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

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 grating silver 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 emission 3. What are the practical difficulties in realizing SP-enhanced LEDs?

Key

notes ^{p g}

4. Summary of the five lectures notes

Remind!

The next chip-scale technology Three light-design regimes

λ limit

Light extraction

WAVE DESIGN ( d ~ λ )

e limit

LED ^{RAY DESIGN}

LED

( d > λ )

Internal QE

PHOTON DESIGN ( d < λ )

Power conversion efficiency of III-Nitride LEDs

E l

Example:

λ=530nm, I=350mA PCE ~ 12%

External efficiency of LEDs External efficiency of LEDs

η η ⎛ R ⎞

= ⎜ ⎟

:extraction efficiency

external

nr e

extrac

xtracti

tio

on

η

η R R η

= ⎜ ⎝ + ⎟ ⎠

[ ]

sin 2 ) ( 2 1

1

, 0 ⎟

⎠

⎜ ⎞

⎝

− ⎛

⎟⎠

⎜ ⎞

⎝

=⎛

∑ ∫

extraction ^c R

θ θ

θ

η

:nonradiative-recombination rate :spontaneous-emission rate

R

R

i (1 0) G N(2 5)

f

% 4

) / ( 4

1

≈ 2

⎠

⎝

⎠

⎝

g

f n

n

air(1.0) -

GaN(2.5) for

%

= 4

Wave Design for efficient extraction of the guided light

-. Geometric optics

extract external

nr ion

η R

η ⎛ R R ⎞

= ⎜ ⎝ + ⎟ ⎠

-. Random scattering g

in surface textured structure

APL 63, 2174 (1993)

Photon Design for increasing the emission rate

nr

η η

R R R

⎛ ⎞

= ⎜⎝ + ⎟⎠

What determines spontaneous emission rate of radiating source?

electron

E

Energy of EM field

( n 1/ 2) ω +

=

N b f h t V fl t ti

E

Number of photon (Stimulated emission)

Vacuum fluctuation (Spontaneous emission)

f

1 1

Fermi’s Golden Rule

2

0

1 1

( ) 2 ( )

R f i ρ ω

τ ω ε

= = p E ⋅

SE Rate : =

eMD Lab.

Microoptics Lab –Hanyang University

Dipole moment of radiation source

Electric field strength

of half photon (vacuum fluctuation)

Photon Design for increasing the emission rate

⎛ ⎞

external extraction

nr

η η

R R R

⎛ ⎞

= ⎜ ⎝ + ⎟ ⎠

2

2

)

1

( ) 1 (

R f i

τ ω ρ

ε ω

= = ⋅

= p E

Ag

n GaN

Quantum Quantum WellWell

p

p--GaNGaN

g

n-GaN

Atoms in microcavity

• High Q

Photonic crystal cavity

• Moderate Q Wid Δ

Surface plasmon coupling

• Low Q

• Narrow Δν

• F_p~ 1 – 5

• Low volume filling factor

• Wider Δν

• F_p(Quantum wells) ~ 3

• F_p(Quantum dots) ~ 5 –100

• Off-resonant and

• Narrow Δν

• F_p~ 5 – 100

• lossy and off-resonant complicated fabrication

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

Photonic-crystal approach

external extraction

nr

η η

R R R

⎛ ⎞

= ⎜ ⎝ + ⎟ ⎠

2

2

)

1

( ) 1 (

R f i

τ ω ρ

ε ω

= = ⋅

= p E

⎝

⎠ ( )

Baba

Limited 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 i

Good 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

( ) 1 (

R f i

τ ω ρ

ε ω

= = ⋅

= p E

2

τ ω ( ) ε =

Very small increase in E, ρ !

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

Surface-plasmon approach

R

η

=

p nr

R R η =

+

'

p sp nr

R

R R

R η = R ⁺

+ +

Surface Plasmons

The SP approach was started for organic LEDs

Conventional Structures:

ITO glass (anode) Organic molecules

Strongly coupled to SPPs Main issue:

SPP Î Radiation coupling

Cathode & Mirror SPP quenching (~40%)

SPP Î Radiation coupling

Metallic mirror Metallic thin film

SPP1

SPP2 SPP1

(Λ >

π

( Λ ~ π / k

)

Direct coupling

SPP band gap SPP cross-coupling

1 2

( Λ = π /[ k

− k

])

Effect of SPP band gap on PL

11411

Angle resolved PL Angle resolved PL of dye molecule (DCM)

1^st and 2^nd order diffraction of SPPs d act o o S s

Tracing 1^storder peaks shows SPP band gap.

Modification of Spontaneous Emission Rate of Eu

Main emission of Eu

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

SPP hi

( h k )

SPP quenching

( spacer thickness )

τ

TRPL at 614nm

Self-driven dipole (CPS) modeling

d

p Metal interface

2 2

d d

e

p b + p + ω p = E / 1 e

Im{ }

b b = + E

0 0

2

( / 2) ( / 2)

0

,

r

i ib t i ib t

r

p b p p E

dt m

dt

p p e

E E e

ω

− − − −

+ + =

= =

0 0

0 0

/ 1 Im{ }

b b E

m p b ω

= +

2 2

0 Re{ }

bb

b e

ω E

⎛ ⎞

⎜Δ ≈ ⎟

14

0

0 0 0

Re{ }

8 4 2 E

m p

ω ω ω ω

Δ ≈ − −

⎜ ⎟

⎝ ⎠

2 unknowns and 2 equations

Dipole Decay Calculation Test : Metal Mirror Cavity

10²

10^-4

10¹ 10²

wer

10^-1 10⁰

pated pow

10^-3 10^-2

perpendicular dipole parallel dipole

dissi

15

0.0 0.5 1.0 1.5 2.0

10^-4

k_x / 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 Spectrum

No guided mode TM

TM

+TE

TM

+TE

+TM

1

Emission Spectrum

70nm 100nm 200nm 390nm

3 0

2.0 2.5 3.0

total emission rate air emission

emission to substrate guided modes

te (R 0)

cover (medium c)

1.0 1.5

g

emission to active layer guided modes

adiation rat

h hc

dipole active material (medium a)

0 50 100 150 200 250 300 350 400

0.0

ra 0.5 h^s ( )

substrate (medium s) (h_a = h_s +h_c)

16

active layer thickness (nm)

Comparison with an experiment

90 100

%) ⁹⁰

100

80 90

iency (%

50 60 70 80

ratio (%)

60 70

PL Effic

10 20 30 40

P_air+P_sub+1.0P_guided P_air+P_sub+0.4P_guided P_air+P_sub+0.8P_guided P_air+P_sub+0.2P_guided P_air+P_sub+0.6P_guided P_air+P_sub+0.0P_guided

power

100 200 300 400 500 60

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 Eu

Ion

SE rate

90% SPP li Dipole-SPP

90% SPP coupling 25 times SE rate Dipole-SPP

coupling 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 QW

Most cited paper

Un-processed

(a)

Half-processed

(b)

Fully-processed

(c) 480nm period (2

order coupling)

(d) 250nm period (1

order coupling)

(160nm gap)

1 ^st Result of SPP enhanced PL from InGaN QW

Nature Materials VOL 3 p 601 605 2004

external extraction

η η ⎛ R ⎞

= ⎜ ⎟

2

2 )

1

( ) 1 (

R = = f ⋅ i ρ ω

= p E

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

external extraction

nr

η η

R

⎜ R + ⎟

⎝ ⎠

2

)

( ) f (

τ ω ρ

ε = p

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

1 ^st Result of SPP enhanced PL from InGaN QW

Nature 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

x2 x14 x28 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?

1 1

0

1 1 ( )

( ) 2

R f i ρ

τ ω ω

= = ε p ⋅ E

= _{High DOS}

due to decrease in Field enhancement

near the source layer

due to decrease in group velocity

eMD Lab.

Microoptics Lab –Hanyang University

1

1 ( ) ( ) 2

R f i ρ

τ ω ω

= = ε p ⋅ E

0

= ( ) 2 τ ω ε ⁼

Field enhancement

High DOS

due to decrease in group velocity

near the source layer

Requirements for enhancing SE rate

-. slow group velocity slow group velocity,

high loss

g p y B

-. 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

original additional

1

original original

R R R

F R R

≡ + = +

For a cavity mode:

3 2

mode volume

3 ( / ) 4

cav c

p

free

R Q n

F R V

λ

= = π

/

1 R

1 1 k

k

F _{⎛ ⎞} λ

⎜ ⎟

f _

0 0

1 1

2 /

SP SP

p

SP

F R π L υ c

= + = + ⎛ ⎞ ⎜ ⎟ ⎝ ⎠

( )

∂ For a SP mode :

2

2

( )

( )

SP

,

SP

at dipole

dz z

d L

dk

ω ωε ω

υ

∞

−∞

∂

= = ∫ ∂ ^E

E

We need a slow and confined mode!

Factors influencing Purcell Enhancement

Factors influencing Purcell Enhancement F F _{p p} ((ω ω))

Si l Q t W ll Si l Q t W ll

GaN ~ GaN ~ ζζ

Ag ~ z

GaN

Single Quantum Well Single Quantum Well

Variation with Ag thickness Variation with GaN thickness

Variation with Ag thickness Variation with GaN thickness

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

cover cover

Cover = 1.0

C 2 0

Cover = 1.5

Cover = 2.0

Î Need a very thin p-GaN layer !!

Improvement I-L curve

2.68 at 10 K F ⎛

= ⎜

1.75 300 F

at K

= ⎜ ⎝

I-V curve

“… the enhanced F_p… 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 k

under a certain thickness

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

• 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 Nanopatterning SP propagation length

1

2

3

ε

ε ε

ω

Nanopatterning

m] 4000

PL_SPs k

= ′′

2 1

)2

(

2 _m

m d

m d m

k c

ε ε ε

ε ε ε ω

⎟⎟ ′

⎠

⎜⎜ ⎞

⎝

⎛

′ +

′′=

2 5

Λ = λsp, 2λsp, 3λsp, …

2500 3000 3500

of SPs [nm Surface Plasmon on the Ag/GaN Interface

1 5 2.0 2.5

πc/μm) 460nm

530nm ^{λsp~70 nm}

1000 1500 2000 2500

tion Length

1.0 1.5

quency (2π 530nm

SP-dispersion

λsp 70 nm λsp~140 nm

450 500 550 600 650 700 750 800 0

500 1000

Propagat

0.0 0.5

0 2 4 6 8 10 12 14

I l W t (2 / )

Freq

S d spe s o

on Ag/GaN

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

2^nd 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

cExciton generation eRadiation

Metal (Ag-based) p-GaN

n-GaN cExciton generation

dSurface plasmon excitation InGaN MQW

e-h

Metal (Ag-based)

Silicon submount

dSurface plasmon excitation

Silicon submount

Λ Λ

D

h D

h

High output directionality g p y

by grating with non-even fill-factor

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

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

Extraction efficiency of a metal grating

• Data sampling at λ = 530 nm / w = 5 nm 1 η

γ

η = ^{+ ⋅}

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

int

1

nr sp

η ⁼ + γ ⋅ γ

1

FDTD

+ η ⋅ γ

1 1

(1 ) 1

FDTD

η

⁺ γ ⁻ 1

FDTD p int

sp

η ⁼ + γ

0 0

η

(1 ) 1

int sp

ext

sp

η γ

η γ

= +

10

180

60 100

γ

η

: nonradiative re-comb. rate : internal quantum eff

Max ~ 80% (at 140 nm / 40 nm)

int ext

η η

: internal quantum eff.

: extraction efficiency of metal grating

γ

단일 원기둥 구조 계산

Two-dimensional silver-grating (2 ^nd order)

1.1 1.2

Normalized LifeTime

Internal Quantum Efficiency 2 0 2.2

0 8 0.9 1.0

y Upward Emitted Power

nternal QE

1 4 1.6 1.8 2.0 Upward em

0.6 0.7 0.8

zed LT / In

1.0 1.2

1.4 mitted pow

0.3 0.4 0.5

Normaliz

0.4 0.6 0.8

wer (a.u.)

Λ = 250nm

Grating depth = 50nm Gap to QW = 30 nm

50 100 150 200 250 300 350 400 450 500 0.3

Diameter (nm)

0.2 p

169 nm

Optimum gap distance between metal and QW

2.0 2.5

cement

λ = 530 nm

d = 20 nm

1.0 1.5

d enhanc

0 0 0.5

Upward

0 5 10 15 20 25 30

0.0

Distance [nm]

coupling to surface plasmons coupling to lossy surface wave coupling to surface plasmons coupling 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 Mirro r L-Shape Substrate

mount

Z θ

• Little damage to p-GaN

• Enlarged surface area for

otat o

stage L-Shape

mount X

low contact resistance

EL Measurement

0.004 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

C u r r e n t ( A )

Sample images

An Optimistic Estimation for SP-enhanced LEDs

FDTD l l ti

At green (530 nm) with a 1

order grating

10 nm

epth

20 nm

FDTD calculation

5 nm

MQW

grating de

2.3 times more Photons 5 nm

60 nm

100 nm 180

140 nm

g

ti i d

Photons generated

0.8 1.0

ed

100 nm grating period 180 nm

82 %

Good directionality

0 2 0.4 0.6

hotons escap

34.1% within 20^o after escape

400 500 600 700 800

0.0

Ph 0.2

Wavelength (nm)

1/(2n²) = 8 % Surface plasmon

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

( Optimized LED with 50 % extraction) Î (82 % / 50 %) x 2.3 ~ 4 times Brighter

Nanocavity lasers

Nanocavity lasers

Nanocavity 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 emission 3. 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 lectures notes

External Efficiencies

p p

nr p

E R

R R

η

+

E R E R

Conventional LED

'

nr p SP

E R E R R R R

η = ⁺

+ +

SP LED

An Optimistic Estimation for SP-enhanced LEDs

10 nm FDTD calculation

At green (530 nm) with a 1

order grating

10 nm

depth

20 nm

2 3 ti 5 nm

MQW

grating d

2.3 times more Photons

ti

60 nm

100 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

D_cor

e 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.