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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.