Reliability investigations Lifetime measurements … investigations Lifetime measurements and degradation mechanisms Bernd Sumpf, Ute Zeimer, Karl Häusler, Andreas Klehr Ferdinand-Braun-Institut

...translating ideas into innovation

Reliability investigations Lifetime measurements anddegradation mechanisms

Bernd Sumpf, Ute Zeimer, Karl Häusler, Andreas KlehrFerdinand-Braun-Institut für HöchstfrequenztechnikGustav-Kirchhoff-Straße 4, D-12489 Berlin, Germany

Jens W. TommMax-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie BerlinMax-Born-Str. 2 A, D-12489 Berlin, Germany

Tutorial - WWW.BRIGHT.EU Plenary Meeting – 5. & 6. July 2006, Cambridge, UK

OutlineI. Aging tests for high power diode lasers

1. Degradation measurements: tasks and objectives2. Statistical basics, acceleration factors3. Experimental set-up4. Aging test – selection of samples, accompanying measurements 5. Statistical analysis of results

II. Analytical work for understanding gradual degradation mechanisms: Strain measurement

1. Approach and Definitions2. Theory 3. Experimental methods and results

3.1 Micro-Photoluminescence (µPL) 3.2 Photocurrent spectroscopy (PCS)3.3 Degree-of-polarization PL or R (DoP-PL)

4. Summary


Aging tests for high power diode lasers

1. Degradation measurements: tasks and objectives

2. Statistical basics, acceleration factors

3. Experimental set-up

4. Aging test – selection of sample, accompanying measurements

5. Statistical analysis of results


Degradation measurements: tasks and objectives

• Degradation MeasurementsDefined aging of the diode laserAccompanying diagnostic measurements to understand aging

• Aim of the measurementQualification of laser diode structureLong term stability of

- Epitaxial structure- Surfaces- Contacts- Facets

Estimation of life time of the laser diodeSuggestions for the improvement of the structure









Statistics: reliability – definitions

Reliability= Probability of a reliable operation until time t.

Failure probability:= Probability for a device failure until time t.

Mean time to failure (MTTF)= Expectation of failure time.

)(tR

)(1)( tRtF −=

dttdt

tdFMTTF )(0∫∞

=


Statistics: assumption – exponential distribution

)exp(-)( ttR ⋅= λReliability:

Failure probability:λ: Failure rate = const. („hazard failure rate“)

Mean time to failure:

Example:Demanded reliability R = 0.98 over an operating time

top = 4 years (35040 h)

)exp(--1)( ttF ⋅= λ

)ln(-1 op

Rt

MTTF ==λ

h107.1 6⋅=MTTF

MIL-HDBK-217F, 6.13 „Optoelectronics, Laser Diode“, 6-21 (1991)


Design of experiments – unknown failure time

Determination of the failure probability of samples:

n – number of test samples in the lotr – number of samples with failure within the testF – failure probability – unknown value

Solution: Beta-distribution

∫−+

=F

n-rr xxxrrn

nFB0

d)-1(!)!(

1)!()(



Assuming a certain confidence level (1 - α)

Determination of α−=−+

= ∫ 1d)-1(!)!(

1)!()(0

Fn-rr xxx

rrnnFB

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0n = 5r = 1

F

Confidence level1−α = 0.6

B(F

, r+1

, n−r

+1)

Beta-distribution for n = 5 and r = 1i.e.: 5 samples in test, 1 failure

B(F, r + 1, n − r + 1) = 0.6

⇒ F = B0.6(r +1, n − r +1)

e.g. F = B0.6 (2;5) = 0.309



Calculation of the MTTF:

Aging time of the experiment: taging = 10000 h (417 days)

60% probability that the MTTF ≥ MTTF0.60Under nominal conditions requested MTTF not verifiable!

⇒ Accelerated lifetime tests

)]1,1( - [1lnMTTF

-1

aging1 +−+

−=− rnrBt

αα

h27055].3090 - [1ln

h10000MTTF1 =−=−α


Reasons for degradation

Point defects, DislocationsAbsorb the laser light and convert it into heatMove during operation and accumulate in the active region

recombination enhanced defect motion – REDM-process

Facet degradationBand gap deformation within the cleaved facetsAbsorption and non-radiative recombination – COD

Local stressLocal changes in the band gap – absorption and heatingPoint defects - recombination enhanced defect reaction – REDR

Processes accelerated at high power and high temperature


Acceleration factors

Overstressing of devices to proof longer lifetimes by• Higher output power, i.e. higher current• Higher temperature

Failure rate for accelerated lifetime test:

op: operational conditionsaging: conditions of the aging test

πT: acceleration factor caused by temperatureπI: acceleration factor caused by higher currentπP: acceleration factor caused by higher output power

PITopaging πππλλ ⋅⋅⋅=

MIL-HDBK-217F, 6.13 „Optoelectronics, Laser Diode“, 6-21 (1991) and Bellcore GR-468-CORE Iss. 1 (1998)


Acceleration factor – temperatureTemperature:

Activation energy Ea = 0.3 - 0.5 eVempirical value, different values from literature

Heating during operatione.g. at Rth = 10 K/W; Popt = 10 W; ηC = 0.5

temperature increase of about 100 K

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−=

opaging

aT

11-expTTk

Eπ

⎟⎟⎠

⎞⎜⎜⎝

⎛−⋅=∆ 11

cth η

PRT



Acceleration factors – power and currentPower:

Derating exponent β = 2 … 6; P – optical power

CurrentCoefficient x = 0 … 2

β

π ⎟⎟⎠

⎞⎜⎜⎝

⎛=

op

agingP P

P

x

op

agingI ⎟

⎟⎠

⎞⎜⎜⎝

⎛=

II

π



Example – accelerated lifetime test

Target MTTF ≥ 1.7 Mio hAssumption: EA = 0.5 eV, β = 2.3, x = 0; 1− α = 0.6; n = 5; taging = 10000 h

Conditions of the accelerated test: Paging = 2 x Pop and Taging = 65°C; Top = 25°C

πT x πP = 66, i.e. 66 times increase of the failure rate

Number of failures r = 0, 1 or 2:To detect the target MTTF – only one diode out of five is allowed to fail !

]-6 1,(-ln[1-h10000MTTF

6.06.0 rrB

TP+

××=

ππ

r MTTF0.6

0 4.3 x 106 h1 1.8 x 106 h2 1.0 x 106 h









Experimental set-up: Requirements

• Measurement of degradation rates below 10-5 h-1

Accuracy better than 1% within 1000 h

- Assumption 1 A current, 10 mA changes in 1000 h

- in 24 h changes of 0.24 mA

- 10 bit resolution of 1 A: 0.976 mA



Aging tests at temperatures 15°C ≤ T ≤ 80°C for more than 1000 h

- Temperature stability better 0.1 K

No degradation of the set-up


Experimental set-up: Test Environment

• Test chambers with stabilized temperature• Measurement of power between 100 mW … 15 W with high accuracy

Challenge: attenuation of power- RW laser diodes: emission power up to P = 500 mW,- High-power BA laser diodes up to P = 15 W- Laser bars P ≥ 100 W

• Computer controlled current supply and measuring system• Selection criteria for devices:

Power-voltage-current characteristicsFacet inspectione.g. longitudinal mode analysis

• Failure analysise.g. cathodoluminescence


Experimental set-up

Geometrical attenuation of lightin test chamber

Test Chamber

LiTHERMTemperature

Controller15°C ≤ T ≤ 75°C

Photodiode Laserdiode

Current supplye.g. Thorlabs - Profile

LDC 80808 A


Experimental set-up

Test chamber for five laser diodes


Experimental set-up

Test chamber

Temperature controller


Attenuation of light withan integrating sphereExperimental set-up

GPIB

MotionController

Currentsupply

Power meter

travelling detector

40 laser diodes at 8 banks

Current andmotion controller,e.g. Newport


Experimental set-up

Electronics

Aging boxwith integrating sphere inside

Holder for 5 laser diodes


Experimental set-up

SM35-45

Test Chamber

TemperatureController

15°C ≤ T ≤ 75°C

Power meter

Measurement of optical power with a detector

capable to measure 100 We.g. Gentec

LaserdiodeCurrent supply

e.g. delta electronica


Experimental set-up 19" Rack system

Test chamber

Heat sinksHolderfor detector









Selection of samples

0.0 0.4 0.8 1.20

100

200

300

400

500

Out

put p

ower

P /

mW

Current I / A

before aging

Power-current-characteristics:• Selection of devices• Definition of operational current

Example:9 devices; λ = 650 nmGeometry: 100 µm x 750 µmT = 15°C

Measurement before 10000 h of aging test



Measurement of near field(optional)

• Detection of facet failures

Example:650 nm device100 µm x 750 µm; T = 15°C, P = 500 mW

Near Field: Top-hat shape; W1/e2 = 100 µm

-100 -50 0 50 100

rela

tive

inte

nsity

/ ar

b. u

nits

position x / µm



Measurement of near field(optional)

• Detection of facet failures

Example:730 nm tapered laser2.75 mm longT = 25°C, P = 2 W

Near Field: W1/e2 = 160 µm

-200 -100 0 100 200re

l. in

tens

ity /

arb.

uni

ts

position x / µm

Facet failure



60 µm

Visual inspection of the facet:

Example 650 nm BA-laser60 µm x 750 µm

Electroluminescence image:• Current below threshold• Optical microscope 60 µm



Visual inspection of the facet:

Example 650 nm BA-laser60 µm x 750 µm

Electroluminescence image:• Current below threshold• Optical microscope

• Failure in the IR Image

60 µm

60 µm



Analysis of the longitudinal mode spectrumSpacing of longitudinal

modes ∆λ depends on Wavelength λLaser length LRefractive index n

Defect change local refractive indexFormation of sub-cavitiesModulation of the envelope of the mode spectrum

Crystal defect

Lzdz

0

Front-facet

Rear-facet

Measurement of spectrum below thresholdFourier-Transformation delivers position of defect

gnL ⋅⋅=∆

2

2λλ



Analysis of the longitudinal mode spectrum – simulation

Mode-Spectrum of a laser diode with L = 1 mm Defects at 125 µm and 350 µmmeasured from front facet

Fourier-Transformation of Mode spectrum

1010 1015 1020 1025 10300

2

4

6

8 R = 10-4

Pow

er P

/ arb

. uni

ts

Wavelength λ / µm

0 2 4 6 8 100.00

0.05

0.10

1000µm

∆ = 3.06µm

348.6µm128.5µm

Position inside the cavity / arb. units.

Am

plitu

de /

arb.

uni

ts


Aging test - 650 nm DQW broad area lasers

100 µm x 750 µmReliable operation at T = 15°C

400 mW (4 mW/µm): 9(9) - t = 1000 h

500 mW (5 mW/µm): 7(9) - t = 10000 hMTTF – 40000 h No COD failures

Lifetime sufficient for medical applications (1000 h)and close to the demands for display application ( > 10000 h) 0 2500 5000 7500 10000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

500 mW

400 mW

Cur

rent

I / A

Aging time t / h


Measurements after aging test

Power-current-characteristics:• Test of devices• Comparison with aging result

Example:Nine 650 nm devicesGeometry: 100 µm x 750 µmT = 15°C

Measurement after 10000 h of aging test

All devices still operational.No COD-Failures

0.0 0.4 0.8 1.20

100

200

300

400

500

Out

put p

ower

P /

mW

Current I / A

before aging after aging



Device without degradation

before 10000 h aging test

after 10000 haging test




600 mW (6 mW/µm): 7(7) - t ≥ 1100 h

Lifetime sufficient for medical applications (1000 h)

0 1000 20000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Cur

rent

I/ A

Aging Time t / h

600 mW at 15°C



Device with degradation

before aging test

after aging test Defects


Cathodoluminescence (CL): Principle• Penetration depth of

electron beamabout 3 µm

• Generation of electron-hole-pairs

• Radiative recombination

• At defectsnon-radiativerecombination

Measurements:• Spectra• Images

Lateral resolution ≤ 1 µmSpectral resolution ≤ 0.5 nm

Electron beam, UB : 5 - 35 kV

Sample, T = 80 K

Monochromator

Detector

Electronics,Monitor


Preparation of samples for CL

• Unsoldering from mount

• Grinding of the n-side metallization

• Wet chemical removal of GaAs substrate

Thickness of sampleabout 3 – 4 µm

CuW

CuW

CuW

AuSubstrateEpi-Layers


CL-measurement: facet failure930 nm,10 W, 25°C, 3700 h, 100 µm stripe width

850 855 860 865 870 875 880

0

1x105

2x105

3x105

4x105

5x105

6x105

outside stripe front facet centre rear facet

Inte

nsity

/ ar

b. u

.

λ / nm

∆λ = ± 1 nm

SE-image


CL-measurements: Internal defects I

934.0 934.5 935.02

4

6

8

920 930 940 9500

2

4

6

8

10

pow

er p

/ a.

u.wavelenghtλ / nm

T = 25°Cλmax = 933.79nmL = 4000µm

pow

er P

/ a.

u.

wavelength λ / nm 0 1000 2000 3000 40000.00

0.01

0.02

0.03

90µm

2928µm1063µm

position inside cavity / µm

ampl

itude

/ a.

u.

100 µm 100 µm

Analysis of the longitudinal mode spectrum – comparison with CL


CL-measurements: Internal defects II980 nm, 8 W, 25°C, 2410 h, 100 µm stripe width

EL-image facet

100 µm


CL-measurements: Internal defects III

[110] – DLDs (dark line defects)

15° - branching in [112] – direction

Confined to stripe

No DSDs (dark spot defects)

930 nm, 8 W, 25°C, 1400 h, 100 µm stripe width

No facet damage, electroluminescence image without failure

[110]









Statistical analysis

Input: • n devices per lot• r failures are observed• Assumption: exponential distribution of failure function • Total test time t• Failure time for the failed diode ti

• α-quantile of χ2-function χ2α:

2/)22(

)(

:level confidence 1

)(1

:likelihoodmaximum

21

1

1

+

+−=

−

+−==

∑

∑

=−

=

r

ttrnMTTF

rttrn

MTTF

rj j

rj j

αα

χ

α

λ

Ref: Reliability and Degradation of Semiconductor Lasers and LEDs,M. Fukuda, Artec House, 1991




400 mW (4 mW/µm): 9(9) - t = 1000 h

500 mW (5 mW/µm): 7(9) - t = 10000 hMTTF – 40000 h No COD failures

Lifetime sufficient for medical applicationsand close to the demands for display application 0 2500 5000 7500 10000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

500 mW

400 mW

Cur

rent

I / A

Aging time t / h



Maximum likelihood estimation:Aging experiment – 500 mW, total test time t = 10000 h• 9 devices per lot; 2 failures are observed• Assumption: exponential distribution of failure function • Failure time for the failed diode tj: 3703 h, 9041 h

h413722

h) 9041 h (3703 h10000)29(

)(1 1

=++⋅−

=

+−==

∑ =

MTTF

rttrn

MTTFrj j

λ




1−α confidence level:Aging experiment – 500 mW, total test time t = 10000 h• 9 devices per lot; 2 failures are observed• Assumption: exponential distribution of failure function • Failure time for the failed diode tj: 3703 h, 9041 h

[ ] h266936.2

h9041h3707 h10000)29(2

2/)22(

)(

6.0

21

1

=++⋅−⋅

=

+

+−=

∑ =−

MTTF

r

ttrnMTTF

rj j

αα

χ





600 mW (6 mW/µm): 7(7) - t ≥ 1100 h

Lifetime sufficient for medical applications

0 1000 20000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Cur

rent

I/ A

Aging Time t / h

600 mW at 15°C



Maximum likelihood estimation:Aging experiment – 600 mW, 2377 h: • 7 devices per lot; 7 failures are observed• Assumption: exponential distribution of failure function • Failure time for the failed diode ti: 1175 h, 1328 h, 1800 h, 1822 h,

1822 h, 2309 h, 2377 h

h18057

h 2377)23091822182218001328(1175

)(1 1

=++++++

=

+−==

∑ =

MTTF

rttrn

MTTFrj j

λ




Maximum likelihood estimation:Aging experiment – 600 mW, 2377 h: • 7 devices per lot; 7 failures are observed• Assumption: exponential distribution of failure function • Failure time for the failed diode ti: 1175 h, 1328 h, 1800 h, 1822 h,

1822 h, 2309 h, 2377 h

[ ] h150616.8

h23772309182218221800132811752

2/)22(

)(

6.0

21

1

=++++++⋅

=

+

+−=

∑ =−

MTTF

r

ttrnMTTF

rj j

αα

χ



SummaryLifetime measurements for laser diodes and bars

Deliver:• Information on quality of materials, processes, mounting, etc.• Hints for the improvement of technology• Data for reliable lifetime of devices, MTTF, reliability

Require:• Enough experimental data• Statistical analysis of data• Highly stable measurement set-up• Accompanying measurements• Non-destructive failure analysis• Destructive failure analysis

Documents

Reliability investigations Lifetime measurements … investigations Lifetime measurements and degradation mechanisms Bernd Sumpf, Ute Zeimer, Karl Häusler, Andreas Klehr Ferdinand-Braun-Institut