Microelectronics Reliability 52 (2012) 804–812

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Microelectronics Reliability

journal homepage: www.elsevier .com/locate /microrel

Chip and package-related degradation of high power white LEDs

Matteo Meneghini a,⇑, Matteo Dal Lago a, Nicola Trivellin a, Giovanna Mura b, Massimo Vanzi b,Gaudenzio Meneghesso a, Enrico Zanoni a

a Department of Information Engineering of the University of Padova, via Gradenigo 6/B, 35131 Padova, Italyb Department of Electrical and Electronical Engineering, University of Cagliari, Italy

a r t i c l e i n f o

Article history:Received 13 January 2011Received in revised form 26 April 2011Accepted 27 July 2011Available online 25 August 2011

0026-2714/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.microrel.2011.07.091

⇑ Corresponding author. Tel.: +39 049 827 7664; faE-mail address: matteo.meneghini@dei.unipd.it (M

a b s t r a c t

With this paper we present an analysis of the degradation of state-of-the-art high power LEDs. Three dif-ferent kinds of commercially available samples, from leading manufacturers, were submitted to stressunder various current and temperature levels. Based on an accurate estimation of the thermal resistanceof the devices, iso-thermal and iso-current stress tests have been carried out, with the aim of separatelyevaluating the role of current and temperature in determining the degradation of the LEDs. Results indi-cate that state-of-the-art LEDs can show a significant degradation of their electrical and optical charac-teristics, when they are operated close to their current/temperature limits. In particular, data revealthe presence of two different degradation mechanisms: (i) the degradation of the blue semiconductorchip, due to the increase in non-radiative recombination, or to the decrease in the acceptor dopant con-centration at the p-side of the diodes; (ii) the chemical degradation of the package, with subsequentworsening of its optical properties. Results suggest that even high-performance LEDs can suffer from lim-ited lifetime: thermal management and driving conditions must be carefully optimized with the aim ofachieving high reliability for LEDs to be adopted in high efficiency lighting systems.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Since the development of the first GaN-based Light-EmittingDiodes (LEDs) in the early 1990s, the technology of LEDs has shownimpressive advancements. Early blue LEDs, commercialized in1993, had a very low efficiency (less than 1%), and luminous power(125 lW at 20 mA for the LEDs demonstrated by Nakamura et al. in[1]). Thanks to the research efforts carried out by many researchgroups worldwide, it is now possible to fabricate blue LEDs withexternal quantum efficiency in excess of 75% [2], thus clearingthe way for the adoption of solid-state light sources in high powerapplications. Similar advancements were possible thanks to theintroduction of a number of improvements in the growth, process-ing and packaging steps of the production process. Blue LEDs arethe core for the realization of white Light-Emitting Diodes, basedon phosphor conversion: in white LEDs, light is generated thanksto the combined use of a blue-emitting LED and a yellow phospho-rous layer (which in most of the cases is based on YAG). Phosphorscan be placed in direct contact with the chip (chip-level conver-sion, CLC), or incorporated into the encapsulating material.Phosphor-converted LEDs were commercialized for the first timein 1996: the incredible progress in LED technology can be under-stood by considering that early white LEDs had a very low efficacy

ll rights reserved.

x: +39 049 827 7699.. Meneghini).

(around 10 lm/W, see for instance [3]), while nowadays it is possi-ble to fabricate devices with record performance in excess of170 lm/W [4]. Similar efficacy values are significantly higher thanthose of conventional light sources, such as incandescent lamps(around 10 lm/W) and fluorescent lamps (around 90 lm/W): whiteLEDs can therefore be considered as excellent candidates for therealization of the next-generation illumination devices. Otheradvantages of LEDs are their small dimension, the high robustnessto atmospheric agents and shocks, and the fast modulation speed.This last property can be effectively exploited to achieve a linearcontrol of the luminous output of the devices, by means of PulseWidth Modulation (PWM).

Another important feature of white LEDs is their high expectedreliability: expected lifetimes of these devices can be in excess of50,000 h [5], and this can be a strong argument to convince cus-tomers to adopt LEDs instead of incandescent or fluorescent lampsfor lighting applications. However, over the last few years, severalauthors [6–27] have demonstrated that the lifetime of white LEDscan be shorter than expected: this is due to the existence of a num-ber of physical mechanisms that can determine the degradation ofLEDs, when they are submitted to high current or high temperaturestress. State-of-the-art high-power LEDs are based on semiconduc-tor chips with an area of 1 mm2, that can be operated at currentlevels in the range between 350 mA and 1 A, depending on the spe-cific model and manufacturer. These current levels correspond tocurrent densities in the range 35–100 A/cm2, which were demon-strated to be sufficiently high to induce a degradation of the blue

Fig. 1. Schematic description of the setup adopted for the stress and measuring procedure.

M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812 805

semiconductor chip [17,21,22]. Furthermore, state of the art LEDsare rated for maximum junction temperatures in excess of130–150 �C [27]. These temperature levels can in principle be sohigh to induce a severe degradation of the optical properties ofthe package material [8,14,21,23]. Since many manufacturers areclaiming that their LEDs can withstand high current and tempera-ture levels, designers and customers are significantly interested inunderstanding which is the reliability of white LEDs under highoperating conditions.

The aim of this paper is to describe an extensive analysis of thedegradation of high-brightness LEDs submitted to stress at varioustemperature and current levels. Measured LEDs are state-of-the-art devices, fabricated by three leading manufacturers: the studywas carried out on off-the-shelf LEDs, and not on a selected set ofdevices, in order to provide data on the real performance and reli-ability of commercially-available LEDs. Data therefore are represen-tative of the current status of the reliability of high power LEDs.

Before carrying out the reliability tests, devices were submittedto electrical and optical characterization. For the stress tests, theindividual devices were mounted on identical heat sinks, by meansof a proper thermal interface material, in order to ensure a goodheat dissipation. A schematic description of the experimental setupis given in Fig. 1. Devices were then submitted to stress at differenttemperature and current levels. The thermal resistance of each ofthe measured LEDs was separately evaluated: thanks to this mea-surement overhead, the junction temperature of each of the ana-lyzed devices – under the adopted stress conditions – can beexactly known. Iso-thermal and iso-current stress tests werecarried out in order to completely describe the role of temperatureand current in determining LED degradation. Results of the stressexperiment indicate the following: (i) the three series of analyzedsamples show significantly different degradation kinetics. Two (ofthe three) series show a significant optical power decrease whenthey are stressed within or close to the recommended operatinglimits. (ii) Two different degradation processes have been identi-fied: chip degradation and package degradation. Chip degradationoccurs during operation at high junction temperature and currentlevels, and can determine a significant decrease in the opticalpower emitted by the devices, and increase in the operating

Table 1summary of the electro-optical characteristics of the devices analyzed within this work.

Device group Typicalluminous flux

CIE 1931 chromaticity coordinates CIE 1temp

x y

A 73.9 lm (350 mA) 0.36 0.36 4500B 100 lm (1 A) 0.35 0.35 4750C 85 lm (350 mA) 0.30 0.28 8000

voltage. Package degradation, on the other hand, is a purelythermally-activated effect, i.e. takes place even when no currentis flowing through the devices. Package degradation can determineboth a decrease in the luminous efficacy of the LEDs, and a worsen-ing of the chromatic properties of the devices. Results are de-scribed in detail in the following.

2. Experimental details

The study was carried out on three different sets of LEDs, fabri-cated by three leading manufacturers. The three groups of LEDs arenamed as A, B, and C in the following. In Table 1 we report asummary of the electrical and optical characteristics of the devices.All the individual analyzed devices were mounted on heat-sinks,by means of a thermally conductive insulator: heat sinks weredesigned in order to guarantee a good heat dissipation, and therepeatable positioning of the LEDs at the entrance of an integratingsphere (Fig. 1). Each device was electrically accessible by a 4-wireKelvin connection, that allows reliable current–voltage (I–V) char-acterization. Before stress, all the LEDs were submitted to electricaland optical characterization, by means of I–V, Optical Power vsCurrent (L–I) and Electroluminescence (EL) measurements. In addi-tion, the thermal resistance of each of the analyzed samples wasmeasured by means of the forward-voltage method [28].

After the preliminary characterization, a stress plan wasdefined: in particular, we carried out the following.

� Constant-junction temperature (iso-thermal) stress tests:several sets of devices with identical characteristics weresubmitted to stress in thermally-controlled environment. Theindividual sets of samples were stressed at different current lev-els, in the range 0.5 A–1.5 A dc. For each set of LEDs (i.e. for eachstress current level) ambient temperature was adjusted in orderto have a junction temperature of 160 �C (on average) on all thestressed devices (junction temperature was evaluated by meansof the method described in [28])� Constant-device current (iso-current) stress tests: several sets

of devices with identical characteristics were submitted tostress tests in thermally-controlled environment, with a con-

931 colorerature

Typical forwardvoltage

Maximum DCforward current

Maximum junctiontemperature

K 3.3 V (350 mA) 1000 mA 150 �CK 3.72 V (1000 mA) 1500 mA 135 �CK 3.2 V (350 mA) 1000 mA 135 �C (175 �C for short

term application)

0.4 0.8 1.2 1.6 2.0 2.4 2.8404550556065707580859095

100105

Device A Device B Device C

Junc

tion

Tem

pera

ture

(°C

)

Joule Power (W)

14.25 °C/W

19.84 °C/W

26.42 °C/W

20 40 60 80 100 1203.0

3.1

3.2

3.3

3.4

3.5 Device A Device B Device C

Forw

ard

volta

ge a

t 350

mA

(V)

Junction temperature (°C)

(a) (b)

806 M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812

stant current of 1 A. The individual sets of samples werestressed at different ambient temperature levels, in the rangebetween 60 �C and 120 �C. For each stress condition, and foreach stressed LED, the corresponding junction temperaturewas evaluated by means of the method described in [28]� Purely-thermal stress: several sets of devices with identical

characteristics were submitted to stress tests in thermal cham-bers, with zero current. The individual sets of samples werestressed at different ambient temperature levels, in the rangebetween 85 �C and 200 �C

For each stress condition, we analyzed 4 samples, in order tohave statistically relevant results.

Fig. 2. (a) Dependence of forward voltage on junction temperature, for represen-tative LEDs of series A, B and C. (b) Junction Temperature vs Joule Power diagram forrepresentative LEDs of series A, B and C.

3. Results and discussion

3.1. Results of thermal characterization

Before carrying out the stress tests, the devices of groups A, Band C were submitted to thermal characterization: this analysisprovides information on the thermal resistance of the individualLEDs (each was mounted on a heat sink, as described above) andon the temperature reached by the junction during operation at agiven current/ambient temperature level. Thermal characteriza-tion was carried out by means of the forward-voltage methoddescribed in detail in [28]: representative results of the thermalanalysis are shown in Fig. 2. Fig. 2a reports the dependence offorward voltage on the junction temperature (V–T curves) for threerepresentative devices. Measurements were taken at 350 mA, bymeans of short current pulses, in a thermal chamber. For all theanalyzed devices, voltage has a nearly linear dependence on junc-tion temperature: the V–T curves of the three series of devices havesimilar slopes, around �3.2 mV/�C, which is close to the valuesreported in the literature (see for instance [28,29]) and by LEDmanufacturers [30–32]. In Fig. 2b we report the Junction Temper-ature vs Joule Power (T–P) diagrams for three representativedevices of groups A, B and C. Joule Power is defined as the differ-ence between the input electrical power and the optical powergenerated by the LEDs. It represents the portion of the input powerthat is not converted into light, and therefore contributes to self-heating. The thermal resistance of the samples can be extrapolatedfrom the slope of the T–P diagrams: typical values for devices ofgroups A, B and C are reported in Fig. 2b. In Fig. 3 we report a sum-mary of the results of the thermal analysis carried out within thiswork: histograms report the thermal resistance values measuredon all the devices which were submitted to high current stress.Solid lines represent Gaussian fit of the experimental data. Ascan be noticed, the three groups of devices have different averagethermal resistances (14 �C/W, 17 �C/W and 23 �C/W for groups A, Band C respectively), even if the same heat sink, thermal interfaceand mounting procedure was used for all the analyzed devices.

3.2. Results of the stress tests

In this section we report on the results of the stress experimentscarried out within this work. Fig. 4 shows the optical power (OP)degradation of LEDs of groups A, B and C submitted to stress at 1A, with an ambient temperature Ta = 100 �C. From this figure,devices of the three groups can be directly compared in terms ofreliability. As can be noticed, while devices of group A are quitestable over a stress time of 1000 h (less than 10% degradation),LEDs of group B and C can show a considerable decrease in opticalpower during operation (around 20% and 43% for devices of seriesB and C). In the remaining part of the paper, we mostly concentrateon the analysis of the degradation kinetics and mechanisms of

LEDs of groups B and C: devices of group A showed a moderateOP decrease only under high current and temperature stress, andare therefore considered to be stable during normal operatingconditions. LEDs of series B and C showed significantly differentdegradation kinetics and modes and will be treated separately inthe following.

3.3. Degradation of LEDs of group B

In Fig. 5 we report the results of iso-thermal stress tests carriedout on devices of group B. Three sets of LEDs were aged at threedifferent current levels (500 mA, 700 mA and 1 A). Each of thethree sets of samples was stressed at a specific ambient tempera-ture level, in order to have an average junction temperature (eval-uated by means of the method described in [28]) of 150 �C on allthe analyzed devices. For each stress condition we stressed fourdevices. As can be noticed, devices aged at different current levels(same junction temperature) showed similar degradation kinetics.This result suggests that for the LEDs of group B, degradation kinet-ics are significantly determined by the temperature reached by thejunction during operation, with no strong dependence on the oper-ating current level.

In order to better understand the role of temperature in deter-mining the degradation of the LEDs of group B, we have carried outa set of iso-current stress tests: we have submitted four sets ofdevices to constant current stress (1 A dc), with different ambienttemperature levels, in the range 60–120 �C. Average junction tem-perature levels were estimated to be equal to 108 �C, 146 �C, 160 �Cand 192 �C for the four sets of devices. Results are summarized inFig. 6: as can be noticed, an increase in junction temperatureinduced an increase in the degradation rate. Time to Failure(TTF90%, time necessary for a 10% decrease in optical power) wasfound to have an Arrhenius-like dependence on temperature. InFig. 7 we report the Arrhenius plot for this set of devices: each datapoint represents one of the analyzed samples, with its TTF90% andthe estimated junction temperature level. As can be noticed, theArrhenius plot can be divided in two different regions, dependingon the temperature reached by the junction during stress. At lowertemperatures, TTF90% has only a weak dependence on junctiontemperature: for stress (junction) temperature levels smaller than140 �C, the activation energy of the degradation process isEa = 0.27 eV. Similar values of Ea were previously reported in[33–35] for stress at moderate temperature levels: the opticaldegradation of the devices was attributed to the increase in thenon-radiative recombination rate in the active region of the blueLED chips, activated by the flow of current through the devices.This mechanism takes place at moderate junction temperature

12 13 14 15 16 170

1

2

3

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5

6

7

Freq

uenc

y co

unts

Thermal resistance (°C/W)14 16 18 20 22 24 18 20 22 24 26 28 30

Group A Group B Group C

Fig. 3. Thermal resistance of the samples analyzed within this work. Solid curves represent Gaussian fit of the experimental data.

0 200 400 600 800 1000

556065707580859095

100105110115

Nor

mal

ized

opt

ical

pow

er (%

)

Stress Time (h)

Stress condition: I=1A; Ta=100°C Group A Group B Group C

Fig. 4. Optical Power measured (at 700 mA) during stress at 1 A, Ta = 100 �C, ondevices of groups A, B and C. Each point is the average obtained on 4 identicalsamples. Average junction temperatures under adopted stress conditions are150 �C, 160 �C and 180 �C for samples of groups A, B and C respectively.

0 200 400 600 800 1000 1200 140070

75

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100

Nor

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ized

opt

ical

pow

er (%

)

Stress Time (h)

Stress Conditions (Tj~150 °C):

I = 500mA

I = 700 mA

I = 1 A

Fig. 5. Optical Power degradation measured (at 700 mA) on devices of group Bsubmitted to stress at different current levels (500 mA, 700 mA and 1 A), with thesame junction temperature (150 �C). Each data point is the average of four identicalsamples, standard deviation of data points is 2% on average.

0 500 1000 1500 2000 250065

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105

Nor

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opt

ical

pow

er (%

)

Stress Time (h)

Stress Conditions: I = 1 A, Tj = 108 °C I = 1 A, Tj = 146 °C I = 1 A, Tj = 160 °C I = 1 A, Tj = 192°C

Fig. 6. Optical Power degradation measured (at 700 mA) on devices of group Bsubmitted to stress at different junction temperature levels (between 108 and192 �C), with the same current level (1 A). Each data point is the average of fouridentical samples.

24 26 28 30 32

100

1000 Ea=0.27 eV

TTF

90% (h

)

q/kT (C/J)

Ea=1.1 eV

500 480 460 440 420 400 380

Junction Temperature (K)

Fig. 7. Arrhenius plot for the TTF90% (time necessary for a 10% decrease in opticalpower) for devices of group B aged at 1 A, different junction temperature levels.Each data point is referred to an individual LED, with its TTF90% and junctiontemperature level.

M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812 807

2.50 2.75 3.00 3.25 3.50 3.750.0

0.1

0.2

0.3

0.4

0.5

0.6

Cur

rent

(A)

Voltage (V)

Stress time: 0h 4h 32h 500h 1000h

Stress conditions:I=1A; Ta=100°C

0 500 1000

70

80

90

100N

orm

aliz

ed o

ptic

al p

ower

(%)

Stress time (h)

Fig. 8. Current–voltage characteristics measured on one LED of group B submittedto stress at 1 A, Ta = 100 �C (junction temperature �160 �C). Inset: variation of theoptical power measured (at 700 mA) on the same sample during stress time.

24 26 28 30

0.0

0.1

0.2

0.3

0.4

Incr

ease

in O

pera

ting

Volta

ge V

ON (V

)

q/kT (C/J)

500 480 460 440 420 400 380

Junction Temperature (K)

Fig. 9. Increase in the operating voltage (difference between the values after andbefore stress) measured (at 700 mA) on devices aged at 1 A, different junctiontemperature levels. Each data point is referred to an individual LED, with itsoperating voltage increase (measured at 700 mA) and junction temperature level.

0 250 500 750 1000 1250

88

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102

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ave

rage

vol

tage

(%)

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opt

ical

pow

er (%

)

Stress time (h)

Optical power at 700mA Voltage at 700mA

100

101

102

103

104

Fig. 10. Variation of the optical power and of the forward voltage measured (at700 mA) during stress at 1 A, Ta = 80 �C, on one of the analyzed samples of group B.Each data point is the average of four identical samples.

0 200 400 600 800 100070

75

80

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95

100 Stress Conditions (Tj~160 °C): I = 0 A I = 500 mA I = 700 mA I = 1 A I = 1.5 A

Nor

mal

ized

opt

ical

pow

er (%

)

Stress Time (h)

Fig. 11. Optical Power degradation measured (at 700 mA) on devices of group Csubmitted to stress at different current levels (500 mA, 700 mA, 1 A, and 1.5 A),with the same average junction temperature (160 �C). We report also the curveobtained by aging samples at 160 �C, with no current, for comparison. Each datapoint is the average of four identical samples.

808 M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812

levels, due to the generation of lattice defects (point defects, vacan-cies, . . .) activated by highly accelerated carriers.

On the other hand, Fig. 7 shows that devices can show a signif-icantly higher degradation during stress at high temperatures. Forjunction temperature levels greater than 140 �C, an higher activa-tion energy was extrapolated for the TTF90% parameter(Ea = 1.1 eV), indicating the presence of a second degradationprocess. In order to better understand the origin of this degrada-tion process, we have analyzed the Current–Voltage characteristicsof the devices during stress time. Results of this analysis indicatethat stress at high junction temperature levels can induce a signif-icant increase in the operating voltage of the devices (see forinstance the I-V curves in Fig. 8). On the contrary, modificationsin I-V characteristics were found to be significantly less pro-nounced in LEDs stressed at junction temperatures lower than140 �C. Results of the analysis of I-V curves before/after stress aresummarized in Fig. 9, that reports the increase in the operatingvoltage VON measured on LEDs stressed at 1 A, different junctiontemperature levels, as a function of the (reciprocal of) stresstemperature. As can be noticed (Fig. 7 and 9), stress at high tem-perature levels (>140 �C) can induce both a decrease in OP andan increase in VON: during stress time, the two parameters vary

with similar kinetics, indicating a strong correlation (Fig. 10). Thiscorrelation can be interpreted based on the results presented in[18,20]: during stress at high temperature levels, the semiconduc-tor material and/or ohmic contact at the p-side of the diodes candegrade due to the interaction between the acceptor dopant (Mg)and the hydrogen present in the semiconductor or passivationlayers, or in the encapsulating material. This can determine areduction of the effective hole concentration, with subsequentincrease in the series resistance (and operating voltage) of the sam-ples, and decrease in the optical power. The activation energy valueextrapolated from the high-temperature region of the Arrheniusplot in Fig. 7 (Ea = 1.1 eV) is consistent with previous reports onhydrogen-related degradation of LEDs (Ea = 1.3 eV, see [18,20]),thus supporting the hypothesis on the degradation mechanism.

3.4. Degradation of LEDs of group C

In Fig. 11 we report the results of iso-thermal stress testscarried out on devices of group C. Several sets of identical deviceswere submitted to stress at different current levels, with the samejunction temperature of 160 �C. Contrary to what observed forgroup B (see Fig. 5), devices of group C aged at different current

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

14

16

18

20

22

24

26

28

30D

egra

datio

n R

ate

(%)

Stress Current (A, Tj=160 °C)

Slope = 0.145 (100/A)

Fig. 12. Degradation rate (degradation after 1000 h) for devices of group C stressedat different current levels, same junction temperature (160 �C). Solid line representsa linear fit of the experimental data.

0 25 50 75 100 12582

84

86

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102

Stress condition: Ta=85°C Ta=130°C Ta=160°C Ta=180°C Ta=200°C

Nor

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ized

opt

ical

pow

er (%

)

Stress Time (h)

Fig. 13. Optical Power degradation measured (at 700 mA) on devices of group Csubmitted to stress at different temperature levels (in the range 85–200 �C), with noapplied bias.

24 25 26 27 28 29

10

100

1000

10000

TTF

90% (h

)

q/(kT) (C/J)

Ea=1.8 eV

Fig. 14. Arrhenius plot for the TTF90% (time necessary for a 10% decrease in opticalpower) for devices of group C aged at different junction temperature levels (noapplied bias). Each data point is the average of 4 identical samples.

Fig. 15. Optical micrograph of one untreated and one stressed LED of group C(1000 h at 160 �C). Picture was taken after the removal of the lens, of theencapsulating material, and of the phosphors, by means of chemical etching.

M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812 809

levels showed different degradation kinetics, even if the junctiontemperature was the same. Fig. 11 reports also the optical powerdegradation measured on samples of group C aged at 160 �C, withno bias (purely thermal stress). As can be noticed, also thermalstress was found to induce a significant optical power decrease:the degradation rate for purely thermal stress was found to besmaller than for stress with both current and temperature, forthe same junction temperature level. Results reported abovesuggest that both current and temperature contribute to the degra-dation of the devices: to analyze the role of current and tempera-ture in determining the degradation kinetics, we have carried outboth iso-thermal and purely-thermal stress tests. Results of iso-thermal stress tests are summarized in Fig. 12, that reports thedependence of the degradation rate on the stress current level,for the devices aged at same junction temperature level (differentcurrent levels): degradation rate has a linear dependence on thestress current level. This result indicates that the flow of carriersthrough the active region of the devices can play a significant rolein determining the optical power decrease [10,17,36], and providesimportant information for the extrapolation of the lifetime of thedevices under specific stress conditions.

The graph in Fig. 11 indicates that current is not the only drivingforce for degradation: also purely thermal stress can induce a sig-nificant optical power decrease. To analyze the role of temperature

in determining the degradation kinetics, we have carried outpurely-thermal stress tests, by submitting several sets of LEDs tostress at high temperature, with no bias. Results are summarizedin Fig. 13: increasing stress temperature determines an increasein the degradation rate. TTF90% was found to have an Arrhenius-likedependence of temperature, with an activation energy of 1.8 eV(Fig. 14).

The analysis of the degradation kinetics in Figs. 11 and 13suggest that two different mechanisms can be involved in the deg-radation: one of these is activated by temperature, while the otherone is activated by the flow of current through the active layer. Tobetter understand the origin of the two degradation processes, wehave carried out an extensive analysis of the properties of thepackage and of the electrical characteristics of the samples duringstress time.

Optical investigation of the package of samples submitted topurely-thermal stress revealed that high temperatures can inducea significant browning of the white material of the package. Thiseffect is considered to be activated by the high temperaturesreached by the devices during stress, with a considerably highactivation energy (1.8 eV, see Fig. 14), which is consistent with pre-vious reports [37,38]. Fig. 15, that was taken after the removal ofthe lens, of the encapsulating material and of the phosphors (bychemical etching), shows the comparison between an untreatedand an aged sample. The browning of the package can significantlyreduce the ability of the package to reflect the radiation emitted by

0.10

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0 10 2030

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Chr

omat

ic c

oord

inat

e x

Before aging

Aged at 160°C for 1250 hours

Angle (°)

0.000.050.100.150.200.250.300.350.400.450.50

0 10 2030

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Angle (°)

Chr

omat

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oord

inat

e y

(a) (b)

(c)

Before stress After stress

After stress, the surfaceof the package is

carbonized: this blockslight reflection for high

emission angles

Before stress, photonsemitted in this direction can be reflected by the

package

LED CHIP PACKAGE

Photons emitted in thisdirection can directly

escape from the package

Fig. 16. Angular variation of the chromatic coordinates x and y measured (a) before and (b) after high temperature stress on devices of group C. (c) Schematic drawingdescribing the effect of package carbonization on light extraction efficiency.

Untreated After stress

Fig. 17. Emission micrograph (detail) of an untreated LED and a stressed (with hightemperature, no bias) LED taken at after the removal of the lens, of theencapsulating material, and of the phosphors.

2.0 2.5 3.0 3.5 4.0 2.0 2.5 3.0 3.5 4.0

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rent

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0 50 1009596979899

100101102

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(%)

Stress Time (h)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Cur

rent

(A)

Stress time: 0h 2h 8h 16h 32h 64h 128h

(a) (b)

Fig. 18. Current–voltage characteristics measured during stress time on one of theLEDs of group C submitted to purely-thermal stress (160 �C, no bias). Curves areplotted both in (a) linear scale and (b) semi-logarithmic scale. Inset: OP degradationof the same device during stress time.

810 M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812

the chip, thus determining a reduction of the emitted opticalpower [27]. Degradation of the package can also result in modifica-tions of the chromatic coordinates of the white LEDs, especially forhigh emission angles (with respect to the vertical of the chip). Thisis summarized in Fig. 16a and b, that reports the results of gonio-metric measurements carried out before and after high-tempera-ture stress on one of the analyzed devices. Stress induced anincrease in the chromatic coordinates for high emission angles:this effect can be ascribed to the darkening of the surface of thepackage, and can significantly limit the color stability of highpower solid-state lighting systems (see the schematic drawing inFig. 16c).

This kind of degradation is related to the specific choice of thematerials adopted for the package of the analyzed devices, i.e. acolorless silicone resin. This material can become dark as a conse-quence of high temperature treatment, thus leading to a severedegradation of the optical and chromatic properties of LEDs. Possi-ble ways of improving the thermal robustness of the devices are:(i) to select package materials with high robustness during thermal

stress; (ii) to improve heat extraction from the package, by enlarg-ing the thermal pad, and by reducing the thermal resistancebetween junction and solder point; (iii) to design suitable thermalmanagement strategies, capable of limiting device self-heatingduring operation in a specific application.

It is worth noticing that purely-thermal stress did not induceany degradation of the optical properties of the blue semiconduc-tor chips contained in the white LEDs. As a representative example,in Fig. 17 we report the emission micrographs taken on anuntreated and on a (severely) stressed white LED after the removalof the lens, encapsulating and phosphorous material. No significantdifference in the intensity of the two blue LED chips can bedetected, despite the important degradation shown by the stressedLED before the removal of the phosphors and of the encapsulatingmaterial (degradation data are reported in Fig. 11). A furtherconfirmation of the fact that purely-thermal stress did not induce

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100

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10-5(a) (b)

Fig. 19. Current–voltage characteristics measured during stress time on one of theLEDs group C submitted to stress at 1 A, with a junction temperature of 160 �C.Curves are plotted both in (a) linear scale and (b) semi-logarithmic scale. Inset: OPdegradation of the same device during stress time.

M. Meneghini et al. / Microelectronics Reliability 52 (2012) 804–812 811

any significant degradation of the blue-semiconductor chip wasobtained by means of I–V measurements. The electrical character-istics of LEDs submitted to high temperature stress – with nocurrent – did not show any change: as shown in Fig. 18, no varia-tion of the forward voltage (Fig. 18a) nor modification in thedefect-related current components (Fig. 18b) was detected. Resultstherefore support the hypothesis that the electrical properties ofthe ohmic contacts and the concentration of defects within theactive region are stable during purely-thermal stress.

The I–V curves of LEDs aged with both current and temperatureshowed a different behavior: while on one hand the operating volt-age (and therefore the electrical properties of the ohmic contacts,see Fig. 19a) was found to be stable over stress time (as alreadyfound for purely thermal stress, see Fig. 18), on the other handstress was found to induce a significant increase in the defect-re-lated current components (representative data are shown inFig. 19b, see the region for V < 2.5 V) of LEDs aged with highapplied bias. This result is the signature of the fact that stressinduces an increase in the concentration of defects within theactive layer of the devices stressed with both temperature andcurrent [17,39]. Defect generation may proceed through a sub-threshold process (as previously described in [7]), activated bythe accelerated electrons injected through the active layer. Thiscan result in an increased non-radiative recombination rate, withsubsequent degradation of the optical properties of the devices.It is worth noticing that only devices aged with both temperatureand current show an increase in the defect-related current compo-nents after stress (see the comparison of Figs. 18 and 19). Thiseffect – that is indicative of the degradation of the blue semicon-ductor chip – does not take place in LEDs aged at high temperaturelevels, with no applied bias. Degradation of LEDs of group C there-fore occurs due to two different processes: (i) the optical degrada-tion of the package, which is activated by temperature (asconfirmed by the Arrhenius-like dependence of the TTF90% param-eter on temperature, see Fig. 14), and (ii) the degradation of theblue semiconductor chip, which is activated by the flow of currentthrough the active region (as confirmed by the linear dependenceof degradation rate on stress current, see Fig. 12). The superposi-tion of these two mechanisms explains the differences betweenthe degradation kinetics of LEDs aged with the same junctiontemperature levels, and different current levels.

4. Conclusions

In summary, with this paper we have described an extensiveanalysis of the degradation of state-of-the-art white LEDs for

lighting applications. The study is based on the analysis of three dif-ferent groups of LEDs, from three leading manufacturers. LEDs weremounted on similar heat sinks, and completely characterized bymeans of electrical, optical and thermal measurements. In orderto fully characterize the degradation kinetics of the devices, andto understand the role of temperature and current in determiningthe optical power decrease, we have carried out iso-thermal, iso-current and purely-thermal stress tests. Results indicate that, ofthe three groups of LEDs, groups B and C showed a considerabledegradation during stress at moderate/high current and tempera-ture levels. Degradation of LEDs of group B is strongly tempera-ture-dependent, and originates from the worsening of the electro-optical properties of the blue semiconductor chip, possibly due (i)to the increase in the defectiveness of the active layer, and (ii) tothe increase in the resistivity of the ohmic contacts/semiconductormaterial at the p-side of the diode (this last mechanism occurs onlyat high temperature levels). On the other hand, devices of group Cshowed a different behavior: two different mechanisms weredetected, i.e. the browning of the package, activated bytemperature, and the generation of defects within the activeregion, activated by current. Results described within this paperindicate that the reliability of LEDs can be limited by a number ofmechanisms, and provide a complete description of the activationenergies of the identified degradation processes. Results stronglysuggest that the operating conditions of LEDs (junction tempera-ture, current, thermal resistance, . . .) must be carefully optimizedin order to obtain a satisfactory reliability of state-of-the-art LEDs.

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