High-power white LEDs for outdoor lighting were submitted to accelerated lifetime stresses to evaluate their robustness and reliability. LEDs featured a 2 mm2 chip with 2 A absolute maximum current (Iabs) at 135 °C junction temperature. A first high-temperature, high-current robustness stress was performed at 1, 1.2, 1.4, 1.6 times Iabs for 50 hours. This stress caused a heavy decrease of optical power and a degradation of colorimetric properties of the LEDs stressed at currents exceeding Iabs. Optical analysis showed darkening of the phosphors and silicone and cracking of the lens. A second, long-term, stress was performed at 0.8 times Iabs at 45, 65, 85, 105 °C. This stress showed almost no lowering in flux for the samples stress at 45 and 65 °C, whereas samples stressed at 85 and 105 °C showed a decrease in flux from 2500 and 200 hours of stress, respectively, estimating a L90 lifetime of 5500 and 1500 hours. xy coordinates shifted proportionally to stress temperature. LEDs stressed at 85 °C and 105 °C eventually failed catastrophically, similarly to the high-current stress, with silicone and phosphors darkening and lens cracking. Raman analysis on high-current stressed LED lenses showed that poly(methyl,phenyl)siloxane was used as lens material. Stress induces higher luminescence of the silicone under Raman analysis. The cause of degradation is attributed to thermomechanical stress (cracking) and high-temperature silicone decomposition (darkening), possibly due to phosphors thermal quenching, causing a hot-spot just above the chip (confirmed by thermography), even if the junction temperature was within manufacturer specifications.
The mechanisms of porous gallium phosphide formation by anodic etching are studied. Gallium phosphide porous samples <100> oriented were prepared in sulfuric acid solution with different concentrations of NaF. The current-voltage characteristic curve depends on the NaF concentrations and shows the typical behavior for porous semiconductors formation. Four regions can be distinguished in the I-V curve: a pore formation zone, a dielectric layer growth, a transition region in which the two processes compete for the control of the surface morphology and finally the GaP surface electropolishing. The oxide formation shifts to higher potential and the pore formation zone is widened by adding fluorides in the etching solution. Raman spectroscopy is applied to investigate the surface morphology of samples prepared in different anodizing current density conditions and in different acid solutions. As regards the dielectric growth, the direct observation of the sample surface and the analysis of the vibrational spectra indicate that in different potential regimes two chemically different oxides can be formed.
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