Nano Technology drives LED Advancements

Transcription

1 Frontiers in Materials Science & Technology Nano Technology drives LED Advancements Dr. Norbert Stath Osram Opto Semiconductors GmbH, Regensburg Outline Progress of LEDs Material quality and nano structures Thin-Film LEDs Phosphor and high flux concepts Applications Conclusion Fig. 1 Int. Mat. For. ; Stath Seite: 1

2 Brightness Evolution of LEDs Since InGaAlP White LED 100 lm/w? LED Efficacy [Lumen / Watt] 10 1 Modern Lamps GaP:N GaAsP:N GaAlAs InGaAlP GaAlAs InGaN InGaN InGaN InGaN Edison GaAsP Fig. 2 SiC 0, Int. Mat. For. ; Stath Seite:

3 White LEDs are on a Steep Improvement Curve 100 Light source efficiency Lumen/Watt Standard Light Sources Year of invention Metal halide White Power LED 2010 Fluorescent Fig // 1959 Incandescent Int. Mat. For. ; Stath Seite: 3 Mercury Halogen 1981 CFL

4 High Brightness LEDs: a Multi-billion Dollar Business Fig. 4 Int. Mat. For. ; Stath Seite: 4

5 High Brightness LED Process Chain Keys for High Wall Plug Efficiency Metal Organic Vapor Phase Epitaxy Package Chip Technology Epitaxy Substrate Light extraction Electrical losses Internal Efficiency Heat dissipation Light extraction λ-conversion η Wall plug = η. int η. extr η electr. η 20 ma AlGaInP (red): 40% = 90% 50% 97%. 90% GaInN (white): 22% = 50% 75% 91% 65% 105 lm/w 70 lm/w Fig. 5 Int. Mat. For. ; Stath Seite: 5

6 Bandgap Engineering for Highly Efficient (AlGa)InP LEDs Advancements in MOVPE technology have been enabling Bandgap Engineering on a Nano scale GaAs substrate Lattice matched active BG structure Conduction Band AlInP AlGaInP Photons Bandenergy (ev) 3.5 (AlGa) 0.5 In 0.5 P/GaAs AlP GaP GaAs InP Lattice Constant (A) Wavelength (nm) Valence Band Q-wells and barriers 5-10 nm thick Wavelength tuned by Al/Ga-ratio 90% int. Q.E. for red emission (high band offsets, low thermal carrier overflow) Fig. 6 Int. Mat. For. ; Stath Seite: 6

7 Thin-Film Technology for LEDs to Free the Photons Conventional LED Thin-Film LED bondpad air n 1 =1 Original substrate (removed) window layer n 2 =3.4 active region LED material θ c Thin epi-layer buffer layer contact (absorbing) substrate Θ escape cone GaAs air: c 1 = sin ( n1 / n2) Θ = 17 c Θ η ex = 2.2% carrier Metal mirror Thin-Film Technology pioneered by Osram OS World-record performance AlGaInP LEDs (amber, red) Fig. 7 Int. Mat. For. ; Stath Seite: 7

8 Thin-Film (TF) LED: Principle of Operation and Realization Multiple passes through the active material Structured interface randomizes angles of reflection Metal mirror eliminates substrate absorption carrier Micro-prism Bond pad Light generation Significantly enhanced light extraction isolation metal (solder + mirror) contact carrier Local current injection and generation of light Micro-prism reflectors High reflective dielectric/metal mirror High extraction efficiency Fig. 8 Int. Mat. For. ; Stath Seite: 8

9 Scalability of TF to High Flux w/o Efficiency Loss Large chip area for high operation currents Comparable current density at operation current for small and large chips 300µm (50mA) Requires an optimized power package to dissipate the heat -> Golden Dragon 1mm (1 A) 150 µm 300 µm AlGaInP chip size Light is extracted only through the top surface LED efficiency is independent of device area 500 µm 1 mm Golden Dragon Fig. 9 Int. Mat. For. ; Stath Seite: 9

10 Evolution of Red AlGaInP-LED Brightness by Applying Nanotechnology Red LED efficiency record! Luminous Flux (lm) 108 lm/w 13,8 lm Current (ma) Efficiency (lm/w) Chip size 300 x 300 µm 2 Luminous Flux (lm) brightness evolution Osram OS.opj nm PTopLED Current (ma) Thinfim chip technology Standard chip technology Fig. 10 Int. Mat. For. ; Stath Seite: 10

11 InGaN LEDs are Applied Nano Technology Light extraction & λ-conversion through Photonic band gap structure Nanophosphors LED Nano structure Planar surface InGaN p-n-junction in bulk Micrometer scale GaN 2-D quantum films Nanometer scale 3-D quantum dots Nanometer scale Surface plasmons Light generation in the crystal through in the past today in the future Fig. 11 Int. Mat. For. ; Stath Seite: 11

12 Material Quality: The Key to Improve Internal Q.E. of InGaN-based LEDs InGaN - Nano Structures TEM-Analysis: Prof. Dr. J. Zweck, Uni Regensburg InGaN 5 nm Transparente contact 250 µm 4 µm Bondpad P- GaN p- GaAlN/GaN n- GaAlN/GaN SiC -Substrate UV - blue - green InGaN- Quantum wells Buffer layer InGaN GaN GaAlN SiC GaN Bandgap Energy [ev] GaN Indium-Cluster 7,0 AlN 6,0 5,0 4,0 GaN 3,0 SiC 2,0 Sapphire InN 1,0 3,0 3,2 3,4 3,6 Lattice Parameter [Å] Fig. 12 Int. Mat. For. ; Stath Seite: 12

13 Learning from Violette-Blue InGaN Laser: Frontier of Material Technology 405 nm InGaN ridge waveguide laser The success factors: controlled monolayer deposition well defined abrupt interfaces very low defect levels ~ 2 µm Quantum well structure university of Regensburg Fig. 13 Int. Mat. For. ; Stath Seite: 13

14 Impact of Defect Density on Laser Performance Laser structure grown on GaN InGaN Laser characteristics with GaN-Sub. vs. SiC-Sub. Laser structure grown on SiC Defect den..: 5x10 6 cm -2 Defect den..: 2x10 9 cm -2 p-gan p-algan waveguide with MQW n-algan buffer U (V) U f GaN-Sub. I th SiC- Sub I (ma) P opt (mw) Improvements of InGaN-Laser on GaN significant lower threshold currents lower forward voltages much higher lifetimes Fig. 14 Int. Mat. For. ; Stath Seite: 14

15 Epitaxial Lateral Overgrowth (ELOG) a Method to Reduce Epi-Defects Epi growth with ELOG TEM-Analysis on ELOGgrown GaN-Structures TEM-Analysis: Prof. Dr. J. Zweck, Uni Regensburg Schema Bending of dislocations SiO 2 -mask Fig. 15 Int. Mat. For. ; Stath Seite: 15

16 In-situ SiN x -Masking and Overgrowth to Reduce Defect Density in InGaN Epi Epi process steps I. GaN growth II. SiN x masking III. GaN cluster growth (reduced reflection signal) IV. Coalescence of GaN clusters (interferring reflection signal) TEM - Analysis In-situ reflection signal Atomic Force Microscopy (AFM): Flatness reached with 180nm growth (start of interferrence) - cluster hight: 10-20nm SiN x Method allows a reduction of dislocation density by factor of 10 (down to 5x10 7 cm -2 ) Fig. 16 Int. Mat. For. ; Stath Seite: 16

17 New Chip Designs for Blue InGaN LED Brightness Advancements External Quantum Efficiency [%] ma Standard η extr = 25% ATON η extr = 50% ThinGaN η int ~ 50% η extr ~ 75% NOTA η extr = 60% Optical 20 ma [mw] 0 Fig Int. Mat. For. ; Stath Seite: 17 0

18 ThinGaN Provides True Surface Emitting Chips, Scalability to Hi-flux, Low Electrical Losses Process Flow ThinGaN: Saphir Sapphire N 2 GaN GaN GaN GaN GaN-Epi Sapphire Epitaxy on Sapphire Mirror metallization GaN-Epi Carrier-Substrate Soldering on carrier Epi Laser-lift off (M. Stutzmann, WSI München OSRAM Patent) Ga Chip-Processing GaN GaN GaN GaN Chip dicing on foil Roughness 200nm rms AFM (Surface text.) 4 µm Epi on carrier 180 True surface emitter 0 Fig. 18 Int. Mat. For. ; Stath Seite: 18

19 ThinGaN: Why is Surface Structuring so Important? ThinGaN concept: reduced internal absorption thin layers no losses on the backside highly reflective mirror prevent waveguiding surface texturing Texturing: Give photons multiple chances to find an escape cone planar surface textured surface epifilm reflector epifilm reflector EL-Intensity [a.u.] planar textured nm Emission spectra Fig. 19 Int. Mat. For. ; Stath Seite: 19 Nano texturing future Phot. bandgap structures Enabling resonant cavities and directed light extraction

20 ThinGaN and Phosphors for White and Colored LEDs Standard light conversion by volume casting blue chip emission white light phosphor homogenously dispersed in silicone casting material Golden DRAGON + yellow phosphor emission nm semiconductor chip (1mm 2 ThinGaN) insufficient color homogeneity and luminance conversion on chip level needed Fig. 20 Int. Mat. For. ; Stath Seite: 20

21 Chip Level Conversion (CLC) for White and Color on Demand LEDs > ma Golden DRAGON Today: µm sized YAG:Ce phosphor particles In the future: Nanophosphors? clear silicone casting w/o lens > 60 lm w. lens CLC layer (phosphor + silicone) on surface emitter (1mm 2 ThinGaN chip) 1mm 2 ThinGaN wafer with 20 µm CLC layer excellent color homogeneity high luminance perfectly suitable for optical systems Fig. 21 Int. Mat. For. ; Stath Seite: 21

22 OSTAR -Platform for High Flux Applications OSTAR Thinfilm chips metal core PCB AlN heatsink Special features for very compact RGGB design low thermal resistance high luminance lambertian emitter flexible optics close to chip different applications Head up Display Projection Headlamp Luminous flux (4 chips monochrome): red: 220 lm (750mA per chip) green: 170 lm (500mA per chip) blue: 44 lm (500mA per chip) Solid State Lighting Fig. 22 Int. Mat. For. ; Stath Seite: 22

23 OSTAR for Automotive Forward Lighting conventional tungsten coil in halogen lamp Automotive Forward Lighting OSTAR Headlamp five 1mm 2 CLC ThinGaN chips assembled on OSTAR module > ma Source: Hella primary optics beam shaping second. optics for projection beam pattern on street Fig. 23 Int. Mat. For. ; Stath Seite: 23

24 DRAGON for LCD TV Display (32 46 ) LED Backlighting Application LED LCD Backlight CCFL LCD B/L LED B/L Benefits brilliant colors high contrast no blurring wide color gamut (> 100% NTSC) DRAGON R-G-B Power Dragons Arrangement of RGGB pixels on metal plate with reflector LCD-TV screen Performance of direct B/L concept for 40 Display Power Dragon W electrical power cd/m 2 on screen - 40 mm unit thickness Fig. 24 Int. Mat. For. ; Stath Seite: 24

25 Conclusion LEDs are on a steep improvement curve starting now to outperform conventional light sources Internal quantum efficiency is continously improved by increased material quality and bandgap engineering on a nano scale Nano structured AlGaInP and InGaN Thin-Film LEDs enabling highest light extraction and scalability to hi-flux Hi-flux package and new phosphor concepts drive LED performance due to thermal and color management There are many applications out there, waiting for further LED advancements We gratefully acknowledge the support by BMBF Fig. 25 Int. Mat. For. ; Stath Seite: 25