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CVD SWNTs transfer on plastic substrates
(a)
100?m
50?m
(f)
1?m
1?m
S. H. Hur et al., Appl. Phys. Lett. 86,
243502(2005)
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Flexible SWNTs Thin-film Transistors
a)
b)
S
Epoxy
ITO
1 cm
PET
(c)
d)
Before SiO2(100nm)
After Epoxy(1.7 ?)
W 250?, L 100? Epoxy 1.7 ?
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Bending Test
a)
b)
1 cm
c)
d)
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Sharp Folding Test
a)
b)
50?m
1?m
Ti/Au
200?m
c)
Tubes transferred on 25? PET film
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Multi Transfer
2 times
1 time
1 ?m
1 ?m
3 times
Printing
2 ?m SU 8 on ITO/glass
Metal etching
1 ?m
By the Screening from 1st layer Only off current
increased
4. Flexible SWNTs TFT
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Aligning of SWNTs on Quartz
C. Kocabas et al., Nano letters, submitted(2005)
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Transfer of Aligned tubes
Transferred aligned SWNTs on 1.7 ?m SU 8

• device 6 cm2/Vs
• L 3.5 ?m

Transferred aligned SWNTs on 700nm SU 8

• device 1 cm2/Vs
• L 2 ?m

D
S
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4. Conclusions
? SWNTs TFT fabricated by nTP showed equivalent
performance compared with metals patterned
in conventional ways ? By simple polymer coating,
p/n/ambipolar SWNTs TFT and CMOS inverter can
be fabricated. ? By using metal layer, various
CVD SWNTs can be transferred onto plastic
substrates. ? Flexible SWNTs TFT showed good
bending performances, and even at sharp
folding, it was still active.
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Acknowledgements
Prof. John Rogers Coskun Kocabas Anshu
Gaur Dahl-Young Khang SWNT members
Funding Darpa, DOE, NSF