Photothermal activated failure mechanism in polymer-based packaging of low power InGaN/GaN MQW LED under active storage

GaN-based LEDs often use polymer material as chip coating. The most used polymer coatings are siloxane-based materials such as poly(methyl-phenyl-silixane) - PMPS - or poly(dimethylsiloxane) - PDMS. Although their thermal properties offer great possibilities to justify their integration in optoelectronic devices, pellicular effect may occur. This paper points out a pellicular failure mechanism occurring in MQW GaN-based LED submitted to active storage (1500 h/30 mA/85 °C) determined from their environment stresses. Before aging, an absorption/reemission fluorescence process has been extracted. By performing fluorescence analysis, we have found out the cause of such mechanism coming from silicone oil (polymer coating). Additional physico-chemical analyses, consisting of 1H NMR and MALDI-TOF mass spectrometry, have been investigated to work out the origin of the absorption/reemission process. The presence of low molecular weight molecules (LMWM) playing the role of fluorophore molecules is responsible for it. After aging, 65% optical power losses have been reported. A combination of electro-optical characterizations and physico-chemical analyses has led to the main failure mechanism extraction that is the molecular change of silicone oil activated by photothermal phenomenon. Such pellicular failure mechanism has been suggested to be linked to polymerization or cross-linking of silicone oil usually present in GaN-based LEDs.