Jeffrey Bokor EECS Department - Microlab Home...

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Carbon-Based Electronics:Will there be a carbon age to follow the silicon age?

Jeffrey BokorEECS Department

UC [email protected]

Solid State Seminar

9-13-13

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Outline

• Review of development of Carbon Nanotube (CNT) transistors (for logic)– Issues, progress, prospects

• Advent of graphene– Recognition of promise of graphene nanoribbons (GNRs) for

logic transistors– Issues, progress, prospects

• Summary of prospects for carbon transistors

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C60: Birth of carbon Nanotech era

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Main properties of carbon nanotubes predicted before discovery!

semiconductor

metal

Applied Physics Letters

5

Single-wall carbon nanotubes discovered in carbon ‘soot’ by TEM

Iijima and Ichihashi, Nature (1993) [NEC]

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CNT Transistor

Tans, et al., Nature (1998) [Dekker group, Delft]

Laser vaporization method for CNT synthesis

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Catalytic CVD growth of CNTs on a surface

Kong, et al, Nature (1998) [Dai group, Stanford]

Catalyst: Fe(NO3)3⋅9H2O/alumina/methanol suspensionCVD at 1000C with methane

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-25

-20

-15

-10

-5

I DS (

A)

-0.4 -0.3 -0.2 -0.1 0.0VDS (V)

0.2 V

-0.1 V

-0.4 V

-0.7 V

-1.0 V

-1.3 V

Self-Aligned Ballistic FETs w/High-k

0.5

10-9

10-8

10-7

10-6

10-5

-I DS

(A)

-1.5 -1.0 -0.5 0.0VG (V)

VDS = -0.1,-0.2,-0.3 Vd~1.7 nmL ~ 50 nm

• Pd zero-barrier height contact• > 5 mA/um at Vg = VDS =0.4V

Javey, et al, Nano Lett. (2004) [Dai group, Stanford]

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High Performance p- and n-FETS

Javey, et al, Nano Lett. (2005) [Dai group, Stanford]

• Doping by adsorption• Lg = 80nm

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CNT-CMOS Integration Chip

NMOS binary tree 11-bit decoder 2048 back-gated CNT transistors >4000 Si NMOS transistors, 1 m Microlab

baseline process

Tseng, et al, Nano Lett. (2004) [UCB/Stanford, Bokor/Dai groups]

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Carbon Nanotube + Silicon MOS Integrated Circuit

-15 -10 -5 0 5 10 15

10-8

10-7

10-6

1x10-5

Imin

Ion

Id (A

)

Vgs (V)

0 1 2 3 4 5 6 70

50

100

150

209 Devices

Total: 523 devices

Semiconductingnanotubes only

coun

ts

Log (on/off)

Tseng, et al, Nano Lett. (2004) [UCB/Stanford, Bokor/Dai groups]

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Direct correlation to diameter variation

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

10-6

1x10-5

Ion

(A)

Diameter (nm)

Ion, Vgs-Vt=-7v

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.010-12

10-11

1x10-10

1x10-9

1x10-8

1x10-7

1x10-6

Measurement Limit

Imin

(A)

Diameter (nm)

1 tube per device

-15 -10 -5 0 5 10 1510-11

1x10-101x10-91x10-81x10-71x10-61x10-5

Id (A

)

Vgs (V)

d=2.9nm d=2.2nm d=1.1nm

Tseng, et al, Nano Lett. (2006) [UCB, Bokor group]

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Parallel Tube CNTs

• To get large drive, need to stack multiple tubes in parallel with common contacts, gate

• Important for ultimate circuit application

• Do parallel array currents add?• How close can tubes be stacked?

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10-10

10-8

10-6

10-4

-I DS

(A)

-1 0 1VG (V)

Parallel Array of Self-Aligned Ballistic FETs

-200

-150

-100

-50

I DS

(A

)

-0.6 -0.4 -0.2 0.0VDS (V)

VGS = -0.9 to 0.3 V in 0.2 V steps

D

S

G

D

G

G

G D

S

S

SWNTS

VDS = -0.1,-0.2,-0.3 V

• 1st demonstration of a parallel array• ~200 uA of current for the array of 8 tubes.

Javey, et al.,Nano Lett. (2004)[Stanford, Dai group]

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CNT Array Density Limited by Screening

Wang, et al. SISPAD (2003) [IBM]

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CNT Array Transistor Circuit Performance

Jie, et al., ISSSC (2007)[Stanford/USC, Wong/Mitra/Zhou groups]

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Vision for CNT channel array MOSFETs

• Array of 1D channels, densely packed

• Density 200-250 per m• No metallic tubes• Narrow diameter

distribution

Bulk Dielectric k2

HfO2

LgcLg

pitch

CNTsor

SNWs

Gate Dielectric k1

MetalGate

Substrate

S D

Bulk Dielectric k2

HfO2

LgcLg

pitch

CNTsor

SNWs

Gate Dielectric k1

MetalGate

Substrate

S D

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A Multiple Growth Strategy to High Densities

Hong, et al, Adv. Mat. (2010) [UIUC, Rogers group]

• Single-crystal quartz growth substrate• “Epitaxial” CNT growth• Layer transfer to Si wafer

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Density Scaling by Multiple Transfers

as grown: 15/um transferred: 15/um

2X transfer: 29/um 4X transfer: 55/umWang, et al., Nano Res. (2010)[USC, Chou group]

Removal of m-tubes by ‘breakdown’

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Coat with small molecule film

Induce Joule heating selectively in m‐SWNTsto form trenches by thermocapillarity

Si (p++)

SiO2

S M S M S S

Si (p++)

M MSiO2

O2 plasma etch exposed m‐SWNTs

Remove film and electrodes; build circuits on remaining s‐SWNTs

S S

Si (p++)

SiO2

S S

Si (p++)

SiO2

Selective Removal of m-tubess From Aligned Arrays

J. Rogers group, UIUC

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Dynamics of Thermocapillary Flow

t= 0 0.1 1 10 100 300 s

2 m

Joule Heating by a SWNT (∆T~5-15C)

Jin, et al., Nat. Nano. (2013) [UIUC, Rogers group]

Heating options:• Gated electrical Joule heating• Selective laser absorption• Selective microwave absorption

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Solution phase nanotube ‘sorting’/purification

Arnold, et al., Nat. Nano. (2006)[Northwestern, Hersam group]

“Density gradient” centrifugation

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Electrical results on sorted CNTs

Arnold, et al., Nat. Nano. (2006)[Northwestern, Hersam group]

Percolating network transistor Sorted tube transistor high on/off ratio

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DNA sequence specific wrapping for sorting

Tu, et al., Nature (2009)[Dupont, Zheng group]

“size exclusion” chromatography

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Purified Single Chirality (10,5) SWNTs

200nm

(10,5)

StartingHiPco material

Separated SWNTs

(10,5)

Zhang, et al, JACS (2009), [Stanford/Dupont, Dai/Zheng groups]

DNA used: (TTTA)3T

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FETS with 99% Semiconducting Tubes

10-1410-1310-1210-1110-1010-910-810-710-610-5

-Ids

(A)

-2 -1 0 1 2Vgs(V)

1000mV 500mV 100mV 10mV

100nm

S

D< 2 2 -4 4 -6 > 6

0

1 0

2 0

3 0

4 0

Perc

enta

ge(%

)

lo g (Io n /Io ff)

Mixed

Purelysemiconducting

Average 15 tubes per deviceIon/Ioff >102 : 88%semiconducting tubes: 99% (0.9915 ~ 88%)

Mostly (10,5) SWNTs

Zhang, et al, JACS (2009), [Stanford/Dupont, Dai/Zheng groups]

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Solution phase array assembly by Langmuir-Blodgett technique

Li, et al. JACS (2007)[Stanford, Dai group]

~80/um~70/um

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Solution processed CNTs are as good as CVD tubes at nanoscale Lg

Choi, et al., ACS Nano (2013)[UCB, Bokor/Javey groups]

Franklin and Chen,Nat. Nano. (2010)[IBM

CVD tubes

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Ultimate scaling study

GAA NW

SG AGNR

GAA CNTDG UTB

Also DG AGNR

M. Luisier (Purdue)

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Device Characteristics:- All: Lg=5nm, VDD=0.5 V, EOT=0.64nm (3.3nm of HfO2 with εR=20)- SG and DG AGNR: width=2.2nm, normalization by width- GAA CNT: diameter=1.58, 1.0, and 0.6 nm, normalization by diameter - GAA and -NW: Si, diameter=3nm, transport=<110>, 1% uniaxial strain- DG UTB: Si, body=3nm,, transport=<110>, 1% uniaxial strain

Simulation Approach:- Same quantum transport simulator for all devices based on Non-equilibrium

Green’s Functions (NEGF) formalism with atomistic resolution of simulation domain and finite element method for Poisson equation

- Bandstructure model: single-pz for carbon and sp3d5s* for silicon (tight-binding)- Ballistic limit of transport (no electron-phonon scattering nor interface

roughness taken into account)- Intrinsic device performances (no contact series resistances included)- No gate leakage currents included- No structure optimization for any of the selected devices

Simulation parameters and assumptions

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Id-Vgs at Vds=0.5V in carbon-based Devices

SiO2HfO2

AGNR width: 2.2nm / CNT diameter: 1.58nm / Band Gap Eg=0.56 eV

• Same EOT gives very different electrostatic gate-channel coupling

EOT=0.64nm EOT=0.64nm

M. Luisier (Purdue)

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Gate Dielectric Effect In Carbon-Based Devices

Comparison of Conduction Band Edge and Spectral Current in Single-Gate AGNR with 0.64nm SiO2 (εR=3.9) and 3.3nm HfO2 (εR=20) => same EOT=0.64nm

OFF- stateSiO2 HfO2

• Effective channel length is longer for the thicker HfO2• Barrier widens and tunneling current drops

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Extreme (sub-10 nm S-D Tunneling regime) d=1.58 nm CNT FETs

Transfer Characteristics Sub-threshold swing

Vds=0.5V

• Bandgap 0.56 eV GAA- CNT (d=1.58 nm) scales poorly

5nm≤Lg≤12nm

Vds=0.5V

d=1.58 nm

3.3nm HfO2 EOT=0.64nm

M. Luisier, et al., IEDM (2011)[Purdue/MIT/UCB, Lundstrom/Antoniadis/Bokor groups]

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Transfer Characteristics Sub-threshold Slope

Gate-length trend for 1 nm CNTs

• Bandgap 0.8 eV GAA- CNT (d=1.0 nm) scales better


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Transfer Characteristics

Id-Vgs at Vds=0.5V in CNT FETs with d=0.6nm and 5≤Lg≤12 nm

Gate-length trend for 0.6 nm CNTs

Sub-threshold Slope

• Bandgap 1.4 eV GAA- CNT (d=0.6 nm) scales well


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• CNT with d=0.6nm and NW with d=3nm have same band gap Eg=1.4eV

• CNT with d=1.0nm has band gap Eg=0.817eV

Comparison of different channel materials

Id-Vgs at Vds=0.5V in CNT, NW, and UTB Devices3.3nm HfO2, EOT=0.64nm

• Bandgap 0.8 eV GAA-CNT (d=1.0 nm) scales poorly• Bandgap 1.4 eV GAA-CNT (d=0.6 nm) scales well• Si NW (d=3 nm) scales very well due to high-mass and band-gap

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9 nm CNT transistor

2012

(5 nm)

(20 nm)

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Monolithic 3D CNT Circuits!

Hai, et al., IEDM (2010)[Stanford, Mitra/Wong groups]

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Graphene

Forms of graphene Graphene resistivity

Geim and Novoselov, Nat. Mat. (2007) [Manchester]

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Bandgap Prediction for Graphene Nanoribbons

Son, et al., PRL (2006)[UCB, Louie group]

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Bandgap Measurements of Etched GNRs

Han, et al., PRL (2007) [Columbia, Kim group]

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GNR formation by unzipping CNTs

Jiao, Nat. Nano. (2010) [Stanford, Dai group]

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0.5

0.4

0.3

0.2

0.1

0.0E

g(eV

)50403020100

W (nm)

100

101

102

103

104

105

106

107

I on/

I off

50403020100W (nm)

)/exp( /II offon TkE Bg

nmWeVEg

8.0

Li, et al. Science (2008)[Dai group]

All sub-10nm GNRs are semiconductingIon currents few uA

GNR Bandgap vs. width

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Bottom-up Synthesized GNRs

Cai et al. Nature (2010) [EMPA (Switzerland), Fasel group]

• Atomically perfect edges!

• 7 C atoms wide• W = 0.74 nm!

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Aligned Growth – Bandstructure Measured

Ruffieux, et al. ACS Nano (2012) [Fasel group]

m* = 0.21 meEg = 2.3 eV

~2 nm pitch!

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Synthesized GNR Transferred to SiO2

Bennett, et al., unpublished [UCB, Bokor/Crommie/Fischer groups]

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Synthesized GNR Transistor Results

Bennett, et al., unpublished [UCB, Bokor/Crommie/Fischer groups]

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Wider GNRs Synthesized with 1.4 eV Gap

Chen, et al., ACS Nano (2013)[UCB, Fischer/Crommie groups]

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Single-Molecule Heterostructures

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Summary/Outlook

• CNT and GNR both promising candidates for CMOS channel material for 8 nm gate length

• Why?• High drive at low V• Good scalability• 3D layer stacking:

10 layers = 3 nodes on roadmap!

• More work needed:• Purified chirality for tubes• Longer, wider GNRs• Dense aligned arrays• Low resistance contacts

• GSR opportunities in my group

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