Microelectronics Reliability 51 (2011) 1742–1746

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Microelectronics Reliability

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Degradation mechanisms of high-power white LEDs activated by currentand temperature

M. Dal Lago, M. Meneghini ⇑, N. Trivellin, G. Meneghesso, E. ZanoniDepartment of Information Engineering, University of Padova, v. Gradenigo 6/B, 35131 Padova, Italy

a r t i c l e i n f o

Article history:Received 30 May 2011Received in revised form 21 June 2011Accepted 28 June 2011Available online 27 July 2011

0026-2714/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.microrel.2011.06.057

⇑ Corresponding author. Tel.: +39 0498277664; faxE-mail address: matteo.meneghini@dei.unipd.it (M

a b s t r a c t

This paper presents a study of the degradation mechanisms that limit the reliability of commercially-available white LEDs. Purely thermal stress and biased iso-thermal stress were carried out for severalthousands hours on 1W-power LEDs, produced by a leading manufacturer. Results reveal that tempera-ture and operating current have different roles in determining the optical degradation of these devices: (i)pure thermal stress induces a short-term optical power decay, strictly correlated to the decrease in thereflectivity of the package/reflector system and with no effects on the electrical characteristics of the bluechip; the activation energy of thermally-induced degradation is equal to 1.8 eV; (ii) constant currentstress induces a long-term degradation process, with a degradation rate which is strongly dependenton the stress current level. In this latter case, optical degradation is ascribed to the degradation of the bluesemiconductor chip: details are provided through the analysis of forward voltage and wavelength shiftduring stress time.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The role of high-power white LEDs in general lighting applica-tions is becoming, day-by-day, increasingly important. The reliabil-ity of these devices, compared to that of conventional light sources,represents one of the keys for their development and their marketpenetration. It is then of fundamental importance to deeply under-stand the various degradation mechanisms that affect the opera-tion of LEDs, in terms of lifetime, chromaticity characteristics andefficiency. The elements that contribute to limit the reliability ofwhite LEDs are of different types and nature, and include chip-levelmodifications, degradation of the optical properties of the encaps-ulant materials, delaminations, phosphor degradation. The twomain driving forces that contribute to LED degradation are temper-ature and current. In recent years, many papers reported studies onthe degradation mechanisms of white LEDs submitted to high-cur-rent and/or high-temperature stress [1–3]. In most of the casesdescribed in the literature, both the flow of current and high tem-peratures contribute to the optical power (OP) decay: from thesestudies it is therefore difficult to separately evaluate the role ofthese two driving forces in determining device degradation.

The aim of this work is to study the degradation mechanisms ofhigh-power white LEDs by using two different sets of experiment:

(i) a set of pure thermal stresses, where devices are exposed tohigh temperatures, with no applied bias;

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: +39 0498277699.. Meneghini).

(ii) a set of iso-thermal biased stresses, where devices arestressed at fixed junction temperature, with different appliedcurrent levels.

This allow us to separately evaluate the thermally-activateddegradation mechanisms, from the current-driven ones.

The results of these degradation tests suggest that high temper-atures have no effects on the electro-optical characteristics of theblue semiconductor chip. On the other hand, thermal storage cansignificantly degrade the optical properties of the package materi-als. Finally, injected current is proven to be the main cause for thegeneration of defects and shunt paths, and for the subsequent lossof optical power.

2. Experimental details

The devices tested in this work are 1 W power white LEDs, pro-duced by a leading manufacturer. The electro-optical characteris-tics of these devices are reported in Table 1.

Tested devices were acquired through commercial suppliers,with no additional selection, in order to study the characteristicsof the LEDs which are currently available on the market for theend user.

All the LEDs were mounted on an aluminum heat sink by meansof a bi-adhesive thermal interface in order to ensure a proper heatdissipation and to avoid that an excessive self-heating of the deviceaffects the results of the reliability tests. Above each LED, wasplaced a shaped mask, that allows a repeatable positioning of theoptical measurement setup.

Table 1Characteristics and maximum ratings of the samples used.

Characteristic Value* Unit

Typical luminous flux at 350 mA 85 lmMaximum junction temperature 160 �CMaximum forward current 1 ATypical forward voltage at 350 mA 3.2 VThermal resistance 6.5 �C/WCoordinate x (CIE 1931) 0.31 –Coordinate y (CIE 1931) 0.32 –Typical color temperature 6500 K

* All data are taken from the manufacturer datasheet.0 250 500 750 1000

70

75

80

85

90

95

100

Nor

mal

ized

opt

ical

pow

er (%

)

Stress Time (h)

Stress Conditions: I = 0, Tj=160 °C I = 500 mA, Tj=160 °C I = 700 mA, Tj=160 °C I = 1 A, Tj=160 °C I = 1.5 A, Tj=160 °C

Fig. 1. Optical power decrease measured at 700 mA.

2.0 2.5 3.0 3.5 4.01E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

Cur

rent

(A)

Voltage (V)

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

Stress condition: I=0A; Toven=160°C

2.0 2.5 3.0 3.50.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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

Cur

rent

(A)

Voltage (V)

(a) (b)

Fig. 2. current–voltage characteristics measured during stress time on one of theLEDs submitted to purely-thermal stress (160 �C, no bias). Curves are plotted bothin (a) linear scale and (b) semi-logarithmic scale.

M. Dal Lago et al. / Microelectronics Reliability 51 (2011) 1742–1746 1743

For each stress condition four identical LEDs were used in orderto obtain statistically relevant data; then, all the results reportedrepresent the average of four samples.

During the tests, the electro-optical characteristics of the sam-ples were monitored through the following equipment:

– Keithley 2612 Source Meter for the current-versus-voltage mea-surements and as a controlled current source during the opticalmeasurements;

– Newport 1830c Optical Power Meter equipped with NewportUV-818 photodiode and a 2’’ integrating sphere for the opticalpower measurements;

– Ocean Optics USB4000 Array Spectrometer for the electrolumi-nescence and reflectivity measurements.

For all the stress tests, the atmosphere inside the climatic cham-bers is air.

2.1. Purely thermal stress tests

Purely thermal stress tests were carried out by submitting LEDsto storage at constant temperature level, with no applied bias.Three temperature levels, greater than the values suggested bymanufacturer were used: 160 �C, 180 �C and 200 �C; a 85 �C ther-mal stress was also performed in order to have a comparison withthe degradation kinetics under moderate operating conditions.

During all the tests, the characteristics of the LEDs were moni-tored by means of optical power versus current (L–I), current ver-sus voltage (I–V) and electroluminescence (EL) measurements. Inthe case of the 200 �C test, the reflectivity of the upper part ofthe package was also measured during stress time.

2.2. Iso-thermal biased stresses

Stress tests at constant current levels in the range 500–1500 mAwere carried out on a large set of samples. For devices aged at dif-ferent current levels, ambient temperature was adjusted in orderto have the same junction temperature level of 160 �C. The temper-ature reached by the junction during operation was estimated bymeans of the forward voltage method described in [4]. In thisway, we could evaluate the effect of current on the stress kinetics,while keeping the same junction temperature on the devices. Foreach stress condition, samples were monitored by of optical powerversus current (L–I), current versus voltage (I–V) and electrolumi-nescence (EL) measurements.

3. Results

Fig. 1 shows the degradation of the optical power of LEDs sub-mitted to iso-thermal stress-tests, and the comparison with thecorrespondent thermal stress at 160 �C (black line); as can benoticed, the degradation kinetics of devices stressed at 160 �C with

no bias stabilizes after 500/750 h. On the other hand, degradationcurves of devices stressed with both bias and temperature (junc-tion temperature equal to 160 �C) showed a stronger degradationfor long stress times.

Results therefore suggest that high temperatures can determinea quick decrease in the optical power of the devices, while on theother hand current flow is responsible for the long-term degrada-tion of LEDs aged with constant current bias. In the following, weanalyze in detail the role of temperature and current in determin-ing device degradation.

3.1. Short-term degradation, purely thermal stress

During pure thermal stress tests, despite the strong optical deg-radation (see Fig. 4), the I–V measurements of the LEDs revealed nomodification on the electrical properties of the devices (Fig. 2): inparticular, no variation of the defect-related current components(Fig. 2a) and of the operating voltage (Fig. 2b) was measured dur-ing stress at high temperature levels.

This result suggests that the electro-optical properties of theblue semiconductor chip are not strongly affected by the high tem-peratures reached during stress. This indicates that the OP degra-dation shown during the thermal stress can be ascribed to thephysical deterioration of the packaging materials.

A confirmation of this hypothesis was obtained by analyzing thepackage by means of microscopy and reflectivity spectroscopy. Asshown in Fig. 3, stress induced a significant darkening of the plasticmaterial of the package, strongly dependent on the temperaturesused for the stress tests; the reflectivity of the circular regionaround the chip is of fundamental importance because it is respon-sible of the extraction of all the light laterally emitted by the chip.

84

88

92

96

100

Stress conditions: Toven=85°C Toven=160°C Toven=180°C Toven=200°C

Nor

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opt

ical

pow

er (%

)

Stress Time (h)

lens break

24 25 26 27 28 29

101

102

103

104

TTF

90%

(h)

q/(kT) [C/J]

Ea=1.8 eV

(a) (b)

0 500 1000 1500

Fig. 4. (a) Normalized optical power degradation during thermal stresses atdifferent temperatures; (b) Arrhenius plot of the TTF90%. Each data point representsthe average on two samples.

Fig. 3. Optical images of four devices before and after 64 h of stress at differenttemperatures.

-20 0 20 40 60 80 100 120 140

20

40

60

80

100Stress conditions: T

oven=200°C

Reflectivity Normalized optical power

Ref

lect

ivity

(%)

Stress time (h)

80

85

90

95

100

105

Opt

ical

Pow

er (

%)

Fig. 6. Comparison between the reflectivity at 435 nm (black line) and thenormalized OP (red line) trends during the pure thermal stress at 200 �C. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

2.0 2.5 3.0 3.51E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1Stress condition: I=1A; T J =160°C

Cur

rent

(A)

Voltage (V)

Stress time: 0h 1h 2h 4h 8h 16h 32h 64h 128h

Fig. 7. Current–voltage characteristics of a sample stressed at 1 A, Tj = 160 �C(logarithmic scale).

Stress condition: I=1A; TJ =160°CStress time:

1744 M. Dal Lago et al. / Microelectronics Reliability 51 (2011) 1742–1746

In order to better characterize the degradation of the package,we have analyzed the variation of the reflectivity of the packageof LEDs submitted to stress at 200 �C. Reflectivity of the packagewas measured by means of an halogen light source, a reflectomet-ric standard and an array spectrometer. The variation of the reflec-tivity during the 200 �C stress test is shown in Fig. 5; during stresstime, the decrease in the reflectivity of the package was found to bestrongly correlated to the optical power decrease, as shown inFig. 6.

In addition, during stress tests no significant variation of theoptical properties of the phosphorous layer and of the siliconencapsulant was detected; this results confirms that high-temper-ature degradation of the devices can be ascribed to the worseningof the reflectivity of the white package.

Package degradation was found to be thermally activated:Fig. 4a summarizes the OP degradation of LEDs submitted to stress

450 500 550 600 650 700 750 8000

20

40

60

80

100

120

Ref

lect

ivity

(%)

Wavelength (nm)

0 h 2 h 4 h 8 h 12 h 20 h 36 h 64 h 128 h

Fig. 5. Package reflectivity during the 200 �C pure thermal stress.

at different temperature levels. Time-to-failure to the 90% of theinitial OP level has an Arrhenius-like dependence on temperature,with an activation energy of the process of 1.8 eV (Fig. 4b).

3.2. Long-term degradation, current-driven degradation

While on one hand purely thermal stress did not significantlymodify the electro-optical properties of the blue semiconductorchip [5], on the other hand, constant current stress induced signif-

2.8 3.0 3.2 3.4 3.60.0

0.2

0.4

0.6

Cur

rent

(A)

Voltage (V)

0h 2h 4h 16h 64h 125h 250h 500h 750h 1000h

Fig. 8. Current–voltage characteristics of a sample stressed at 1 A, Tj = 160 �C(linear scale).

0 250 500 750 1000-80

-60

-40

-20

0

20

40Vo

ltage

var

iatio

n at

600

mA

(mV)

Stress time (h)

Stress conditions: I=0 A; Tj=160°C I=500mA; Tj=160°C I=700mA: Tj=160°C I=1500mA; Tj=160°C

Fig. 9. Voltage variation at 600 mA. Each data point represents the average of foursamples. Fig. 11. Degradation rate (degradation after 1000 h) for devices stressed at different

current levels, same junction temperature (160 �C). Solid line represents a linear fitof the experimental data.

M. Dal Lago et al. / Microelectronics Reliability 51 (2011) 1742–1746 1745

icant modifications of the electrical characteristics of the samples.I–V measurements show an increase in the parasitic current com-ponents (Fig. 7) and also a decay in the forward voltage for highcurrent values (Fig. 8).

The decrease in operating voltage appears to be closely corre-lated to the current value used for the stress as shown in Fig. 9,which reports the voltage decay measured at 600 mA for threebiased stress tests and the corresponding purely 160 �C thermalstress (black line).

The variation in forward voltage measured during high currentstress was found to be correlated to the decrease in optical power(Fig. 10a) and also to a variation in emission wavelength, as shownin Fig. 10b. This result indicates that stress induced significantmodifications in the characteristics of the active region of the de-vices, as discussed in the following.

The correlation between the optical power decay and the cur-rent values shows a linear behavior, with an acceleration factorof 0.145 1/A (of the degradation rate after 1000 h; see Fig. 11).

4. Discussion and conclusion

With this paper we have presented an extensive analysis of thedegradation of 1W-power white LEDs submitted to several stressconditions.

Devices aged at high temperature, with no applied bias, showeda remarkable optical power decrease, with no variation in theirelectrical characteristics. Optical degradation was found to be sig-nificantly correlated to the decrease in the reflectivity of the pack-

0 500 1000

2.46

2.63

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.u.)

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Stress conditions:I=1.5 A ; Tj= 160°C

3.42

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ge (V

)

0 500 10003.42

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3.51 Stress conditions:I=1.5 A ; Tj= 160°C

Stress time (h)

435.4

435.6

435.8

436.0

436.2

436.4

Wav

elen

gth

(nm

)

(b)(a)

Fig. 10. (a) Comparison between the forward voltage decay and the normalizedoptical power degradation. (b) Comparison between the forward voltage decay andthe shift of the blue emission peak wavelength. Each data point represents theaverage on four samples.

age (Figs. 3–5), with subsequent worsening in the extractionefficiency of the LEDs. Degradation kinetics of LEDs submitted topurely thermal stress have a nearly exponential shape (Fig. 4a),with a time constant that strongly depends on the stress tempera-ture level. Time-To-Failure of package-related degradation wasfound to have an Arrhenius dependence on temperature, with anactivation energy of 1.8 eV (Fig. 4b).

Furthermore, several sets of identical LEDs were stressed at dif-ferent current levels, with the same junction temperature, toachieve a consistent comparison. Results indicated that the flowof current through the active region of the devices can significantlyaccelerate degradation kinetics with respect to purely thermalstress (Fig. 1). When both current and temperature are appliedduring stress, the operating voltage of the LEDs can significantlydecrease (Fig. 8); decrease in operating voltage is clearly correlatedto the optical degradation (Fig. 10a), and to a slight increase in theemission wavelength of the blue semiconductor chip (Fig. 10b).

Results strongly suggest that iso-thermal stress induces a se-vere degradation in the electro-optical properties of the blue In-GaN-based LED, contrary to what observed for purely-thermalstress. In particular, decrease in forward voltage can be ascribedto a significant generation of parasitic shunt paths [6] (seeFig. 7), that can be attributed to an increase in the vertical epitaxialdefects density (i.e. threading dislocations). As a consequence, afraction of the injected carriers can flow through the junction with-out undergoing recombination, and this reduces device efficiency.

Fig. 12. Schematic representation of the Quantum Confined Stark Effect: afterbiased stress, due to the reduced current injection, the internal electric field is lesscompensated and so photons are emitted at lower energies.

1746 M. Dal Lago et al. / Microelectronics Reliability 51 (2011) 1742–1746

This interpretation is also consistent with the measured increase inthe emission wavelength: in fact, after stress a smaller amount ofcarriers is injected in the quantum wells (QWs). This can deter-mine a significant decrease in the screening of the piezoelectricand spontaneous fields in the QWs, with subsequent red-shift ofthe peak wavelength of the blue semiconductor chip [7] as showedby the scheme of Fig. 12.

In summary, results indicate that temperature and current cansignificantly contribute to the degradation of high-power LEDs.An accurate analysis is necessary to separately evaluate the contri-bution of these two driving forces in determining device degrada-tion. In a real application, in most cases, high junctiontemperatures are reached due to the high current levels and soall the observed degradation mechanisms contribute in limitingthe lifetime of the devices; it is then important to recognize the ef-fects of operating conditions in order to develop a reliable anddurable solid-state light source.

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[3] Buso S, Spiazzi G, Meneghini M, Meneghesso G. Performance degradation ofhigh brightness light emitting diodes under DC and pulsed bias. IEEE TransMater, Dev Reliab 2008;8(2):312–22. doi:10.1109/TDMR.2008.916549.

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