UVC LEDs are of fundamental importance for many applications, including sterilization and disinfection, thanks to their high efficiency and low environmental impact. However, several physical processes still limit the lifetime and reliability of these devices. We present recent case studies in the field of UVC LED reliability. Initially, we review the performance/efficiency of state-of-the-art commercial devices, and discuss the issues related to LED self-heating, and the related electro-optical transient behavior. Then, we discuss the impact of defects on LED degradation, based on combined deep-level transient spectroscopy (DLTS) and deep-level optical spectroscopy (DLOS) measurements, and Technology Computer-Aided Design (TCAD) simulations. We show that, during prolonged operation, UVC LEDs can show considerable changes in the electrical characteristics: a) an increase in the sub-turn on leakage, that can be reproduced by TCAD as due to an increase in trap-assisted tunneling, related to deep traps located in the interlayer between the last barrier and the EBL; b) an increase in the turn-on voltage, that is explained by the degradation of the metal/p-GaN contact, due to a decrease in the active magnesium concentration. Electro-optical measurements reveal that a stronger degradation is detected at low measuring current levels, confirming an important role of defect-mediated recombination. Remarkably, degradation kinetics do not follow an exponential trend, but can be fitted by using the Hill’s formula. A higher Mg doping in the EBL mitigates the degradation rate. Results are interpreted by considering that degradation is due to the de-hydrogenation of point defects, which increases the density of non-radiative recombination centers.
This presentation will provide an overview of the state-of-the-art in the development of AlGaN-based far-UVC-LED technologies. We will explore origins for the observed decline in the external quantum efficiency (EQE) with decreasing emission wavelength and present different approaches to improve the RRE, CIE, and LEE of UV light emitters. We will also discuss design aspects for far-UVC irradiation systems and provide an outlook of future prospects of far-UVC-LED device technology as well as the potential for a wider use of far-UVC sources in applications like room air disinfection.
Light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) are of great interest for monitoring gases, pollutants in water as well as the in-vivo inactivation of multi-drug-resistant bacteria. This paper reviews advances in development of AlGaN-based DUV-LEDs, including the realization of low defect density AlN on sapphire. DUV-LEDs near 230 nm with output powers of more than 3 mW will be demonstrated and the root causes for the efficiency drop at shorter UV wavelength will be explored, including changes in the polarization of light emission, the role of point defects as well as carrier injection in AlGaN MQWs.
The market of Ultraviolet (UV) Light Emitting Diodes (LEDs) is expected to expand substantially in the coming years, thanks to the disinfection properties of the UV light; however, a detailed study on the reliability-limiting processes is a fundamental step, for an effective deployment of this technology. We investigated the degradation mechanisms of AlGaN-based UV Single Quantum Well (SQW) LEDs, with a nominal emission wavelength of 265 nm, an area of 0.1 mm2 and a nominal current density of 100 A·cm-2. By means of constant current stress test and Capacitance Deep Level Transient Spectroscopy (C-DLTS) we studied the main electrical, optical, spectral and capacitance characteristics of the devices, in order to understand the dominant causes of degradation. For aged devices the electrical characterization shows increased subthreshold leakage currents, due to the increase in Trap Assisted Tunneling (TAT) components, as well as an increase in drive voltage, which is ascribed to contact degradation or a decrease in injection efficiency. The optical output power showed a decrease especially at low current levels, which has been ascribed to an increase in non-radiative recombination and suggests the generation of defects in the LED active region. C-DLTS measurements showed in unaged devices the presence of two defects in the structure, both ascribed to magnesium (Mg), located at 475 meV and 150 meV from the respective band. Moreover, we detected the increase in concentration of a third defect during the stress test with an activation energy of 700 meV, that acts as a point defect, and could be ascribed to gallium vacancies or nitrogen antisites.
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