Degradation Mechanisms of High-Power LEDs for Lighting Applications: An Overview

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<ul><li><p>78 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY 2014</p><p>Degradation Mechanisms of High-Power LEDs forLighting Applications: An OverviewMatteo Meneghini, Senior Member, IEEE, Matteo Dal Lago, Nicola Trivellin,</p><p>Gaudenzio Meneghesso, Fellow, IEEE, and Enrico Zanoni, Fellow, IEEE</p><p>AbstractThis paper reports on the degradation mechanismsthat limit the reliability of high-power light-emitting diodes(LEDs) for lighting applications. The study is based on the ex-perimental characterization of state-of-the-art LEDs fabricatedby leading manufacturers. We demonstrate that, despite highpotential reliability, high-power LEDs may suffer from a numberof degradation mechanisms that affect the stability of the bluesemiconductor LED chip and of the phosphor layer used for thegeneration of white light. More specifically, we describe the follow-ing relevant mechanisms: 1) the optical degradation of LEDs, dueto an increase in the nonradiative recombination rate, which canbe correlated to modifications in the forward-bias currentvoltagecharacteristics; 2) the variation in forward voltage, due to the in-crease in series resistance; 3) the optical degradation of phosphorlayers used for blue-to-white light conversion; and 4) the failureof LEDs submitted to hot plugging, which is the direct connec-tion of an LED chain to an energized power supply, due to thegeneration of high current spikes. Results provide an overview onthe failure mechanisms that limit the reliability of state-of-the-artLEDs and on the role of current and temperature in determiningthe failure of the devices.</p><p>Index TermsDegradation, light-emitting diode (LED), light-ing, reliability.</p><p>I. INTRODUCTION</p><p>OVER the last few years, light-emitting diodes (LEDs)based on gallium nitride (GaN) have demonstrated tobe excellent devices for the realization of high-efficiency lightsources. The GaN material system (and, more specifically, theInGaN and AlGaN alloys) can be used for the fabricationof LEDs emitting in the blue, green, and ultraviolet spectralregions. Due to the research of many industries and academiclaboratories and to improvement in technology, the externalquantum efficiency of monochromatic GaN-based LEDs has</p><p>Manuscript received March 4, 2013; revised April 23, 2013; acceptedApril 23, 2013. Date of publication June 21, 2013; date of current versionJanuary 16, 2014. Paper 2012-ILDC-714.R1, presented at the 2012 Interna-tional Symposium on the Science and Technology of Lighting, Troy, NY, USA,June 2429, and approved for publication in the IEEE TRANSACTIONS ONINDUSTRY APPLICATIONS by the Industrial Lighting and Display Committeeof the IEEE Industry Applications Society. This work was supported in part bythe University of Padova (Progetto Giovani Ricercatori), in part by the Cassadi Risparmio di Padova e Rovigo (CARIPARO) Foundation, and in part by theItalian Ministry of Research through the PRIN program.</p><p>The authors are with the Department of Information Engineering, Univer-sity of Padova, 35131 Padua, Italy (e-mail: matteo.meneghini@dei.unipd.it;dallagom@dei.unipd.it; ntrivell@dei.unipd.it; gauss@dei.unipd.it; zanoni@dei.unipd.it).</p><p>Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.</p><p>Digital Object Identifier 10.1109/TIA.2013.2268049</p><p>Fig. 1. (a) Schematic of an LED with chip-level conversion. (b) Schematic ofan LED light source based on the RP approach.</p><p>increased from 25% (in early 2000) to over 70% for state-of-the-art devices [1].</p><p>The most convenient method to generate white light startingfrom a monochromatic LED is to use phosphor conversion, i.e.,blue LEDs (with a peak wavelength in the range between 450and 470 nm) are covered with a phosphor layer (usually yttriumaluminum garnets doped with rare earths), which converts theshort-wavelength radiation emitted by the LEDs into a broadyellow-green spectrum. The sum of the blue peak emitted by thedevices and of the luminescence of the phosphors is perceivedas white light; the chromatic properties (color rendering indexand correlated color temperature) of the white-light source canbe tuned by varying the composition of the phosphors. Forinstance, phosphors based on aluminates can be used to achievegreen emission, whereas phosphors based on nitrides generatea luminescence signal in the red spectral region. In an LED-based light source, phosphors can be directly deposited on theLED chip [chip-level conversion; see Fig. 1(a)] or placed somecentimeters away from the blue LEDs [remote phosphor (RP)approach; see Fig. 1(b)]. The latter approach significantly in-creases the dimensions of the light source but has the importantadvantage of minimizing the self-heating of the phosphors, thusimproving the reliability of the systems.</p><p>0093-9994 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.</p></li><li><p>MENEGHINI et al.: DEGRADATION MECHANISMS OF HIGH-POWER LEDs FOR LIGHTING APPLICATIONS 79</p><p>Due to improvement in the technology of blue LEDsand phosphors, white LEDs with efficiencies in excess of150 lm/W are commercially available [1], and those with ef-ficiencies of 200 lm/W are becoming feasible [2]. White LEDs,therefore, represent excellent candidates for the realization ofnext-generation light sources, which can compete with conven-tional fluorescent and incandescent lamps. (The latter has beenbanned in several countries.)</p><p>Despite the high potential of LED technology, the total LEDlighting market share was 9% in 2011 [3], mostly generatedin the architectural lighting segment. Several factors may beresponsible for the poor market penetration of LEDs, as fol-lows: 1) the relatively high cost of LED-based light sources,which can be reduced by adopting innovative technologies andsubstrates for the growth of GaN (e.g., GaN-on-silicon [4]and thin-film technology [5]); 2) several factors still limit theefficiency of LEDs, including nonradiative recombination [6]and the efficiency droop [7]; by minimizing these problems,it will be possible to further increase the efficiency of LED-based light sources; and 3) under high-stress conditions, LEDscan show significant degradation, which is activated by highcurrent densities and/or temperatures reached during operation[8][16]. As a consequence, the lifetime of the devices can beshorter than 50 000 h, which is usually considered as a targetfor LED technology [17].</p><p>With this paper, we analyze the main factors that limit thereliability of high-power LEDs for lighting applications. Basedon experimental results obtained on state-of-the-art devices, weprovide a detailed description of 1) the gradual degradationof the blue semiconductor chip due to increased nonradiativerecombination, 2) the degradation of electrical characteristics ofthe LEDs due to the increased series resistance or the generationof parasitic shunt paths, 3) the degradation of the phosphor layerdue to thermal effects, and 4) the effects of hot plugging(the connection of an LED chain to an energized power sup-ply), which may result in electrical overstress, with subsequentcatastrophic failure of the devices. The results presented withinthis paper provide a complete description of the main mech-anisms that limit the reliability of state-of-the-art LEDs andprovide information for the optimization of solid-state lightingdevices.</p><p>II. EXPERIMENTAL DETAILS</p><p>The results described in the subsequent sections have beenobtained on high-power LEDs, fabricated by three leadingmanufacturers. (The different sets of samples are referred toas groups A, B, and C hereafter.) The devices have a typicaloperating current in the range of 350700 mA (correspondingto 3570 A/cm2), and the characteristics are summarized inTable I. The samples were submitted to constant-current stress,with current levels in the range between 0.5 and 1.5 A. MostLED manufacturers recommend to use a maximum junctiontemperature (Tj) of 125 C150 C; to slightly accelerate thedegradation kinetics, we stressed the devices close to and abovethis limit (i.e., with Tj in the range between 100 C and 175 C).At the different stages of the stress experiments, an electricaland optical characterization of the devices was carried out, with</p><p>TABLE ISUMMARY OF THE CHARACTERISTICS OF THE</p><p>DEVICES ANALYZED WITHIN THIS PAPER</p><p>Fig. 2. OP versus current characteristics measured on a power LED (group A)before and during stress at 1.2 A, with a junction temperature of 150 C. (Inset)Schematic of nonradiative recombination (involving a defective state located inthe bandgap) and of radiative (band to band) recombination. While the firstprocess generates heat (thus decreasing device efficiency), the latter generatesa photon (represented by the wavy line). If stress induces an increase in non-radiative recombination, the efficiency of the devices significantly decreases,particularly at low (measuring) current levels.</p><p>the aim of achieving a complete description of the degradationprocess. The study of the degradation of the phosphors wascarried out on RP plates, with a circular shape, a diameter of61.5 mm, and a thickness of 2.1 mm, which were submitted tothermal stress (with temperatures in the range between 85 Cand 145 C), for 9000 h.</p><p>Finally, we analyzed the effects of hot plugging, i.e.,of the direct connection of an LED module to an energizedpower supply; more specifically, we studied the current spikesinduced by hot plugging, the dependence of in-rush currenton the number of LEDs in a module, and, by scanning electronmicroscopy (SEM), the origin of the failure of the LEDs.</p><p>III. RESULTS</p><p>A. Gradual Degradation of LEDs: Role of theSemiconductor Chip</p><p>Here, we analyze the gradual degradation of high-powerLEDs, induced by operation and high temperature/current lev-els. In Fig. 2, we report the optical power (OP) versus current(L I) characteristics of a power LED submitted to stress at1.2 A, with a junction temperature of 150 C. These stressconditions are very close to the limits specified by the manu-facturer. Stress induced a significant decrease in the OP of the</p></li><li><p>80 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY 2014</p><p>Fig. 3. Variation in OP measured (on one of the analyzed devices, group A)during stress at 1.2 A, with a junction temperature of 150 C. The differentcurves report the OP decrease measured at different (measuring) current levelsduring the same stress experiment.</p><p>devices (14% after 1000 h). Fig. 3 provides a description ofthe degradation kinetics, for the same LED (stressed at 1.2 A,Tj = 150</p><p>C); the various curves describe the variation in OPmeasured at several current levels during the stress experiment.Results indicate that the OP decrease is more prominent at lowmeasuring current levels; this behavior suggests that degrada-tion is due to an increase in the nonradiative recombination ratewithin the active region of the devices [18], possibly due tothe generation of lattice defects [19] (see the inset in Fig. 2for a schematic representation of radiative and nonradiativerecombination). According to this hypothesis, at low measuringcurrent levels (e.g., at 0.05 A), the (few) carriers injected intothe junction can be captured by nonradiative centers, and thismay result in a strong decrease in the efficiency of the devices.On the other hand, at higher (measuring) current levels, thenonradiative recombination centers are saturated by the largeamount of free carriers [18], and for this reason, they have alower impact in limiting the efficiency of the LEDs. Previousreports (see, for instance, [19] and references therein) studiedthe nature of the lattice defects responsible for the OP decreaseby using spectroscopic techniques, such as deep-level transientspectroscopy and photocurrent spectroscopy. Preliminary re-sults in [19] indicate that the optical degradation of LEDs maybe related to the increase in the concentration of a deep levelthat is commonly referred to as E2 [20][23], which has anactivation energy in the range of 0.50.6 eV, depending on theproperties of the analyzed devices. This defective level can berelated to nitrogen antisite (NGa) defects [20], to impurities(such as carbon [21]), to the residual Mg concentration in GaN[22], or to the presence of gallium vacancies [23]. During stress,the concentration of defects may increase as a consequence ofthe injection of high densities of carriers, through a subthresh-old defect generation process, as described in [8].</p><p>In many cases, optical degradation was found to be signifi-cantly correlated with modifications in the electrical character-istics of the devices. In Fig. 4, we report the currentvoltagecharacteristics measured before and during a constant-currentstress experiment. Results indicate that stress may induce an in-crease in current in the low-forward-bias region, well correlated</p><p>Fig. 4. Currentvoltage characteristics measured on a power LED (group A)before and during stress at 1.2 A, with a junction temperature of 150 C.</p><p>Fig. 5. OP decrease and increase in forward current measured during stress ofone of the analyzed devices (group A).</p><p>to the OP decrease (see Fig. 5). These important modificationsmay be due either to an increase in the concentration of point(nonradiative) defects within the active region of the devices,as described in [18], and/or to the generation of parasitic shuntpaths in parallel to the junction. Both mechanisms can modifythe amount of carriers that are effectively captured by thequantum well region, thus being responsible for the measureddecrease in OP.</p><p>Another relevant mechanism was detected on LEDs stressedat high-junction-temperature levels (&gt; 150 C), i.e., an increasein the series resistance (see Fig. 6). This effect was typical forthe early generations of LEDs (see, for instance, [13] and [14])and is usually ascribed to the degradation of the conductivity ofthe p-type material, due to the passivation of the Mg acceptors,induced by the formation of MgH bonds [14]. Even if severalmanufacturers have solved this issue (e.g., LEDs in Group C inthis paper did not show any modifications in forward voltage),one of the groups of LEDs studied within this paper (Group B)was found to show a considerable increase in series resistancewhen operated at high-temperature levels (see Fig. 6). Thiseffect can be critical for lighting applications; in fact, it leads toa considerable increase in the operating voltage of the devices,which may result in a decrease in the luminous flux, when the</p></li><li><p>MENEGHINI et al.: DEGRADATION MECHANISMS OF HIGH-POWER LEDs FOR LIGHTING APPLICATIONS 81</p><p>Fig. 6. Currentvoltage characteristics measured on one of the analyzeddevices (group B) before and during stress at 0.7 A, Tj = 168 C.</p><p>voltage drop of the diodes exceeds the maximum output voltageof the driver used to bias the LEDs. To understand this problem,we can consider a simple example as follows: LED modules areusually...</p></li></ul>

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