© MuAnalysis 2014 1

Shining a light on LED technology

M. Simard-Normandin

MuAnalysis Ottawa

MuAnalysis

2

Who is MuAnalysis • Private electronic materials

laboratory in Ottawa – 12 years as independent

company – Serving >350 companies world-

wide – Focusing on discrete active

components • LEDs and lasers • Diodes • Power transistors

– FA, Rel and DPA of all electronic components

• ICs and discretes

• Passives and displays • We are registered under the

Controlled Goods (ITAR) program

3

LEDs, a new disruptive technology • Just a few years ago LEDs were dim and used as indicator lights

and decorations and in other non critical applications. • 20% of all energy used is for lighting, using devices that waste

80% of this energy. • LEDs are changing that. They are changing the way we live. • They are everywhere. • Let’s find out what they really are.

4

Outline • Introduction

– What are LEDs – History

• Physics of the semiconductor die – Band diagrams – Types of LEDs (UV to IR) – Quantum wells – Contact metallurgy

• Packaging – Heat, heat, heat – Lens or no lens – Phosphors – A19 format

• Reliability consideration – Failure modes – LM79 and LM80

Each point could be the topic of a one hour tutorial, this will be an overview of what is happening in commercially available LED technology today.

5

What are LEDs : light emitting diodes

• These special diodes emit light from UV to IR while other diodes don’t.

6

History • http://www.electronicsweekly.com/Articles/18/02/20

11/49501/50-year-history-of-the-LED.htm • First real LEDs appeared in early 60’s (IR). • Soon followed by red, orange and yellow. • Green eluded for a long time.

– GaP based green LEDs were dim and of a dull color.

• Blue GaN LEDs appeared in the early 90s. • Green GaN followed. • UV LEDs are still very expensive ( >100x visible). • Blue GaN LEDs: Nobel prize 2014 • Since they were first invented, LEDs have been used

in small appliances. • High brightness consumer applications (TVs, light

bulbs) started just 3-4 years ago.

7

Band diagram of Si (indirect band gap) and GaAs (direct band gap)

In Si, the low point of

the conduction band

does not line up with

the high point of the

valence band. In

GaAs and GaN it

does.

Eg

This representation

ignores whether the

gap is direct or

indirect.

Energy band structures of Si and GaAs. Circles

indicate holes in the valence bands and dots indicate

electrons in the conduction bands.

From Sze, Semiconductor Devices,

Physics and Technology, Wiley 1985

8

Traditional GaN LED on sapphire substrate (UV to green)

GaN

sapphire

GaN layer is 3 to 10um thick

Sapphire is 100-200um thick

Usually the sapphire surface is

patterned to allow dislocation

free growth of the GaN layer.

9

InGaAlP red LED top emitting (yellow to red)

7um

Light emitting

MQW

10

AlGaAs red LED side emitting (red to IR)

1.48µm Thickness

varies with

emission

wavelength. No

fine structure.

Low Al in light

emitting region

11

GaAs IR LED at 920nm

Optically and

in SEM,

device is

featureless

unless etched.

12

LED dice are the size of a grain of sugar

sugar

Tip of

ballpoint pen

rice

13

Turn-on voltage and emitted wavelength depend on band gap. This green LED was in older GaP technology White LEDs are really blue LEDs with a phosphor

I-V Curves of Individual Diodes

-0.005

0

0.005

0.01

0.015

0.02

0.025

1 2 3 4

Voltage (V)

I (a

mp

s)

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LED geometry, more than a p-n junction. In a simple p-n junction the newly created photons can decompose into e-h pairs since the materials are the

same. A heterojunction is needed.

N-contact

P-contact

Growth substrate:

sapphire, SiC or Si

P-contact N-contact

N-GaN

GaN/InGaN

MQW

ITO P-GaN

Most In, Ga, Al, As, P,

emit yellow to IR Emits UV

to green

Heterojunction

15

Real life view of GaN LED in cross-section

P-GaN

Light

emitting

N-GaN

16

Cross-section of GaN LED • Electrons are needed to produce photons

– Need to be stopped from getting to the positive contact: role of p-AlGaN Electron Blocking Layer

• Light is produced in the GaN-InGaN Multiple Quantum Well

• The SuperLattice reduces dislocations and serves as an electron reservoir. The more electrons supplied, the brighter the light emitted (up to a point, then droop sets in).

p-AlGaN EBL P-GaN (Mg doped)

MQW

SL

N-GaN (Si doped)

17

TEM cross sections: blue GaN LEDs from different manufacturers

ITO

P-GaN

P-GaN

ITO

P-GaN

dislocation

The MQWs are

clearly seen, the

SLs are faint

(less In) Bar is 50nm in

all 3 images

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110329

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

1E+23

0 1000 2000 3000 4000 5000

Depth:[nm]

Concentration:[atoms/cm3]

1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

Intensity:[c/s]

Mg

Si

27Al

69Ga

115In

Ga->

Al->

In-> Si

Mg

SIMS profile to 5um depth

N-contact depth

p-AlGaN n-AlGaN

19

SIMS profiles to ~1um (different LEDs)

110343

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

1E+23

0 200 400 600 800 1000

Depth:[nm]

Concentration:[atoms/cm3]

1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

Intensity:[c/s]

Mg

Si

27Al

64Zn

69Ga

115In

Ga-

>

Al-

>

In-> Si

Mg

Zn->

120101

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

1E+23

0 100 200 300 400 500 600 700

Depth:[nm]

Concentration:[atoms/cm3]

1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

Intensity:[c/s]

Mg

Si

27Al

69Ga

115In

Ga->

Al->

In->

Si

Mg

20

Anodes are more complex

Anode Cathode

Cr adhesion to Au

Au, In, Si

EBL layer. p-GaN is thick and roughened

10µm

ITO is patterned with

6µm diam. exclusion

circles

10µm

21

Packaging

• Two main concerns:

– Getting the light out, where it is wanted

– Getting rid of the heat

• Contrary to popular belief, LEDs operate very hot

• The heat goes into the printed circuit.

• LEDs are electronic systems, they lose 80% of the energy to heat

– So do tungsten light bulbs! So, what gives?

– LED bulbs deliver 50 to 60 lm/W and use less than 13W. (so do CFLs)

• The LEDs themselves deliver close to 100lm/W

– Tungsten bulbs deliver 12 lm/W at 40 to 100W

22

LED packages

23

Types of lenses

Monolithic

overmolded silicone

Silicone-filled glass shell

with metal holding ring

Silicone outer shell

filled with silicone gel

24

Side emitting LEDs debunked

• Side emitting LEDs are not special semiconductor devices

• They are regular LEDs mounted in a very small package with solder pads on the side. – Used in small screens in

cell phones, cameras etc..

Top emitting

Side emitting

25

Getting the light out

26

Getting rid of the heat

Alumina

ceramic

Copper

The GaN layer is <10um thick and

the MQW is about 0.1um thick

Heat sink

Copper

27

GaN on SiC

Doped SiC

cathode

SiC not

electrically

connected

SiC on top

28

GaN on GaN

29

Flipped LEDs Luxeon’s approach

4x4 array

5x5 array

A

C

30

Cross-section view

package

Underfill between bumps

GaN cathode anode

31

Cree’s approach

Anode conductive substrate

cathode

Highly

doped Si

Silver +

complex

metal

interface

GaN metal

32

Cree’s new approach, leave the SiC growth substrate on.

Anode

SiC

Cathode

33

Osram’s approach

Anode bond wire

Cathode to

conductive substrate

(can be a metal

composite)

34

Phosphors

• Particles that convert the blue light of GaN LEDs to a full visible spectrum

33.3µm

35

Phosphor debunked • The phosphor layer is a

thin coat of particles applied to the surface of blue LEDs – Partially removed in SEM

image – Can be particles in a glass

or ceramic plate glued to the GaN surface

• There is no phosphor on the inside of the bulb – It’s just frosted glass or

frosted plastic • No vacuum • Could be removed

36

Common phosphors

• YAG (Yttrium Aluminum Garnet) – Ce or Gd doped

• Barium thiogallate • Eu-doped barium strontium silicate. • Lutetium oxide • Aluminum Cerium Lutetium Oxide • Strontium carbonate • Calcium silicate

• No Phosphorous!

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Phosphors

Warm white 3000K Cool white 6500K

38

Phosphor

limited to

die

Phosphor

under lens

Phosphor plates

39

40

Remote phosphor

Blue LEDs

Phosphor particles

embedded in clear

plastic give an

overall yellow

appearance

41

A19 Light Bulbs Where are the LEDs in the light bulb?

A: pear shape

19: 19 1/8th inch, diameter of bulge

Diffuser LEDs heat sink neck with circuit heat shield socket

42

A19 light bulbs, circa 2011 mostly 8W and not commodity items

Philips

(12W)

Sylvania GE Feit Pharox

Lighting

Science Samsung Sharp LG Luminus

43

Sylvania

Philips

GE Pharox

Feit

44

A19 evolution 2011 18 months later

X6

8W 430lm 54lm/W 12W 830lm 68lm/W

45

Inside the neck

Daughter card

LED driver chip:

ON Semi HS01G

half bridge

resonant controller

Power transistor

46

Incandescent

100W A19 10.5W A19

CFL 7.5W A19 Fluorescent tube

PAR30 LED

120Hz ripple

3.5W LED

chandelier

1.8W LED

chandelier

47

LED HF ripple

Par30

10.5 W A19

3.5W 1.8W

7.5W A19

Top row

V: 50mV/div

H: 5µs/div

Bottom row

V: 20mV/div

H Left: 5µs/div

H Right: 10µs/div

48

Reliability wire bonds and protection diode

Good quality Poor quality

49

Failure mode: dielectric breakdown

A

C

0.6µm

50

Delamination: overheating and loss of electrical contact

20kV x150 200µm

51

Two failure mechanisms

Overheating decomposes the silicone of the inner lens which becomes corrosive

Overdriving carbonizes the inner lens above the LED

52

V-pits, not an immediate failure but they accelerate degradation

Relaxation of crystal-dislocations

reaching down to the active zone.

Allow contaminants to reach the

active region 750nm

53

IES LM-79

• What is it? – An approved method describing procedures and

precautions in performing electrical and photometric measurements of solid state lighting products.

• Scope – Applies to complete LED luminaires incorporating

control electronics plus heat sinks. – Excludes LED products requiring external control

circuitry e.g. bare LED chips, packages and modules.

54

Required Measurements

Outputs Conditions Techniques

Total luminous flux Ambient: 25° C +/- 1° C limited heat transfer and air flow Power: AC waveshape limit to harmonic RMS of 3% Voltage regulation to +/- 2% Seasoning & Stabilization Test orientation Electrical settings Instrumentation

Integrated sphere systems with spectral radiometer

Electric Power Goniophotometer system for measuring luminous intensity distribution where the total luminous flux is derived, plus color characteristics CCT and CRI are obtained in conjunction with the spectral radiometer.

Luminous intensity distribution

Chromaticity

Spectral mismatch

Correlated color Temperature (CCT)

Color Rendering Index (CRI)

55

IES LM-80

• What is it?

– An approved method for measuring Lumen depreciation of solid state ( LED) light sources, arrays and modules.

• Doesn’t cover measurement of luminaries and isn’t a method of estimation of life

• Most lamp sources burn out. LED’s typically don’t fail.

– LED’s continue to degrade – eventually beyond useful light output

56 Source: Lighting Research center. Rea 2000 Bullough 2003

Operating time (hrs)

0 5000 10000 15000 20000

57

Summary

• Direct band gap material is needed for light emission.

• The band gap determines the emission wavelength.

• Heterojunctions prevent the reabsorption of photons.

• The light emission region is a MQW.

• There is very little GaN in GaN LEDs.

• LEDs get very hot and need massive heat sinks.

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Summary (cont.) • Phosphors are composed of small particles of various

rare-earth minerals embedded in silicone. They do not contain phosphorous.

• A19 light bulbs have expensive electronic components.

• Thermal expansion causing broken wire bonds or die attach separation are common failure modes.

• Silicone can decompose and become corrosive if

overheated • V-pits are related to premature degradation

• LM79 and LM80 are standards regulating LED

performance

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Contact Information

Dr. Martine Simard–Normandin

President & CEO

MuAnalysis

2301 St. Laurent Blvd., Suite 500

Ottawa, ON K1G 4J7 Canada

Telephone: (613) 721-4664 x227

Facsimile: (613) 721-4682

Email: martine@muanalysis

Website: www.muanalysis.com