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Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm−2 achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation. [ABSTRACT FROM AUTHOR]

The hole transport and the carrier distribution in AlGaN-based far-ultraviolett (UVC) light emitting diodes (LEDs) emitting around 233 nm was investigated. Temperature-dependent electroluminescence measurements on dual wavelength AlGaN multiple quantum well (MQW) LEDs show a strong shift in the spectral power distribution from 250 to 233 nm with decreasing temperature. Comparing experimental data with simulation shows that the hole mobility and the electron to hole mobility ratios have a significant influence on the carrier injection efficiency (CIE) and that the change in the spectral power distribution is originating from a change in the hole distribution in the MQWs. Poor hole injection and charge carrier confinement in the AlGaN MQW active region was identified as one of the main reasons for the low CIE in far-UVC LEDs. [ABSTRACT FROM AUTHOR]

The COVID‐19 pandemic underscored the crucial importance of enhanced indoor air quality control measures to mitigate the spread of respiratory pathogens. Far‐UVC is a type of germicidal ultraviolet technology, with wavelengths between 200 and 235 nm, that has emerged as a highly promising approach for indoor air disinfection. Due to its enhanced safety compared to conventional 254 nm upper‐room germicidal systems, far‐UVC allows for whole‐room direct exposure of occupied spaces, potentially offering greater efficacy, since the total room air is constantly treated. While current evidence supports using far‐UVC systems within existing guidelines, understanding the upper safety limit is critical to maximizing its effectiveness, particularly for the acute phase of a pandemic or epidemic when greater protection may be needed. This review article summarizes the substantial present knowledge on far‐UVC safety regarding skin and eye exposure and highlights research priorities to discern the maximum exposure levels that avoid adverse effects. We advocate for comprehensive safety studies that explore potential mechanisms of harm, generate action spectra for crucial biological effects and conduct high‐dose, long‐term exposure trials. Such rigorous scientific investigation will be key to determining safe and effective levels for far‐UVC deployment in indoor environments, contributing significantly to future pandemic preparedness and response. [ABSTRACT FROM AUTHOR]

The electro‐optical characteristics of deep ultraviolet light‐emitting diodes (DUV LEDs) emitting at 265 nm and grown on AlN/sapphire templates with different threading dislocation densities, i.e., high‐temperature annealed (HTA) AlN, epitaxially laterally overgrown (ELO) AlN, and HTA‐ELO AlN are analyzed. The external quantum efficiency of each individual device is separated into maximum radiative recombination efficiency, carrier injection efficiency, and light extraction efficiency. This is achieved by combining an ABC‐model‐based fit of the current‐dependent external quantum efficiency together with calibrated Monte Carlo ray‐tracing simulations. A maximum radiative recombination efficiency between 50% and 60% is estimated for DUV LEDs grown on ELO and HTA‐ELO AlN/sapphire, whereas the values for devices grown on HTA AlN/sapphire are around 45%. The extracted radiative recombination efficiency does not scale with the measured threading dislocation density (TDD), even when accounting for the inhomogeneous TDD in the AlN base layers. This discrepancy is attributed to the formation of dislocation half‐loops introduced by additional compressive strain caused by the HTA process and may result in the formation of additional nonradiative recombination centers in the AlGaN multi‐quantum well region. In addition, the carrier injection efficiency values ranging from 45% to 55% are determined for devices grown on all three templates. [ABSTRACT FROM AUTHOR]

AlGaN-based far-UVC light emitting diodes (LEDs) with an emission wavelength of 233 nm were fabricated in the form of micro-LED arrays with emitter diameters ranging from 1.5 to 50 μm. The mesa was plasma etched with a sidewall angle of 45°–50°, and insulator layers made of SiNx or SiO2 were deposited. While the external quantum efficiency (EQE) of the LEDs with SiNx showed only a small dependency on the micro-LED diameter, the LEDs using SiO2 showed an increase in the peak EQE by a factor of four as compared to large area devices. This enhancement is attributed to a strong increase in the light extraction efficiency due to total internal reflection and re-direction at the inclined mesa, allowing TM-polarized light emitted in the plane of the quantum well to be extracted through the sapphire backside of the chips. [ABSTRACT FROM AUTHOR]

We compare the optical properties and device performance of unpackaged InGaN/GaN multiplequantum-well light-emitting diodes (LEDs) emitting at ~430 nm grown simultaneously on a high-cost small-size bulk semipolar (112 2) GaN substrate (Bulk-GaN) and a low-cost large-size (1122) GaN template created on patterned (1012) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ~105 cm-2-106cm-2 and basal-plane stacking fault (BSF) density of 0 cm-1, while the PSS-GaN substrate has the TDD of ~2 x 108 cm-2 and BSF density of ~1 x 10³ cm-1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density. [ABSTRACT FROM AUTHOR]

The effect of bulk AlN single‐crystal substrate thickness on the light extraction efficiency and external quantum efficiency of far ultraviolet‐C light emitting diodes (far‐UVC LEDs) has been investigated. AlGaN multi‐quantum well LEDs designed for emission around 235 nm are grown by metal organic vapor phase epitaxy on low defect density bulk AlN substrates as well as on AlN/sapphire templates. As the AlN substrate is thinned step‐by‐step from 550 to 70 μm by chemo‐mechanical polishing, the output power of LEDs increased from 3 μW for a thickness of 550 to 110 μW for a thickness of 70 μm (on‐wafer at a dc current of 100 mA). From the Lambert–Beer law an effective absorption coefficient of (84 ± 4) cm−1 is derived for the AlN substrate. Monte Carlo ray‐tracing simulations for an AlN substrate thickness of 70 μm yield an on‐wafer LEE of (0.9 ± 0.1)% and the internal quantum efficiency was estimated to be (5.6 ± 1.5)%. [ABSTRACT FROM AUTHOR]