applied sciences

Article

Multi-Azimuth Failure Mechanisms inPhosphor-Coated White LEDs by CurrentAging Stresses

Zhangbao Peng 1,†, Ziquan Guo 1,†, Tingzhu Wu 1, Peng Zhuang 2, Zhicheng Ye 1, Yuan Shi 2,

Tien-Mo Shih 1,3, Yijun Lu 1, Hao-Chung Kuo 4,5,* and Zhong Chen 1,*

1 Department of Electronic Science, Fujian Engineering Research Center for Solid-State Lighting,Xiamen University, Xiamen 361005, China; zbpeng@stu.xmu.edu.cn (Z.P.); zqguo@xmu.edu.cn (Z.G.);wutingzhu@gmail.com (T.W.); andy.ye@wenano.cn (Z.Y.); tmshih@xmu.edu.cn (T.-M.S.);yjlu@xmu.edu.cn (Y.L.)

2 Xiamen Products Quality Supervision & Inspection Institute, National Testing Center for LED ApplicationProducts, Xiamen 361004, China; zhuangpeng@xmzjy.org (P.Z.); shiyuan@xmzjy.org (Y.S.)

3 Changtai Tianming Physics Research Institute, Changtai 363900, China4 Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan5 Department of Electronic Engineering, Xiamen University, Xiamen 361005, China* Correspondence: hckuo@faculty.nctu.edu.tw (H.-C.K.); chenz@xmu.edu.cn (Z.C.)† These authors contributed equally to this work and should be considered co-first authors.

Received: 14 March 2018; Accepted: 2 April 2018; Published: 12 April 2018

Abstract: We have experimentally analyzed multi-azimuth degradation mechanisms thatgovern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W)light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging(CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermaland packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversionefficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coatedreflective layer deposited with extraneous chemical elements (e.g., C, O, Si, Mg, and Cu, respectively)contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts,thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging inthree cases. Particularly at the chip level, the formation of point defects increases the number ofnon-radiative recombination centers, and thus decreases the optical power during aging stages.Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealingeffect facilitate the increase of the optical power in two specific aging stages (192 h~384 h and768 h~1536 h). This work offers a systematic guidance for the development of reliable LED-basedlight sources in general-lighting areas.

Keywords: light-emitting diodes; failure mechanisms; current-stress aging; optical power; reliability

1. Introduction

The solid-state lighting (SSL) technology is regarded as the next-generation lighting approach,and exhibits great advantages of energy-saving, environmental-friendliness and smart lighting amongothers [1,2]. It is typically represented by white light-emitting diodes (LEDs), especially for thehigh-power phosphor-coated white (hppc-W) LEDs, which enjoy additional advantages, such as longlifetime, color-tunable property and high luminous efficiency [3,4].

However, room for improvement remains, including the internal quantum efficiency of theactive region [5,6], the light-extraction technology [7], the current-flow design [8], the minimization ofresistive loss [9], the electrostatic discharge stability [10], and the color-rendering property via the color

Appl. Sci. 2018, 8, 610; doi:10.3390/app8040610 www.mdpi.com/journal/applsci

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mixing [11]. Aside from these improvements, various stress-inducing degradation tests, by means oftemperature, current, static charge, optical radiation, and moisture, have been developed [12–14].In some cases, two or more types of stresses are combined to accelerate the aging process forhppc-WLEDs [15,16].

As state-of-art failure-generating methods, high-temperature stresses and high-current stressesare regarded as the most commonly-used approaches that accelerate the optical degradation [17,18].The former under various environmental temperatures have been carried out by severalauthors [19–21]. Among them, thermal stresses can primarily lead to the optical power decay forthe degradation of the packaging system, such as the detachment of the contact metallization [19],the yellowing and fracture of the optical lens [20], and the degradation of phosphor mixtures [21].Although current stress has also been applied to testing the degradation of LEDs, they may result inthe optical degradation due to the deterioration at the chip level, such as the crystal defect formationin the epitaxial layer [22] and the reactivation of Mg dopants [23]. Since the current stress is usuallyaccompanied by spontaneous thermal effects especially under high current stresses, investigating thedegradation of hppc-WLEDs has become significant and practical. Furthermore, studies of currentstresses that include photometric, chromatic, electrical, thermal, and packaging characteristics havebeen rarely reported in literatures.

In this article, comprehensive multi-azimuth failure analyses on hppc-WLEDs with peanut-shapedlenses at the chip level and the packaging level under 350 mA, 550 mA and 750 mA arepresented. Particularly, peanut-shaped lenses offer the optical uniformity and large-view-angle lightspatial distributions.

2. Experimental Methodology

Each of hppc-WLEDs consists of a blue InGaN LED chip (with an area of 1 mm × 1 mm),yellow YAG: Ce3+ yellow phosphors, a silicone epoxy lens, a silver-coated reflective layer, and otherpackaging materials, as schematically shown in Figure 1 [24]. Three CSAconditions (T = 25 ◦C,I = 350 mA (CSA-1), T = 25 ◦C, I = 550 mA (CSA-2), and T = 25 ◦C, I = 750 mA (CSA-3), respectively)are considered in these experiments. Thirty samples are divided into three groups (each group contains10 samples) corresponding to three aging conditions. After aging tests, all samples are cooled down inthe air for more than 5 h to release the remaining thermal energy prior to following measurements [25].

Figure 1. The schematic structure of peanut-shaped hppc-WLEDs.

All instruments used in the experimental study are listed in Table 1. Specifically, a spectrometer(Spectro-320e, Instrument Systems Inc.), with a 500 mm integrating sphere (ISP-500, InstrumentSystems Inc.) and an angular distribution analyzer (LEDGON 100, Instrument Systems Inc.),is employed to measure integral and angular spectral power distributions (SPDs) of these hppc-WLEDs.Therefore, photometric and chromatic analyses can be performed after relevant parameters areobtained from SPDs. The thermal resistance can be measured by a transient thermal tester (T3Ster

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2000/100, MicReD Ltd.) with an accuracy resolution within 0.1 ◦C. An electric source meter (Keithley2611, Keithley Inc.) is used to drive samples, and to measure their electrical properties, such ascurrent-voltage (I-V) characteristics and series resistances. Transmissivities of silicone epoxy lensesand reflectivities of silver-coated reflective layers are measured by adopting a 150 mm integratingsphere (ISP-150, Instrument Systems Inc.) with the same spectrometer mentioned above. Duringmeasurements, a white LED, as the light source, is placed on a heat sink regulated by the temperaturecontroller (Keithley 2510, Keithley Inc.). In addition, a field emission scanning electron microscopy(FE-SEM, Sigma-HD, Zeiss Ltd.) attached with an energy dispersive spectrometer (EDS, X-MaxN,Oxford Instruments Ltd.) is employed to detect and analyze microstructural and compositionalchanges on surfaces of silver-coated reflective layers.

Table 1. Various items and their characteristics of instruments used in the experimental study.

Instrument Model Characteristic

Spectrometer Spectro-320e Wavelength: 170–1700 nmIntegrating Sphere ISP-500, ISP-150 Sizes: 500 mm, 150 mmAngular Distribution Analyzer LEDGON 100 Angle range: 360◦; Angle accuracy: 1◦Transient Thermal Tester (T3Ster) T3Ster 2000/100 Testing currents: 0–500 mA; Resolution: 0.1 ◦CElectric Source Meter Keithley 2611 Maximum current: 1.5 A; Maximum voltage: 200 VTemperature Controller Keithley 2510 Temperature range: 0–80 ◦C; Resolution: 0.001 ◦CField emission Scanning ElectronMicroscopy (FE-SEM)

Sigma-HD Magnifications: 10–1,000,000×; Resolution: 1.0 nm

Energy Dispersive Spectrometer(EDS)

X-MaxN SDD detector: 50 mm2; Accuracy: 5 nm

3. Results and Discussions

3.1. Photometric and Chromatic Analyses

Figure 2 depicts optical decays of hppc-WLEDs, measured at 350 mA and 25 ◦C of the heat-sinktemperature, for CSA-1, CSA-2, and CSA-3, respectively. As observed from Figure 2, significant opticaldecays occur in CSA-3 after aging for 3840 h, indicating that high current stresses will induce severeoptical decays. Average optical decays after aging for 3840 h in three cases are estimated as 10.2%,11.0%, and 15.2%, respectively.

Figure 2. The normalized optical power versus aging time in three cases.

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To illustrate the change of the correlated color temperature (CCT) [26], we desire to introduce aparameter, δCCT, which can be written as

δCCT = CCT[a f ter]− CCT[be f ore], (1)

where CCT[be f ore] and CCT[a f ter] denote the CCT value before and after aging, respectively. Figure 3aplots the variation of CCT as a function of time. As observed, values of δCCT increase as time elapses,but with mild variations. Specifically, after aging for 3840 h, all values of δCCT in three cases liewithin 200 K. Figure 3b–d show measured SPDs in three cases prior to and after aging. Both blueand yellow emissions decrease simultaneously after 3840 h aging. Optical decays of blue and yellowemissions are separately illustrated in these figures. In all three cases, severe optical degradations canbe observed from yellow emissions relatively to blue counterparts. Especially in CSA-3, the opticaldecay reaches 12.2% for the blue emission and 17.1% for the yellow emission. For clear illustration,we calculate the ratio of the yellow optical power to the blue optical power, namely Y/B=Pyellow/Pblue,which indirectly describes the phosphor-conversion efficiency (PCE) of hppc-WLEDs [27]. Values ofY/B are also shown in Figure 3b–d. Changes of Y/B are estimated as 2.0%, 4.3%, and 5.6% for CSA-1,CSA-2, and CSA-3, respectively. They imply that PCE slightly decreases after aging, because of thewavelength shift of blue LED chips after 3840 h aging [28]. The higher the aging current becomes,the more decrease PCE exhibits. Therefore, we conclude that the decrease in PCE during CSA testsleads to optical decays for these hppc-WLEDs.

Figure 3. (a) The CCT variation versus aging time in three cases; SPDsof representative samples forthree cases: (b) CSA-1; (c) CSA-2; and (d) CSA-3, respectively, prior to and after aging.

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3.2. Electrical Analyses

Prior to and after aging for 3840 h, I-V curves of hppc-WLEDs nearly consolidate in all three casesfor Vf > 2.5 V (Figure 4a–c). These phenomena attest that indistinguishable deterioration of Ohmiccontacts is observed in all aging stages. However, when Vf < 2.5 V, leakage currents at reverse biasand low forward bias are found to increase after aging processes, manifesting the generation of shuntpaths [29]. This undesirable generation originates from the birth and the propagation of nitrogenvacancies [30,31], which behave as non-radiative recombination centers, and weaken the optical power.Ratios of the current after 3840 h aging to the current before aging (Ia/Ib) at three representativevoltages (−5 V, −4 V, and −3 V, respectively) are calculated in insets of Figure 4a–c. Large changesof Ia/Ib occur in CSA-2 (Figure 4b) and CSA-3 (Figure 4c), indicating that numerous shunt paths aregenerated after the high-current aging.

Figure 4. The I-V curves of representative hppc-WLEDs before and after 3840 h aging in cases of(a) CSA-1; (b) CSA-2; and (c) CSA-3, respectively; (d) Optical decays at various currents in three cases.

We also compare optical decays at various currents, which range from 10 mA to 100 mA in threecases. Results, depicted in Figure 4d, reveal that the optical decay increases as the testing currentincreases. Because both spectral shifts of the blue emission and the Joule heating in LED devicesbecome prominent at high currents, further PCE decreases emerge [32]. Therefore, as PCE decreases,optical decays of total white lights increase.

Forward voltages measured at 350 mA and series resistances (according to the equation in theinset in Figure 5, where Vf denotes the forward voltage, h the Planck constant, hv the photon energy,e the elementary charge, Rs the series resistance, and I the electrical current) remain almost the sameduring aging periods in three cases (Figure 5), indicating again that Ohmic contacts remain unchanged

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after aging. However, at the same aging stage, voltages in CSA-2 exhibit larger values than thosein other two cases. Apart from Ohmic contacts, it is generally believed that high current stresseswill induce severe degradations in the active region of LED chips [33]. Nevertheless, considering thechange of normalized optical power in Figure 2, where the optical power in CSA-2 also appears largerthan other two cases except in two aging stages (384 h and 3840 h), we conjecture that the degradationof LED chips in CSA-2 should be mitigated in comparison with that of LED chips in CSA-1 and CSA-3.

Figure 5. The forward voltage versus aging time in three cases.

To further study degradation mechanisms in chips from I-V characteristics, we select the result ofdevices in CSA-2 as an example. Figure 6 illustrates the degradation of the optical power measuredat 350 mA (black square) and the corresponding change of the ideality factor n (red circle), which iscomputed via the I-V curve fitting by using the Sah-Noyce-Shockley model [34]. In general, pointdefects, such as nitrogen and gallium vacancies, inevitably assist electrons to enter quantum wells viathe tunneling effect involving the non-radiative recombination process [35], thus increasing the valueof n. Specifically, the normalized optical power overall decreases except in two anomalous periods(192 h~384 h and 768 h~1536 h), whereas the trend of the ideality factor appears almost oppositely.

Figure 6. For the case of CSA-2: the normalized optical power versus aging time (black square,left vertical axis) and the ideality factor versus aging time (red circle, right vertical axis) with respectiveerror bars.

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Based on this reciprocity, it is plausible to create a reciprocal relationship between the normalizedoptical power and the ideality factor, exemplified by P = C1 + C2/n, where P denotes the normalizedout power; n the ideality factor; and C1 and C2 are constants to be determined. However, the detailedstudy of this plausibility belongs to the future work and will be omitted here. In the first anomalousperiod (192 h~384 h), the normalized optical power undergoes an increase due to the improvementof the effective carrier concentration reactivated by Mg dopants in the p-GaN [36]. In the secondanomalous period (768 h~1536 h), the optical power increases inversely due to the annealing effect [37],which eliminates a portion of defects after the administration of the long-term high-current stress.Therefore, the annealing effect dominates this period, leading to the increase in the normalized opticalpower as well as the decrease in the ideality factor.

3.3. Thermal Analyses

We also perform thermal analyses on these hppc-WLEDs. Thermal resistances of samples aremeasured by the transient thermal tester (T3Ster). Based on the cumulative structure function byT3Ster software, plotted in Figure 7a, we can discern different thermal resistances in components ofLEDs, including the die, the die attach, the silver adhesive, and the copper, respectively. Figure 7b–dshow cumulative structure functions of hppc-WLEDs before and after aging for 3840 h in threecases. In comparison with respective initial values of thermal resistances, only small alterations(δ(Rth) < 1 K/W) can be observed, indicating that the thermal property remains unchanged inthree cases.

Figure 7. (a) The cumulative structure function of samples; and those of samples before and after3840 h aging in cases of (b) CSA-1; (c) CSA-2; and (d) CSA-3, respectively.

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3.4. Packaging Analyses

Figure 8a depicts photographs of four representative samples corresponding to before and afteraging for 3840 h in three cases, with sample number of analyzed samples as #6 (CSA-1), #16 (CSA-2),and #23 (CSA-3), respectively. Ratios of the optical power after 3840 h aging to the optical powerbefore aging for #6, #16, and #23 are computed as 90.8%, 90.2%, and 80.7%, corresponding to 9.2%,9.8%, and 19.3% optical decay, respectively. From the appearance in Figure 8a, the sample in CSA-2exhibits remarkable yellow-browning in the optical lens, whereas that in CSA-3 demonstrates distinctdarkening on the surface of the silver-coated reflective layer. Considering the optical decay and thesurficial change, we rationalize that the darkening on the surface of the silver-coated reflective layerinduces more serious degradation of the optical power than the yellow-browning in the silicone-basedoptical lens does.

Figure 8. (a) Photographs of four representative samples; and (b) angular intensity distributionsof hppc-WLEDs.

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Angular intensity distributions look like a pair of bat wings for peanut-shaped LEDs withmaximum intensities at ±60◦ (Figure 8b). The largeness of view angles in these peanut-shapedLEDs clearly differs from that in conventional hemisphere-shaped LEDs [38]. Angles at which themaximum intensity occurs remain unchanged for all cases, whereas intensities in three aged casesdisplay monotonic decreases at all tested angles as the current stress increases. These decreasesmay be attributed to the systematic degradation in hppc-WLEDs, including the yellow-browningof optical lenses, the darkening of silver-coated reflective layers, and the decomposition ofphosphor-silicone mixtures.

We employ a white LED (with the current driven at 350 mA and the heat-sink temperaturemaintained at 25 ◦C) to measure and compare transmissivities of silicone epoxy lenses and reflectivitiesof silver-coated reflective layers in an unaged device and three 3840 h-aged devices in CSA-1, CSA-2,and CSA-3, respectively. As shown in Figure 9a, all transmissivities of aged lenses experience adecrease in comparison with the unaged one at measured wavelengths ranging between 380 nmand 780 nm. Clearly, the transmissivity of the lens in CSA-2 lowers most severely among those oflenses in all aged cases. The reason for this loss lies in that the device in CSA-2 exhibits a prominentyellow-browning silicone epoxy lens. Besides, variations of reflectivities on surfaces of silver-coatedreflective layers before and after aging are shown in Figure 9b. The reflectivity in CSA-3 is generallylower than those in CSA-1 and CSA-2 within the visible wavelength range due to the most severedarkening of the silver-coated reflective layer. Therefore, both decreases of the transmissivity of thesilicone epoxy lens and the reflectivity of the silver-coated reflective layer are found to be stronglyrelated to the optical power decays after aging.

Figure 9. Before and after 3840 h aging in CSA-1, CSA-2, and CSA-3, respectively: (a) transmissivitiesof silicone epoxy lenses; and (b) reflectivities of silver-coated reflective layers.

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To further investigate the degradation of silver-coated reflective layers, we employ a field emissionscanning electron microscopy (FM-SEM) on surfaces of silver-coated reflective layers to observemicrostructural changes of these surfaces before and after 3840 h aging, as shown in Figure 10. Resultsindicate that, in comparison with the surface of the un-aged sample (Figure 10a), dark areas aregenerated on those of aged silver-coated reflective layers (Figure 10b–d). Generally, the formation ofdark areas can be primarily attributed to the carbonization effect of plastic encapsulation materials onsurfaces of silver-coated reflective layers [39,40]. This effect reduces the intensity of the reflected lightbecause the impinging light is absorbed by these dark areas. Compositional changes of silver-coatedreflective layers have also been examined with an energy dispersive spectrometer (EDS). As shownin Figure 11a–d, some extra chemical elements, such as Cu, Mg, and Si, are found on surfaces ofaged silver-coated reflective layers. By contrast, these elements are found absent on the unagedsurface. Especially in CSA-3 (Figure 11d), the element C increases from 0.65 wt % to 21.40 wt %,and similarly, the element O increases from 0.63 wt % to 15.88 wt %. We deduce that increases ofelements C and O can be associated with the degradation of silicone epoxy lenses. The element Si with13.22 wt % in CSA-3 appears one order of magnitude higher than that with 1.05 wt % in CSA-1 and1.83 wt % in CSA-2 only. Again, we conjecture that most of the element Si may deposit on surfaces ofsilver-coated reflective layers from phosphor-silicone mixtures. These significant increases of threeelements can collectively demonstrate the highest degree of seriousness of the carbonization effect inCSA-3. In addition, small weight percentages of element Cu (CSA-1: none, CSA-2: 2.32 wt %, CSA-3:3.11 wt %) and element Mg (CSA-1: 0.01 wt %, CSA-2: 0.05 wt %, CSA-3: 0.12 wt %) emerge on surfacesof aged silver-coated reflective layers. The emergence of the former stems from the exposure of thecopper slug, whereas that of the latter stems from (a) the diffusion of the Mg dopant in p-GaN layeror/and (b) the cross-contamination during manufacturing and encapsulating processes of LEDs [41].Conceivably, both elements Cu and Mg are highly correlated with high-current stresses.

Figure 10. The SEMimages on surfaces of silver-coated reflective layers: (a) one unaged sample;and three representative aged samples in (b) CSA-1; (c) CSA-2; and (d) CSA-3, respectively (after3840 h aging).

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Figure 11. The EDS chemical element contents on surfaces of silver-coated reflective layers: (a) oneunaged sample, and three representative aged samples in (b) CSA-1; (c) CSA-2; and (d) CSA-3;respectively (after 3840 h aging).

4. Conclusions

We have investigated multiple degradation mechanisms of hppc-WLEDs under three currentstresses (CSA-1, CSA-2, and CSA-3, respectively). Results indicate that the optical degradation isstrongly correlated to the current stress. However, high current stresses do not necessarily causesevere optical decays. In the active layer of the LED chip, the Mg dopant reactivation and theannealing effect play positive roles in enhancing the optical power, whereas the increase of pointdefects plays a negative role in reducing the optical power. Packaging materials, such as silver-coatedreflective layers, silicone epoxy lenses, and phosphor-silicone mixtures, degrade constantly whenbeing subjected to the high-current stress. In addition, Ohmic contacts, thermal resistances, and anglesof the maximum intensity remain unchanged, and contribute insignificantly to the degradation ofLEDs during aging tests. Therefore, in this study, we have presented a comprehensive investigation ofphotometric, chromatic, electrical, thermal, and packaging analyses of hppc-WLEDs in current-stressaging processes. In practical applications, these experimental findings can provide LED communitieswith a guidance to develop reliable light sources.

Acknowledgments: This work was supported in parts by the International Science and Technology CooperationProgram of China (2015DFG62190), the Hong Kong, Macao, and Taiwan Special Science and TechnologyCooperation Program (2015DFT10120), the National Natural Science Foundation of China (61504112, 11604285,and 51605404), the Science and Technology Project of the State Administration of Quality Supervision, Inspectionand Quarantine (2014QK020), the Technological Innovation Project of Economic and Information Commission ofFujian Province, and the Strait Postdoctoral Foundation of Fujian Province.

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Author Contributions: Hao-Chung Kuo and Zhong Chen proposed the main idea; Zhangbao Peng and ZiquanGuo performed all experiments and wrote the manuscript; Tingzhu Wu, Peng Zhuang, and Zhicheng Ye analyzedthe data; Yuan Shi, Tien-Mo Shih, and Yijun Lu have contributed to the final revision.

Conflicts of Interest: The authors declare no conflict of interest.

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