Optical Spectroscopy of Carbon Nanotube p-n Junction Diodes
Transcript of Optical Spectroscopy of Carbon Nanotube p-n Junction Diodes
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Optical Spectroscopy ofCarbon Nanotube p-n Junction Diodes
Ji Ung Lee
College of Nanoscale Science and EngineeringUniversity at Albany-SUNY
6th US-Korea Forum on Nanotechnology April 28-29, 2009
p n
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The College of Nanoscale Science & Engineering and The College of Nanoscale Science & Engineering and Albany NanoTech Complex at the University at AlbanyAlbany NanoTech Complex at the University at Albany
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State-of-the-Art Infrastructure
$50M, 150K ft2
32K CleanroomCompleted: 03/04
NanoFab 300S
$175M, 228K ft2
60K Cleanroom
Completion: 10/08
NanoFab 300N
NanoFab 300E$100M, 250K ft2
Completion: 1Q/09 $16.5M, 70K ft2
4K Cleanroom
Completed: 06/97
NanoFab 200
750K ft2 cutting-edge facilities (96,000 ft2 300mm Wafer Cleanrooms).$4.5B investments and 2500 R&D jobs on site.
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ANT/CNSE will house over 125 state-of-the-art 300mm wafer tools when build out is completed.
Designed for 32nm node & beyond but compatible with previous generations.
• Unit process, module integration, and full flow capability.
• Facility will have a 45nm baseline process for use by partners.
Facility capable of 25 integrated wafer starts (WSD)
per day. • 24/7 operation, wafer release 6 Days / Week
300 mm Wafer Processing Capability
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Device fabrication on 300mm wafers
>1000 devices/die
~100 nm features
Advanced processes
70nm70nm
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Why study the p-n diode:
• The p-n junction diode is the most fundamental of all the semiconductor devices – it is the basis for the majority of solid state devices.
• For fundamental understanding of semiconductors: Example: Hall-Shockley-Read Theory.
For any new semiconductor, a proper characterization of the p-n diode is important.
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Interplay between transport and optical properties:
• SWNT Diode Fabrication and DC Characteristics
• Optical Properties:Photovoltaic Effect Enhanced Optical Absorption - Excitons
• Origin of the Ideal Diode Behavior (BGR-BandgapShrinkage)
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Bulk p-n junction diode basics:
EC
EV
EFEquilibrium
EC
EV
Forward Bias(Recombination)
I=Io(eqV/nKT-1)I
V
Diode Equation:(ideal if n=1)
N-type(electrons) P-type(holes)V
I
Reverse Bias(Generation)
EC
EV1
2
3
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Electrostatic doping:
CarrierConcentration
Split gates VG1,2 J.U. Lee et. al., APL: July 5, 2004
p n
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J.U. Lee et. al., APL: July 5, 2004
CNT diode/rectifier:(p-n or n-p diode devices)
-1 10-6
-5 10-7
0 100
5 10-7
1 10-6
-1.5 -1 -0.5 0 0.5 1 1.5
VDS
(Volts)
pS D
p nS D
pnS D
-10V -10V
-10V +10V +10V -10V
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Nearly Ideal Diode Characteristics with n~1 (1.2)
)1( −= TnKqV
oBeII
p
n
pn
10 -11
10 -10
10 -9
10 -8
10 -7
-0.4 -0.2 0 0.2 0.4
VGS1,2=+/-10VFit
VDS
(Volts)
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Series Resistance Limits Current:
10 -11
10 -10
10 -9
10 -8
10 -7
-0.4 -0.2 0 0.2 0.4
VDS
(Volts)
Rs
Rs: measured from the resistive mode – due to n-type to metal contact resistance.
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(a) (b)
1 µm
Suspended SWNT Diodes:
p n
Suspended tube formed based on a self-registering technique
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Ideal Diodes with Ideality Factor n=1.0 for Suspended Diodes
VDS(V)10-13
10-12
10-11
10-10
10-9
10-8
10-7
-0.5 0 0.5 1
FitData
Rs
IDS
(Am
ps)
n=1.0
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
-0.5 0 0.5
SWNTs are perfect, substrates are not.
J.U. Lee, Appl. Phys. Lett. 87, 073101 (2005)
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PVPD
LED
-8x10-12
-4x10-12
0
4x10-12
8x10-12
-0.2 -0.1 0 0.1VDS(V)
IDS
(Am
ps)
pn
Photovoltaic Effect
(λ =1.5 µm)
Isc
Voc
Voc and Isc:Completely define PV properties for an ideal diode
Increase Intensity
J.U. Lee, Appl. Phys. Lett. 87, 073101 (2005)
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-0.10 -0.05 0.00 0.05 0.1010-15
10-14
10-13
10-12
10-11
IDS
(A)
VDS(V)
Exciton Peaks in the Photocurrent Spectra(similar to SWNTs in solution)
0.5 1.0 1.50
1x10-14
2x10-14
3x10-14
4x10-14
I SC (A
)
Energy (eV)
1
2
3
4
5
1
3
J.U. Lee et.al., Appl. Phys. Lett. 90, 053103 (2007)
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DOS: One Electron ModelDOS: One Electron Model
3DBulk Semiconductor
2DQuantum Well
1DQuantum Wire
0DQuantum Dot
E
D. O
. S.
D. O
. S.
D. O
. S.
D. O
. S.
E E E
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Heh = −ε |re−rh |e 2
Electron-Hole Coulomb Interaction
EXCITONS IN CARBON NANOTUBES
results in the electron-hole binding that forms the exciton states below the conduction subband edge
Exciton Hydrogenic Levels n=1,2,3…
continuum
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Sommerfeld Factor: Coulomb Interaction
Absorption
Energy
2D:CoulombEffects
Absorption
Energy
E
3D:CoulombEffects
Excitons
Absorption
Energy
1D:CoulombEffects
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T. Ogawa and T. Takaghara, Phys. Rev. B 43, 14325 (1991)
Sommerfeld Factor in 1D -> 0 at Eg
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0.6 0.8 1.0 1.2 1.4
0.5
1.0
1.5
2.0 I S
C (N
orm
aliz
ed)
Energy (eV)
Spectra with similar first energies
EB
2
3 = E221 = E11
Lack of any features at Egdue to Sommerfeldfactor <1
Side bands measure dark exciton
J.U. Lee et.al., Appl. Phys. Lett. 90, 053103 (2007)
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1.0 1.2 1.4 1.6 1.8 2.00.4
0.8
1.2
1.6
2.0
100 200 300
Inte
nsity
(a.u
.)
Raman frequency (cm-1)
Ene
rgy
(eV
)
Diameter (nm)
Comparison to Photoluminescent Data:
+: Emperical KatauraWeisman et.al. Nano Lett. 3, 1235 (2003)
- E11 and E22– Exciton-phonon ▲ - Quasipaticle Bandgap
Continuum:1.55eV/nm
E11: 1.01eV/nm
EB: 0.54 eV/nm
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0.4 0.5 0.60.5
0.6
0.7
0.8
0.9
1.0
0.5 1.0 1.50
10
20
30
40
4
1 = E11
2
I SC (f
A)
Energy (eV)
3 = E22
5 = E33
E11
(eV
)
Ea(eV)
E11=Ea
Origin of the Ideal Diode Behavior and Exciton Dissociation:
Ea < E11 ??
-0.10 -0.05 0.00 0.05 0.10 0.15 0.2010-15
10-14
10-13
10-12
10-11
10-10
10-9
10-8
IDS
(A)
VDS (V)
Ideal Diodes:n=1.0
Two mechanism for n=1.0:1) Direct Band-to-Band2) Diffusion of Minority Carriers
from the doped regions
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Isc
D
pn
np
S
p
n
Isc
E111
2
3
EB Ea
L
EF
EC
EVEa
Many-Body Renormalization of Band structure (BGR – band gap renormalization) and Proposed Mechanism for Exciton Dissociation:
Formation of heterointerfaces along a homogenous material
J.U. Lee, Phys. Rev. B 75, 075409 (2007)
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Device Ideal for Studying BGR:
-0.10 -0.05 0.00 0.05 0.10 0.15 0.201E-15
1E-14
1E-13
1E-12
1E-11
1E-10
1E-9
1E-8 6V 8V 11V
IDS
(A)
VDS (V)
Variable Doping with VG1,2: • Diode follows ideal relation with doping.
• Evidence of strong BGR: Io when Doping . w/o BGR Io when Doping .
p
SiO2
VG1 VG2
S D
L
n
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Ef Ef
w/o BGR: minority carrier decreases
Ef
w/ BGR: minority carrier increases!
Origin of increase in Io with Doping:
Increase Doping
Minority Carriers
No shrinkage of the band gap
Shrinkage of the band gap
P type semiconductor
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Conclusions:
• Bipolar devices are more fun to study.
• How do neutral excitons dissociate to generate large photocurrents?
• Window to the study of many-body effects: BGR, biexctions, etc…
Funding: NSF, NRI/INDEX, IFC, IBM and UAlbany
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Split Gates
1,2...layer graphene flake
nn--typetype pp--typetypenn--typetype pp--typetype
Future Work: Graphene p-n junctions: Optics-like manipulation of electrons