Light Emitting Diodes and Solid-State Lighting E. Fred Schubert Department of Electrical, Computer, and Systems Engineering Department of Physics, Applied Physics, and Astronomy Rensselaer Polytechnic Institute, Troy, NY 12180 Phone: 518-276-8775 Email: [email protected] Internet: www. LightEmittingDiodes. org 1 of 164 Solid-state lighting Inorganic devices: • Semiconductor plus phosphor illumination devices • All-semiconductor-based illumination devices Organic devices: • Remarkable successes in low-power devices (Active matrix OLED monitors, thin-film transistors, etc.) Comp. Semiconductors, 2006 • Substantial effort is underway to demonstrate high-power devices • Anticipated manufacturing cost and luminance of organic devices are orders of magnitude different from inorganic devices Predicted growth of LED market 2 of 164 1 OLED versus LED OtOpto Tec hCorp. OCOsram Corp. OLEDs are area sources LEDs are point sources They do do not blind They are blindingly bright Suitable for large-area sources Suitable for imaging-optics applications Luminance of OLEDs: 102 –104 cd/m2 Luminance of LEDs: 106 – 107 cd/m2 Luminance of OLEDs is about 4 orders of magnitude lower OLED manufacturing cost per unit area must be 104 lower OLEDs LEDs Low-cost reel-to-reel manufacturing Expensive epitaxial growth 3 of 164 Quantification of solid-state lighting benefits Energy benefits • 22 % of electricity used for lighting • LED-based lighting can be 20 more efficient than incandescent and 5 more efficient than fluorescent lighting Environmental and economic benefits • Reduction of CO2 emissions, a global warming gas • Reduction of SO2 emissions, acid rain • Reduction of Hg emissions by coal-burning power plants • Reduction of hazardous Hg in homes Financial benefits • Electrical energy cost reduction, but also savings resulting from less pollution, global warming Hg Cause: SO Cause: CO2 2 CO2 ,SO2, NOx, Hg, U Cause: Waste heat and acid rain Antarctica Czech Republic Switzerland United States 4 of 164 2 Quantification of benefits Global benefits enabled by solid-state lighting technology over period of 10 years. First numeric value in each box represents annual US value. The USA uses about ¼ of world’s energy. Savings under “11% scenario” Reduction in total energy consumption 43.01 1018 J 11% 4 10 = = 189.2 1018 J Reduction in electrical energy consumption 457.8 TWh 4 10 = = 18,310 TWh = 65.92 1018 J Financial savings 45.78 109 $ 4 10 = = 1,831 109 $ Reduction in CO2 emission 267.0 Mt 4 10 = 10.68 Gt Reduction of crude-oil consumption (1 barrel = 159 24.07 106 barrels 4 10 = = 962.4 106 liters) barrels Number of power plants not needed 70 4 = 280 et al Reports on Progress (*) 1.0 PWh = 1000 TWh = 11.05 PBtu = 11.05 quadrillion Btu “=” 0.1731 Pg of C = 173.1 Mtons of C Schubert ., 1kgofC“=”[(12amu+2 16 amu) / 12 amu] kg of CO2 = 3.667 kg of CO2 in Physics 69, 3069 (2006) 5 of 164 History of LEDs Henry Joseph Round (1881 – 1966) 1907: First observation of electroluminescence 1907: First LED LED was made of SiC, carborundum, an abrasive material Henry Joseph Round 6 of 164 3 Light-Emitting Diode – 1924 – SiC – Lossev Oleg Vladimirovich Lossev (1903 – 1942) Brilliant scientist who published first paper at the age of 20 years The Lossevs were noble family of a Russian Imperial Army officer Lossev made first detailed study of electroluminescence in SiC Lossev concluded that luminescence is no heat glow (incandescence) Lossev noted similarity to vacuum gas discharge Oleg Vladimirovich Lossev SiC – Carborundum 7 of 164 Light-Emitting Diode – 1924 – SiC – Lossev Oleg V. Lossev noted light emission for forward and reverse voltage Measurement period 1924 – 1928 First photograph of LED Lossev’s I-V characteristic 8 of 164 4 Light emission in first LED First LED did not have pn junction! Light was generated by either minority carrier injection (forward) or avalanching (reverse bias) “Beginner’s luck” 9 of 164 History of AlGaAs IR and red LEDs There is lattice mismatch between AlGaAs and GaAs Growth by liquid phase epitaxy (LPE) Growth technique to date: Organometallic vapor Phase epitaxy 10 of 164 5 One of the first application of LEDs LEDs served to verify function of printed circuit boards (PCBs) LEDs served to show status of central processing unit (CPU) 11 of 164 History of GaP red and green LEDs There are direct-gap and indirect-gap semiconductors GaAs is direct but GaP is indirect Iso-electronic impurities (such as N and Zn-O) enable light emission 12 of 164 6 Red GaP LEDs N results in green emission Zn-O results in red emission However, efficiency is limited 13 of 164 Application for GaP:N green LEDs Dial pad illumination Telephone company (AT&T) decided that green is better color than red 14 of 164 7 LEDs in calculators LEDs were used in first generation of calculators Displayed numbers could not be seen in bright daylight LEDs consumed so much power that all calculators had rechargeable batteries 15 of 164 History of GaN blue, green, and white light emitters Blue emission in GaN in 1972, Maruska et al., 1972 However, no p-doping attained Devices were developed by RCA for three-color flat-panel display applications to replace cathode ray tubes (CRTs) Nichia Corporation (Japan) was instrumental in blue LED development Dr. Shuji Nakamura lead of development 16 of 164 8 Applications of green LEDs High-brightness LEDs for outdoor applications 17 of 164 History of AlGaInP visible LEDs Hewlett-Packard Corporation and Toshiba Corporation developed first high-brightness AlGaInP LEDs AlGaInP suited for red, orange, yellow, and yellow-green emitters 18 of 164 9 Recent applications High power applications 19 of 164 Radiative and nonradiative recombination Recombination rate is proportional to the product of the concentrations of electrons and holes R= B n p where B = bimolecular recombination coefficient n = electron concentration p = hole concentration 20 of 164 10 Radiative electron-hole recombination n n n p p p 0 and 0 n free electron concentration n0 equilibrium free electron concentration n excess electron concentration n p R d d Bnp dt dt R recombination rate per cm3 per s B bimolecular recombination coefficient 21 of 164 Carrier decay (low excitation) B n p t n t n ()0 0 ( )0 e B n p 1 0 0 carrier lifetime B bimolecular recombination coefficient 22 of 164 11 Radiative recombination for low-level excitation Radiative lifetimes determine switch-on and switch-off times 23 of 164 Recombination lifetime: Theory versus experiment Radiative lifetime decreases with doping concentration 24 of 164 12 Nonradiative recombination in the bulk Generation of heat competes with generation of light This is a very fundamental issue 25 of 164 Recombination mechanisms Recombination via deep levels Auger recombination Radiative recombination 26 of 164 13 Visualization of defects • DkDark spo ts are c lus ters o dfftfdefects • Dark spots are dark because lack of radiative recombination 27 of 164 Shockley-Read recombination p n n p np R 0 0 SR N v 1 n n n N v 1 p p p t p p 0 1 t n n 0 1 p n n 1 0 0 N v 1 n n n N v 1 p p p t p p 0 1 t n n 0 1 C p n S F E E V 1 1 C T Fi S n D1 T n 1 cosh D T i 0 n 0 HG E kT UXW E 2 i U • Mid-gap levels are effective non-radiative recombination centers 28 of 164 14 Nonradiative recombination at surfaces Surface recombination Surface recombination velocity 29 of 164 Surface recombination F S x L V n exp( /n ) n() x n n() x n n G1 W 0 0 LS H n n X S surface recombination velocity x distance from semiconductor surface Ln carrier diffusion length Surface recombination velocities of semiconductors GaAs S = 106 cm/s GaP S = 106 cm/s InP S = 103 cm/s GaN S = 103 cm/s Si S = 101 cm/s 30 of 164 15 Demonstration of surface recombination • Making surface recombination “visible” 31 of 164 Competition: radiative & nonradiative recombination 1 1 1 r nr 1 r int 1 1 r nr carrier lifetime nr nonradiative carrier lifetime r radiative carrier lifetime int internal quantum efficiency 32 of 164 16 LED basics: Electrical properties Shockley equation for p-n junction diodes C D S D p D T eV kT I e A p n n e – 1 D n0 p0 T E p n U C D 2 2 S D p n D n T eV kT e A i n i e – 1 D N N T E p D n A U I eV kkT s e – 1 I where s is the saturation current 33 of 164 P-n junction band diagram 34 of 164 17 Diode current-voltage characteristics Forward voltage is approximately equal to Eg / e 35 of 164 Diode forward voltage UV-LEDs frequently show excess forward voltage 36 of 164 18 Deviations from ideal I-V characteristic eV n kT II ()ideal s e VIR e V IR n kT I s I e ()()s ideal R s p nideal ideality factor Rs parasitic series resistance Rp parasitic parallel resistance 37 of 164 Non-ideal I-V characteristics Problem areas of diode can be identified from I-V characteristic 38 of 164 19 Methods to determine series resistance Method shown above suitable for series resistance measurement At high currents, diode I-V becomes linear due to dominance of series resistance. Diode series resistance can be extracted in linear regime 39 of 164 Carrier distribution in pn homo- and heterojunctions Double heterostructure enables high carrier concentrations in active region • A high efficiency results 40 of 164 20 Carrier overflow in double heterostructures Carrier leakage out of active region 41 of 164 Saturation of output power due to leakage A larger number of QWs reduce leakage 42 of 164 21 Electron blocking layers Electron blocking layer Hole blocking layer Which of the two would be more effective? 43 of 164 Diode forward voltage V h e E e = / g / E EE EE V g IR C 0 V 0 e s e e I R s resistive loss E –E C 0 electron energy loss upon injection into quantum well E –E V 0 hole energy loss upon injection into quantum well 44 of 164 22 LED basics: Optical properties Internal, extraction, external, and power efficiency P h number of photons emitted from active region per second int /() int number of electrons injected into LED per second I / e number of photons emitted into free space per second extraction number of photons emitted from active region per second P h number of photons emitted into free space per sec.