GaN-based light-emitting diodes (LEDs) have recently emerged as excellent devices for the fabrication of the next generation light sources: thanks to the efforts of the scientific and industrial communities, white LEDs with efficacy in excess of 150 lm/W have already been demonstrated, thus clearing the way for a massive adoption of LEDs in various fields (lighting, automotive, biomedical, optogenetics, …).

However, several factors still limit the performance and the reliability of InGaN-based LEDs, as briefly described in the following:

GaN-based LEDs are usually grown on foreign substrates (silicon carbide, sapphire and silicon), due to the lack of GaN substrates of reasonable size and cost; this fact has a negative impact on the crystalline quality of the devices, resulting in high densities of dislocations, whose presence limits the efficiency and the reliability of the LEDs.
At low current densities, the efficiency of InGaN LEDs is limited by Schockley-Read-Hall (SRH) recombination; on the other hand, at high current levels efficiency is governed by the so-called efficiency droop, whose physical origin is still under discussion.
In normal operation, LEDs are supposed to work at high current density (>70-100 A/cm2) and temperature (Tj up to 150 °C) levels.

This may favor the occurrence of several degradation mechanisms (generation of defects, degradation of the contacts, creation of leakage paths, …) that limit the lifetime of LEDs well below the targets of this technology (50 000 h).

We present an extensive analysis of the physical mechanisms that limit the performance and the reliability of InGaN-based light-emitting diodes. Based on recent experimental results, we analyze the following relevant issues:

Origin of non-radiative (SRH) losses in InGaN LEDs: based on combined optical measurements, differential lifetime investigation, and deep level transient spectroscopy (DLTS), we demonstrate that the radiative efficiency of LEDs is strongly dependent on the crystalline quality of the devices. We discuss the role of threading dislocations in favoring the efficiency losses, and we present a detailed description of the properties of the deep levels responsible for SRH recombination in GaN-based LEDs.
Physical mechanisms responsible for the degradation of LEDs: we discuss the most critical degradation mechanisms, namely the generation of defects induced by high current densities, the degradation of the ohmic contacts, the generation of parasitic leakage paths; the discussion is based on recent results obtained by various analytical techniques, including electroluminescence (time-resolved and spatially resolved), DLTS, AFM, … The role of the main driving forces (current, temperature, optical power) and the related acceleration factors will be described and discussed.

The experimental data will be critically compared to previous literature reports throughout the presentation, to provide an exhaustive overview of the topic.

Matteo Meneghini

Appearances