Raphael Baillot

Effects of silicone coating degradation on GaN MQW LEDs performances using physical and chemical analyses

This work presents a physics of failure (POF) methodology coupling failure signatures with physico-chemical analyses. The aim is to work out electro-optical failure signatures located in packaged InGaN/GaN Multiple Quantum Wells Light Emitting Diodes (MQW LEDs). Electrical and optical characteristics performed after accelerated ageing tests (30 mA/85 °C/1500 h), confirm a 65% drop of optical power and an increase of one decade of leakage current spreading at the silicone oil/chip interfaces. Through measurements of silicone coating fluorescence emission spectra, we demonstrate that the polymer enlarges the LED emission spectrum and shifts central wavelength. This shift is related to silicone oil spectral instability and the central wavelength of packaged LED appears to be temperature insensitive. In this paper, we discriminate the degradation of bulk silicone oil responsible for optical losses from the polymer/chip interface inducing larger leakage current.

Failure Mechanisms in Packaged Light-Emitting Diodes Under Gamma Radiations: Piezoelectric Model Based on Stark Effect

Degradation of packaged light-emitting diodes (LEDs) under gamma irradiations has been investigated using usual electro-optical characterizations. Failure mechanisms have been revealed by an accurate degradation model taking into account the Stark effect in electroluminescence spectrum and piezoelectric field in the active zone. Gamma irradiations have induced an additional mechanical stress close to 0.25 MPa in the active layer resulting from the change of the polymer mechanical properties used on the LED package.

Optical performances degradation of InGaN/GaN MQW LEDs related to fluorescence shift of copolymer-based silicone coating

Accelerated ageing tests (1500h/85°C/30mA) performed on packaged InGaN/GaN MQW LEDs have reported a fluorescence shift of silicone oil responsible for optical losses. Electrical and optical characteristics highlight a 65% loss of optical power. Through measurements of the copolymer silicone coating fluorescence emission spectra, we demonstrate that the polymer fluorescence (induced from the blue light emitted from the chip) enlarges the LED emission spectrum (7%) and shifts central wavelength (5 to 7 nm). To understand such a fluorescence shift, Attenuated Total Reflection, Nuclear Magnetic Resonance (NMR), mass spectrometry and Differential Scanning Calorimetry (DSC) have been performed. The copolymer molecular structure has been affected after ageing. Actually, NMR and Mass spectrometry evidences the disappearance of low molecular weight molecules and the presence of high molecular weight molecules after ageing. Such a mechanism is associated with the polymerization of the silicone oil after ageing. Indeed, DSC has confirmed that silicone oil polymerization process is activated by temperature. Finally, both polymerization of the silicone oil due to temperature and fluorescence shift activated by photothermal process have been identified as the main failure mechanisms responsible for optical power degradation.

Integration of the Methodology Starting from Component Design

Power light-emitting diodes (LEDs) for public lighting are distinguished from other LEDs by the need to obtain a color yield index (CYI) greater than 85, the highest possible optical power (>> 1 W), high luminous efficiency (>> 100 lm.W −1 ), excellent thermal management with thermal resistances of assemblies lower than 4°C.W −1 , and the lowest possible manufacturing cost (<

Failure Analysis Methodology of Blue LEDs

4/lamp).

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

Focused on the description of the basic tools required for failure analysis of optoelectronic components for an InGaN/GaN MQW structure. In particular, we have presented two methods (electrical and optical) for measuring the junction temperature. We have shown that this parameter, as well as information provided by the physicochemical analyses, represents major points in the preparation of electrical and optical models of both types of the studied structures. Thermal, electro-optical and physicochemical analyses have demonstrated the wealth of information provided in order to develop a complete physical model.

Overview on Sustainability, Robustness, and Reliability of GaN Single-Chip LED Devices

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.

Methodology of failure analysis applied to packaged LEDs

This paper gave an actual overview of sustainability, robustness, and reliability of GaN-based LED devices. This overview deals with low- and medium-power LED composed of a single chip and details the main functional parameters to regard to estimate performance drifts of the technology related to application.