Temperature induced degradation of InAlGaN multiple-quantum well UV-B LEDs
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Temperature induced degradation of InAlGaN multiple-quantum well UV-B LEDs Johannes Glaab1, Christian Ploch1, Rico Kelz1, Christoph Stölmacker1, Mickael Lapeyrade1, Neysha Lobo Ploch1, Jens Rass1, Tim Kolbe1, Sven Einfeldt1, Frank Mehnke2, Christian Kuhn2, Tim Wernicke2, Markus Weyers2, Michael Kneissl1,2 1
Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany 2 Technische Universität Berlin, Institut für Festkörperphysik, Hardenbergstr. 36, 10623 Berlin, Germany ABSTRACT The reliability of InAlGaN multiple quantum well LEDs emitting around 308 nm has been investigated. The UV-B LEDs were stressed at constant current and current density, while the heat sink temperature was varied between 15°C and 80°C. The results reveal two different modes of the decrease of the optical power during aging. First, a fast reduction of the optical power within the first 100 h (mode 1) can be observed, followed by a slower degradation for operation times >100 h (mode 2). Mode 1 can be described as an initial degradation activation process which saturates after a certain time, whereas the second degradation mode can be described by a square-root time dependence of the optical power, suggesting a diffusion process to be involved. Both degradation modes are accompanied by changes of the I-V characteristic, particularly the reverse-bias leakage current and the drive voltage. Furthermore, the degradation behavior is strongly influenced by the temperature. Both, the maximum reduction of the optical power and the increase of the leakage current become stronger at higher temperatures. INTRODUCTION Research and development effort on InAlGaN multiple quantum well (MQW) ultraviolet (UV) light emitting diodes (LED) have significantly intensified in the last few years. This growing interest has been triggered by a wide range of application of UV-B LEDs (280 nm – 320 nm), such as UV curing1, phototherapy2, and plant-growth lighting3, which can benefit from the advantages of UV-B LEDs over other UV-B sources. In contrast to medium-pressure mercury gas-discharge lamps, UV-B LEDs offer high spectral purity, low operation voltage, fast on/off switching without warmup, and are environmentally friendly. However, in many cases, even for commercially available devices, the optical power of UV-B LEDs degrades relatively fast, sometimes in the course of a few hundred hours4, 5, 6. This fact limits a faster market penetration and has motivated various studies on the degradation behavior of deep UV LEDs. Especially the influence of the operation parameters, such as drive current and temperature on the degradation behavior, and a better understanding of the physical mechanism behind the optical power degradation have been subjects of investigation. Gong et al. investigated the optical power reduction over time of 280 nm LEDs and found a strong correlation to the operation current and the temperature in the active region, with activation energies of the temperature driven degradation process of 0.23
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