Efficient light emission from LEDs, OLEDs, and nanolasers via surface-plasmon resonance Seok Ho Song, Hanyang University, http://optics.anyang.ac.kr/~shsong silver grating Key notes 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?
emind! The next chip-scale technology limit e limit Three light-design regimes WAVE DESIGN ( d ~ ) Light extraction LED AY DESIGN ( d > ) Internal QE PHOTON DESIGN ( d < ) Today we focus on the photon design regime based on surface plasmon resonance.
Power conversion efficiency of III-Nitride LEDs Example: λ=530nm, I=350mA PCE ~ 12%
External efficiency of LEDs c extraction η η η sp, 0 external extraction internal Extraction efficiency 1 1 ( ) sin d 2 2 1 2 4( n / n ) f g 4% for GaN(2.5)-air(1.0) We need a wave design tech. Internal quantum efficiency η int nr nr : nonradiative (loss) rate : spontaneous-emission rate We need a photon design tech.
Wave Design for efficient extraction of the guided light -. Geometric optics η external ηextraction nr -. andom scattering in surface textured structure APL 63, 2174 (1993)
Photon Design for increasing the emission rate η external ηextraction nr What determines spontaneous emission rate of radiating source? E i Energy of EM field ( n 1/2) electron Number of photon (Stimulated emission) Vacuum fluctuation (Spontaneous emission) E f SE ate : Fermi s Golden ule 1 1 2 f i ( ) ( ) p 2 E 0 Photon DOS (density of states) Dipole moment Electric field strength of radiation source of half photon (vacuum fluctuation) Microoptics Lab Hanyang University emd Lab. 6
Photon Design for increasing the emission rate 1 1 f p E i ( ) 2 0 2 ( ) E, increase η external η extraction nr Ag p-gan Quantum Well n-gan Atoms in microcavity High Q Narrow F p ~ 1 5 Low volume filling factor Photonic crystal cavity Moderate Q Wider F p (Quantum wells) ~ 3 F p (Quantum dots) ~ 5 100 Off-resonant and complicated fabrication Surface plasmon coupling Low Q Narrow F p ~ 5 100 lossy and off-resonant 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 1 1 f p E i ( ) 2 0 2 ( ) E, increase η external ηextraction nr Baba LumiLed Limited by surface recombination Good scheme!!! 100 um m device size achievable. Several layer of PC for extraction. Good internal quantum efficiency Needed (>90%). Multiple pass limits device size (~10um). Small volume needed. Not so good for lighting. Surface recombination limited Surface recombination limited. Noda
Photonic-crystal assisted LEDs 1 1 f p E i ( ) 2 0 2 ( ) Very small increase in E, Look like an effect of wave design rather than photon design!
Surface-plasmon approach 1 1 f p E i ( ) 2 0 2 ( ) E, increase η external ηextraction nr Surface Plasmons p int p nr p int p p sp p sp nr sp
The SP approach was started for organic LEDs ITO glass (anode) Organic molecules Cathode & Mirror SPP quenching (~40%) Strongly coupled to SPPs Main issue: SPP adiation coupling Metallic mirror Metallic thin film SPP2 SPP1 SPP band gap ( ~ / ) k SPP Direct coupling ( / k SPP ) SPP cross-coupling ( /[ k k ]) SPP1 SPP2
Effect of SPP band gap on PL 11411 Angle resolved PL of dye molecule (DCM) 1 st and 2 nd order diffraction of SPPs Tracing 1 st order peaks shows SPP band gap.
Modification of Spontaneous Emission ate of Eu 3+ Main emission of Eu 3+ (614nm) SPP quenching ( spacer thickness) TPL at 614nm
Self-driven dipole (CPS) modeling d p Metal interface 2 2 d d 2 e pb 2 0 p0 p E dt dt m p p e, E E e i( ib/ 2) t i( ib/ 2) t 0 r 0 2 unknowns and 2 equations r 2 e b/ b 1 Im{ E } 0 0 m p0b0 2 2 b bb0 e 8 4 2m p 0 0 0 e{ E0} 14
Dipole Decay Calculation Test : Metal Mirror Cavity 10 2 10-4 10 2 dissipated power 10 1 10 0 10-1 10-2 10-3 perpendicular dipole parallel dipole 10-4 0.0 0.5 1.0 1.5 2.0 k x / k 1 J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, 725-741, 2000 15
CPS Model Calculation for Spontaneous Emission ates of an OLED Emission Spectrum No guided mode TM 0 TM 0 +TE 0 TM 0 +TE 0 +TM 1 70nm 100nm 200nm 390nm 3.0 radiation rate ( 0 ) 16 2.5 2.0 1.5 1.0 0.5 0.0 total emission rate air emission emission to substrate guided modes emission to active layer guided modes 0 50 100 150 200 250 300 350 400 active layer thickness (nm) dipole h c h s ( h h h ) a s c cover (medium c) active material (medium a) substrate (medium s)
Comparison with an experiment PL Efficiency (%) 100 90 80 70 60 100 200 300 400 500 Film Thickness (nm) power ratio (%) 100 90 80 70 60 50 40 30 20 10 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 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 active layer thickness (m) (measured) (calculated) 17
SPP Enhanced Spontaneous Emission of Eu 3+ Ion SE rate 90% SPP coupling 25 times SE rate Dipole-SPP coupling fraction Maximum internal efficiency
ole 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 nd order coupling) (d) 250nm period (1 st order coupling) (160nm gap)
1 st esult of SPP enhanced PL from InGaN QW Nature Materials, VOL 3, p.601-605, 2004 1 1 f p E i ( ) 2 0 2 ( ) E, increase η external p sp ηextractio n p sp nr Nature Materials, VOL 3, p.601-605, 2004
1 st esult of SPP enhanced PL from InGaN QW Nature Materials, VOL 3, p.601-605, 2004 40x100nm 2 133nm wide, 400nm period grating (no enhancement for 200nm wide, 600nm period grating) 0.42 0.18 x2 x28 x14 0.06 Average internal quantum efficiency estimation
TPL of SPP enhanced InGaN QW emission
How does the surface-plasmon resonance contribute to emission rate? 1 1 2 f i ( ) ( ) p 2 E 0 Field enhancement near the source layer High DOS due to decrease in group velocity emd Lab. Microoptics Lab Hanyang University 26
1 1 2 f i ( ) ( ) p 2 E 0 Field enhancement near the source layer High DOS due to decrease in group velocity equirements for enhancing SE rate -. slow group velocity -. tight confinement of mode -. low ohmic loss -. large field enhancement A B fast group velocity, low loss slow group velocity, high loss A B Q.W. Q.W.
Purcell factor defining enhancement of the spontaneous emission F p 1 original additional additional original original η int η p sp Fp F / min int nr p sp nr p nr p p max, ηint 1 Fp p nr F 1 p For a cavity mode: F p cav 3 Q( c / n) 2 4 V free 3 mode_volume For a SP mode : F p SP 1 ksp / k 1 1 2 L / c SP p SP ( ) dz ( z) d E SP, L 2 dk E at dipole 0 2 We need a slow and confined mode!
Factors influencing Purcell Enhancement F p () GaN ~ Single Quantum Well GaN Ag ~ z Variation with Ag thickness Variation with GaN thickness
Purcell enhancement factor: A numerical factor (F-1) estimation cover Cover = 1.0 Cover = 1.5 Cover = 2.0 Need a very thin p-gan layer!!
Improvement I-L curve F p 2.68at10 K 1.75at 300 K No improvement 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? Practical Barriers (especially for InGaN/GaN devices) Thin p-gan leads to abrupt occurrence of leakage current under a certain thickness SP propagation length in blue wavelength along the Ag/GaN interface is extremely short Nanopatterning becomes a huge burden at short wavelength 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 Propagation Length of SPs [nm] PL SPs 4000 3500 3000 2500 2000 1500 1000 500 1 2 k k m d c m d 3 2 Surface Plasmon on the Ag/GaN Interface 0 450 500 550 600 650 700 750 800 Wavelength of Photon [nm] ( ) 2 m m 2 Frequency (2c/m) 2.5 2.0 1.5 1.0 0.5 = sp, 2sp, 3sp, 530nm sp~140 nm 460nm sp~70 nm SP-dispersion on Ag/GaN 0.0 0 2 4 6 8 10 12 14 In-plane Wavevector (2 /m) Green LEDs might be possible. 2 nd order gratings (~280nm) might be readily fabricated by Holo litho at Green.
Schematic structure Photon Sapphire n-gan Exciton generation adiation InGaN MQW p-gan Metal (Ag-based) e-h Surface plasmon excitation Silicon submount
High output directionality 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 int FDTD int 1ext 1 nr sp sp 1ext 1 sp sp ext 1 1 ext FDTD int (1 ) 1 sp sp 0 180 0 10 nr int ext sp : nonradiative re-comb. rate : internal quantum eff. : extraction efficiency of metal grating : re-comb. rate to surface plasmon 60 100 Max ~ 80% (at 140 nm / 40 nm)
Two-dimensional 단일원기둥구조 silver-grating 계산 (2 nd order) = 250nm Grating depth = 50nm Gap to QW = 30 nm Normalized LT / Internal QE 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Normalized LifeTime Internal Quantum Efficiency Upward Emitted Power 50 100 150 200 250 300 350 400 450 500 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Upward emitted power (a.u.) Diameter (nm) 169 nm
Optimum gap distance between metal and QW Upward enhancement 2.5 2.0 1.5 1.0 0.5 λ = 530 nm d = 20 nm 0.0 0 5 10 15 20 25 30 Distance [nm] coupling to lossy surface wave coupling to surface plasmons 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 Substrate mount Aperture Little damage to p-gan otation stage X Z θ Mirro r L-Shape mount Enlarged surface area for low contact resistance Linear stage Y
EL Measurement Power(arb.) 0.0045 0.004 0.0035 0.003 0.0025 0.002 0.0015 0.001 Higher output power up to 70 % ref 250A_3 250B_2 250C_2 270A_4 270B_2 270C_3 290A_3 290B_2 0.0005 0 0 0.1 0.2 0.3 0.4 Current(A)
Sample images
An Optimistic Estimation for SP-enhanced LEDs At green (530 nm) with a 1 st order grating MQW 5 nm 10 nm grating depth FDTD calculation 20 nm 2.3 times more Photons generated 60 nm 140 nm 100 nm grating period 180 nm Good directionality Surface plasmon Photons escaped 1.0 0.8 0.6 0.4 0.2 0.0 82 % 34.1% within 20 o after escape 1/(2n 2 ) = 8 % 400 500 600 700 800 Wavelength (nm) (Bare-chip LED with 8 % extraction) ( Optimized LED with 50 % extraction) (82 % / 8 %) x 2.3 ~ 24 times Brighter (82 % / 50 %) x 2.3 ~ 4 times Brighter
Nanocavity lasers
Nanocavity lasers
Final comments Summary External Efficiencies Conventional LED SP LED ' p Ep nr p E E p p SP SP nr p SP An Optimistic Estimation for SP-enhanced LEDs At green (530 nm) with a 1 st order grating MQW 5 nm 10 nm grating depth FDTD calculation 20 nm 2.3 times more Photons generation 60 nm 140 nm 100 nm grating period 180 nm