Tutorial T5 Will Carbon Replace Silicon? The future of graphitic electronics.

1:30-2:20 Jim Meindl Nanoelectronics in Retrospect and Prospect

2:20-3:10 Millie Dresselhaus From Graphene to Graphite to Nanotubes to Graphene.

3:10-3:30 Break

3:30-4:20 Phillip Kim The Physics of Graphene

4:20-5:10 Walt de Heer Epitaxial Graphene:

Designing a new electronics material from the ground up

5:10-5:30 Panel Discussion

Outline

• The problem

• The carbon solution

• Why epitaxial graphene

• Properties

• Patterning

• Prototype Devices

• Chemistry: opening a gap

The first transistor (Bell labs 1947)

The “crystal triode” John Bardeen, Walter Brattain and William Shockley

The Shockley Building

The silicon revolution

The end.

The carbon solution

Carbon is prominent!

Carbon electronics

Carbon nanotubes: Room temperature ballistic conductors

T. Ando, T. Nakanishi and R. Saito

J. Phys. Soc. Jpn. 67, 2857 (1998)

“The absence of backward scattering is

shown to be ascribed to Berry's phase

which corresponds to a sign change of the

wave function under a rotation of a

neutrino-like particle* in the wave vector

space in a two-dimensional graphite”

*i.e obeying the Dirac-Weyl equation

Quantized ballistic conductance

Nanotube fiber!

- L (!m)

G (

2e

2/h

)

L

V

reservoir

reservoir

dissipation in reservoir

no dissipation in channelreservoir

reservoir

dissipation in river bed

Diffusive transport Ballistic transport

Carbon nanotube transistors

Band structure of graphene: (used for graphite , Wallace 1947, McClure 1957,

and for nanotube transport , Ando 1998)

• Linear dispersion

• Symmetry electrons - holes

• Pseudo spin

)ˆ,ˆ,ˆ(ˆzyx

!!!! =

!

" x =0 1

1 0

#

$ %

&

' ( ;" y =

0 )i

i 0

#

$ %

&

' ( ;" z =

1 0

0 )1

#

$ %

&

' (

!

H = vF " ˆ # " p

!

E = ±vF p

T. Ando, J. Phys. Soc. Jpn

67 (1998) 2857

Graphene ribbons resemble nanotubes

metallic bands edge states

gap!1/width

semiconducting

mettalic

EF

Phys. Rev. Lett. 98, 206805 (2007)

Quantum confinement gap in exfoliated graphene ribbons

Philip Kim Egap= 0.2 eV /(W-20 nm)

Exfoliated Graphene flakes

Graphene’s advantage: cut-a-structure

Seamless connection between

graphene components

EF

Semiconducting strip

Von Voff

Simple ballistic FET

Quantum interference ring

valence band

conduction band

! Quantum dot

Epitaxial graphene

Production of Epitaxial graphene

“Standard” UHV method

Charrier et al.

Epitaxial graphene “Typical” UHV system

STM

Production of Epitaxial graphene

Georgia Tech Vacuum Furnace method (2003)

STM Vacuum Furnace

C-Face termination

Si-Face termination

SiC SiC

Thick graphene films

4 - 100 ML

Thin graphene films

1 - 5 ML

Epitaxial Graphene Growth

Furnace Grown Si-Face UHV Grown Si-Face Furnace Grown C-Face

C

Si

U29.00001 Joanna Hass J.Milan,L31.00010

10X10 !m 10X10 !m 7.5X7.5 !m

LEED: 72.2eV

20!m

Epitaxial graphene, C-face

AFM

10 Å

STM

Epitaxial graphene pleats

Epitaxial graphene, Si-face

400 Å

G. M. Rutter, et al. Science 317, 219-222 (2007).

20!m

UHV grown

AFM

STM

LEED: 78.3eV

!""#$!#

%"""#$!#

!"#$%$&'$!$%"""#$!#

10!m

Furnace grown

AFM

STM

Properties

Magnetotransport: graphene like;

non-trivial Berry’s phase,

1!m x 6.5!m R= 502 !/sq != 9500 cm/Vs

Solid State Com. 2007,Grenoble/GIT collaboration!

"R

("

) R

xx ("

/sq)

Field (T)

1/B (T-1)

"R

/R=

4%

1/B (T-1) 100 mK

Hall bar C-face

Shubnikov de Haas oscillations

Landau level index

Landa

u le

vel i

ndex

Xioasong Wu Y3.00002 Berger, D29.00006

Sadawski, Potemski, Martinez, Berger de Heer. PRL 97, 266405 (2006);

IR cyclotron resonance spectroscopy: Dirac cone

!B dependence of Landau levels

v =1.03 106 m/s

EF

Wavenumber (cm)-1 100 200 300 400 500 600 700

1.5T

1.5T

1.4T

0.8

1.0

1.0

1.0

0.8

0.8

0.8

Graphite ~ 1!m

50 layers

5-7 layers

9-10 layers

1.0 B=1.5T

Tra

nsm

issi

on

(B) line

Multi-layer

Graphene

Rela

tive tra

nsm

issi

on Infrared absorption spectrum in a magnetic field

!

B(T1/ 2)

Tra

nsi

tion e

nerg

y (m

eV

) Field dependence of Landau level transitions

en

erg

y

Field

Dirac cone measured within 10 meV of the Dirac point

Gerard Martinez P30.00014

Raman of C-face EG: Graphene-like

(Graphitic residue)

(SiC substrate)

5-10 layers

70-90 layers

Monolayer on SiO2

Epitaxial multilayer graphene

K K’

q~K

Graphene

Faugeras, Nerriere, Potemski, Mahmood, Dujardin, Berger, de Heer

APL 2008

Graphite (HOPG)

K K’

q~K

Epitaxial graphene

J. Hass et al. cond-mat/07062134!

STM/XRD: rotational stacking of C-face EG

Density functional theory

2 graphene layers rotated 2.2˚; supercell 46.1˚

L. Magaud, F. Varchon, CNRS

STM : moiré pattern graphene

AB stack graphene bi-layer

R30/R2 stack graphene bi-layer

Rotational stacking yields same electronic structure as isolated sheet

U29.00001 Joanna Hass

Graphene on Si-face: gap is observed;

Gap closes as the number of layers increases.

n=1 n=2 n=3 n=#

Zhou, Gweon, Fedorov, First, de Heer, Lee, Guinea,. Castro Neto, Lanzara

Nature Materials 6, 770-775 (2007)

Substrate-induced band gap in Si-face EG

Patterning

Process

Hall bar FET Quantum Interference loop

e-beam lithography

Devices

Confinement and Coherence

Berger, D29.00006

T=4,6, 9, 15, 35 and 58 K; -9 T!B!9 T.

Landau levels:

En(B)="(2neBv02)

Confinement:

En(W)=n#v0/W

Confined Landau levels:

En(B,W)$ [En(W)4+En(B)4]1/4

Magneto-transport of a narrow graphene Hall bar

!*=2.7 m2/Vs.

•A reversible, reproducible, drop in the conductivity is observed at 200K.!

• The resistance is at its theoretical minimum . !

•Transport is phase coherent over the entire structure (0.5X5 µm).!

• Resistance is at its theoretical minumum (no boundary scattering!)!

• Oscillations periodic in the magnetic field are seen.!

180 200 220 240 260 280 3000

200

400

600

800

1000

1200

Temperature (K)

Anomalous Conductance Transition

Gating Epitaxial Graphene

Graphene transistors (Conventional FETs)

Side gate structure

Resi

stance

(k!

/sq

)

-1 1 0

-10 -10 0

-6 -4 -2 0 2

20

30

10

0.3 0.4 0.5

0.6

0.2

0

20

22

24

26

Gate Voltage (V)

Si-face Top gate

C-face Top gate

C-face Side gate

1.5x12.5!m

3.5x12.5!m

0.1x1!m

Top and side gated FETs

s d

g1

g3

g2

S

d

g1

g2

Top gated FET

Side gated FET

The first epitaxial graphene transistors

Xuebin Li

Q29.00006

EF!

The Dirac point

!#

xy (Hall)(k

")$

5

0

-5

20

10

15

5

0 -5 -4 -3 -2 -1 0

Vg(V) -6

280 K!

-5 Tesla!

1.5!mx12!m Positive Hall (electrons)

Negative Hall (holes)

#xx

(R

esi

sta

nce

)(k"

)$

Dirac point ED

S

G

G

D

Top gated Hall bar, Si face

Jakub Kedzierski,

Craig Keast, Peter Wyatt,

Paul Healey, Pei-Lan Hsu

MIT Lincoln Labs

Mike Spinkle, Claire Berger, Walt de Heer, Georgia institute of Technology

Large scale patterned epitaxial graphene FET’s

Production process

SiC blank

Furnace Graphitize

Device Integration

Hydrogen etch

Pattern

Final Device Geometry

•! Device description and cross section –! Nominal device – Graphene/SiC active layer (C-side), L = 10um, W = 5 um,

ridge parallel, 50nm HfO2 dielectric, Al gate

–! Microscope image shown before gate lift-off

W=5um

L=10um

Al gate (next level)

Pt Drain

Pt Source SiC

Gr

SiC

Al

SiC Al

HfO2

Graphene

Set of Identical Devices (Si-face)

•! Minimum conductivity –! 130uS to 250uS

•! Field Effect Mobility values –! 400-1000 cm2/Vs

•! Ion/Ioff ~ 5

Drain current vs. gate voltage at Vd = 0.5V

- 5 - 4 - 3 - 2 - 1 0 1 2 3 0 . 0

2 0 0 . 0 !

4 0 0 . 0 !

6 0 0 . 0 !

8 0 0 . 0 !

1 . 0 m

1 . 2 m

1 . 4 m

% d (

1/O

hm

s)

V g ( V )

G r a p h C W 2 S i T

H f O 2 = 4 0 n m W = 5 µ m , L = 1 0 µ m

Quantum Interference Device

Standing wave; (Destructive interference); no transmission!

Propagating wave; unit transmission!

V=0!

Chemistry: opening a gap

Ruoff et al. Nature 448, 457 (2007)

Graphite Graphene oxide (GO)

oxidation

Reduction of GO

Preparation: Hummer’s method

graphene oxide

2 !m$2 !m

AC electrophoresis 2-3 Volts, 20-50 kHz

Separation between electrodes: 400 nm, 800 nm, 1400 nm

Graphene oxide suspension from N. Kovtyukhva and T. Mallouk, Penn State University

Device made of GO flakes

Xiaosong Wu, Mike Sprinkle, Xuebin Li, Fan Ming, Claire Berger, Walt de Heer

L29.00012 Mike Sprinkle; L29.00009, Fan Ming

Reverse

Forward

&b

•! Asymmetric in bias voltage

•! Asymmetry correlates with the lengths of the contact edges

•! There is no systematic dependence on the width of the gap

For over 20 samples studied: Ionized donor density at 300K "d=2.2*1010 to 6.1*1011 cm-2

Barrier : &b = 0.5 to 0.7 eV

Typical I-V curve

dI/dV=74 k!$

GO flake

“Burnt”

+

-

breakdown

GO mobility

! = 850 cm2/Vs at breakdown

%d=1X1011/cm2

#b decreases from 0.7 to 0.55 eV "d increases from 3.8*1011 to 9.1*1010 cm-2

180! for 16 hours

as deposited

Tuning the gap

Optical image of a graphene cross after RIE etching.

HSQ mask with window: in situ oxidation

-15 -10 -5 0 5 10 15-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

Vsd

(V)

I (

µ A)

100K

In situ patterned oxidation of graphene structure

Chemical Oxidation

Graphene oxide

(2)

Deposit dielectric and metal gate electrode

Source Drain Gate

(3)

SiC

HSQ

Graphene ribbon

(1) Pattern epitaxial graphene ribbon

insert afm image here

EG-GO-EG transistor

Examples of patterned Epitaxial Graphene structures!

Quantum Interference Device

Side gated ribbon (FET)

gate

gate

drain

source

Hall bar (various ribbon widths)

I I

V

V

The temperature dependence of the device parameters. a) The temperature dependence of the Schottky barrier height. b) The area density of ionized donors as a function of inverse temperature. Circle: experiment; Line: A fit to an exponential law (Nd=N0exp(-Ei/2kBT)) gives the ionization energy: Ei=62 meV.

Band structure of graphene: (used to explain graphite , Wallace 1947)

“DIRAC CONE” Near ED, E=±v|p|= ± v|hk|

v~ 108 cm/s

Xiaosong Wu, Mike Sprinkle, Xuebin Li, Fan Ming, Claire Berger, Walt A. de Heer

Submitted to PRL, Dec. 2007 (ConMat: 0712.0820v1)

The epitaxial-graphene/graphite-oxide junction,

an essential step towards epitaxial graphene electronics

Vsd > 2 V

320K 300K 270K 240K 200K 150K 100K 77K

EG-GO-EG, a 400nm gap

For over 20 samples studied: Ionized donor density at 300K "d=2.2*1010 to 6.1*1011 cm-2

Barrier : &b = 0.5 to 0.7 eV

Optical absorbance of Graphene Oxide

GO MSM device a) A bilayer rectangular GO flake over a 400nm Au gap. b) A pentagon-shaped GO flake bridges two MEG electrodes. The bright spots on MEG are residue of e-beam resist: PMMA, while the bright lines are wrinkles that are often seen in C-face EG. c) I-V characteristics of an 800 nm device. d) An energy band diagram for a EG-GO-EG device. Two Schottky barriers form at the contact edges. i) zero bias. ii) finite bias.

Figure 2. I-V characteristics of a 400 nm device at various temperatures: 320, 300, 270, 240, 200, 150, 100, 77K. The sample was annealed at 180 C for 16 hours. a) Nonlinear I-V. The inset: I-V before (blue) and after (red) curing. b) I/T3/2 as a function of Vsd

1/4/T.

Production of patterned structures

A Raman scattering study of epitaxially-grown graphite on silicon carbide;

pyrolitic graphite and graphene: C. Faugeras,

A. Nerriere,M. Potemski,A Mahmood,E. Dujardin,C. Berger ,and W. A. de Heer

cond-mat0709.2538v3 19 Sep 2007

(Graphitic residue)

(SiC substrate)

5-10 layers

70-90 layers

Monolayer on SiO2

Epitaxial multilayer graphene Graphite (HOPG)

K K’

q~K

K K’

q~K

Graphene

#

5

40

60

50

n ≈

Substrate-induced band gap opening in epitaxial graphene

S.Y. Zhou, G.-H. Gweon, A.V. Fedorov, P.N. First, W.A. de Heer, D.-H. Lee,F. Guinea,A.H. Castro Neto,

and A. Lanzara

Nature Materials 6, 770-775 (2007)