[PPT] - Optical Spectroscopy of Carbon Nanotube p-n Junction Diodes Ji Ung PowerPoint Presentation

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jlee1@uamail.albany.edu

Optical Spectroscopy of Carbon Nanotube p-n Junction Diodes

Ji Ung Lee

College of Nanoscale Science and Engineering University 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 Albany Albany NanoTech Complex at the University at Albany

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State-of-the-Art Infrastructure

$50M, 150K ft2 32K Cleanroom Completed: 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

70nm 70nm

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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

f 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-Bandgap

Shrinkage)

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Bulk p-n junction diode basics:

EC EV EF

Equilibrium

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 EV 1 2 3

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Electrostatic doping:

Carrier Concentration Split gates VG1,2

J.U. Lee et. al., APL: July 5, 2004

p n

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20µm VG1 VG2 D S

2 gate device 3 and 4 gate devices

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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

0.5 1 1.5

V

DS(Volts)

p S D p n S D p n S D

10V
10V
10V

+10V +10V

10V

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Nearly Ideal Diode Characteristics with n~1 (1.2)

) 1 ( − =

T nK qV

B

e I I

p n p n

10 -11 10 -10 10 -9 10 -8 10 -7

0.4
0.2

0.2 0.4

VGS1,2=+/-10V Fit V

DS (Volts)

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Series Resistance Limits Current:

10 -11 10 -10 10 -9 10 -8 10 -7

0.4
0.2

0.2 0.4

V

DS (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

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10

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10

10

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10

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10

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0.5

0.5 1

Fit Data

Rs

IDS (Amps)

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

0.5

SWNTs are perfect, substrates are not.

J.U. Lee, Appl. Phys. Lett. 87, 073101 (2005)

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PV PD LED

8x10
12
4x10-12

4x10 -12 8x10 -12

0.2
0.1

0.1

VDS(V) IDS (Amps)

p n 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 0.10 10

15

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10

13

10

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10

11

IDS (A) VDS(V)

Exciton Peaks in the Photocurrent Spectra

(similar to SWNTs in solution)

0.5 1.0 1.5 1x10

14

2x10

14

3x10

14

4x10

14

ISC (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 Model DOS: One Electron Model

3D Bulk Semiconductor 2D Quantum Well 1D Quantum Wire 0D Quantum 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:

Coulomb Effects Absorption Energy

E

3D:

Coulomb Effects Excitons Absorption Energy

1D:

Coulomb Effects

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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

ISC (Normalized)

Energy (eV)

Spectra with similar first energies

EB 2 3 = E22 1 = E11

Lack of any features at Eg due to Sommerfeld factor <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 2.0 0.4 0.8 1.2 1.6 2.0

100 200 300

Intensity (a.u.) Raman frequency (cm

1)

Energy (eV) Diameter (nm)

Comparison to Photoluminescent Data:

+: Emperical Kataura Weisman 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 0.6 0.5 0.6 0.7 0.8 0.9 1.0

0.5 1.0 1.5 10 20 30 40

4 1 = E11 2 ISC (fA) 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 0.20 10

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IDS (A) VDS (V)

Ideal Diodes: n=1.0

Two mechanism for n=1.0: 1) Direct Band-to-Band 2) Diffusion of Minority Carriers from the doped regions

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Isc

D

pn np

S

p n

Isc

E11

1 2 3

EB Ea

L

EF EC EV Ea

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 0.20 1E-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

f the

band gap Shrinkage

f 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 Split Gates 1,2...layer 1,2...layer graphene graphene flake flake

n n-

type

type p p-

type

type n n-

type

type p p-

type

type