Light Emitting Diodes

Lezione per il corso di Fisica dello Stato Solido (prof. Mara Bruzzi)

Francesco Biccari

biccari@gmail.com

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

Artificial light sourcesLED: Light Emitting Diode

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)

Physical quantities. Energy

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

Physical quantities. Colors

Black body spectrum

Solar spectrum

Physical quantities. Colors

Color temperature and Color Correlated Temperature

Color Rendering Index (CRI)

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

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

Artificial light sources. Comparison

Natural

Artificial light sources. Evolution

LED’s history

Haitz’s law: the luminous flux per

Package doubled every 18-24 months!

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)

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.

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

We needdirect gapsemiconductors!

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

Measuring lifetimes

Time resolved photoluminescence. Usually at very lowtemperature (remember that non-radiative recombinationprobability decreases by lowering temperature)

Exercise

Most used semiconductors for LEDs

InGaN material system

• High density of dislocations in InGaN/GaN not detrimental

• Impossible to cover entirespectrum due to indiumre-evaporation

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.

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!

pn junctions

• Shockley equation for the ideal pn diode

• Threshold voltage

Diode forward voltage

Diode emission spectrum

White light LEDs

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

DH and series resistance

Sources ofseries resistance:

contactsbarriersbulk

High internal efficiency design

DH. Carrier losses. Escape from DH

Leakage of carriers from confinement regionincreases exponentially with temperature!

Obviously ∆E >> kT

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

DH. Electron blocking layers

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)

LEDHigh extraction efficiency design

Design for high extraction efficiency

• <"# =#)!I+&JK-JJ#L*+#+&%+M

#)!I+&JK+N+$&J#L"#O+$+M=

PQRS/(TU)

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#

• Light escape cone

Few percents!!!Francesco Biccari – LED – Corso di Fisica dello Stato Solido (prof.ssa Mara Bruzzi) 43/64

Emitter geometry

Pochi l/W

25 l/W

125 l/W

• About 50% of light is absorbed in the substrate for geometrical reasons.

• Reflectors

LEDTemperature effects

Drive circuitsPackaging

Temperature effects

• Internal efficiency depends on temperature(SRH recombination, barrier overcoming)

Temperature effects

• High temperatures shorten the lifetime of the devices

• High temperatures degrades the encapsulant

• Change of band gap -> resistance, emission spectrum, forward voltage.

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

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.

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.

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.

Growth techniques. MOCVD/MOVPE

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

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!

Manufacturing of GaN LED

Turnkey lines

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

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: biccari@gmail.com

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