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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 [email protected] S.Iijima, Nature 354 (1991) 56.

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Carbon Nanotube Field-Effect Transistors: An Evaluation

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Carbon nanotube field effect transistors an evaluation

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

[email protected]


Carbon nanotube field effect transistors an evaluation

S.Iijima, Nature 354 (1991) 56

Single-wall and multi-wall NANOTUBES

Compare: flaxen hair - 20,000 nm


Carbon nanotube field effect transistors an evaluation

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)


Single walled carbon nanotube

2p orbital, 1e-(-bonds)

Single-Walled Carbon Nanotube

Hybridized carbon atom  graphene monolayer  carbon nanotube


Carbon nanotube field effect transistors an evaluation

Structure (n,m):

(5,2) Tube

VECTOR NOTATION FOR NANOTUBES

Chiral tube

Adapted from Richard Martel


Carbon nanotube field effect transistors an evaluation

E-EF (eV) vs. k|| (1/nm)

Eg/2

(5,0) semiconducting

(5,5) metallic


Carbon nanotube field effect transistors an evaluation

Doping

  • Substitutional unlikely

  • Adsorbed possible

  • e.g., K, O

Tubes are naturally intrinsic

  • Interior possible


Carbon nanotube field effect transistors an evaluation

Phonons

  • Acoustic phonons (twistons) mfp  300 nm

  • Optical phonons

  • mfp  15 nm

Ballistic

transport

possible


Fabricated carbon nanotube fets

Nanotube

Fabricated Carbon Nanotube FETs

  • Few prototypes

    • [Tans98]: 1st published device

    • [Wind02]: Top-gated CNFET

    • [Rosenblatt02]: Electrolyte-gated


Carbon nanotube field effect transistors an evaluation

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


Carbon nanotube field effect transistors an evaluation

E

kz

kx

kx

MODE CONSTRICTION

and

TRANSMISSION

Doubly degenerate lowest mode

T

CNT (few modes)

METAL (many modes)


Carbon nanotube field effect transistors an evaluation

Eb

Quantum Capacitance Limit

gate

Cins

insulator

CQ

nanotube

source


Carbon nanotube field effect transistors an evaluation

Quantum Capacitance and Sub-threshold Slope

High k dielectrics:

zirconia - 25

water - 80

70 mV/decade !

- Javey et al., Nature Materials, 1, 241, 2002


Carbon nanotube field effect transistors an evaluation

AMBIPOLAR CONDUCTION

Experimental data:

M. Radosavljevic et al.,

arXiv: cond-mat/0305570 v1

Vds= - 0.4V

Vgs=

-0.15

+0.05

+0.30


Carbon nanotube field effect transistors an evaluation

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


General non equilibrium case

E

E

1D DOS

E

EFS

g(E)

EFD

0.5

f(E)

f(E)

0.5

General non-equilibrium case

Non-equilib f(E)

Q(z,E)=qf(E)g(E)

Solve Poisson iteratively


Carbon nanotube field effect transistors an evaluation

CURRENT in 1-D SYSTEMS


Carbon nanotube field effect transistors an evaluation

Quantized Conductance

In the low-temperature limit:

Interfacial G: even when transport is ballistic in CNT

155 S for M=2


Carbon nanotube field effect transistors an evaluation

Measured Conductance

G  0.4 Gmax

at 280K !!

A. Javey et al., Nature, 424, 654, 2003

  • No tunneling barriers

  • Low R contacts (Pd)


Carbon nanotube field effect transistors an evaluation

Drain Saturation Current

VGS

Eb

EF

If T=1

Get BJT behaviour!

Zero-height Schottky barrier


Carbon nanotube field effect transistors an evaluation

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


Carbon nanotube field effect transistors an evaluation

Predicted Drain Current

-ve

0

+ve

Vgs=Vds=0.4V

70mA/m !!


Carbon nanotube field effect transistors an evaluation

Transconductance

Low VDS: modulate for G

High VDS: modulate VGS for gm


Carbon nanotube field effect transistors an evaluation

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


Carbon nanotube field effect transistors an evaluation

CNFET Logic

A.Javey et al., Nature Materials, 1, 241, 2002

Gain=60

0,0

1st OR-gate


Carbon nanotube field effect transistors an evaluation

Recognition-based assembly

CNTs Functionalized with DNA

Williams, Veenhuizen, de la Torre, Eritja and Dekker Nature,420, 761, 2002.


Carbon nanotube field effect transistors an evaluation

Self-assembly of DNA-templated CNFETsK.Keren et al., Technion.


Carbon nanotube field effect transistors an evaluation

Self-assembly of DNA-templated CNFETsK.Keren et al., Technion.


Carbon nanotube field effect transistors an evaluation

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.


Carbon nanotube field effect transistors an evaluation

Extra Slides


Carbon nanotube field effect transistors an evaluation

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)

  • Let’s make transistors!


Carbon nanotube field effect transistors an evaluation

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.


Carbon nanotube properties

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


Carbon nanotube field effect transistors an evaluation

Density of States

k|| or kz


Carbon nanotube field effect transistors an evaluation

Tight Binding

David John, UBC

Wolfe et al., “Physical Properties of Semiconductors”


Carbon nanotube field effect transistors an evaluation

David John

Density of States

(5,0) tube

E(eV) vs. DOS (100/eV/nm)

E(eV) vs. k|| (1/nm)


Carbon nanotube field effect transistors an evaluation

Tuning the Bandgap

T. Odom et al., Nature, 391, 62, 1998

Eg < 0.1 eV for d > 7 nm

“zero bandgap” semiconductor


Carbon nanotube field effect transistors an evaluation

The Ideal Structure

nanotube

oxide

gate

Coaxial

Planar


Carbon nanotube field effect transistors an evaluation

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


Carbon nanotube field effect transistors an evaluation

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


Carbon nanotube field effect transistors an evaluation

Nanotube

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


Carbon nanotube field effect transistors an evaluation

Phenomenological treatment of metal/nanotube contacts

Evidence of work function-dependence of I-V: A. Javey et al., Nature, 424, 654, 2003

Zero holebarrier


Schr dinger poisson model

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


Schr dinger poisson model1

Schrödinger-Poisson Model

L.C. Castro,

D.L. John

S

CNT

D

Unbounded plane waves


Carbon nanotube field effect transistors an evaluation

Increasing the Drain Current

Vgs=Vds=0.4V

70mA/m !!


Carbon nanotube field effect transistors an evaluation

Array of vertically grown CNFETs

W.B. Choi et al., Appl. Phys. Lett., 79, 3696, 2001.

2x1011 CNTs/cm2 !!


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