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Exceptional ballistic transport in
epigraphene
Walt de Heer
Georgia Institute of Technology
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Program Objective
First formulated in 2001 and patented in 2003, our objective is to develop nanoelectronics based on epitaxial graphene on silicon carbide.
Sprinkle et al. Nature Nano, vol. 5, pp. 727-731,. 2010
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Si crystal Si wafer
SiC wafer
Si nanoelectronics
200 GHz epitaxial graphene transistor (GaTech)
3
Intel funded the GIT Graphene project in 2003
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Epitaxial graphene, by the CCS method reviewed in PNAS 108, 16900 (2011)
First CCS Induction Furnace
(2004)
The graphene sheet continuously covers the entire surface. Its major disadvantage is, that it cannot be back gated.
STM (Courtesy J. Stroscio)
pleat
50 nm
400 nm
0.5 nm
0.05 nm Atomically flat
Epitaxial graphene produced by vacuum sublimation of Si from electronics grade SiC.
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Important achievements
• Record breaking high- speed analog FETs.
• Demonstration of high quality semiconducting epigraphene and related FETs.
• Demonstration of novel spintronic devices.
• Record breaking room temperature exceptional (not understood) single channel ballistic transport, involving new physics.
Gate
Sou
rce
Dra
in
1µm
Gate
Sou
rce
graphene
S D G graphene
SiC
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Exfoliated graphene nanoribbons have transport gaps
Transport gap (not a band-gap) “Edge disorder”
This realization essentially ended exfoliated nanoelectronics research!
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for W=40 nm and L=1 µm E1,0/kB=600 K, E0,1/kB=23 K,
Ribbons as waveguides
Caveat: Micron length graphene ribbons are quantum dots!
c* ≈ 106 m/s
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Structured graphene growth on sidewalls
~1500C, 10 min
Graphene ribbon
Photo-lithography defined Ni mask Plasma etched SiC step
Preferential graphene growth on the recrystallized (1-10n) facets
Selective growth on sidewalls etched into SiC Avoids disorder at edges
M.Sprinkle, Nature Nano 2010
Graphene grows rapidly on substrate steps and sidewalls. The sidewall first re-crystallizes (facets) and then graphitizes
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High device density
M.Sprinkle, Nature Nano 2010
More than 10,000 FETs per chip (6 x 4 mm2) Scalable method
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27°
S
Sample bias (V) Sample bias (V) Sample bias (V)
0
0.1
dI/
dV
(ar
b.u
nit
s)
dI/
dV
(ar
b.u
nit
s)
dI/
dV
(ar
b.u
nit
s)
Antonio Tejeda, Muriel Sicot, CNRS, France
STM - STS: graphene on side wall - buffer on Si-face
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Ribbons and beyond: Structured graphene
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.
Exceptional Room temperature Ballistic Transport in Epitaxial Graphene Nanoribbons
( Nature 506, 349, 2014)
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Multiprobe, in-situ transport measurements
SEM image of 4 probes positioned on a sidewall ribbon Outer probes supplying a current. Inner probes measure potential.
Omicron nanoprobe system (Tegenkamp group Hannover)
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0 1 2 3 4 5 Inner probe spacing L(µm)
R4
pp (
h/e
2)
0
1
2
3
Multiprobe, in-situ transport measurements
Four-point (R4pt) and two-point (R2pt) resistances as a function of probe spacing L.
4.2
28
16
58
l(µm)
(-100)
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Mo
bili
ty (
cm
2 V
-1 s
-1)
Ribbon width (nm)
Resis
tivity (
µW
-cm
)
Sh
ee
t re
sis
tan
ce
(W
)
100 1000 10 10-2
10-1
100
101
102
103
107
102
103
104
105
106
104
100
101
102
103 R = r
Length
Width.Thickness
r =¶R
¶LW.T
How to determine resistivity unambiguously
Measuring dR/dL eliminates contact resistances
Room temperature resisitivies and mobilities
0 1 2 3 4 5
Inner probe spacing L(µm)
R4
pp (
h/e
2)
0
1
2
3
Theoretical limit
Typical 2D exfoliated
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Ribbon conductance versus probe spacing and temperature
Two channels (0+, 0-)
One channel (0+)
0- turns off
0+ turns off
17 µm L0=200 nm
G=1+ exp(-T/T0) (L≤L0) G=1+ exp(1-L/L0) exp(-T/T0) (L>L0) T0=hc*/kBL
The 0- mode is the lowest longitudinal excitation of the ribbon, i.e. the n=0, m=1 state
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Longitudinal excitations of quasi particles
n=0, m=1
n=0, m=0
• What are the quasi particles? • They are Fermions and appear to have large magnetic moments. • They seem to have finite, temperature independent lifetimes! • Could they be composite particles (charged excitons)?
E0,1=hc*/2L
E0,0=0
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Summary Transport in neutral epigraphene nanostructures is exceptional,
with no counterpart in any other material system.
A new kind of quasiparticle is involved whose properties are ideal
for a new form of electronics.
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Conductance is ballistic at room temperature. G≈G0=2e2/h (≈1/13kW). Theoretically it should be 2G0
Current densities exceeding 10,000 µA/µm are sustained
Nanotube
fiber
- L (µm)
G (
2e2
/h)
L
V