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The aim of this paper is to investigate the degradation mechanisms of UV-B AlGaN-based light-emitting diodes (LEDs) submitted to constant current stress beyond the typical application conditions. We demonstrate the existence of two main degradation mechanisms that significantly impact the relative amplitude of the main quantum well (QW) peak at 310 nm and of a parasitic peak at ~340 nm related to the electron overflow toward the last quantum barrier before the p-side. These mechanisms are thoroughly investigated by means of electrical and optical measurements, and by photocurrent (PC) spectroscopy. We demonstrate that in the first 50 h of stress, degradation is caused by an increased defect density in the last quantum barrier and/or carrier escape from the QWs, which results in a decrease in the QW emission and in an increase in the parasitic peak at 340 nm. For longer stress times, degradation is dominated by a diffusion process, causing an increase in defect density and nonradiative recombination in the LED. This has a direct impact on both the QW peak and the parasitic peak, which show a significant decrease for long stress times. PC spectroscopy demonstrates that the defects responsible for degradation are most likely located next to mid-gap, thus acting as efficient nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

Highly reflective metal contacts for the p-layers of AlGaN-based deep ultraviolet light emitting diodes (DUV LEDs) emitting at 305 nm have been investigated. Different Pd-Al metal stacks have been examined and a reflectivity of 82% was obtained after annealing, which is about two times the value measured for conventional Pd contacts. The high reflectivity is attributed to the formation of an Al4Pd phase besides an Al phase in the annealed contact. DUV LEDs with highly-reflective Pd-Al contacts exhibited an increase of the light output power by 30% at a dc current of 20 mA. [ABSTRACT FROM PUBLISHER]

This paper describes an extensive analysis of the degradation of (InAlGa)N-based UV-B light-emitting diodes (LEDs) submitted to constant current stress. This paper is based on combined electrical characterization, spectral analysis of the emission, deep-level transient spectroscopy (DLTS) and photocurrent (PC) spectroscopy. The results of this analysis demonstrate that: 1) UV-B LEDs show a gradual degradation when submitted to constant current stress; the decrease in optical power is stronger for low measuring current levels, indicating that degradation is related to the increase in Shockley–Read–Hall (SRH) recombination; 2) the current–voltage characteristics measured before/during stress show an increase in the current below the turn-on voltage, that is ascribed to the increase in trap-assisted tunneling (TAT) components; and 3) DLTS analysis and PC spectroscopy measurements were carried out to identify the properties of the defects responsible for the degradation of the optical and electrical characteristics. The results indicate that stress induces or activates defects centered around 2.5 eV below the conduction band edge. These defects, close to midgap, can explain both the increased SRH recombination and the increase in TAT components detected after stress. Moreover, the DLTS measurements allowed to identify the signature of Mg-related acceptor traps. [ABSTRACT FROM PUBLISHER]

We investigated the effect of bow reduction of AlInGaN-based deep-ultraviolet light-emitting diode (DUV LED) wafers using internally focused laser patterning. The laser-induced stress inside of the sapphire substrate was found to increase with increasing density of laser lines and decreasing distance of the laser lines to the back surface of the sapphire substrate. By adjusting the laser process, the convex bow of DUV LED wafers could be almost completely eliminated. This bow reduction resulted in an improved uniformity of the LED forward voltage across the wafer compared to the reference wafer without laser patterning. [ABSTRACT FROM AUTHOR]

The spectral performance of solar-blind Al xGa1- xN based metal-semiconductor-metal ultra-violet photodetectors has been measured for top- as well as bottom-illumination at different bias voltages. In the bottom-illumination case the external quantum efficiency spectra can be tuned between a peak or a broad wavelength spectrum by adjusting absorber layer thickness and applied bias voltage. For thin absorber layers the external quantum efficiency is enhanced by a factor of three, reaching 20% quantum efficiency at 20 V bias, compared to the front-illumination case. Results of two-dimensional device simulations are well in agreement with the experimental findings. From these simulations it can be concluded, that the different spectral response for top- and bottom-illumination results from the different overlap of optical carrier generation by absorption and carrier transport by the electric field. Spectra of external quantum efficiency of solar-blind metal-semiconductor-metal photodetectors upon top-illumination (dashed) and bottom-illumination (continuous) at 1 V, 5 V, and 20 V bias voltage. The Al0.5Ga0.5N absorber layers of different thickness dAbs were grown on AlN/sapphire templates. [ABSTRACT FROM AUTHOR]

The effect of the active region growth temperature (TMQW) on the external quantum efficiency (EQE) of AlGaN‐based far‐ultraviolet‐C light‐emitting diodes (far‐UVC LEDs) emitting near 235 nm is investigated. AlGaN multi‐quantum well (MQW) active regions are grown at temperatures between 850 and 1100 °C by metal‐organic vapor‐phase epitaxy, while special care is taken to keep aluminum mole fractions and thicknesses constant for all MQWs. Temperature‐ and excitation‐power‐dependent photoluminescence spectroscopy reveal a more than tenfold increase of the radiative recombination efficiency (RRE) when the growth temperature increases from 850 to 1020–1060 °C. The output powers for mounted far‐UVC LEDs at 0.2 A increase from 0.5 mW for TMQW of 900 °C to 2.5 mW for TMQW of 1020 °C, corresponding to an increase in EQE from 0.04% to 0.23% at 0.2 A. However, lifetime measurements reveal a reduction of the L70 lifetime from 400 to 1 h when TMQW increases from 900 to 1060 °C. In this investigation, it is shown that optimizing the growth conditions provides a promising approach to further increase the RRE and EQE and lifetime of far‐UVC LEDs. [ABSTRACT FROM AUTHOR]