Light Emitting Diodes
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1 Light Emitting Diodes Lezione per il corso di Fisica dello Stato Solido (prof. Mara Bruzzi) Francesco Biccari
2 Introduction to artificial lighting
3 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 3/64 World electrical consumption by lighting TWh worldwide consumption of electrical energy in TWh worldwide for lighting (19%)
4 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 4/64 Artificial light sources Incandescent lamps conventional, halogen Gas discharge lamps light directly from the gas light converted by a fluorescent materials
5 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 5/64 Artificial light sources LED: Light Emitting Diode
6 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 6/64 Physical quantities. Energy Radiometry Φ : Radiant power (W) I : Radiant intensity (W/sr) Φ = I dω L : Radiance (W/sr/m 2 ) I = L cosθ da Photometry (related to vision!) Φ v : Luminous flux (lm) I v :Luminous intensity (lm/sr=cd) L v : Luminance (lm/sr/m 2 =cd/m 2 ) E v : Illuminance (lm/m 2 = lux) E : Irradiance (W/m 2 ) dφ = E da cosθ
7 Physical quantities. Energy Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 7/64
8 Physical quantities. Efficacy Luminous efficacy: luminous flux/electrical power (lm/w) Luminous efficiency: useful power/electrical power Cost ( /lm) Losses in a fluorescent lamp Example of not visible light Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 8/64
9 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 9/64 Physical quantities. Colors Black body spectrum Solar spectrum
10 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 10/64 Physical quantities. Colors Color temperature and Color Correlated Temperature Color Rendering Index (CRI)
11 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 11/64 Artificial light sources. Comparison If all lamps in USA were substituted with LED lamps, USA will save: 35 TWh electrical energy 3.9 billion $/year 20 millions of tons of CO 2
12 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 12/64 Artificial light sources. Comparison Incandescent Standard Incandescent Halogen CFL Efficacy (lm/w) Cost (c /lm) 0,04 0,15 0,5 2,8 LED Lifetime (hr) CCT CRI Others Mercury, bulky, electronics Electronics, monocromaticity in a wide range, high shock resistance, compact
13 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 13/64 Artificial light sources. Comparison Natural Cold
14 Artificial light sources. Evolution Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 14/64
15 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 15/64 LED s history Haitz s law: the luminous flux per Package doubled every months!
16 Los Angeles International Airport
17 LED Basic principles
18 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 18/64 Radiative recombination In general, at equilibrium n 0 p 0 = n i 2 R 0 = G 0 The radiative recombination rate is R = Bnp (even out of equilibrium) The rate equation is =. = +, = + The minority radiatiave carrier lifetime in low injection is = ( ) The minority radiative carrier lifetime in high injection is =+ (non linear)
19 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 19/64 Non-radiative recombination SRH (Shockley-Reed-Hall). In low injection regime: =!"#.$%&&. ' SHR recombination increases with temperature. SHR recombination is higher for trap levels near the mid-gap. Auger. ()*+& =, -. and ()*+& =, #.., 10 '.2 cm 5 /s Surface recombination.
20 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 20/64 Internal radiative quantum efficiency For a LED I want that most of the carriers recombine radiatively: 8 (: or equivalently 8 (: The internal quantum efficiency is given by < "# = = > = A EF BCD =?@ A EF BCD AEF GB We need direct gap semiconductors!
21 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 21/64 Direct band gap materials for LEDs Direct allowed transitions Indirect transitions e.g. GaAs : E = g 1.42 ev Absorption and emission are related e.g. Si : Eg = 1.12 ev But impurities can be used, for example in GaP
22 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 22/64 Measuring lifetimes Time resolved photoluminescence. Usually at very low temperature (remember that non-radiative recombination probability decreases by lowering temperature)
23 Exercise Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 23/64
24 Most used semiconductors for LEDs Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 24/64
25 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 25/64 InGaN material system High density of dislocations in InGaN/GaN not detrimental Impossible to cover entire spectrum due to indium re-evaporation
26 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 26/64 Most used semiconductors for LEDs Material Present usage Typical emission wavelengths GaAs Low brightness (infrared LEDs) 860 nm AlGaAs Both low and high brightness red LED nm GaP (GaP:N, GaP:Zn-O, AlGaP) Low brightness (green LED) 555 (565, 700) nm GaAsP:N AlInGaP Low brightness (yellow, orange, red LEDs) nm High brightness (yellow, orange, red LEDs) nm InGaN High brightness (green and blu LEDs) nm All these materials are called III-V : an element of group III and an element of group V. GaAs and many others cannot be found in nature: postulated and demonstrated during by H. Welker. Many of them are alloys of two or more III-V materials.
27 Most used semiconductors for LEDs Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 27/64
28 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 28/64 pn junctions The idea of LED is to create a zone of a semiconductor where there is an out of equilibrium condition between holes in VB and electrons in CB. A simple piece of semiconductor cannot transform electrical energy in e.m. energy by e-h recombination. The solution is using a pn junction!
29 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 29/64 pn junctions Shockley equation for the ideal pn diode Threshold voltage
30 Diode forward voltage Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 30/64
31 Diode emission spectrum Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 31/64
32 White light LEDs Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 32/64
33 LED High internal efficiency design
34 Double heterostructure The initial indicator LEDs: 20 lm/w. L n in p type GaAs about 15 µm Double Heterostructures (DH): increase carrier concentration and therefore R (lifetime decreases) Problem of lattice mismatch nm Active region (lightly doped or undoped) and confinment region (doped) Moreover the emitted photons can escape more easily! Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 34/64
35 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 35/64 DH and series resistance Sources of series resistance: contacts barriers bulk
36 High internal efficiency design Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 36/64
37 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 37/64 DH. Carrier losses. Escape from DH Leakage of carriers from confinement region increases exponentially with temperature! Obviously E >> kt
38 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 38/64 DH. Carrier losses. Carrier overflow. Double heterostructure Quantum well At very high injection current, the quasi Fermi level can overcome the top of the barrier. Especially in low volume active regions (quantum wells or quantum dots) Solution: use multiple quantum wells
39 DH. Electron blocking layers Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 39/64
40 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 40/64 High internal efficiency design Contatto semitrasparente (high p-doping) InGaN tipo p (doping Mg) Zona attiva MQW (pozzi da qualche nm) InGaN tipo n (doping Si) Substrato conduttore (per es. SiC) Contatto
41 LED High extraction efficiency design
42 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 42/64 Design for high extraction efficiency < "# = #)!I+& JK -JJ#L*+#+&%+M = P QRS/(TU) #)!I+& JK +N+$&J#L "#O+$+M V/W Typical values 70% 95% < +X&%$"J# = #)!I+& JK -JJ#L +X%$+M = P/(TU) #)!I+& JK -JJ#L*+#+&%+M P YZ[ /(TU) Typical values 50% 60% < +X = #)!I+& JK -JJ#L +X%$+M = P/(TU) #)!I+& JK +N+$&J#L "#O+$+M V/W =< "# < +X&%$"J#
43 Design for high extraction efficiency Light escape cone Few percents!!! Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 43/64
44 Design for high extraction efficiency Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 44/64
45 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 45/64 Design for high extraction efficiency Emitter geometry 125 l/w 25 l/w Pochi l/w
46 Design for high extraction efficiency Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 46/64
47 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 47/64 Design for high extraction efficiency About 50% of light is absorbed in the substrate for geometrical reasons. Reflectors
48 Design for high extraction efficiency Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 48/64
49 LED Temperature effects Drive circuits Packaging
50 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 50/64 Temperature effects Internal efficiency depends on temperature (SRH recombination, barrier overcoming)
51 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 51/64 Temperature effects High temperatures shorten the lifetime of the devices High temperatures degrades the encapsulant Change of band gap -> resistance, emission spectrum, forward voltage.
52 Drive circuits Diode current depends exponentially on the voltage Threshold voltage depends on temperature Constant voltage (not for high power): Constant current (for high power): Luminous efficacy decreases with temperature Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 52/64
53 Packaging Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 53/64
54 Packaging Semiconductor die : This is the light emitting diode itself formed from the semiconductor. Lead frame: This houses the die and acts as the connection to it. Encapsulation: This surrounds the assembly and acts as protection as well as dispersing the light. Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 54/64
55 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 55/64 Communication LEDs The spontaneous lifetime of carriers in LEDs in directgap semiconductors is of the order of ns depending on the active region doping concentration (or carrier concentrations) and the material quality. Thus, modulation speeds up to 1 Gbit/s are attainable with LEDs.
56 LED Growth techniques and fabrication
57 Growth techniques. MOCVD/MOVPE Metal Organic Chemical Vapor Deposition (MOCVD) known also as Metal Organic Vapour Phase Epitaxy (MOVPE) The wafer (substrate, sapphire for GaN) is exposed to one or more volatile precursors (trimethylgallium, ammonia and H 2 for GaN), which react and/or decompose on the substrate surface, because of its high temperature (600 C), to produce the desired deposit. Pressure mbar Volatile by-products are removed by gas flow through the reaction chamber. Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 57/64
58 Growth techniques. MOCVD/MOVPE Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 58/64
59 Growth techniques. MBE In a ultra high vacuum chamber, the elements sublime from effusion cells (Ga and N for GaN). The atoms slowly (3000 nm/h) deposit, and sometimes react among each other, on the heated substrate forming a thin film. Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 59/64
60 LED manufacturing Substrate: Al 2 O 3 (Sapphire), SiC, GaAs Epitaxy: MOCVD FEOL (front end of line): cleaning, litography, etch, metallization, deposition, annealing BEOL (back end of line): cutting, testing, sorting, die attachment, wire bonding, encapsulation A GaN substrate is very difficult to fabricate! OSRAM commercially use Si! Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 60/64
61 Manufacturing of GaN LED Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 61/64
62 Turnkey lines Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 62/64
63 References E. F. Schubert. Light-Emitting Diodes. 2nd edition. (2006). ISBN: , F. Bisegna, F. Gugliermetti, M. Barbalace, L. Monti. Stato dell arte dei LED (Light Emitting Diodes) Report RdS/2010/238. ENEA and Sapienza. High Brightness Light Emitting Diodes. Volume 48 of Semiconductors and semimetals. Academic Press, ISBN , Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 63/64
64 Thanks to prof. Anna Vinattieri for providing several figures, numbers and slides of this presentation
65 Francesco Biccari LED Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 65/64 Disclaimer This presentation is not for profit but only for spreading the knowledge of LED technologies. Many figures of this presentation were taken from books (in particular from E. F. Schubert. Light-Emitting Diodes. 2nd edition. (2006)), from the Internet and from scientific papers. The owners of copyrights can contact me to remove them from this presentation at the following address:
Hello and Welcome to this presentation on LED Basics. In this presentation we will look at a few topics in semiconductor lighting such as light
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