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
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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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.
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)
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
Electrostatic doping:
CarrierConcentration
Split gates VG1,2 J.U. Lee et. al., APL: July 5, 2004
p n
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
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)
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.
(a) (b)
1 µm
Suspended SWNT Diodes:
p n
Suspended tube formed based on a self-registering technique
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)
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)
-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)
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
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
Sommerfeld Factor: Coulomb Interaction
Absorption
Energy
2D:CoulombEffects
Absorption
Energy
E
3D:CoulombEffects
Excitons
Absorption
Energy
1D:CoulombEffects
T. Ogawa and T. Takaghara, Phys. Rev. B 43, 14325 (1991)
Sommerfeld Factor in 1D -> 0 at Eg
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)
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
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
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)
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
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
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
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
