Review on failure mechanisms InGaN/GaN MQW LED for public light applications

Abstract

This paper gave an actual overview of sustainability, robustness and reliability of GaN LED devices. The overview deals with low, medium and high power GaN LEDs and detailed the main functional parameters drift to estimate performances of the technology regarding application. Different LEDs are considering and coming from different manufacturer and different years of development: LED type I(2005), II(2010) and type III(2015). 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 different Multi Quantum Wells (MQW) GaN-based LEDs coming from different manufacturer and submitted to active storage. Before ageing, an absorption/reemission fluorescence process has been extracted from some LEDs. By performing fluorescence analysis, we have found out the cause of such mechanism coming from silicone oil (polymer coating). Additional physics-chemical analyses, consisting in 1H NMR (Nuclear Magnetic Resonance) and MALDI-TOF (Matrix Assisted Laser Desorption Ionisation/Time Of Flight) 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 fluorophor molecules is responsible for it. After ageing, some LEDs have loss from 3%(type III) to 65%(type I) of optical power showing the technological evolution of LED assembly. 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.