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

Transcription

1 Efficient light emission from LEDs, OLEDs, and nanolasers via surface-plasmon resonance Seok Ho Song, Hanyang University, 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?

2 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.

3 Power conversion efficiency of III-Nitride LEDs Example: λ=530nm, I=350mA PCE ~ 12%

4 External efficiency of LEDs c extraction η η η sp, 0 external extraction internal Extraction efficiency 1 1 ( ) sin d ( 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.

5 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)

6 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 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

7 Photon Design for increasing the emission rate 1 1 f p E i ( ) ( ) 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) ~ Off-resonant and complicated fabrication Surface plasmon coupling Low Q Narrow F p ~ lossy and off-resonant photonic/website/surf-plasmon-ohps-f.ppt Department of Physics, University of North Texas, Denton, Texas 76203

8 Photonic-crystal approach 1 1 f p E i ( ) ( ) 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

9 Photonic-crystal assisted LEDs 1 1 f p E i ( ) ( ) Very small increase in E, Look like an effect of wave design rather than photon design!

10 Surface-plasmon approach 1 1 f p E i ( ) ( ) E, increase η external ηextraction nr Surface Plasmons p int p nr p int p p sp p sp nr sp

11 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

12 Effect of SPP band gap on PL 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.

13 Modification of Spontaneous Emission ate of Eu 3+ Main emission of Eu 3+ (614nm) SPP quenching ( spacer thickness) TPL at 614nm

14 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 e{ E0} 14

15 Dipole Decay Calculation Test : Metal Mirror Cavity dissipated power perpendicular dipole parallel dipole k x / k 1 J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, ,

16 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 ) total emission rate air emission emission to substrate guided modes emission to active layer guided modes 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)

17 Comparison with an experiment PL Efficiency (%) Film Thickness (nm) power ratio (%) 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 active layer thickness (m) (measured) (calculated) 17

18 SPP Enhanced Spontaneous Emission of Eu 3+ Ion SE rate 90% SPP coupling 25 times SE rate Dipole-SPP coupling fraction Maximum internal efficiency

19 ole of Preferred Orientation of the Dipole Source Adv. Mater Angle integrated EL

20 Enhanced PL by Coupled SPP

21 Cross-Coupled vs Coupled SPP (1) (2) (3) (4)

22 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)

23 1 st esult of SPP enhanced PL from InGaN QW Nature Materials, VOL 3, p , f p E i ( ) ( ) E, increase η external p sp ηextractio n p sp nr Nature Materials, VOL 3, p , 2004

24 1 st esult of SPP enhanced PL from InGaN QW Nature Materials, VOL 3, p , x100nm 2 133nm wide, 400nm period grating (no enhancement for 200nm wide, 600nm period grating) x2 x28 x Average internal quantum efficiency estimation

25 TPL of SPP enhanced InGaN QW emission

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

27 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.

28 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 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!

29 Factors influencing Purcell Enhancement F p () GaN ~ Single Quantum Well GaN Ag ~ z Variation with Ag thickness Variation with GaN thickness

30 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!!

31 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.

32 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

33 SP propagation length Nanopatterning Propagation Length of SPs [nm] PL SPs k k m d c m d 3 2 Surface Plasmon on the Ag/GaN Interface Wavelength of Photon [nm] ( ) 2 m m 2 Frequency (2c/m) = sp, 2sp, 3sp, 530nm sp~140 nm 460nm sp~70 nm SP-dispersion on Ag/GaN In-plane Wavevector (2 /m) Green LEDs might be possible. 2 nd order gratings (~280nm) might be readily fabricated by Holo litho at Green.

34 Schematic structure Photon Sapphire n-gan Exciton generation adiation InGaN MQW p-gan Metal (Ag-based) e-h Surface plasmon excitation Silicon submount

35 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

36 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 nr int ext sp : nonradiative re-comb. rate : internal quantum eff. : extraction efficiency of metal grating : re-comb. rate to surface plasmon Max ~ 80% (at 140 nm / 40 nm)

37 Two-dimensional 단일원기둥구조 silver-grating 계산 (2 nd order) = 250nm Grating depth = 50nm Gap to QW = 30 nm Normalized LT / Internal QE Normalized LifeTime Internal Quantum Efficiency Upward Emitted Power Upward emitted power (a.u.) Diameter (nm) 169 nm

38 Optimum gap distance between metal and QW Upward enhancement λ = 530 nm d = 20 nm 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)]

39 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

40 EL Measurement Power(arb.) Higher output power up to 70 % ref 250A_3 250B_2 250C_2 270A_4 270B_2 270C_3 290A_3 290B_ Current(A)

41 Sample images

42 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 % 34.1% within 20 o after escape 1/(2n 2 ) = 8 % 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

43 Nanocavity lasers

44 Nanocavity lasers

45 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