Light Emitting Diodes

(1)

Light Emitting Diodes

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

Francesco Biccari

[email protected]

(2)

(3)

World electrical consumption by lighting

• 17982 TWh worldwide

consumption of electrical

energy in 2005

• 3418 TWh worldwide for

lighting (19%)

(4)

Artificial light sources

Incandescent lamps

conventional, halogen

Gas discharge lamps

light directly from the gas

light converted by a fluorescent materials

(5)

Artificial light sources

LED: Light Emitting Diode

(6)

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

• E : Irradiance (W/m

2 ₎

dΦ = E dA

cosθ

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)}

(7)

Physical quantities. Energy

(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

(9)

Physical quantities. Colors

Black body spectrum

Solar spectrum

(10)

Physical quantities. Colors

Color temperature and Color Correlated Temperature

Color Rendering Index (CRI)

(11)

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)

Artificial light sources. Comparison

Incandescent

Standard

Incandescent

Halogen

CFL

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

Others

Mercury, bulky,

electronics

Electronics,

monocromaticity in a wide

range, high shock

resistance, compact

(13)

Artificial light sources. Comparison

Cold

Natural

(14)

Artificial light sources. Evolution

(15)

LED’s history

Haitz’s law: the luminous flux per

Package doubled every 18-24

months!

(16)

(17)

LED

(18)

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)

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)

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

_BCDEF

A

_BCDEF

A

_GBEF

We need

direct gap

semiconductors!

(21)

eV

42 .

1 :

GaAs

e.g.

_E

_g

₌

Direct band gap materials for LEDs

Direct allowed transitions

Indirect transitions

eV

12 .

1 :

Si

e.g.

_E

_g

₌

But impurities can be used,

for example in GaP

Absorption and emission are related

(22)

Measuring lifetimes

Time resolved photoluminescence. Usually at very low

temperature (remember that non-radiative recombination

probability decreases by lowering temperature)

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Exercise

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Most used semiconductors for LEDs

(25)

InGaN material system

• High density of dislocations

in InGaN/GaN not detrimental

• Impossible to cover entire

spectrum due to indium

re-evaporation

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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

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.

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Most used semiconductors for LEDs

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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!

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pn junctions

• Shockley equation for the ideal pn diode

• Threshold voltage

(30)

Diode forward voltage

(31)

Diode emission spectrum

(32)

White light LEDs

(33)

LED

(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

• Active region (lightly doped or undoped) and confinment

region (doped)

• Moreover the emitted photons can escape more easily!

10 – 1000 nm

(35)

DH and series resistance

Sources of

series resistance:

contacts

barriers

bulk

(36)

High internal efficiency design

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DH. Carrier losses. Escape from DH

Leakage of carriers from confinement region

increases exponentially with temperature!

Obviously ∆E >> kT

(38)

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

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DH. Electron blocking layers

(40)

High internal efficiency design

Contatto

Substrato conduttore

(per es. SiC)

InGaN

tipo n (doping Si)

Zona attiva MQW

(pozzi da qualche nm)

InGaN

tipo p (doping Mg)

Contatto

semitrasparente

(high p-doping)

(41)

LED

(42)

Design for high extraction efficiency

• <

_"#

=

#)!I+& JK - J J#L *+#+&% +M

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

=

P

_QRS

/(TU)

V/W

Typical values 70% – 95%

• <

_{+X &%$ "J#}

=

#)!I+& JK - J J#L +X %$ +M

#)!I+& JK - J J#L *+#+&% +M

=

P/(TU)

P

_YZ[

/(TU)

Typical values 50% – 60%

• <

_+X

=

#)!I+& JK - J J#L +X %$ +M

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

=

P/(TU)

V/W

= <

"#

<

+X &%$ "J#

(43)

Design for high extraction efficiency

• Light escape cone

Few percents!!!

(44)

Design for high extraction efficiency

(45)

Design for high extraction efficiency

Emitter geometry

Pochi l/W

25 l/W

125 l/W

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Design for high extraction efficiency

(47)

Design for high extraction efficiency

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

geometrical reasons.

• Reflectors

(48)

Design for high extraction efficiency

(49)

LED

Temperature effects

Drive circuits

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Temperature effects

• Internal efficiency depends on temperature

(SRH recombination, barrier overcoming)

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Temperature effects

• High temperatures shorten the lifetime of the devices

• High temperatures degrades the encapsulant

• Change of band gap -> resistance, emission spectrum,

forward voltage.

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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

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Packaging

(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.

(55)

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.

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LED

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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 30 – 1000 mbar

• Volatile by-products are

removed by gas flow

through the reaction chamber.

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Growth techniques. MOCVD/MOVPE

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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.

(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!

(61)

Manufacturing of GaN LED

(62)

Turnkey lines

(63)

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