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.
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.
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.
Driven by applications like monitoring of combustion engines, toxic gases, nitrates in water, as well as the inactivation of multi-drug-resistant germs, the development of AlGaN-based light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) has markedly intensified. This paper will provide a review of recent advances in development of DUV-LEDs, including the realization of low defect density AlGaN heterostructures on sapphire substrates. The performance characteristics of DUV LEDs emitting in the wavelength range between 260 nm and 217 nm will be discussed and milli-Watt power LEDs near 233 nm will be demonstrated.
We investigate the spectral broadening in deep ultraviolet (UV) multi quantum well light emitting diodes (LED) by modeling the emission spectra. Experimental emission spectra of deep UV LEDs exhibit a at tail towards lower energies and a steep decrease towards high energies that cannot be explained by convolution of the spectrum with a broadening function. We devise a luminescence model based on the broadening of the density of states (DOS) function which is consistent with the experimental spectra. The broadening of the DOS also explains the emission red shift with respect to the quantum well subband transitions. In addition, we investigate the in uence of the DOS broadening on the carrier and luminescence in the active region.
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