Reliabilityand FailureAnalysis ofOptoelectronicDevices ve… · Reliabilityand FailureAnalysis ofOptoelectronicDevices Matteo Meneghinia, Massimo Vanzib, µE-LAB, Reliability and

Reliability and Failure Analysis

of Optoelectronic Devices

Matteo Meneghinia, Massimo Vanzib,

µE-LAB,

Reliability and failure analysis of optoelectronics devices

Matteo Meneghini , Massimo Vanzi ,

Gaudenzio Meneghessoa and Enrico Zanonia

aUniversity of Padova, Department of Information Engineering,

via Gradenigo 6/B, 35131 Padova, Italy

E-mail: [email protected] of Cagliari, Italy

Outline

•Operating principles and structure of LEDs and laser diodes

•Degradation of optoelectronic devices

•Degradation of the heterostructure of LEDs and laser diodes

by means of electro-optical techniques

•Analysis of the degradation of the properties of ohmic

contacts of optoelectronic devices

•Degradation processes related to Dark Line Defects

•Degradation of the facets of laser diodes

µE-LAB,


•Degradation of the facets of laser diodes

•Conclusions

Recombination processes in semiconductors

µE-LAB,


After EF Schubert, Light Emitting Diodes, 2nd

Edition, Cambridge University Press

Surface recombination

µE-LAB,


After EF Schubert, Light Emitting Diodes, 2nd

Edition, Cambridge University Press

Light Extraction from LEDs

Light output

LightPlastic dome

Domedsemiconductor

(a) (b) (c)

p

Electrodes Electrodes

pn Junctionn+

n+

(a) Some light suffers total internal reflection and cannot escape. (b) Internal reflections

Substrate

µE-LAB,


(a) Some light suffers total internal reflection and cannot escape. (b) Internal reflectionscan be reduced and hence more light can be collected by shaping the semiconductor into adome so that the angles of incidence at the semiconductor-air surface are smaller than thecritical angle. (b) An economic method of allowing more light to escape from the LED isto encapsulate it in a transparent plastic dome.

Structure of a power LED

µE-LAB,


Thin-film LEDs

Thin Film technology (ThinGaN) (4 basic steps)

1) High reflectivity p-contact: 2) Wafer bonding

substrate

epitaxy (≈6 µm)

substrate

carriersolder

metal

3) Substrate removal

substrate

Laser Lift Off (for InGaN on sapphire)

4) Surface roughening

µE-LAB,


substrate

carrier

basic process patents owned by OSRAM

carrier

increased light extraction

Thin-film LEDs

PowerThinGaN LED structure:• low internal absorption ⇒ thin epi layers

Thinfilm principle:

• prevent absorption in substr. ⇒ highly reflecting mirror

Present actions:

PowerThinGaN LED structure:• low internal absorption ⇒ thin epi layers

GaN

textured surfacecontact

^

Solder layer

QW

• prevent waveguiding ⇒ optimize surface roughness

Mirror layer

1mm

µE-LAB,


Carrier substrate

Solder layer

PowerThinGaN top viewPower thin film LED; scematic side view

Light extraction of 75% is reached

White LEDs

Phosphors

µE-LAB,


Spontaneous vs Stimulated Emission

µE-LAB,


Gain - LASER

µE-LAB,


Structure of a laser diode

µE-LAB,


Optical characteristics of Laser Diodes

LaserOptical Power

LEDOptical PowerStimulated

Optical Power

λ

Laser

λ

Optical Power

I0

λ

Ith

Spontaneousemission

Stimulatedemission

µE-LAB,


Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

LEDs vs Laser Diodes

Light powerLaser diode

LED10 mW

)(2

)1(2

0 thph JJqn

RWhcP −

−=

λτ

Current0

LED

100 mA50 mA

5 mW JP ∝0

µE-LAB,


Typical optical power output vs. forward currentfor a LED and a laser diode.

100 mA50 mA

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)


L

WCleaved reflecting surface

Oxide insulator

Stripe electrode

SubstrateElectrode

p-GaAs (Contacting layer)

n-GaAs (Substrate)

p-GaAs (Active layer)

Currentpaths

L

p-AlxGa

1-xAs (Confining layer)

n-AlxGa

1-xAs (Confining layer) 12 3 Substrate

µE-LAB,


Schematic illustration of the the structure of a double heterojunction stripecontact laser diode

Active region where J > Jth.(Emission region)

Cleaved reflecting surfaceEllipticallaserbeam


µE-LAB,


Degradation of optoelectronic devices

µE-LAB,


Critical factors for LED degradation

Defects in the

active layer

(non-rad rec., Degradation of the

Detachment of the

bonding wire

b)b)

Degradation of

ohmic contacts (non-rad rec.,

impurity

diffusion, In-

related effects)

Degradation of the

phosphorous layer

bonding wire

Aluminum plate

Browning of the lens

and package material

ohmic contacts

(Mg-related)

µE-LAB,


Aluminum plate

Detachment from the

copper frame

Critical factors for laser diode degradation

µE-LAB,


After M. Fukuda, ESREF 2007

Optical Characteristics of LEDs and Laser Diodes

OptoelectronicOptoelectronic devicesdevices::

•High densities of injected carriers

•LEDs Light output power is nearly proportional to injected current

•Laser Diodes Above threshold optical power has a linear dependence on

Lig

ht

po

we

r

StimulatedStimulated

emissionemission High High

gaingain

Lig

ht

po

we

r

LED Laser

•Laser Diodes Above threshold optical power has a linear dependence on

injected current

µE-LAB,


Current

SpontaneousSpontaneous emissionemission

((L~carrierL~carrier density)density)

Current

EECC

EEVV

Stress conditions for LEDs and Laser Diodes

DrivingDriving methodologymethodology

••LED LED ConstantConstant currentcurrent drivingdriving ((dimmingdimming) ) DegradationDegradation parameterparameter: : OpticalOptical powerpower

••Laser Laser ConstantConstant OpticalOptical PowerPower drivingdriving ((egeg disk disk writingwriting) ) DegradationDegradation parameterparameter: :

operatingoperating currentcurrent

LED Laser

Lig

ht

po

we

r

Gradual (slow)

Sudden

Rapid

Gradual (slow)

SuddenRapid

Cu

rre

nt

LED Laser

µE-LAB,


Aging time

Rapid

Aging time

Degradation of Laser Diodes

SuddenSudden degradationdegradation::

DislocationDislocation growthgrowth in the in the innerinner regionregion

RapidRapid ((afterafter gradualgradual):):

Op

era

tin

gO

pe

rati

ng

Cu

rre

nt

Cu

rre

nt Stress at Stress at constantconstant

opticaloptical powerpower

RapidRapid ((afterafter gradualgradual):):

--facetsfacets (COD)(COD)

--dislocationdislocation growthgrowth in the in the activeactive regionregion

--bondingbonding damagedamage

--electrodeelectrode damagedamage

GradualGradual::

--pointpoint defectsdefects in the in the activeactive regionregion

Op

era

tin

gO

pe

rati

ng

µE-LAB,


GradualGradual::

--pointpoint defectsdefects in the in the activeactive regionregion

--facetsfacets

Stress Stress timetime

Gradual degradation of the Active Region

-

Generation of defects

µE-LAB,


Generation of defects

A case study: degradation of the active region of Blu-Ray LDs

n-GaN Guiding Layer

InGaNInGaN/GaN MQWs/GaN MQWs

p-AlGaN e- Blocking Layer

p-GaN Contact Layer DevicesDevices characteristicscharacteristics::

•Vertical structure devices (on GaN)•Emission wavelength = 405 nm•Slope efficiency = 1.6 W/A•Threshold current density = 3.2 kA/cm2 (29 mA)

GaN substrate and buffer

AlGaN Cladding Layer

n-GaN Guiding Layer •Threshold current density = 3.2 kA/cm2 (29 mA)

Stress Stress fixturesfixtures::

•Devices mounted on Peltier fixtures•Junction-to-case thermal resistance ~ 52 K/W•Maximum self-heating during stress 15 °C

AdoptedAdopted stress stress conditionsconditions::

µE-LAB,


24

AdoptedAdopted stress stress conditionsconditions::

Fixed parameter Varying parameter

Fixed current (70 Fixed current (70 mAmA dc)dc) TemperatureTemperature in the range 30in the range 30--7070 °°CC

Fixed Fixed TTcc (180 (180 °°C)C) NNo biaso bias

Fixed Fixed TTcc (70 (70 °°C)C) Current in the range 200 µACurrent in the range 200 µA--80 80 mAmA dcdc

A typical stress setup

µE-LAB,


Thermal characterization – Calibration phase

•Samples on a Peltier-fixture (30<T<90°C)CalibrationCalibration

Goal To evaluate junction temperature during operation

Method Xi and Schubert, Appl. Phys. Lett., 85, 2163, 2004.

•Samples on a Peltier-fixture (30<T<90 C)

•Short current pulses Voltage measurements

CalibrationCalibrationphasephase

Im (mA)

80 mA

4.2

4.4

4.6

4.8

Vol

tage

(V

)

Im=20 mA, I

m=30 mA,

Im=40 mA, I

m=50 mA,

Im=60 mA, I

m=70 mA,

Im=80 mA

µE-LAB,


26

0/)( TTmf BeAIV −+=

ExtrapolationExtrapolation ofof A, B and TA, B and T00 forfor the the calculationcalculation ofof RRthth

t

1 mA

30 40 50 60 70 80 90 1003.4

3.6

3.8

4.0

4.2

Vol

tage

(V

)

Fixture temperature TF (°C)

Thermal characterization – Rth extrapolation

••FixedFixed temperature (70 temperature (70 °°C)C)

••FixedFixed currentcurrent ((IImm))

••VoltageVoltage measurementmeasurement afterafter self self heatingheatingtransienttransient JunctionJunction temperature temperature evaluationevaluation

JunctionJunctiontemperature temperature evaluationevaluation

transienttransient JunctionJunction temperature temperature evaluationevaluation

•Rth = 52 K/W

•During stress at 70 mA,

TF=70 °C, Tj~80 °C (within 76

78

8070 mA

60 mA

50 mA

40 mA

Junc

tion

tem

pera

ture

(°C

)

Thermal resistance = 52 K/W(case temperature = 70 °C)

µE-LAB,


TF=70 °C, Tj~80 °C (within

recommended limits)

50 100 150 200 25070

72

7430 mA

25 mA

Junc

tion

tem

pera

ture

(°C

)

Joule Power = Total Power - Optical Power (mW)

20 mA

20

30

40

50 Before stress After stress

Opt

ical

pow

er (

mW

)

Threshold

L-I Characteristics before/after stress

•L-I curves measured before and after stress at 40 mA, 70 °C

AfterAfter stress: stress: •Increased Ith

•Same slope efficiency

Stress at 40 mA (4.4 kA/cm2), 70 °C,

1000 h

101

102

Opt

ical

Pow

er (

mW

)

Before stress After stress

20 25 30 35 40 45 50 55 60

0

10

Opt

ical

pow

er (

mW

)

Injected current (mA)

Threshold current increase

•Same slope efficiency

Linear scale

Semilog.

µE-LAB,


20 25 30 35 40 45 50 55 6010-2

10-1

100

Opt

ical

Pow

er (

mW

)

Injected current (mA)

Sub-thresholdemissiondecrease

Semilog.

scale•Stress induces also the decrease of

sub-threshold emission

(see semilog. plot)

Stress at 40 mA (4.4 kA/cm2), 70 °C, 1000 h

Ith increase vs OPsub decrease

1)( −=∆ th tI

I

ComparisonComparison betweenbetween::

ThresholdThreshold currentcurrent increaseincrease

0

5

10

I th in

crea

se (

%)

1)0(

−=∆th

thth I

I

)0(

)(1

sub

subsub OP

tOPOP −=∆

SubSub--thresholdthreshold OP OP decreasedecrease(at 28 (at 28 mAmA))

0 200 400 600 800 100060

70

80

90

100

0

OP

sub

varia

tion(

%)

I

Stress at 40 mA (4.4 kA/cm2), 70 °C, 1000 h

µE-LAB,


)0(subOP

The two parameters vary with the same kinetic Correlation

0 200 400 600 800 1000OP

Stress time (h)

Correlation between Ith increase and OPsub decrease

BNA +=τ1

n

Carriers recombination lifetime τ is determined

by the balance between the non-radiative (A) and

radiative (B) recombination

I is proportional to the ratio between threshold )( thth

thth BnAn

nI +=∝

τIth is proportional to the ratio between threshold

carrier density (nth) and the carriers recombination

lifetime τ

SinceSince::

•Both OPsub and Ith depend on the balance between radiative and non-radiative recombination events (τ)

µE-LAB,

Reliability and failure analysis of optoelectronics devices30

radiative recombination events (τ)•The variations of these two parameters have similar kinetics•Coefficient A is related to the defectiveness of the active layer (increasing during stress, Tomiya 2004)

Ith and OPsub variation can be attributed to the increase of the non-

radiative recombination rate A

Measurement details: τnr extrapolation

Purpose: Extrapolate non-radiative lifetime during ageing

Method: Proposed by Van Opdorp in 1981

Based on subthreshold Optical measurement

Non radiative

recombination

Radiative

recombination

Total currentLeakage current

Inj. current

Pint

electrons

Pout

p region

µE-LAB,


recombinationPint

holes

n region

40

50

60 After 400 h

Non

-rad

. rec

ombi

natio

n ra

te (

%)

Increasing stre

ss times

τnr during stress time

Stress at 120 mA, 70 °C

Linear correlation

0 2 4 6 8 10 12

0

10

20

30

40

Non

-rad

. rec

ombi

natio

n ra

te (

%)

Before stress

Increasing stre

ss times

)( ththth BnAnI +∝

Linear correlation

between the increase in

Ith and A

0 2 4 6 8 10 12

Non

-rad

. rec

ombi

natio

n ra

te (

%)

Threshold current increase (%)

Previous reports (Tomiya2004, Marona2006) The decrease in τnr can

be correlated to the increase of the concentration of defects in the

active region of the devices

Which is the driving force for degradation?

Measurement results: Correlation of Ith and τnr

1.0

1.1

1.2

1.3

1.4

Nor

mal

ized

Par

amet

er v

aria

tion

Stress @ 4.44 KA/cm2

Ith variation

ComparisonComparison

betweenbetween::

Constant current ageing,

Fixed temperature = 70°C

Stress at 40 mA dc

0.5

0.6

0.7

0.8

0.9

1.0

τnr variation

Nor

mal

ized

Par

amet

er v

aria

tion

Stress @ 4.44 KA/cm Stress @ 6.66 KA/cm2



IIthth increaseincrease

ττττττττnr nr decreasedecrease

Parameters vary

according to the

Stress at 40 mA dc

Stress at 60 mA dc

Stress at 80 mA dc

Stress at 100 mA dc

µE-LAB,


0 200 400 600 800 1000 1200 1400 1600 1800 20000.4

nr

Nor

mal

ized

Par

amet

er v

aria

tion

Stress Time (h)Ith and ττττnr are correlated during device degradation

Which is the effects of different chaning ageing current?

according to the

square root of

time

Measurement results: Effects of ageing current

The correlation is confirmed40mA 60mA

Plot of Ith increase as a function of 1/τnr

140

130

140

var

iatio

n (%

)

80mA 100mA

100 120 140 160 180 200

100

110

120

130

130

140

var

iatio

n (%

)

130

140

100 120 140 160 180 200

100

110

120

130

I th v

aria

tion

(%)

( )[ ]2/1 ththnrth BnnqVI += τ

Compatible with

trasparency condition

µE-LAB,


The two mechanisms have the same

dependence on the driving currents

100 120 140 160 180 200

100

110

120

I th v

aria

tion

(%)

1/τnr variation (%)

100 120 140 160 180 200

100

110

120

1/τnr variation (%)

Trivellin et al, ESREF 2009

Effect of stress current on the degradation kinetics

Stress conditions:

•Case temperature=70 °C

•Stress current=200µA-80 mA

•Maximum junction temperature

= 82 °C0.5

1.0

4

6

8

80 mA

Thr

esho

ld c

urre

nt in

crea

se (

%)

60 mA

Thr

esho

ld c

urre

nt in

crea

se (

%)

Tc=70 °C

= 82 °C

Increasing stress current determines

faster degradation

•Degradation kinetics are strongly determined by the stress current level

0 50 100 150 200

0.0

0.5

0 50 100 150 200

0

2

Thr

esho

ld c

urre

nt in

crea

se (

%)

Stress time (h)

Stress at 200 µA,T

c=70 °C

40 mA

Thr

esho

ld c

urre

nt in

crea

se (

%)

Stress time (h)

20 mA

µE-LAB,


•Degradation kinetics are strongly determined by the stress current level

•At 70 °C Threshold current is 40 mA

•Degradation occurs also below lasing threshold (and at 200 µA!), i.e. without strong

optical field and self-heating Role of current?

Degradation Rate vs Stress Current

6

8

incr

ease

afte

r 20

0 h

(%) At the stress

temperature (70 °C),

threshold current is

40 mA

In this region, stress

current is < Threshold

current

Ith = 4

0 m

A

0 20 40 60 800

2

4

I th in

crea

se a

fter

200

h (%

)

+ 0.1% / mA

40 mA

µE-LAB,


•Degradation occurs also below threshold

•Degradation rate is strongly determined by stress

current level

0 20 40 60 80

Stress current (mA)

Degradation rate = Ith

increase after 200 h of

stress

Study of the degradation of the active region by C-V measurements

p-type

Anode

n-type

Cathode

µE-LAB,


The area for the C-V

measurements is close to

500 x 75 µm2

Constant current stress – Output power during stress

Stress at 32 A/cm2

µE-LAB,


••Stress Stress conditionsconditions

85 A/cm85 A/cm22, RT (, RT (TjTj<100 <100 °° C)C)

Current ageing – Current – Voltage Characteristics


85 A/cm85 A/cm22, RT (, RT (TjTj<100 <100 °° C)C)85 A/cm85 A/cm22, RT (, RT (TjTj<100 <100 °° C)C)

µE-LAB,


••Stress Stress inducesinduces significantsignificant modificationsmodifications in the in the

electricalelectrical characteristicscharacteristics ofof the the devicesdevices ((increaseincrease in in

reverse and reverse and generationgeneration--recombinationrecombination componentscomponents))

Heterostructure degradation C-V measurementsInformation about

charge distributionDepleted region


85 A/cm85 A/cm22, RT (, RT (TjTj<90 <90 °° C)C)

ApparentApparent chargecharge distributiondistribution

p+-GaN n-GaN

µE-LAB,

Reliability of Reliability of GaNGaN--based optoelectronic devices: state of the art and perspectivesbased optoelectronic devices: state of the art and perspectives

••Detection Detection ofof changeschanges ofof the the

chargecharge profileprofile inducedinduced byby stressstress

••A. A. CastaldiniCastaldini, IWN 2004, IWN 2004

••PhysPhys. . StatStat. Sol. (c) 2, 2862 (2005). Sol. (c) 2, 2862 (2005)

••J. J. ApplAppl. . PhysPhys. 99, 053104 (2006). 99, 053104 (2006)

Correlation between OP loss and C-V data

••CorrelationCorrelation betweenbetween

OP OP decreasedecrease and and chargecharge

variationvariation

••ChangesChanges

in the Cin the C--V V

profilesprofiles

1.0

1.2

1.4

Con

cent

ratio

n (x

1018

cm

-3) ••LocalizedLocalized chargecharge

increaseincrease in the in the

activeactive regionregion

1.1. Stress Stress inducedinduced a a significantsignificant increaseincrease in in

the Cthe C--V curve (V curve (concentratedconcentrated in the in the

activeactive regionregion))

µE-LAB,


40 60 80 100 120 140 160 180 200 2200.0

0.2

0.4

0.6

0.8

Con

cent

ratio

n (x

10

Depth (nm)

Before stress After 5h After 100h


2.2. Strong Strong correlationcorrelation betweenbetween OP OP

decreasedecrease and and chargecharge variationvariation

3.3. DLTS DLTS analysisanalysis indicatedindicated thatthat stress stress

inducesinduces the the increaseincrease ofof a a DLDL peakpeak

((detecteddetected DLsDLs havehave EaEa in the in the rangerange

250250--900 900 meVmeV))

Correlation between OP loss and C-V data

••CorrelationCorrelation betweenbetween

OP OP decreasedecrease and and chargecharge

variationvariation

••ChangesChanges

in the Cin the C--V V

profilesprofiles

0.000

A

••DLTS DLTS analysisanalysis in in

the the activeactive regionregion1.1. Stress Stress inducedinduced a a significantsignificant increaseincrease in in

the Cthe C--V curve (V curve (concentratedconcentrated in the in the


µE-LAB,


50 100 150 200 250

-0.012

-0.008

-0.004

C

B

Untreated 2 hours 10 hours 20 hours 50 hours 100 hours

Temperature (K)

∆C/C


2.2. Strong Strong correlationcorrelation betweenbetween OP OP

decreasedecrease and and chargecharge variationvariation

3.3. DLTS DLTS analysisanalysis indicatedindicated thatthat stress stress

inducesinduces the the increaseincrease ofof a a DLDL peakpeak

((detecteddetected DLsDLs havehave EaEa in the in the rangerange

250250--900 900 meVmeV))

Gradual degradation

-

Degradation of the contacts

µE-LAB,


Degradation of the contacts

A case study: degradation of the ohmic contacts of LEDs

p-Side

Anode golden pad SiN passivation

•Devices with vertical structure grown on SiC

•Bare LED chips mounted on InGaN/GaN MQW

n-Side

SiC bulk

Cathode contact

•Bare LED chips mounted on TO18 package

•LEDs covered with SiN passivation to reduce surface leakage and encapsulate the

devices

µE-LAB,


devices

•LEDs submitted to stress at high temperatures

(180<T<250 °C)

Stress of bare LED chips – OP during stress

75

100

180°CO

P m

easu

red

at 1

0 m

A

(% o

f ini

tial v

alue

)

0 500 1000 1500

25

50

190°C

210°C220°C

230°C

OP

mea

sure

d at

10

mA

(%

of i

nitia

l val

ue)

250°Cτ/100 teKI −⋅−=

µE-LAB,


0 500 1000 1500

Stress time (minutes)

IncreasingIncreasing stress temperature stress temperature determinesdeterminesstrongerstronger degradationdegradation

Stress of bare LED chips – Activation energy

103

Tim

e co

nsta

nt (

τ)

kTEt AeeKI // ,100 ∝⋅−= − ττ

21 22 23 24 25 26

101

102

Tim

e co

nsta

nt (

Ea= 1.3 eV

µE-LAB,


OP OP degradationdegradation isis thermallythermally activatedactivated(Ea=1.3 (Ea=1.3 eVeV))

21 22 23 24 25 26

q/kT

Stress of bare LED chips – L-I curves

10-1

100

Out

put p

ower

(a.

u.)

10-5 10-4 10-3 10-2 10-1

10-4

10-3

10-2

Out

put p

ower

(a.

u.)

Before stress After 90 min at 250 °C

µE-LAB,


10 10 10 10 10

Input current (A)

••OpticalOptical powerpower decreasedecrease waswas more more prominentprominent at at high high measuringmeasuring currentcurrent levelslevels

Stress of bare LED chips – Normalized L-I curves

90

100

110

Rel

ativ

e O

utpu

t Pow

er (

norm

aliz

ed)

Before stress After 90 min at 250 °C

0 5 10 15 2040

50

60

70

80-42 % OP at 1 mA

Rel

ativ

e O

utpu

t Pow

er (

norm

aliz

ed)

-56 % OP at 20 mA

µE-LAB,


••OpticalOptical powerpower decreasedecrease waswas more more prominentprominent at at high high measuringmeasuring currentcurrent levelslevels

0 5 10 15 20

Rel

ativ

e O

utpu

t Pow

er (

norm

aliz

ed)

Input current (mA)

Stress of bare LED chips – EMMI

M. Meneghini et al., IEEE TED 53 (12), 2981-2987 (2006)

Anode golden pad

p-GaN

250µµµµm

• Stress at 250 °C

• I=10 mA

Anode contact

Top view

µE-LAB,


Before stress: uniform pattern

Before stress• I=10 mA


M. Meneghini et al., IEEE TED 53 (12), 2981-2987 (2006)

Anode golden pad

p-GaN

250µµµµm

• Stress at 250 °C

• I=10 mA

Anode contact

Top view

µE-LAB,


Before stress: uniform pattern

• I=10 mAAfter 30 min


6

8CB

In

tens

ity(a

.u.) Before stress

AAnode golden pad

p-GaN

250µµµµm

A B C

0 25 50 75 100 125

0

2

4

6

Inte

nsity

(a.u

.)

Anode contact

Top view

µE-LAB,


0 25 50 75 100 125

X Axis (µm)

BeforeBefore stress stress UniformUniform emissionemission profileprofile in in regionsregions A and BA and B


6

8CB

In

tens

ity(a

.u.) Before stress

After 30 min

AAnode golden pad

p-GaN

250µµµµm

A B C

0 25 50 75 100 125

0

2

4

6

Inte

nsity

(a.u

.)

After 30 min After 60 min After 90 min

Passivated LEDAnode contact

Top view

µE-LAB,


0 25 50 75 100 125

X Axis (µm)

AfterAfter stress stress Strong Strong decreasedecrease ofof the light the light emissionemission in in regionregion A (far A (far fromfrom the the centralcentral pad)pad)

Stress of bare LED chips – I-V curves

15

20

Cur

rent

(m

A) D R

S

2.0 2.5 3.0 3.5 4.00

5

10

Before stress After 10 min After 40 min After 90 min

Cur

rent

(m

A)

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2.0 2.5 3.0 3.5 4.0

Voltage (V)

High temperature treatment High temperature treatment determinesdetermines the the shiftshift ofof the Ithe I--V V curvescurves towardstowards higherhigher voltagesvoltages

Stress of bare LED chips – Fitting of the I-V curves

3.7

3.8

3.9

4.0

23

24

25

26

Idea

lity

fact

or n

Ser

ies

resi

stan

ce R

s (Ω

)

10

15

20

Before stress After 10 min

Cur

rent

(m

A) D R

S

0 20 40 60 80 100

3.5

3.6

21

22

Ideality factor

Idea

lity

fact

or n

Stress time (min)

Ser

ies

resi

stan

ce R

s (

Series resistance

2.0 2.5 3.0 3.5 4.00

5 After 10 min After 40 min After 90 min

Voltage (V)

RSD

nkt

RIVq

eII)(

0

−

∝

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eII 0∝••RsRs IncreaseIncrease IncreasedIncreased resistivityresistivity ofof pp--GaNGaN and and

contactscontacts••n n increaseincrease DegradationDegradation ofof the the propertiesproperties ofof the ohmic the ohmic

contactscontacts

Stress of bare LED chips – Role of Passivation

••PECVD introduces H in the passivation PECVD introduces H in the passivation

layer and/or at the interface with player and/or at the interface with p--GaNGaN

••Generation of MgGeneration of Mg--H complexes H complexes Lowering of effective acceptor concentrationLowering of effective acceptor concentration

pp--GaN layerGaN layer

PassivationPassivation

HH HHHH HH HH HH HH HH HH HH

••Generation of MgGeneration of Mg--H complexes H complexes Lowering of effective acceptor concentrationLowering of effective acceptor concentration

Metal p-GaN

hh++

Ohmic contacts on pOhmic contacts on p--type GaN type GaN Tunnel junctions (very Tunnel junctions (very

high acceptor concentration) high acceptor concentration)

EEFF VBVB

AnalysisAnalysis on on TLMsTLMs Information on the Information on the effectseffects ofof stress on M/S system stress on M/S system

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hh++

Metal p-GaN

Stress Stress lowerslowers activeactive dopant dopant concentrationconcentration at pat p--side side

SchottkySchottky barrierbarrier broadeningbroadening

RectifyingRectifying behaviorbehavior EEFFVBVB

••Passivation deposition introduces H in Passivation deposition introduces H in the passivation layer and/or at the the passivation layer and/or at the interface with pinterface with p--GaN (SiHGaN (SiH44 as as precursor)precursor)

HH HHHHHH

HHHH

HH

HH

HH

HH

HH

HH

HH

HH

HH HHHHHH

HHHH

••Heating at 250 Heating at 250 °°C allows this hydrogen to interact with LEDs surfaceC allows this hydrogen to interact with LEDs surfac e


••S. M. Myers, J. Appl. Phys., 89, 6, 3195, 2001S. M. Myers, J. Appl. Phys., 89, 6, 3195, 2001

••C. H. Seager, J. Appl. Phys., 92, 12, 7246, 2002C. H. Seager, J. Appl. Phys., 92, 12, 7246, 2002

••Generation of MgGeneration of Mg--H complexes and/or degradation of ohmic contacts H complexes and/or degradation of ohmic contacts (worsening of transport properties)(worsening of transport properties)

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••Golden pad obstructs hydrogen Golden pad obstructs hydrogen diffusion towards pdiffusion towards p--GaNGaN

••Degradation takes place mainly out of Degradation takes place mainly out of the pad regionthe pad region

••Emission is concentrated under the padEmission is concentrated under the pad


100

Out

put p

ower

(%

)

••DegradationDegradation isis stronglystronglyrelatedrelated toto the the presencepresence

ofof the the SiNSiN--PECVDPECVD

0 20 40 60 80 100

40

60

80 Without passivation

With passivation

Out

put p

ower

(%

)

Stress at 250 °C

ofof the the SiNSiN--PECVDPECVDpassivation passivation layerlayer

••DuringDuring PECVD PECVD processprocess, , SiHSiH44 and NHand NH33 are are usedused

asas precursorsprecursors(passivation (passivation containscontains

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0 20 40 60 80 100

Stress time (min)(passivation (passivation containscontains

hydrogenhydrogen))

Degradation of the contacts of power LEDs

Aim: characterization of the degradation of the met al/semiconductor contacts

Techniques:

•Scanning Electron Microscopy (SEM)

•Energy dispersive X-ray Spectrometry (EDS)

BackBack--scattered imagescattered image 3D reconstruction3D reconstruction

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Poor adhesion of the metal layer Poor adhesion of the metal layer Related to RRelated to R ss increase?increase?

Degradation of the ohmic contacts of LEDs

••Bare Bare InGaNInGaN LED LED chipschips havehave beenbeen submittedsubmitted toto thermalthermal storagestorage

(180<T<250 (180<T<250 °° C)C)

••OP OP degradationdegradation hashas a a nearlynearly exponentialexponential decaydecay, and , and isis correlatedcorrelated

toto emissionemission crowdingcrowdingtoto emissionemission crowdingcrowding

••ThermallyThermally activatedactivated processprocess, Ea=1.3 , Ea=1.3 eVeV

••OP OP decaydecay isis relatedrelated toto the the degradationdegradation ofof the the electricalelectrical

propertiesproperties ofof the the LEDsLEDs IncreasedIncreased RsRs, , worseningworsening ofof the the

propertiesproperties ofof the ohmic the ohmic contactscontacts, , determiningdetermining emissionemission crowdingcrowding

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••DegradationDegradation isis relatedrelated toto the the presencepresence ofof PECVDPECVD--SiNSiN passivation passivation

RoleRole ofof hydrogenhydrogen in in devicesdevices degradationdegradation

••SputteringSputtering hashas beenbeen proposedproposed asas anan alternative alternative forfor passivationpassivation

Rapid degradation of the active region

-

Dark Line Defects

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Dark Line Defects

Dark line defects

•The structure of the DLDs has been the object of many studies.

Transmission Electron Microscopy (TEM) analyses of DLDs present them as

dense three-dimensional networks of dislocation loops and dipoles

•These clusters of crystal defects develop around a threading or a misfit

dislocation crossing the active layer, and are primarily found at the QW

and the neighbouring cladding layersand the neighbouring cladding layers

•Threading dislocations can be present in the

heterostructure as a consequence of the crystal

growth process. Most of the threading dislocations

emerge from the substrate and thread through the

epitaxial multilayer structure

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•Misfit dislocations arise from internal stresses

associated with the differences between the lattice

parameters of the layers forming the multilayer

structure. They can also be introduced by handling

and mounting processes

Dark line defects

•DLDs appear as region of very low or even dark luminescence efficiency

•DLDs can appear as oriented dark structures in CL and EL images

•They are located in the active region and are constrained to the waveguide

regionSEMSEM

EBIC

EBIC detail

Another specimen

EL reference

EL common to COD and DLD

•EBIC images

also show a

dark constrast,

which reveals a

high charge-

trapping

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EBICtrapping

efficiency in

those region

Dark line defects

•When studied by TEM DLDs appear as clusters of dislocations loops and dipoles

•The dislocation motion leading to the formation of DLDs proceeds by

Recombination-enhanced dislocation climb/glide

•Consists of an increase in the dislocation length mediated by either the

absorption or emission of point defects

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Dislocation climb in the active layer

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Dark line defects

DLDs appear as a network of dislocations and of dislocation loops, evolving from native defects at the epitaxial AlGaAs/GaAs

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native defects at the epitaxial AlGaAs/GaAs interfaces under the effect of temperature (and recombination, as demonstrated years later)

Dark line defects

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Rapid degradation of the active region

-

Facet Degradation

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Facet Degradation

Facet degradation

While the lifetime of low power laser diodes is limited by gradual degradation, the

maximum optical power of high power laser diodes is mostly limited by the catastrophic

optical mirror damage (COMD). This failure consists in the destruction of the mirror

facetslight absorption at the facet

electron-hole pair --- bond breaking generation

facet oxidation

nonradiative recombination

defect increase

dislocation growth

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heating

reduction of band-gap energy

catastrophic optical damage (COD)

dislocation growth

current concentrationCourtesy of Prof.

M. Fukuda

Facet degradation

The key parameters controlling the COMD are:

•the Surface Recombination Velocity (SRV);•the Surface Recombination Velocity (SRV);

•the density of defects at the facet;

•the facet treatment and coating, which partially determine the

previous parameters;

•the temperature dependence of the band gap of materials

forming the active region;

•the optical power and the current injection;

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•the optical power and the current injection;

•the thermal conductivities of the different layers forming the

laser structure

Facet degradation

EBIC reveals lattice-oriented dark stripes at the “burned” mirrors

COD (Catastrophic Optical

Damage) affects the laser mirrors

SEM EBIC

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detail detail

Facet degradation

Newton rings

POUT(mW)

monitor

front facet

Optical, on a nearby zone IL(mA)

IL(mA)

front facet

reference

leakydiode

Degradation after THB tests.Mechanism: detachmentof the mirror coating

optical

SEM

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VL(V)

reference

EBIC

Facet degradation

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Facet degradation

Photoinduced oxidation of the facet. The watermolecules near the facet can be cracked to H+ and OH− and to further radicals by photolysis and trigger by photolysis and trigger the oxidation

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Catastrophic Degradation

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ESD-related failure

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ESD-related failure

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ESD-related failure

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ESD-related failure

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ESD-related failure

Reverse-bias EL distribution

(before any ESD stress)

Micrograph after ESD failure (failed

region is indicated by an arrow)

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Sudden failure related to EOS

Poor definition of the mesa

Failure analysis by means of SEM images

Before AgeingBefore Ageing

Spike

of the mesa borders

After degradation

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Catastrophic Failure

Conclusions

With this presentation we have given an introduction on the

operating principles and degradation mechanisms of LEDs and laser

diodes

Based on a number of case studies, we have presented guidelines forBased on a number of case studies, we have presented guidelines for

the investigation of:

•The degradation of the heterostructure of LEDs and laser diodes by

means of electro-optical techniques

•The analysis of the degradation of the properties of ohmic contacts

of optoelectronic devices

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of optoelectronic devices

•Degradation processes related to Dark Line Defects

•The degradation of the facets of laser diodes

Documents

Reliabilityand FailureAnalysis ofOptoelectronicDevices ve… · Reliabilityand FailureAnalysis ofOptoelectronicDevices Matteo Meneghinia, Massimo Vanzib, µE-LAB, Reliability and