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Light Emitting Diodes
Lezione per il corso di Fisica dello Stato Solido (prof. Mara Bruzzi)
Francesco Biccari
2013-12-18
Introduction to artificial lighting
World electrical consumption by lighting
• 17982 TWh worldwide consumption of electrical energy in 2005
• 3418 TWh worldwide for lighting (19%)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 3/64
Artificial light sources
Incandescent lampsconventional, halogen
Gas discharge lamps
light directly from the gas
light converted by a fluorescent materials
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 4/64
Artificial light sourcesLED: Light Emitting Diode
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 5/64
Physical quantities. Energy
Radiometry
• Φ : Radiant power (W)
• I : Radiant intensity (W/sr)
Φ = ∫ I dΩ
• L : Radiance (W/sr/m2)
I = ∫ L cosθ dA
• E : Irradiance (W/m2)
dΦ = E dA cosθ
Photometry (related to vision!)
• Φv : Luminous flux (lm)
• I v :Luminous intensity (lm/sr=cd)
• L v : Luminance (lm/sr/m2=cd/m2)
• E v : Illuminance (lm/m2 = lux)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 6/64
Physical quantities. Energy
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 7/64
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
Physical quantities. Colors
Black body spectrum
Solar spectrum
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 9/64
Physical quantities. Colors
Color temperature and Color Correlated Temperature
Color Rendering Index (CRI)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 10/64
Artificial light sources. Comparison
If all lamps in USA were substitutedwith LED lamps, USA will save:
35 TWh electricalenergy
3.9 billion $/year
20 millions of tonsof CO2
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 11/64
Artificial light sources. ComparisonIncandescent
StandardIncandescent
HalogenCFL LED
Efficacy (lm/W) 14 17 70 85
Cost (c€/lm) 0,04 0,15 0,5 2,8
Lifetime (hr) 1000 2000 10000 20000
CCT 2200-2900 2700-3200 2000-6000 2700-3000
CRI 100 95-100 75-85 75-85
Others Mercury, bulky, electronics
Electronics,monocromaticity in a wide
range, high shock resistance, compact
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 12/64
Artificial light sources. Comparison
Cold
Natural
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 13/64
Artificial light sources. Evolution
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 14/64
LED’s history
Haitz’s law: the luminous flux per
Package doubled every 18-24 months!
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 15/64
Los Angeles International Airport
LEDBasic principles
Radiative recombination
In general, at equilibrium n0p0 = ni2 R0 = G0
The radiative recombination rate is R = Bnp (even out of equilibrium)
The rate equation is
= − . = + ∆, = + ∆
The minority radiatiave carrier lifetime in low injection is =
()
The minority radiative carrier lifetime in high injection is = +
∆(non linear)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 18/64
Non-radiative recombination
SRH (Shockley-Reed-Hall).
In low injection regime: = !"#.$%&&.'
SHR recombination increases with temperature.SHR recombination is higher for trap levels near the mid-gap.
Auger. ()*+& = ,-. and ()*+& = ,#
.. , ≈ 10'.2cm5/s
Surface recombination.
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 19/64
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<"# ==>
=?@=
ABCDEF
ABCDEF AGB
EF
We needdirect gapsemiconductors!
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 20/64
eV42.1 :GaAs e.g. g =E
Direct band gap materials for LEDs
Direct allowed transitions Indirect transitions
eV12.1 :Si e.g. g =E
But impurities can be used, for example in GaP
Absorption and emission are related
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 21/64
Measuring lifetimes
Time resolved photoluminescence. Usually at very lowtemperature (remember that non-radiative recombinationprobability decreases by lowering temperature)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 22/64
Exercise
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 23/64
Most used semiconductors for LEDs
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 24/64
InGaN material system
• High density of dislocations in InGaN/GaN not detrimental
• Impossible to cover entirespectrum due to indiumre-evaporation
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 25/64
Most used semiconductors for LEDs
Material Present usageTypical emissionwavelengths
GaAs Low brightness (infrared LEDs) 860 nm
AlGaAs Both low and high brightness red LED 680–860 nm
GaP (GaP:N, GaP:Zn-O, AlGaP) Low brightness (green LED) 555 (565, 700) nm
GaAsP:N Low brightness (yellow, orange, red LEDs) 580–650 nm
AlInGaP High brightness (yellow, orange, red LEDs) 590–625 nm
InGaN High brightness (green and blu LEDs) 450–530 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 1952-1953 by H. Welker. Many of them are alloys of two or more III-V materials.
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 26/64
Most used semiconductors for LEDs
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 27/64
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!
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 28/64
pn junctions
• Shockley equation for the ideal pn diode
• Threshold voltage
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 29/64
Diode forward voltage
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 30/64
Diode emission spectrum
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 31/64
White light LEDs
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 32/64
LEDHigh internal efficiency design
Double heterostructure
• The initial indicator LEDs: 20 lm/W.
• Ln in p type GaAs about 15 µm
• Double Heterostructures (DH): increase carrier concentration and therefore R (lifetime decreases)
• Problem of lattice mismatch
• Active region (lightly doped or undoped) and confinmentregion (doped)
• Moreover the emitted photons can escape more easily!
10 – 1000 nm
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 34/64
DH and series resistance
Sources ofseries resistance:
contactsbarriersbulk
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 35/64
High internal efficiency design
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 36/64
DH. Carrier losses. Escape from DH
Leakage of carriers from confinement regionincreases exponentially with temperature!
Obviously ∆E >> kT
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 37/64
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
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 38/64
DH. Electron blocking layers
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 39/64
High internal efficiency design
Contatto
Substrato conduttore (per es. SiC)
InGaNtipo n (doping Si)
Zona attiva MQW (pozzi da qualche nm)
InGaNtipo p (doping Mg)
Contatto semitrasparente (high p-doping)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 40/64
LEDHigh extraction efficiency design
Design for high extraction efficiency
• <"# =#)!I+&JK-JJ#L*+#+&%+M
#)!I+&JK+N+$&J#L"#O+$+M=
PQRS/(TU)
V/W
Typical values 70% – 95%
• <+X&%$"J# =#)!I+&JK-JJ#L+X%$+M
#)!I+&JK-JJ#L*+#+&%+M=
P/(TU)
PYZ[/(TU)
Typical values 50% – 60%
• <+X =#)!I+&JK-JJ#L+X%$+M
#)!I+&JK+N+$&J#L"#O+$+M=
P/(TU)
V/W= <"#<+X&%$"J#
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 42/64
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
Design for high extraction efficiency
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 44/64
Design for high extraction efficiency
Emitter geometry
Pochi l/W
25 l/W
125 l/W
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 45/64
Design for high extraction efficiency
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 46/64
Design for high extraction efficiency
• About 50% of light is absorbed in the substrate for geometrical reasons.
• Reflectors
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 47/64
Design for high extraction efficiency
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 48/64
LEDTemperature effects
Drive circuitsPackaging
Temperature effects
• Internal efficiency depends on temperature(SRH recombination, barrier overcoming)
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 50/64
Temperature effects
• High temperatures shorten the lifetime of the devices
• High temperatures degrades the encapsulant
• Change of band gap -> resistance, emission spectrum, forward voltage.
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 51/64
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
Packaging
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 53/64
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
Communication LEDs
• The spontaneous lifetime of carriers in LEDs in direct-gap semiconductors is of the order of 1–100 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.
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 55/64
LEDGrowth techniques and fabrication
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 morevolatile precursors (trimethylgallium, ammonia and H2 for GaN), which react and/ordecompose on the substrate surface, because of its high temperature (600°C), to produce the desired deposit.
• Pressure 30 – 1000 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
Growth techniques. MOCVD/MOVPE
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 58/64
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
LED manufacturing
• Substrate: Al2O3 (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 isvery difficult to fabricate!OSRAM commercially use Si!
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 60/64
Manufacturing of GaN LED
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 61/64
Turnkey lines
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 62/64
References
• E. F. Schubert. Light-Emitting Diodes. 2nd edition. (2006). ISBN: 978-0-521-86538-8, 978-0-511-34476-3
• F. Bisegna, F. Gugliermetti, M. Barbalace, L. Monti. Stato dell’arte dei LED (Light Emitting Diodes) . 2010. Report RdS/2010/238. ENEA and Sapienza.
• High Brightness Light Emitting Diodes. Volume 48 of Semiconductors and semimetals. Academic Press, 1998. ISBN 0080864457, 9780080864457
• http://www.light-measurement.com
• http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/Sample-Chapter.pdf
• http://fp.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm
• http://www.madehow.com/Volume-1/Light-Emitting-Diode-LED.html
• http://www.omslighting.com/ledacademy/570/
• http://pubs.rsc.org/en/content/articlehtml/2009/ee/b821698c
• http://active-semi.com/sheets/PSG_LED_General-Lighting-Applications_Released.pdf
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 63/64
Thanks to prof. Anna Vinattieri
for providing several figures,
numbers and slides of this
presentation
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 scientificpapers.
The owners of copyrights can contact me to remove themfrom this presentation at the following e-mail address: [email protected]
Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 65/64