Efficient light emission from LEDs, OLEDs, and

(Fifth Lecture) Techno Forum on Micro-optics and Nano-optics Technologies

Efficient light emission from LEDs, OLEDs, and nanolasers via surface-plasmon resonance

송 석 호, 한양대학교 물리학과, http://optics.anyang.ac.kr/~shsong

silver gratingsilver grating

1. How does the surface plamon resonance enhance the internal quantum efficiency of light source?2. Understand the Fermi-Golden rule and Purcell enhancement factor in spontaneous emission3. What are the practical difficulties in realizing SP-enhanced LEDs?

Key notes p g

4. Summary of the five lecturesnotes

Remind!

The next chip-scale technology Three light-design regimes

λ limit

Light extraction

WAVE DESIGN

( d ~ λ )

e limit

LED RAY DESIGNLED( d > λ )

Internal QEPHOTON DESIGN

( d < λ )

Power conversion efficiency of III-Nitride LEDs

E lExample:λ=530nm, I=350mAPCE ~ 12%

External efficiency of LEDsExternal efficiency of LEDs

Rη η⎛ ⎞

= ⎜ ⎟

:extraction efficiency

externalnr

e

extrac

xtracti

tio

on

nη R Rη

η

= ⎜ ⎟+⎝ ⎠[ ]

2sin)(1

21

, 0⎟⎠⎞

⎜⎝⎛−⎟

⎠⎞

⎜⎝⎛= ∑ ∫psextraction dR

c θθθηθ

:nonradiative-recombination rate:spontaneous-emission ratenrRR

i (1 0)G N(2 5)f%4

)/(41

2≈

⎠⎝⎠⎝

gf nn

air(1.0)-GaN(2.5)for %4=

Wave Design for efficient extraction of the guided light-. Geometric optics

extractexternalnr

ionRη

R Rη

⎛ ⎞= ⎜ ⎟+⎝ ⎠

-. Random scattering gin surface textured structure

APL 63, 2174 (1993)

Photon Design for increasing the emission rate external extractionnr

η ηRR

R⎛ ⎞

= ⎜ ⎟+⎝ ⎠

What determines spontaneous emission rate of radiating source?electron

iEEnergy of EM field

( 1/ 2)nω +

N b f h t V fl t ti

fE

Number of photon(Stimulated emission)

Vacuum fluctuation(Spontaneous emission)

f

1 1Fermi’s Golden Rule

2

0

1 1 ( )( ) 2

R f i ρ ωτ ω ε

= = ⋅p ESE Rate : Photon DOS(density of states)

eMD Lab. 6Microoptics Lab –Hanyang University

Dipole moment of radiation source

Electric field strengthof half photon (vacuum fluctuation)

Photon Design for increasing the emission rate⎛ ⎞

external extractionnr

η ηRR

R⎛ ⎞

= ⎜ ⎟+⎝ ⎠

2

02)1

( )(1R f i

τ ωρ

εω= = ⋅Ep E, ρ increase

Ag

n GaNQuantum Quantum WellWell

pp--GaNGaN

g

n-GaN

Atoms in microcavity• High Q

Photonic crystal cavity• Moderate Q

Wid Δ

Surface plasmon coupling• Low Q

• Narrow Δν• Fp ~ 1 – 5

• Low volume filling factor

• Wider Δν• Fp(Quantum wells) ~ 3

• Fp(Quantum dots) ~ 5 –100• Off-resonant and

• Narrow Δν• Fp ~ 5 – 100

• lossy and off-resonant

complicated fabrication

www.phys.unt.edu/research/ photonic/website/Surf-Plasmon-OHPs-f.pptDepartment of Physics, University of North Texas, Denton, Texas 76203

Photonic-crystal approach

external extractionnr

η ηRR

R⎛ ⎞

= ⎜ ⎟+⎝ ⎠

2

02)1

( )(1R f i

τ ωρ

εω= = ⋅Ep E, ρ increase

nr⎝ ⎠0( )

BabaLimited by surface recombination

G d h !!!

Limited by surface recombination

G d h !

LumiLed

Good scheme!!!100 um device size achievable.

Several layer of PC for extraction.

G d i t l t ffi i

Good scheme!100 um device size achievable.

Several layer of PC for extraction.

G d i t l t ffi iGood internal quantum efficiency Needed (>90%).

Multiple pass limits device size (~10um).

Small volume needed.

Good internal quantum efficiency Needed (>90%).

Multiple pass limits device size (~10um).

Small volume needed.Small volume needed.Not so good for lighting.

Surface recombination limited

Small volume needed.Not so good for lighting.

Surface recombination limited

Noda

Surface recombination limited.Surface recombination limited.

Photonic-crystal assisted LEDs

2

2)1

( )(1R f i

τ ωρ

εω= = ⋅Ep

02( )τ ω ε

Very small increase in E, ρ !

Look like a result of wave design rather than photon design!

Surface-plasmon approach

pRη =intp nrR R

η =+

' sppintp np rs

RR R

RR

η+

=+ +

Surface Plasmons

The SP approach was started for organic LEDs

Conventional Structures:ITO glass (anode)

Organic molecules

Strongly coupled to SPPs

Main issue: SPP Radiation couplingCathode & Mirror SPP quenching

(~40%)SPP Radiation coupling

Metallic mirror Metallic thin film

SPP1

SPP2

SPP1

( / )SPPkπΛ >( ~ / )SPPkπΛDirect couplingSPP band gap SPP cross-coupling

1 2( /[ ])SPP SPPk kπΛ = −

Effect of SPP band gap on PL11411

Angle resolved PLAngle resolved PL of dye molecule (DCM)

1st and 2nd orderdiffraction of SPPsd act o o S s

Tracing 1st order peaks shows SPP band gap.

Modification of Spontaneous Emission Rate of Eu3+

Main emission of Eu3+ (614nm)Main emission of Eu (614nm)

SPP hi

( )h k

SPP quenching

( )spacer thicknessτ

TRPL at 614nm

Self-driven dipole (CPS) modeling

d

p Metal interface

2 22d d ep b p p Eω+ + =

2

/ 1 Im{ }eb b E= +0 02( / 2) ( / 2)

0 0,

r

i ib t i ib tr

p b p p Edt mdt

p p e E E eω ω

ω

− − − −

+ + =

= =

0 00 0

/ 1 Im{ }b b Em p bω

= +

2 20 Re{ }

bbb e Eω⎛ ⎞

Δ ≈⎜ ⎟

14

00 0 0

Re{ }8 4 2

Em p

ωω ω ω

Δ ≈ − −⎜ ⎟⎝ ⎠2 unknowns and 2 equations

Dipole Decay Calculation Test : Metal Mirror Cavity

102

10-4

101

102

wer

10-1

100

pate

d po

w

10-3

10-2

perpendicular dipole parallel dipole

diss

i

15

0.0 0.5 1.0 1.5 2.010-4

kx / k1 J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, 725-741, 2000

CPS Model Calculation for Spontaneous Emission Rates of an OLED

Emission SpectrumNo guided mode TM0 TM0+TE0 TM0+TE0+TM

1

Emission Spectrum

70nm 100nm 200nm 390nm

3 0

2.0

2.5

3.0

total emission rate air emissionemission to substrate guided modeste

(R0)

cover (medium c)

1.0

1.5

g emission to active layer guided modes

adia

tion

rat

hchdipole active material

(medium a)

0 50 100 150 200 250 300 350 400

0.0

0.5ra sh( )

substrate (medium s)

( )a s ch h h= +

16

active layer thickness (nm)

Comparison with an experiment

90

100

%) 90

100

80

90

ienc

y (%

50607080

ratio

(%)

60

70

PL E

ffic

10203040

Pair+Psub+1.0Pguided Pair+Psub+0.4Pguided Pair+Psub+0.8Pguided Pair+Psub+0.2Pguided Pair+Psub+0.6Pguided Pair+Psub+0.0Pguided

pow

er

100 200 300 400 50060

Film Thickness (nm)0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400

active layer thickness (μm)

(measured) (calculated)

17

SPP Enhanced Spontaneous Emission of Eu3+ Ion

SE rate

90% SPP li

Dipole-SPP

90% SPP coupling25 times SE rate

Dipole-SPPcoupling fraction

Maximum internal efficiency

Role of Preferred Orientation of the Dipole Source

Adv. Mater. 14 19 1393

Angle integrated EL

Enhanced PL by Coupled SPP

Cross-Coupled vs Coupled SPP

(1)

(2)

(3)

(4)

SPP Enhanced PL of InGaAs QWMost cited paper

Un-processed(a)

Half-processed

(b)

Fully-processed(c) 480nm period (2nd order coupling)(d) 250nm period (1st order coupling)(160nm gap)

1st Result of SPP enhanced PL from InGaN QWNature Materials VOL 3 p 601 605 2004

external extractionη ηR⎛ ⎞

= ⎜ ⎟2

2)1

( )(1R f i ρ ω= = ⋅Ep E, ρ increase

Nature Materials, VOL 3, p.601-605, 2004

external extractionnr

η ηRR⎜ ⎟+⎝ ⎠02

)( )

(fτ ω

ρε

p

Nature Materials, VOL 3, p.601-605, 2004

1st Result of SPP enhanced PL from InGaN QWNature Materials, VOL 3, p.601-605, 2004

40x100nm2 133nm wide, 400nm period grating

(no enhancement for 200nm wide, 600nm period grating)

0.42

0 06

0.18

x14x2x28

0.06

Average internal quantum efficiency estimatione age te a qua tu e c e cy est at o

TRPL of SPP enhanced InGaN QW emission

How does the surface-plasmon resonance contribute to emission rate?

21 1 2

0

1 ( )1( ) 2

R f i ρτ ω

ωε

= = ⋅EpHigh DOS due to decrease in

Field enhancement near the source layer

due to decrease in group velocity

eMD Lab. 26Microoptics Lab –Hanyang University

21 ( )1( ) 2

R f i ρτ ω

ωε

= = ⋅Ep0( ) 2τ ω ε

Field enhancement

High DOS due to decrease in group velocity

near the source layer

Requirements for enhancing SE rate

-. slow group velocityslow group velocity,high lossBg p y

-. tight confinement of mode-. low ohmic loss-. large field enhancement

g

fast group velocity,l l

A

low loss

A B

Q.W. Q.W.

Purcell factor defining enhancement of the spontaneous emission

R R R+1original additional additionalp

original original

R R RF

R R+

≡ = +

For a cavity mode:3

2mode volume

3 ( / )4

cav cp

free

R Q nFR V

λπ

= =

0/11 1SP SPR k k

F λ⎛ ⎞⎜ ⎟

_f

0

0

1 12 /

SP SPp

SP

FR L cπ υ

⎛ ⎞= + = + ⎜ ⎟⎝ ⎠

( )∂

For a SP mode :

2

2

( ) ( ),SPSP

at dipole

dz zdL

dk

ωεω ωυ

∞

−∞

∂∂= =

∫ EE at dipole

We need a slow and confined mode!

Factors influencing Purcell Enhancement Factors influencing Purcell Enhancement FFpp((ωω))

Si l Q t W llSi l Q t W llGaN ~ GaN ~ ζζ

Ag ~ z

GaNSingle Quantum WellSingle Quantum Well

Variation with Ag thickness Variation with GaN thicknessVariation with Ag thickness Variation with GaN thickness

Purcell enhancement factor (F-1)Purcell factor: A numerical estimationcovercover

Cover = 1.0

C 2 0Cover = 1.5

Cover = 2.0

Need a very thin p-GaN layer !!

ImprovementI-L curve

2.68 10at KF

⎛= ⎜ No improvement1.75 300p

Fat K

= ⎜⎝

No improvementI-V curve

“… the enhanced Fp … can be attributed to an increase in the spontaneous emission rate due to SP-QW coupling.”

Why SP-LED hasn’t been successful yet?y y

Practical Barriers (especially for InGaN/GaN devices)Practical Barriers (especially for InGaN/GaN devices)

• Thin p-GaN leads to abrupt occurrence of leakage current d t i thi kunder a certain thickness

• SP propagation length in blue wavelength along the Ag/GaN interface is extremely shorty

• Nanopatterning becomes a huge burden at short wavelength

• Damageless p GaN patterning has been impossible• Damageless p-GaN patterning has been impossible• SQW devices are prone to leakage current due to carrier overflow• Silver is a nasty material with poor adhesion to GaN

and tends to agglomerate at an elevated temperature

SP propagation length NanopatterningSP propagation length

123

εεεω ′′⎟⎞⎜⎛ ′

Nanopatterning

4000

m]

kPLSPs ′′

=21

2)(2 mm

dm

dm

ck

εε

εεεεω

′⎟⎟⎠

⎞⎜⎜⎝

⎛+′

=′′

2 5

Λ = λsp, 2λsp, 3λsp, …

2500

3000

3500

of S

Ps

[nm Surface Plasmon on the Ag/GaN Interface

1 5

2.0

2.5

πc/μ

m)

460nm530nm λsp~70 nm

1000

1500

2000

2500

tion

Leng

th

1.0

1.5

quen

cy (2

π 530nm

SP-dispersion

λsp 70 nm λsp~140 nm

450 500 550 600 650 700 750 8000

500

1000

P

ropa

gat

0.0

0.5

0 2 4 6 8 10 12 14I l W t (2 / )

Freq

S d spe s oon Ag/GaN

Wavelength of Photon [nm] In-plane Wavevector (2π /μm)

2nd order gratings (Λ~280nm)

i ht b dil f b i t d Green LEDs might be possible.

might be readily fabricated

by Holo litho at Green.

Schematic structure

Photon

n-GaN

Sapphire

Exciton generationRadiation

Metal (Ag-based)

p-GaN

n-GaN Exciton generation

Surface plasmon excitation

InGaN MQW e-h

Metal (Ag-based)

Silicon submount

Surface plasmon excitation

Silicon submountΛΛ

Dh

Dh

High output directionality g p yby grating with non-even fill-factor

1st order grating, fill factor=0.1 1st order grating, fill factor=0.5

2nd order grating, fill factor=0.1 2nd order grating, fill factor=0.7

Extraction efficiency of a metal grating

• Data sampling at λ = 530 nm / w = 5 nm

1 ext spη γη+ ⋅

=

• Data sampling at λ = 530 nm / w = 5 nm

1int nr spη

γ γ=

+ ⋅

1 ext spFDTD η γ+ ⋅

1

1

(1 ) 1FDTDi tη γ+ −

1pFDTD

intsp

ηγ

=+

00

extη

(1 ) 1int spext

sp

η γη

γ+

=

10

180

60 100nrγη

: nonradiative re-comb. rate

: internal quantum effMax ~ 80% (at 140 nm / 40 nm)

int

ext

ηη

: internal quantum eff.

: extraction efficiency of metal grating

spγ : re-comb. rate to surface plasmon

단일 원기둥 구조 계산Two-dimensional silver-grating (2nd order)

1.1

1.2

Normalized LifeTimeInternal Quantum Efficiency 2 0

2.2

0 8

0.9

1.0y

Upward Emitted Power

nter

nal Q

E

1 4

1.6

1.8

2.0 Upw

ard em

0.6

0.7

0.8

zed

LT /

In

1.0

1.2

1.4 mitted pow

0.3

0.4

0.5N

orm

aliz

0.4

0.6

0.8

wer (a.u.)

Λ = 250nmGrating depth = 50nm Gap to QW = 30 nm

50 100 150 200 250 300 350 400 450 500

0.3

Diameter (nm)

0.2p

169 nm

Optimum gap distance between metal and QW

2.0

2.5

cem

ent

λ = 530 nmd = 20 nm

1.0

1.5d

enha

nc

0 0

0.5

Upw

ard

0 5 10 15 20 25 300.0

Distance [nm]

coupling to surface plasmonscoupling to lossy surface wave coupling to surface plasmonscoupling to lossy surface wave

6nm is a theoretical limit given by self-driven dipole (CPS) modeling[W. L. Barens and P. T. Worthing, Optics Communications 162, 16 (1999)]

Grating on p-GaN

Rotation

Aperture

Mirror

L-Shape

Substratemount

Zθ

• Little damage to p-GaN• Enlarged surface area for

otat ostage L-Shapemount

X

Y

low contact resistance Linearstage

EL Measurement0.004

0.0045

Higher output power t 70 %

0.0025

0.003

0.0035

arb.)

r e f

250A_3

250B_2

250C_2

up to 70 %

0.0015

0.002

0.0025

Power(a

270A_4

270B_2

270C_3

290A_3

0

0.0005

0.001 290B_2

0

0 0.1 0.2 0.3 0.4

Cu r re n t (A )

Sample images

An Optimistic Estimation for SP-enhanced LEDsFDTD l l ti

At green (530 nm)with a 1st order grating

10 nm

epth 20 nm

FDTD calculation

5 nmMQW

grat

ing

de 2.3 times more

Photons5 nm

60 nm100 nm 180

140 nm

g

ti i d

Photonsgenerated

0.8

1.0

ed

100 nm 180 nmgrating period

82 %Good directionality

0 2

0.4

0.6

hoto

ns e

scap

34.1% within 20oafter escape

400 500 600 700 8000.0

0.2PhWavelength (nm)

1/(2n2) = 8 %Surface plasmon

(Bare-chip LED with 8 % extraction) (82 % / 8 %) x 2.3 ~ 24 times Brighter( Optimized LED with 50 % extraction) (82 % / 50 %) x 2.3 ~ 4 times Brighter

Nanocavity lasers

Nanocavity lasersNanocavity lasers

Final comments

1. How does the surface plamon resonance enhance the internal quantum efficiency of light source?2. Understand the Fermi-Golden rule and Purcell enhancement factor in spontaneous emission3. What are the practical difficulties in realizing SP-enhanced LEDs?

Key notes 3. What are the practical difficulties in realizing SP enhanced LEDs?

4. Summary of the five lecturesnotes

External Efficienciesp

pnr p

RER R

η =+

E R E R

Conventional LED

' p p SP SPnr p SP

E R E RR R R

η+

=+ +

SP LED

An Optimistic Estimation for SP-enhanced LEDs10 nm

FDTD calculation

At green (530 nm)with a 1st order grating

10 nm

dept

h 20 nm

2 3 ti

5 nmMQW

grat

ing

d 2.3 times more

Photonsti

60 nm100 nm 180 nm

140 nm

grating period

generation

Final comments

Summary of the five lectures

(06/23) Introduction: Micro- and nano-optics based on diffraction effect for next generation technologies(06/30) Guided-mode resonance (GMR) effect for filtering devices in LCD display panels(07/07) Surface-plasmons: A basic(07/14) Surface plasmon waveguides for biosensor applications(07/14) Surface-plasmon waveguides for biosensor applications(07/21) Efficient light emission from LED, OLED, and nanolasers by surface-plasmon resonance

R0 T0

GMR grating

Micros

Dcore SPP mode

metal strip

core

cladding

metal slab

core

cladding

Final comments

Summary of the five lectures

Now, let’s get back to Macros with Nanos and Micros.