The market for UV LEDs is experiencing a rapid growth, also driven by the need for effective and efficient disinfection systems. Before UV LEDs can be widely accepted by the market, they need to demonstrate a high reliability, with lifetimes of several thousands of hours. Several physical processes may limit the reliability of UVB and UVC LEDs, resulting in a loss in efficiency during long term operation.
This paper aims at discussing the most relevant processes that can lead to the degradation of UVB and UVC LEDs, with focus on: (i) instability of the electrical properties, which may result in gradual changes in the turn-on voltage of the devices during long-term operation. (ii) The generation of defects within the active region of the devices, with consequent increase in the Shockley-Read-Hall non-radiative recombination rate. Optical spectroscopy is found to be very effective for the identification of deep (midgap) traps during operation of the devices. (iii) trap states near the junction, with consequent impact on trap-assisted-tunneling of the current-voltage characteristics. (iv) the propagation of point defects, especially impurities, and accumulation of charges at heterointerfaces, that can reduce the carrier injection efficiency, thus leading to a decrease in the emitted optical power.
In the last year the market of ultraviolet (UV) light-emitting diodes (LEDs) had a huge increase due to the higher demand of devices caused by the pandemic crisis. In fact, it was demonstrated that UV LEDs, and in particular UV-B and UV-C LEDs, could be used as efficient sources for the disinfection of surfaces against Sars-CoV-2. In this work, we investigated electrical, optical and spectral degradation mechanisms in a series of commercial UV-C LEDs (275 nm – 280 nm) available on market. We present an exhaustive comparison of the main sample characteristics, studying their evolution when the LEDs are submitted to constant current stress tests (for about 350 h) at the absolute maximum current indicated in the respective datasheets. In particular, we carried out an extensive set of measurements, including current-voltage (I-V), optical power-current (L-I) and power spectral density (PSD) characteristics at various steps of stress, combined with the analysis of the drive voltage during the tests and of the thermal droop of the devices. We found: (i) a set of LEDs with a promising L80 of 10000 min (about 170 h), (ii) the presence of parasitic peaks and bands in all devices, ascribed to radiative recombination outside the QWs, and (iii) a substantial increase in thermal droop in all LEDs due to a decrease in injection efficiency and an increase in SRH recombination events during the stress tests.
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