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Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure

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The influence of the perturbed reference wave in electron holography is considered for the case of static electromagnetic microfields, whose extension around the observed specimen cannot be neglected. These microfields are called ‘‘long-range’’ to distinguish them from the ‘‘short-range’’ ones, whose extension is strictly limited within the object wave and hence do not perturb the reference wave. Optical reconstructions of experimental holograms of simple electrostatic or magnetic long-range fields have been modeled and simulated. The results indicate that perturbation effects must be taken into account when long-range microfields are investigated by electron holography; it is also shown that their influence can be minimized by increasing the interference distance between the object and reference wave.

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This paper presents failure modes observed in long‐term aging of high‐brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non‐radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.