GaN-based optoelectronics

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:

  1. to analyze the transport and emission mechanisms in GaN-based LEDs and lasers;
  2. to identify the failure mechanisms of GaN-LEDs and lasers by means of accelerated-life tests at high temperature and current levels;
  3. to characterize the operating limits of commercially available LEDs for application in solid-state lighting;
  4. 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:

  1. Study of the mechanisms responsible for radiative recombination in LEDs under different excitation conditions: electrical excitation (electroluminescence), optical excitation (photoluminescence) and electronic (cathodoluminescence)
  2. Identification of the failure mechanisms of the LEDs, by means of accelerated life tests: role of high temperatures, low current densities, high current densities
  3. Analysis of the role of the passivation layer in determining devices degradation
  4. 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;
  5. 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.
  6. 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.

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