METHODOLOGIE D'ANALYSE DE DEFAILLANCE POUR L'EVALUATION DE LA FIABILITE DE DIODES ELECTROLUMINESCENTES GaN

Abstract : GaN-based LEDs are currently used in a wide range of applications as solid-state lighting, backlighting or full-color displays. Up to date, polymer-based packaging degradation mechanisms are not fully understood. The purpose of this thesis is to work out a methodology of failure analysis contributing towards reliability estimation of GaN-based LEDs under active storage ageing tests. The methodology consists in extracting electro-optical failure signatures to locate degraded zones. A second step is based on physico-chemical analyses used to both confirm failure mechanisms and reduce the number of components to study. Environmental ageing tests (1500h/85°C/30mA)have been performed on low power InGaN/GaN MQW LEDs (2,5mW) through a project in collaboration with the French Space Agency (CNES). A 65% loss of optical power has been reported after ageing. Through the methodology, we have found out that optical loss is due to the molecular structure modification of silicone oil (i.e. chip coating) activated by photothermal mechanism thereby involving both a 69% fluorescence emission loss and a strong decrease of LED light absorption (90%). A similar failure mechanism has been reported on YAG:Ce/silicone oil mixture located in phosphor converted high power white InGaN/GaN MQW LEDs (CEA-LETI collaboration - Solid-State Lighting project). Fluorescence efficiency has increased (1,2% at 450 nm) despite both strong absorption (94%) and fluorescence emission (85%) losses. Actually, molecular structure modification of silicone oil has induced a 45% loss of optical power and a 3,6% yellow shift of white light. Such drift has been linked to both a 5nm blue shift of UV fluorescence involving a 2nm red shift of LED light.
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Raphaël Baillot. METHODOLOGIE D'ANALYSE DE DEFAILLANCE POUR L'EVALUATION DE LA FIABILITE DE DIODES ELECTROLUMINESCENTES GaN. Electronique. Université Sciences et Technologies - Bordeaux I, 2011. Français. ⟨tel-00673985⟩

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