Carbon%20Nanotube%20Field-Effect%20Transistors:

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Carbon Nanotube Field-Effect Transistors An
Evaluation
D.L. Pulfrey, L.C. Castro, D.L. John
Department of Electrical and Computer
Engineering University of British
Columbia Vancouver, B.C. V6T1Z4,
Canada pulfrey_at_ece.ubc.ca
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Single-wall and multi-wall NANOTUBES
Compare flaxen hair - 20,000 nm
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CNT formation by catalytic CVD
2000nm
5?m islands in PMMA patterned by EBL
LPD of Fe/Mo/Al catalyst
Lift-off PMMA
No field
CVD from methane at 1000C
J.Kong et al., Nature, 395, 878, 1998
A. Ural et al., Appl. Phys. Lett., 81, 3464, 2002
Growth in field (1V/micron)
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Single-Walled Carbon Nanotube

• Hybridized carbon atom ? graphene monolayer ?
  carbon nanotube

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VECTOR NOTATION FOR NANOTUBES
Chiral tube
Adapted from Richard Martel
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E-EF (eV) vs. k (1/nm)
Eg/2
(5,0) semiconducting
(5,5) metallic
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Doping

• Substitutional unlikely

• Adsorbed possible
• e.g., K, O

Tubes are naturally intrinsic

• Interior possible

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Phonons

• Acoustic phonons (twistons) mfp ? 300 nm

• Optical phonons
• mfp ? 15 nm

Ballistic transport possible
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Fabricated Carbon Nanotube FETs

• Few prototypes
• Tans98 1st published device
• Wind02 Top-gated CNFET
• Rosenblatt02 Electrolyte-gated

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CLOSED COAXIAL NANOTUBE FET STRUCTURE
chirality (16,0) radius 0.62 nm bandgap 0.63
eV length 15 - 100 nm oxide thickness (RG-RT)
2 - 6 nm
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MODE CONSTRICTION and TRANSMISSION
Doubly degenerate lowest mode
T
CNT (few modes)
METAL (many modes)
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Quantum Capacitance Limit
gate
Cins
insulator
CQ
nanotube
source
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Quantum Capacitance and Sub-threshold Slope
High k dielectrics zirconia - 25 water - 80
70 mV/decade ! - Javey et al., Nature Materials,
1, 241, 2002
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AMBIPOLAR CONDUCTION
Experimental data M. Radosavljevic et al.,
arXiv cond-mat/0305570 v1
Vds - 0.4V Vgs -0.15 0.05 0.30
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Minimize the OFF Current
?S,D 3.9 eV Increasing ?G ? 3.0, 4.37 eV
?G 4.2 eV Increasing ?S,D ? 3.9, 4.2, 4.5 eV
ON/OFF ?103
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General non-equilibrium case
Non-equilib f(E)
Q(z,E)qf(E)g(E)
Solve Poisson iteratively
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CURRENT in 1-D SYSTEMS
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Quantized Conductance
In the low-temperature limit
Interfacial G even when transport is ballistic
in CNT
155 ?S for M2
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Measured Conductance
G ? 0.4 Gmax at 280K !!
A. Javey et al., Nature, 424, 654, 2003

• No tunneling barriers
• Low R contacts (Pd)

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Drain Saturation Current
If T1 Get BJT behaviour!
Zero-height Schottky barrier
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ON Current Measured and Possible
CQ limit
?S,D 3.9eV ?G 4.37eV
80 of QC limit!
Present world record Javey et al., Nature, 424,
654, 2003
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Predicted Drain Current
-ve
0
ve
VgsVds0.4V
70mA/?m !!
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Transconductance
Low VDS modulate for G High VDS modulate VGS
for gm
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Transconductance Measured and Possible
CQ limit
?S,D 3.9eV ?G 4.37eV
80 of QC limit!
Highest measured Rosenblatt et al. Nano. Lett.,
2, 869, 2002
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CNFET Logic
A.Javey et al., Nature Materials, 1, 241, 2002
Gain60
0,0
1st OR-gate
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Recognition-based assembly
CNTs Functionalized with DNA
Williams, Veenhuizen, de la Torre, Eritja and
Dekker Nature, 420, 761, 2002.
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Self-assembly of DNA-templated CNFETs K.Keren
et al., Technion.
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Self-assembly of DNA-templated CNFETs K.Keren
et al., Technion.
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CONCLUSIONS

• Schottky barriers play a crucial role in
  determining the drain current.
• Negative barrier devices enable
• control of ambipolarity,
• high ON/OFF ratios,
• near ultimate-limit S, G, ID, gm.
• CNFETs can be self-assembled via biological
  recognition.
• CNs have excellent thermal and mechanical
  properties.
• CNFETs deserve serious study as molecular
  transistors.

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Extra Slides
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Compelling Properties of Carbon Nanotubes

• Nanoscale
• Bandgap tunability
• Metals and semiconductors
• Ballistic transport
• Strong covalent bonding
• -- strength and stability of graphite
• -- reduced electromigration (high current
  operation)
• -- no surface states (less scattering,
  compatibility with many insulators)
• High thermal conductivity
• -- almost as high as diamond (dense circuits)
• Lets make transistors!

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CHIRAL NANOTUBES
Armchair
Zig-Zag
Chiral
From Dresselhaus, Dresselhaus Eklund. 1996
Science of Fullerenes and Carbon Nanotubes. San
Diego, Academic Press. Adapted from Richard
Martel.
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Carbon Nanotube Properties

• Graphene sheet 2D E(k//,k?)
• Quantization of transverse wavevectors
• k? (along tube circumference)
• ? Nanotube 1D E(k//)
• Nanotube 1D density-of-states derived from
  ?E(k//)/?k-1
• Get E(k//) vs. k(k//,k?) from Tight-Binding
  Approximation

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Density of States
k or kz
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Tight Binding
David John, UBC Wolfe et al., Physical
Properties of Semiconductors
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Density of States (5,0) tube
E(eV) vs. DOS (100/eV/nm)
E(eV) vs. k (1/nm)
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Tuning the Bandgap
T. Odom et al., Nature, 391, 62, 1998
Eg lt 0.1 eV for d gt 7 nm zero bandgap
semiconductor
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The Ideal Structure
nanotube
oxide
gate
Coaxial
Planar
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CNT formation by catalytic CVD
5?m islands in PMMA patterned by EBL
1000nm
LPD of Fe/Mo/Al catalyst
300nm
Lift-off PMMA
CVD from methane at 1000C
2000nm
J.Kong et al., Nature, 395, 878, 1998
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CNT formation by E-field assisted CVD
V applied between Mo electrodes. CVD from
catalytic islands.
No field
10V applied
A. Ural et al., Appl. Phys. Lett., 81, 3464, 2002
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Bottom-gated Nanotube FETs
1st CNFET S. Tans et al., Nature, 393, 49, 1998
Note very high ID 10mA/?m
A. Javey et al., Nature, 424, 654, 2003
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Phenomenological treatment of metal/nanotube
contacts
Evidence of work function-dependence of I-V
A. Javey et al., Nature, 424, 654, 2003
Zero hole barrier
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Schrödinger-Poisson Model

• Need full QM treatment to compute
• -- Q(z) within positive barrier regions
• -- Q in evanescent states (MIGS)
• -- S ? D tunneling
• -- resonance, coherence

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Schrödinger-Poisson Model
L.C. Castro, D.L. John
S
D
CNT
Unbounded plane waves
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Increasing the Drain Current
VgsVds0.4V
70mA/?m !!
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Array of vertically grown CNFETs W.B. Choi et
al., Appl. Phys. Lett., 79, 3696, 2001.
2x1011 CNTs/cm2 !!