• Concept of SP-LED

(1)

Emission enhancement behaviors in the coupling

between surface plasmon polaritons and light emitters

작성 : 한양대학교 물리학과 최기영

(2)

• Concept of SP-LED

• History

• Theoretical calculations

• PL enhancement of BLUE GaN SP-LED

• PL, EL enhancement of GREEN GaN SP-LED

• Summary

(3)

A Better Light Emitting Diode

Display

Free-Space

Optical Communication

Brighter

General Lighting

Brighter, More Directional

Brighter, More Directional, Faster

(4)

rate ion recombinat ve

nonradiati :

efficiency extraction

:

rate emission s

spontaneou 1 :

nr extrac

extrac ext

Γ

= Γ

Γ + Γ

= Γ

η τ η η η

) % /

( 4

2

1

2

≈

=

g f

extrac

n

η n

(5)

• Geometric optics

• Random scattering in surface textured structure

APL 63, 2174 (1993)

(6)

(7)

(8)

CONCEPT

2 0

1 1

( ) 2 ( )

R f i ρ ω

τ ω ε

= = p E ⋅

SE Rate : =

Dipole moment

of the radiating source Electric field strength

of half photon (vacuum fluctuation)

Photon DOS

(Density of States) Surface Plasmons Field enhancement

Directional extraction

p int

p nr

R R R η =

+

'

_int ^p ^sp

p sp nr

R

R R

R η = R ⁺

+ +

(9)

HISTORY

We have shown for the first time that

strongly directional emission … by SP excited

on a periodically structured surface.

(10)

HISTORY

An SPP energy gap can block this decay channel and significantly inhibit the emission from the sample.

(11)

HISTORY

(12)

HISTORY

… spontaneous emission into surface plasmon is ~ 55 times faster … We have carefully calibrated the internal quantum efficiency of this InGaN SQW and we find it to be > 90%.

(13)

ONLY

There are Photo-luminescences.

From 1999 to 2008

Can it work?

HISTORY

(14)

HISTORY

2.68 10 1.75 300

p

at K

F at K

= ⎜ ⎛

⎝

… the enhanced F_p…

can be attributed

to an increase in the spontaneous emission rate due to SP-QW coupling.

No improvement I-V curve

Improvement

I-L curve

(15)

-. Gain condition

-. FDTD 계산을 통해 반사형 금속 격자 구조 결정 -. Green GaN SP-LED 의 PL, EL 측정

-. Blue GaN SP-LED 의 PL 측정

(16)

Theoretical calculations

(17)

Purcell Factor:

2 0

0

/ ( )

( ) / 1

/ '

SP SP

p SP p

SP SP

k k a

F R R R

V c k U ε

= + = + E

2

0

1 1

( ) 2 ( )

R f i ρ ω

τ ω ε

= = p E ⋅

Spontaneous Emission Rate : =

Green

Purcell’s factor on flat metal surface

(18)

Extraction efficiency condition

( )

int 0 int

0 int

0 0

int int

0 int

1

1 1 1

1

1 1

r sp

sp

r nr sp r nr r sp r sp

sp

r r nr sp r p r

r nr

r sp sp

r nr r nr sp

p r p p r nr

r nr

r nr sp

p r nr

k C k

k k k k k k C k k C k

k k k k k F k

k k

k C k C k

k k k k C k

F k F F k k

k k

k k k k

C

F k k

ξ η η

η

η η

η + ⋅

+ + + + ⋅ + ⋅

= = = ⋅ = ⋅

+ +

+

+ ⋅ ⋅

+ ⎡ ⎤

+ +

= ⋅ = ⋅ = ⋅ +⎢⎣ ⋅ + ⎥⎦

+

+ +

= ⋅ + ⋅ −

+

( )

0 int

1 1 1

r nr

p p

k k k

C F

F η

⎡ ⎛ + ⎞⎤

⎢ ⎜ + ⎟⎥

⎢ ⎝ ⎠⎥

⎣ ⎦

⎡ ⎤

= ⋅ +⎢ ⋅ − ⎥

⎣ ⎦

0 1 int

sp when C

ξ η

∴ ≥ ≥

0 int

r

r nr

k k k η =

+

int

r sp

sp

r nr sp

k C k

k k k

η = ^{+ ⋅} + +

r nr sp

p

r nr

k k k

F k k

+ +

= +

Original IQE :

Modified IQE by SP :

Purcell’s factor :

Extraction efficiency of SP : C

Enhancement factor of IQE : ^int₀

int SP sp

ξ η

=η

(19)

2^nd order coupling (Λ=103nm), T=0.02%, R=0.07%, A=99.91%

n-GaN

p-GaN (20nm) Ag (20nm) PR

(100nm, n=1.6)

Λ

4^th order coupling (Λ=206nm), T=0.03%, R=0.09%, A=99.88%

Case 1

(20)

n-GaN

p-GaN (20nm) Ag (20nm) Λ

Case 2

(21)

n-GaN p-GaN

(20nm, 120nm) Ag (20nm)

Case 3

Λ

(22)

0 40 80 120 160 200 240 0

5 10 15 20 25

Λ=λ_SP Λ=2λ_SP Λ=3λ Λ=4λSP_SP

R e fl ectan ce [ A .U ]

grating depth [nm]

Λ

(23)

SP 1^storder 2^ndorder 3^rdorder 4^th order 1차 회절광이 수직으

로 방출된다.

2차 회절광이 수직으 로 방출된다.

Λ=λ

_SP

=6 λ

₀

Λ=2λ

_SP

Λ=4λ

_SP

k_x k_y

GaN circle Air circle

3차와 2차 회절광이 GaN로 방출되고 3차 회절광 만이 공 기 중으로 방출된다.

Λ=3λ

_SP

Fourier Trans. Fourier Trans.

4차와 3차 회절광이 GaN로 방출되고 4차 회절광 만이 공 기 중으로 방출된다.

(24)

0 10 20 30 40 50 0.0

0.2 0.4 0.6 0.8 1.0

Fraction

Distance [nm]

A

B

C D

A : Lossy surface wave mode B : Surface plasmon mode C : Direct radiation mode D : Balance

h = 20 nm 에서의 비율은, A : 25.1 %

B : 55.9 % C : 19 %

(25)

silver GaN

Λ d

Q.W h

0 0

(a) (b)

400

1000

Wav e le ngth [ n m]

Angle (degree) Purcell’s factor

700 500 600

800 900

5 10 15

0.0 1.0

0.5

20 40 60

silver GaN

Λ d

Q.W h

silver GaN

Λ d

Q.W h

0 0

(a) (b)

400

1000

Wav e le ngth [ n m]

Angle (degree) Purcell’s factor

700 500 600

800 900

5 10 15

0.0 1.0

0.5

20

20 4040 6060

(26)

100 200 300 400 500 600 100

200 300 400 500 600

1.5 2 2.5 3

200 400 600 800 1000 1200

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

10 20 30 40 50 60 110

120 130 140 150 160

10 20 30 40 50 60 110

120 130 140 150 160

Grating depth [nm]

Grating period [nm ]

Grating depth [nm]

Gra ting per iod [nm]

7.5

3.0 1.5

3.0

100 200 300 400 500 600

1.5 2 2.5 3

100 200 300 400 500 600

1.5 2 2.5 3

200 400 600 800 1000 1200

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

200 400 600 800 1000 1200

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

10 20 30 40 50 60 110

120 130 140 150 160

10 20 30 40 50 60 110

120 130 140 150 160

Grating depth [nm]

Grating period [nm ]

Grating depth [nm]

Gra ting per iod [nm]

7.5

3.0 1.5

3.0

(a) (b)

0 int

0 0

int

1 1 1

sp

int sp

int p

C

F

η η

ξ η η

⎛ ⎞

= ≈ + − ⎜ ⎜ ⎝ − ⎟ ⎟ ⎠

0

int _sp 1

C η for ξ

∴ ≥ ≥

에서, 의 조건이 성립한다.

추출효율이 Green GaN LED 의 original IQE(~20%로 가정) 보다 크므로 SP gain 은 1보다 크다.

Purcelll’s factor SP gain

(27)

Refractive indices :

GaN = 2.44, Al₂O₃=1.77157, Silicon encapsulant = 1.5 Al₂O₃로 방출되기 위한 GaN 내에서의 내부 방사각도 = 46.5°

Encapsulant 로 방출되기 위한 GaN 내에서의 내부 방사각도 = 37.9 ° (*Encapsulant 는 형태가 다양할 수 있어 계산하지 않음.)

GaN Al₂O₃ Encap.

Angular distribution

(28)

SP gain 에 의한 IQE 증가와 extraction efficiency 를 고려해서 External efficiency 를 계산.

(Encap. 까지 도달한 빛은 모두 추출된다고 가정) A : total external efficiency

B : collimated radiation 되는 비율 (GaN 에서 9° / Encap. 내에서는 14.7° )

Dashed line : bare chip 상태의 conventional green GaN LED 의 external efficiency.(4.2%)

(original IQE : 20 % / randomly free dipole 에서 radiation 된 빛의 encap. 까지의 추출효율 : 21 %)

A B

120 130 140 150 160

0 10 20 30 40 50 60 70

Efficiency [%]

Grating period [nm]

External quantum efficiency

(29)

GREEN SP-LED EXPERIMENTS

(30)

400 500 600 700 800 0.0

0.2 0.4 0.6 0.8 1.0

Photons escaped

Wavelength (nm) 34.1% within 20^o

after escape 81.9%

13.9%

1/(2n²) = 7.7%

Why Green?

OPTICS EXPRESS 16, 1269 (2008) JKPS 53, 1715 (2008)

± 10

^D

± 90

^D

(31)

Why Green?

SP propagation length

450 500 550 600 650 700 750 800 0

500 1000 1500 2000 2500 3000 3500 4000

Propagation Length of SPs [nm]

Wavelength of Photon [nm]

Surface Plasmon on the Ag/GaN Interface PL_SPs k

= ′′

2 1

2 2

3

) ( 2

_m

m d

m d m

k c

ε ε ε

ε ε ε ω

′

⎟⎟ ′′

⎠

⎜⎜ ⎞

⎝

⎛

′ +

= ′

′′

0.0 0.5 1.0 1.5 2.0 2.5

0 2 4 6 8 10 12 14

In-plane Wavevector (2π /μm)

Frequency (2πc/μm)

460nm

530nm

SP-dispersion on Ag/GaN

λsp~70 nm λsp~140 nm

2

^nd

order gratings (

Λ~280nm)

might be readily fabricated by Holo litho at Green.

Λ = λsp, 2λsp, 3λsp, …

Nanopatterning

Green LEDs might be possible.

(32)

Schematic structure

Metal (Ag-based) p-GaN

n-GaN

Silicon submount

Photon

Sapphire

c Exciton generation

d Surface plasmon excitation e Radiation

InGaN MQW

e-h

D

Λ

h D

Λ

h

(33)

Purcell’s factor calculation by FDTD

bulk

spon cavity

p cavity

spon bulk

F P

P τ

= τ =

Y. Xu et al., J. Opt. Soc. Am. B 16, 465 (1999) 30nm

80nm 30nm 80nm radiation flux detecting plane

bulk

τ

spon cavity

τ

spon cavity

P

bulk

: spontaneous emission lifetime in bulk material : spontaneous emission lifetime in microcavity

: radiation power for a dipole in a microcavity

: radiation power for a dipole in a bulk dielectric material

F

p : Purcell’s enhancement factor

(34)

FDTD calculation results

100 200 300 400 500

1.0 1.5 2.0 2.5 3.0 3.5

Purcell's factor

Diameter [nm]

▶ 주기:250nm, 높이:50nm, 지름:170nm인 원기둥 격자 Z방향 수평다이폴 30nm거리

▶ 높이:50nm, 지름:170nm인 단일 원기둥

FluxTop FluxBot FluxX FluxY Sum N_Lifetime Nine pillars 1.968 -1.422 0.006 0.010 3.405 0.336

One pillar 2.107 -1.344 0.008 0.021 3.480 0.329

(35)

Etched GaN – Ag 2D grating Etched GaN – Ag

2D grating

Holographic Litho or NIL

P-GaN dry etch damage recovery

Holographic Litho or NIL

P-GaN dry etch damage recovery

GaN – SiO

2

/Ag 2D grating GaN – SiO

2

/Ag

2D grating

Holographic Litho

Posi & double exposure

Dry & wet hybrid etch

Holographic Litho

Posi & double exposure

GaN – SiO

₂

/Ag 1D grating GaN – SiO

₂

/Ag

1D grating

Direct e-beam writing

GaN – ITO/Ag 2D grating GaN – ITO/Ag

2D grating

Holographic litho (image reversal & double

exposure)

ITO Lift-Off

Holographic litho (image reversal & double

exposure)

ITO Lift-Off

GaN Ag GaN

Ag SiO2

GaN Ag SiO2

GaN Ag ITO

1차 샘플

1차 샘플 2차 샘플2차 샘플 최종 목표

최종 목표

<OR>

E-beam litho on SiO₂/GaN

LED fab as in normal 1mm FC Dry & wet

hybrid etch of SiO₂

I-V-L measurement on wafer probe station

장점: 최단시간소요, P-GaN 손상無 단점: 공정비용高, 면적小, 전극저항高

단면도 평면도

Surface Plasmon LED 제작 공정

(36)

Wafer

SP 0001 SP 0002 SP 0003 SP 0004

CD-SEM 으로 grating 확인 함 SiO2 dry etch + wet etch

~130nm

SP 0004

CD-SEM 으로는 grating 확인 못함 SiO2 wet etch

~130nm

SP 0003

CD-SEM 으로 grating 확인 함 SiO2 dry etch + wet etch

~40nm

SP 0002

CD-SEM 으로는 grating 확인 못함 SiO2 wet etch

~40nm

SP 0001

비고 grating 형성방법

p-GaN 두께 wafer Lot

Wet etch로 SiO2 grating을 형성한 것은 최종적으로 grating이 남아있지 않아서 reference와 유사한 특성을 보임

(37)

Surface Plasmon LED EL 특성

0.00E +00 2.00E -04 4.00E -04 6.00E -04 8.00E -04 1.00E -03 1.20E -03 1.40E -03

0.00E +00 2.00E -02 4.00E -02 6.00E -02 8.00E -02 1.00E -01 I (A )

L(W)

40n m p-GaN Re f 40n m p-GaN Gr ati n g

0.00E +00 5.00E -04 1.00E -03 1.50E -03 2.00E -03 2.50E -03

0.00E +00 2.00E -02 4.00E -02 6.00E -02 8.00E -02 1.00E -01 I( A)

L(W)

130n m p-GaN Re f 130n m p-GaN Gr ati n g

GaN Ag SiO2

GaN Ag

27% UP

유의차 없음

vs. vs.

40nm p-GaN

40nm p-GaN 130nm p-GaN

130nm p-GaN

SP Coupling 有 SP Coupling 無

(38)

TEM & SEM images of EPI layer

50nm

250 80

30 nm h nm d nm Λ =

=

(39)

Reference wafer 의 spectral map

• p-layer를 얇게 제작했을 때 정상적으로 녹색 파장이 방출되는지를 확인하기 위해 spectral map 을 측정

(40)

PL enhancement, I-L curve, I-V curve

480 490 500 510 520 530 540

PL intensity [arb. unit]

Wavelength [nm]

Reference SP-LED

480 490 500 510 520 530 540

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

PL enhancement [I spled/I ref]

Wavelength [nm]

(a)

(b)

0.00 0.02 0.04 0.06 0.08 0.10 1

2 3 4 5

Forward current [A]

Luminous flux [lm]

0.001 0.02 0.04 0.06 0.08 0.10 2

3 4 5

V f [V]

Forward current [A]

~1.15

(41)

Decay rate

17 K 300 K ref. 0.12 ns^-1 0.29 ns^-1 SP-LED 0.16 ns^-1 0.31 ns^-1

Purcell’s enhancement factor

1.33 (17 K) 1.07 (300 K)

0 20 40 60 80 100

0.01 0.1 1 10

PL intensity [arb. units]

Time (ns)

reference SP-LED

17 K

300 K

저온-상온 TRPL 결과 비교

nr

r sp

p

r nr

k k

F k k

+ k +

= +

“

저온에서는 non-radiative recombination 이 감소하므로 저온에서의 Purcell’s factor 값이 더 큰 것은 표면 플라즈몬 모드로의 결합이 영향을 미치고 있다고 생각할 수 있다.”

Advanced Materials 20, 1253 (2008)

(42)

Wall-plug efficiency

, ,

,

(

power spled power ref

)

power

power ref

η η

Δ = − η

power spled^,

, power ref

η

: wall-plug efficiency of SP-LED : wall-plug efficiency of ref.

0.00 0.02 0.04 0.06 0.08 0.10

0 1 2 3 4 5

W a ll-plug effi ciency [%]

Forward current density [A/mm

²

]

Reference SP-LED #1 SP-LED #2 SP-LED #3

0.00 0.02 0.04 0.06 0.08 0.10

0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600

Δη

power

Forward current density

(43)

Sample image

(44)

BLUE SP-LED EXPERIMENTS

(45)

(46)

p-GaN

40 nm 30 nm

20 nm 10 nm

n-GaN

(47)

90° 10° 0°

(48)

TRPL result

w=10nm w=20nm

w=30nm w=40nm

400nm 500nm

0ns

100ns

(49)

400 420 440 460 480 500 0.04

0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22

0.24 10nm

20nm 30nm 40nm un-coated

decay rate (1/ns)

wavelength (nm)

400 420 440 460 480 500

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

2.4 10nm

20nm 30nm 40nm un-coated

F p

wavelength (nm)

(a) (b)

400 420 440 460 480 500

0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22

0.24 10nm

20nm 30nm 40nm un-coated

decay rate (1/ns)

wavelength (nm)

400 420 440 460 480 500

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

2.4 10nm

20nm 30nm 40nm un-coated

F p

wavelength (nm)

(a) (b)

Purcell’s enhancement factor

(50)

Sample images

-EL 관측 :

p-GaN층의 두께가 20nm, 40nm인 LED구조에 대하여 모두 current 주입에 의한 광의 발생이 확인 됨.

(51)

• Concept of SP-LED

Emission enhancement behaviors in the coupling

between surface plasmon polaritons and light emitters

CONTENTS

• Concept of SP-LED

• History

• Theoretical calculations

• PL enhancement of BLUE GaN SP-LED

• PL, EL enhancement of GREEN GaN SP-LED

• Summary

A Better Light Emitting Diode

Display

Free-Space

Optical Communication

Brighter

General Lighting

Brighter, More Directional

Brighter, More Directional, Faster

rate ion recombinat ve

nonradiati :

efficiency extraction

:

rate emission s

spontaneou 1 :

Γ

= Γ

Γ + Γ

= Γ

η τ η η η

) % /

( 4

2

1

≈

=

n

η n

• Geometric optics

• Random scattering in surface textured structure

APL 63, 2174 (1993)

CONCEPT

1 1

( ) 2 ( )

R f i ρ ω

τ ω ε

= = p E ⋅

SE Rate : =

Photon DOS

(Density of States) Surface Plasmons Field enhancement

Directional extraction

R R R η =

+

'

R

R R

R η = R +

+ +

HISTORY

We have shown for the first time that

strongly directional emission … by SP excited

on a periodically structured surface.

HISTORY

HISTORY

HISTORY

ONLY

There are Photo-luminescences.

From 1999 to 2008

Can it work?

HISTORY

HISTORY

2.68 10 1.75 300

at K

F at K

= ⎜ ⎛

⎝

can be attributed

No improvement I-V curve

Improvement

I-L curve

-. Gain condition

R η = R ⁺