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High-Field, Thermal & Energy Properties of 2D Devices & Layers

Eric PopElectrical Engineering (EE) and Precourt Institute for Energy (PIE)Stanford University

http://poplab.stanford.edu

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• Our “flatland” (2D) subgroup and alumni:

– Dr. Vincent Dorgan, Dr. Myung-Ho Bae

– Dr. Enrique Carrion, Dr. Zuanyi Li, Dr. Sharnali Islam

– Chris English, Kirby Smithe, Michal Mleczko, Ning Wang

• Undergrads:

– Maryann, Yuan, Andrew, Tim, Justin

• Sponsors:

– Air Force Research (AFOSR)

– National Science Foundation (NSF)

• Collaborators:

– E. Reed, K. Goodson, Y. Nishi, I. Fisher, K. Saraswat, H.-S.P. Wong (Stanford)

– J. Lyding (UIUC), D. Akinwande (UTA), I. Knezevic (Wisc.), R. Sordan (Milano), A. Roy, V. Varshney (AFRL)

Acknowledgements

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http://poplab.stanford.edu

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2D Materials in Our Lab

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CVD growth of graphene, BN and MoS2

20 µm

ZrSe2

HfSe2

100 µmmono-MoS2

A1g 194 cm-1

100 200 300 400Raman Shift (cm-1)

Inte

nsi

ty (

a.u

.)

10 µm

WTe2 (metallic)

MoTe2

BN

CVT growth of MoTe2, WTe2, ZrSe2, HfSe2

50 100 150 200 250 3000

0.5

1

1.5

2

2.5

3

3.5

4x 10

Raman Shift (cm-1)

Inte

ns

ity

(a.

u.)

~ ~~ ~

~ ~

3-6 Layer

Bulk

WTe2

exfoliate

collab. H.S.-P. Wong, Y. Nishi, I. Fisher

HfSe2

Si

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Device (Transistor) Scaling• Problem: 20th century transistors “carved” out of 3D materials (Si)

surface roughness restricts mobility, band gap, scaling of dimensions

• Solution: 21st century transistors with atomically thin 1D and 2D materials (<1 nm) can re-enable LG scaling

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LG ~ (tchtox)1/2

roughness

tch

<1 nm

1 nm

1D carbonnanotube (CNT) 2D TMD (MoS2,

WTe2, ZrSe2)

sources: K. Uchida, A. Kis, IBM, our work tch or d (nm)1 2 3 40

10

102

µ(c

m2V

-1s-1

)

104

103

CNTsgraphene

SOI

MoS2

WSe2

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High-Field Transport in Suspended Graphene

Suspended graphene allows us to study intrinsic coupled electrical-thermal properties

SiO2

Si

VD

VG1 μm

suspendedgraphene

-1000

500

2000

1500

T (K)

DS

0 1 2 30

0.2

0.4

0.6

0.8

1

F (V/m)

I/W

(m

A/m

)

experiments

0 1 2 3F (V/m)

μ0 = 2,500

cm2/V/s

μ0 = 25,000cm2/V/s

cleaner

more disordered

simulations

0 2 4 6 8 10 12 140

1

2

3

4

5

v (1

07 cm

/s)

Sample #

high-fieldintrinsic vsat

300 1,000 2,0000

1000

2000

3000

4000

(W

m-1

K-1)

T (K)

This work (exfoliated)This work (CVD)Graphite (in-plane)

steeper drop-off than graphite

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V. Dorgan, A. Behnam, H. Conley, K. Bolotin, E. Pop, Nano Letters 13, 4581 (2013)

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Continuity

Poisson

Thermal

Charge

Simulation: Ambipolar + Poisson + Heating

0' 0T

A k p g T Tx x

sgn( ) ( )x x x x xI p n qW p n v

, , 10

sgn( )1 /

xx j x j x x

x sat

vV V p n x

v v

2

20 0

1, 4

2ox ox

x x Gx Gx ix

C Cp n V V V V n

q q

iterateitera

te

models “GFETTool” and “S2DS” available on http://nanoHUB.org

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Transport at Graphene Grain Boundaries

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K. Grosse, V. Dorgan, D. Estrada, J. Wood, I. Vlassiouk, G. Eres, J. Lyding, W. King, E. Pop, Appl. Phys. Lett. 105, 143109 (2014)

scanning Joule expansion microscopy (SJEM)

technique:

Goal: Understanding nanometer scale graphene transport, heating and reliability

– Measured heating at graphene grain boundaries (GB)

– Deduced grain boundary resistance; Collaboration with ORNL

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High-Field Transport in MoS2

• Negative differential conductance (NDC) at high-field in MoS2

• Simulations suggest combination of:

– Self-heating and µ(T) ~ T-2

– Intervalley scattering (K to Q valley)

• Results shed light on band structure

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VD

VG

1 µm

Au

Au

MoS2

S

tch

0 2 4 60

1

2

3

4

5

6

7

8

F (V/m)

v d (1

06

cm/s

)

100

10-5

10-10

10-15

-1 0 1-1

0

1

-1 0 1

-1

0

1

kx (nm-1)

0 2 4 60

0.5

1

1.5

2

2.5

3

3.5

4

F (V/m)

80 K

200 K

300 K

500 K

experiment (T0 = 80 K)

Upper Valley

(Q)Lower Valley

(K)

ΔE = 130 meV

k y (nm

-1)

kx (nm-1)

k y (nm

-1)v d

(10

6 cm

/s)

K

QΓ

0 2 4 6 80

50

100

150

200

250

300

VD (V)

I D (A

/ m

)

80 K

200 K

300 K

500 K

120 K 160 K

400 K

VGT ≈ 30 V

A. Serov, V. Dorgan, C. English, E. Pop, Device Research Conf. (2014)V. Dorgan, A. Serov, C. English, E. Pop, submitted (2015)

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Some Recent Results on Monolayer CVD MoS2

• Growth of monolayer MoS2 by CVD on PTAS/SiO2/Si

• Effective mobility (µeff) comparable to exfoliated material

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K. Smithe, C. English, S. Suryavanshi, E. Pop, Device Research Conf. (2015)

Good current achieved (~275 µA/µm) in 80 nm device, but contact resistance remains dominant

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Summary of Challenges in 2D Devices

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Material Quality:

Contact Resistance (RC):

SiO2 (90 or 300 nm)

Si (p++)

VD VSVTG

VBG

L

W

Metals

Oxide

Graphene

Si (p++) VBG

LT

Metals

Si (p++)

High-κ

Substrate

Graphene

VBG

SiCGrowth

CVD[2][1]

Interfaces:

o Pre vac. annealo Post vac. anneal

tOFF

VD

VTG

tON,TGtON,D

Rprobe

RL

50 Ω

VP VTG

GFET

VD

VS

CprobeCpad

Scope

-8 -4 0 4 80.4

0.8

1.2

1.6

VTG [V]

R [

K

]

DC

tON,TG = 100 µs

= 10 µs

= 400 ns

(air)

(b)

0 100 2000

0.5

1

1.5

Time [ns]

V [

V]

d

tON,D

tON,TG

+Variability!E. Carrion, E. Pop et al, IEEE TED, 61, 1583 (2014)

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A Few Words on Thermal 2D Work

• Large in-plane thermal conductivity of graphene, BN (>500 W/m/K)

• Ultra-low cross-plane thermal conductivity of layered WSe2 (<0.1 W/m/K)

– Lower than plastics and comparable to air

• Huge thermal anisotropy in all layered 2D materials (>10-100x)

• MRS Bulletin review with AFRL:

• Large thermopower in TMDs (S ~ 0.5 mV/K) Thermoelectrics?

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E. Pop, V. Varshney, A.K. Roy, "Thermal Properties of Graphene: Fundamentals and Applications," MRS Bulletin 37, 1273 (2012)

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Ballistic Phonons in Graphene Ribbons

First measurement of quasi-ballistic

heat flow in graphene near room

temperature

First measurement of phonon edge

scattering in graphene nanoribbons

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100 200 300 400107

108

109

G/A

(W

m-2K

-1)

T (K)50

long graphene13

(L ~ 10 μm)

W~130 nmW~85 nmW~65 nmW~45 nm

~T1.5

Maximum heat flow alsolimited by edge scattering.

But, “short” graphene devices reach up to ~35% of theoretical ballistic thermal limit at room T. “short”

quasi-ballistic

“long”(diffusive)

graphenenanoribbons

M.-H. Bae, Z. Li, Z. Aksamija, P. Martin, F. Xiong, Z.-Y. Ong, I. Knezevic, E. Pop, Nature Comm. 4, 1734 (2013)

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Looking Ahead: Future OpportunitiesCould we:

– Exploit anisotropy for routing heat? (thermal diode)

– Separate thermal and electrical flow? (thermal transistor)

– Design electronics with built-in thermoelectric cooling?

– Achieve transparent heat spreaders and flexible thermoelectrics?

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collab: E. Reed, K. Goodson, K. Saraswat, H.-S.P. Wong, Y. Cui

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Looking Ahead: Future OpportunitiesCould we:

– Exploit anisotropy for routing heat? (thermal diode)

– Separate thermal and electrical flow? (thermal transistor)

– Design electronics with built-in thermoelectric cooling?

– Achieve transparent heat spreaders and flexible thermoelectrics?

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( )

1

3

9 Pristine MoS2

0 2 4 6 8

G(M

W/m

2 /K

)

Time (min)

Al / MoS2Li

electrolyte LiPF6

Thermal Switching

A. Sood, F. Xiong, […], E. Pop, Spring MRS (2015)

collab: E. Reed, K. Goodson, K. Saraswat, H.-S.P. Wong, Y. Cui

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Summary

• Goals: understand 2D materials and devices for analog (e.g. graphene) and digital (e.g. MoS2) electronics

• High-field transport experiment & models

• Insights into band structure, grains, power dissipation

• Unique 2D thermal properties

• Next:– Quasi-ballistic transport evaluation

– Role of contact geometry

– Exploit heterostructures, thermal anisotropy, thermoelectric properties

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Contact:W: http://poplab.stanford.eduE: epop@stanford.edu

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