Analysis of Strain Effect in Ballistic Carbon Nanotube FETs

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Analysis of Strain Effect in Ballistic Carbon
Nanotube FETs
Nov. 30, 2006
Youngki Yoon
Dept. of Electrical Computer Engineering Univers
ity of Florida
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Outline

• Carbon nanotube field-effect transistor
• Uniaxial strain on CNTs
• Material properties of strained CNTs
• Strain effect on Eg
• Strain effect on band-structure-limited velocity
• Simulated device structure approach
• Simulation results
• I-V characteristics
• Strain effect on Imin
• Strain effect on Ion
• Strain effect on intrinsic delay
• Concluding remarks

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What is CNTFET?
G
D
S
CNTFET
Conventional MOSFET
CNTFET with metal source drain contacts
CNTFET with doped source drain extentions
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Why strained CNTs?
J. Cao et al., PRL (2003)
(a) Tensile uniaxial strain and (b) compressive
uniaxial strain on the channel of a CNTFET.

• Conductance is change by several orders of
  magnitude
• Sentitivity change is available.

T. Tombler et al., Nature (2000)
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Lets apply uniaxial strain!
(16,0) CNT

• Band gap is increased (Egh0.33eV to 0.44eV).
• Slope (band-structrue-limited velocity) is
  decreased.

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Strain effect on CNTs (Variation of Eg and
band-structure-limited velocity)

• Tensile strain
• Eg of (16,0) CNT ? (n3q1 group)
• Eg of (17,0) CNT ? (n3q2 group)
• Compressive strain
• Eg of (16,0) CNT ? (n3q1 group)
• Eg of (17,0) CNT ? (n3q2 group)

Eg vs. uniaxial strain strength
Band-structure-limited velocity

• Tensile strain
• B.S.L. vel. of (16,0) CNT ? (n3q1 group)
• B.S.L. vel. of of (17,0) CNT ? (n3q2 group)
• Compressive strain
• B.S.L. vel. of of (16,0) CNT ? (n3q1 group)
• B.S.L. vel. of of (17,0) CNT ? (n3q2 group)

The lowest subbands of (16,0) CNTs. Solid
lines unstrained (16,0) CNT. Dashed lines 2
strained CNT.
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Device structure approach

• Device Structure
• Coaxially gated Schottky Barrier CNTFET (
  )
• 3nm HfO2 gate oxide with a dielectric constant of
  16
• 40nm strained (16,0) and (17,0) CNT channel
• 0.4V power supply
• Approach
• Self-consistent NEGF formalism with Poisson
  equation
• Mode space approach

Device structure
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Mode space approach
Real space approach
A part of (n,0) zigzag nanotube lattice in real
space
Mode space approach
(n,0) ZNT is decoupled into n one-dimensional
mode space lattice.
Mode space lattice
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ID-VG characteristics
(16,0) CNTFET w/ uniaxial strain
(17,0) CNTFET w/ uniaxial strain

• Device characteristics strongly depend on the
  band gap of the channel material.
• ID-VG characteristics change significantly with
  even a small strain.

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Strain effect on Imin

• Main figure
• Solid line (16,0) CNTFET
• Dashed line (17,0) CNTFET

• Subset band profile vs. channl position at
  VG0.2V
• Solid line unstrained (16,0) CNTFET
• Dashed line 2 strained (16,0) CNTFET

• Imin minimum current delivered (VG0.2V)
• A simple estimation for Imin

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Strain effect on Ion
Ion
Ioff
VDD0.4V

• Solid line (16,0) CNTFET
• Dashed line (17,0) CNTFET

(16,0) CNTFET w/ uniaxial strain

• Ion current at VGVonVoffVDD ,
• where Voff is the voltage at Ioff10-7A.

unstrained
2
unstrained
2 uniaxial
(16,0) CNTFET at on-state
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Strain effect on intrinsic delay
(16,0) CNTFET w/ uniaxial strain
(17,0) CNTFET w/ uniaxial strain
0
2
0
2
Lowest conduction band of (16,0) CNT
Ec vs. X for the same Ion/Ioff
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Summary

• Two important material property changes after
  applying uniaxial strains
• Eg
• Band-structure-limited velocity
• Nominal device approach
• Coaxially gated CNT SBFET with half band gap SB
  height
• Self-consistent NEGF with Poissons eq.
• Mode space approach
• Results
• I-V characteristics are changed a lot with even a
  small strain strength.
• Imin , Ion , and intrinsic delay are affected by
  Eg and B.S.L velocity changes.
• Strain engineering can be effectively used to
  tune up the device performance, but trade-off
  should be carefully considered.