Characterization and reliability of GaN-based Light Emitting Diodes (LED) and Laser Diodes (LD) by means of electrical, optical and microscopical techniques
Summary: a detailed characterization, reliability and failure analysis study has been carried out on Light-Emitting Diodes (LED) and laser diodes (LD) based on gallium nitride (GaN). The activity has been carried out by means of electro-optical measurements and microscopical characterization. The attention has been focused on visible and deep-ultraviolet (DUV) LEDs, and on advanced GaN-based laser structures for Blue-Ray technology.
The aim of this work has been:
- to analyze the transport and emission mechanisms in GaN-based LEDs and lasers;
- to identify the failure mechanisms of GaN-LEDs and lasers by means of accelerated-life tests at high temperature and current levels;
- to characterize the operating limits of commercially available LEDs for application in solid-state lighting;
- to understand the role of the different driving forces (current, temperature, optical power, …) in determining LEDs and LDs degradation.
The most relevant and important results have been:
- Study of the mechanisms responsible for radiative recombination in LEDs under different excitation conditions: electrical excitation (electroluminescence), optical excitation (photoluminescence) and electronic (cathodoluminescence)
- Identification of the failure mechanisms of the LEDs, by means of accelerated life tests: role of high temperatures, low current densities, high current densities
- Analysis of the role of the passivation layer in determining devices degradation
- Definition of failure models, and discussion of the derived models with the manufacturers of the devices. This phase has a strong importance, since it provides to the manufacturer information for the improvement of the growth and processing phases;
- Identification of the failure modes and degradation mechanisms of blue Laser Diodes based on GaN (Blue-Ray technology). Degradation has been attributed to an impurity diffusion process.
- Analysis of the degradation of Deep-UV LEDs based on alGaN technology: the analysis has indicated that the efficiency decay is related to current transport inside the heterostructure, and proceeds by the increase of the non-radiative recombination rate in the quantum-well region.