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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 will give an overview of the progress in ultraviolet-emitting vertical-cavity surface-emitting lasers (VCSELs) and their potential applications in areas such as disinfection and medical therapy. This includes our demonstration of the shortest wavelength VCSEL, emitting at 310 nm under optical pumping, and a detailed analysis of its filamentary lasing characteristics. The UVB-emitting AlGaN-based VCSEL was realized by substrate removal using electrochemical etching, enabling the use of two high-reflectivity dielectric distributed Bragg reflectors. The potential of using this or alternative methods to push the emission to shorter wavelengths will be examined as well as concepts to realize electrically injected devices.

We here demonstrate thin-film flip-chip (TFFC) ultraviolet-B light-emitting diodes (LEDs) fabricated by a standard LED process and followed by a substrate removal based on selective electrochemical etching of an n-doped multilayered Al0.11Ga0.89N/Al0.37Ga0.63N sacrificial layer. The integration of the LEDs to a Si carrier using thermocompression bonding allowed roughening of the N-polar AlGaN side of the TFFC LEDs using TMAH-etching, which increased the light extraction efficiency by approximately 45% without negatively affecting the I-V-characteristics. This resulted in an optical output power of 0.47 mW at 10 mA for an LED with a p-contact area of 0.03 mm2.

The aim of this work is to analyze the degradation in (In)AlGaN-based UV-B LEDs, with a nominal emission wavelength of 310 nm, submitted to constant current stress at a high current density of 350 A/cm2. We observed two main degradation mechanisms that were studied by investigating the evolution of the main emission peak from the quantum well (QW) and of a parasitic peak centered at 340 nm. In the first 50 hours of stress the main peak decreases and the parasitic peak (probably related to radiative recombination in the quantum barrier next to the electron blocking layer) increases at drive currents between 5 mA and 50 mA. Secondly, after 50 hours of stress both the main and the parasitic peak decrease. The first degradation mode could be related to carrier escape from the QWs, since the increase in the parasitic peak is correlated with the decrease in the main peak. After 50 hours of stress, we observed that the current below the turn-on voltage at V = 2 V increases with a square-root of time dependence. This behavior indicates the presence of a diffusion process, probably by point defects causing an increase of non-radiative recombination in the LED.

Light emitting diodes (LEDs) in the UVB (280 nm – 315 nm) spectral range are of particular interest for applications such as plant growth lighting or phototherapy. In fact, LEDs offer numerous advantages compared to conventional ultraviolet light sources such as a tunable emission wavelength, a small form factor, and a minimal environmental impact. State-of-the-art devices utilize p-GaN and low aluminum mole fraction p-AlGaN layers to enable good ohmic contacts and low series resistances. However, these layers are also not transparent to UVB light thus limiting the light extraction efficiency (LEE). The exploitation of UV-transparent p-AlGaN layers together with high reflective metal contacts may significantly increase the LEE. In this paper, the output power of LEDs emitting at 310 nm with a UV-transparent and absorbing Mg-doped AlGaN superlattice is compared. A three-fold increase of the output power was observed for LEDs with UV-transparent p-AlGaN layers. To investigate these findings, LEDs with low reflective Ni/Au and high reflective Al contacts are fabricated and characterized. Together with ray tracing simulations and detailed measurements of the metal reflectivities, we were able to determine the LEE and the internal quantum efficiency (IQE). According to on-wafer measurements, the external quantum efficiency (EQE) increases from 0.3% for an absorbing p-Al0.2Ga0.8N/Al0.4Ga0.6N-superlattice with Ni/Au contacts to 0.9% for a UV-transparent p-Al0.4Ga0.6N/Al0.6Ga0.4N-superlattice with Al contacts. This 3× enhancement of the EQE can be partially ascribed to an improved LEE (from 4.5% to 7.5%) in combination with a 1.8× increase of the IQE when using a p-Al0.4Ga0.6N/Al0.6Ga0.4N-superlattice instead of a p-Al0.2Ga0.8N/Al0.4Ga0.6N-superlattice.

In this paper we report on the influence of the heterostructure design of (InAlGa)N-based UV-B LEDs grown by metalorganic vapor phase epitaxy on sapphire substrates on the degradation behavior of the device. Two types of LEDs with different heterostructure design, resulting in peak-wavelengths of about 290 nm and 310 nm, respectively, were stressed at a constant operation current of 100 mA and a heat sink temperature of 20°C. Electro-optical characterization of the LEDs over 1.000 h of operation shows two different degradation modes with respect to the change of the emission spectrum and leakage current. The first mode during the initial hours (290 nm LED: 0 h - 500 h, 310 nm LED: 0 h – 100 h) of operation is represented by a fast reduction of the quantum well (QW) luminescence, a constant or increasing parasitic luminescence between 310 nm and 450 nm and a fast increase of the reverse- and forward-bias leakage current. These changes are more pronounced (higher degradation rate) in the 290 nm LEDs and can therefore be attributed to the different heterostructure design. In contrast, the second degradation mode at longer operation times (290 nm LED: >500 h, 310 nm LED: >100 h) is marked by a slow reduction of both the QW and the parasitic luminescence, as well as a slow increase of the leakage current which are similar for both types of LEDs. Furthermore, the second mode is marked by a square-root time dependence of the QW luminescence intensity, indicating a diffusion process to be involved.

UV light emitters in the UV-B spectral range between 280 nm and 320 nm are of great interest for applications such as phototherapy, gas sensing, plant growth lighting, and UV curing. In this paper we present high power UV-B LEDs grown by MOVPE on sapphire substrates. By optimizing the heterostructure design, growth parameters and processing technologies, significant progress was achieved with respect to internal efficiency, injection efficiency and light extraction. LED chips emitting at 310 nm with maximum output powers of up to 18 mW have been realized. Lifetime measurements show approximately 20% decrease in emission power after 1,000 operating hours at 100 mA and 5 mW output power and less than 30% after 3,500 hours of operation, thus indicating an L50 lifetime beyond 10,000 hours.