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- ItemA 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser(Washington, DC : ACS Publications, 2021) Hjort, Filip; Enslin, Johannes; Cobet, Munise; Bergmann, Michael A.; Gustavsson, Johan; Kolbe, Tim; Knauer, Arne; Nippert, Felix; Häusler, Ines; Wagner, Markus R.; Wernicke, Tim; Kneissl, Michael; Haglund, ÅsaUltraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically pumped, and all with low Al-content AlGaN cavities and emission near the bandgap of GaN (360 nm). Here, we demonstrate an optically pumped VCSEL emitting in the UVB spectrum (280-320 nm) at room temperature, having an Al0.60Ga0.40N cavity between two dielectric distributed Bragg reflectors. The double dielectric distributed Bragg reflector design was realized by substrate removal using electrochemical etching. Our method is further extendable to even shorter wavelengths, which would establish a technology that enables VCSEL emission from UVA (320-400 nm) to UVC (<280 nm). © 2020 American Chemical Society. All rights reserved.
- ItemImproving AlN Crystal Quality and Strain Management on Nanopatterned Sapphire Substrates by High-Temperature Annealing for UVC Light-Emitting Diodes(Weinheim : Wiley-VCH, 2020) Hagedorn, Sylvia; Walde, Sebastian; Susilo, Norman; Netzel, Carsten; Tillner, Nadine; Unger, Ralph-Stephan; Manley, Phillip; Ziffer, Eviathar; Wernicke, Tim; Becker, Christiane; Lugauer, Hans-Jürgen; Kneissl, Michael; Weyers, MarkusHerein, AlN growth by metalorganic vapor-phase epitaxy on hole-type nanopatterned sapphire substrates is investigated. Cracking occurs for an unexpectedly thin-layer thickness, which is associated to altered nucleation conditions caused by the sapphire pattern. To overcome the obstacle of cracking and at the same time to decrease the threading dislocation density by an order of magnitude, high-temperature annealing (HTA) of a 300 nm-thick AlN starting layer is successfully introduced. By this method, 800 nm-thick, fully coalesced and crack-free AlN is grown on 2 in. nanopatterned sapphire wafers. The usability of such templates as basis for UVC light-emitting diodes (LEDs) is furthermore proved by subsequent growth of an UVC-LED heterostructure with single peak emission at 265 nm. Prerequisites for the enhancement of the light extraction efficiency by hole-type nanopatterned sapphire substrates are discussed. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemStatus and Prospects of AlN Templates on Sapphire for Ultraviolet Light-Emitting Diodes(Weinheim : Wiley-VCH, 2020) Hagedorn, Sylvia; Walde, Sebastian; Knauer, Arne; Susilo, Norman; Pacak, Daniel; Cancellara, Leonardo; Netzel, Carsten; Mogilatenko, Anna; Hartmann, Carsten; Wernicke, Tim; Kneissl, Michael; Weyers, MarkusHerein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light-emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high-temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemThe 2020 UV emitter roadmap(Bristol : IOP Publ., 2020) Amano, Hiroshi; Collazo, Ramón; De Santi, Carlo; Einfeldt, Sven; Funato, Mitsuru; Glaab, Johannes; Hagedorn, Sylvia; Hirano, Akira; Hirayama, Hideki; Ishii, Ryota; Kashima, Yukio; Kawakami, Yoichi; Kirste, Ronny; Kneissl, Michael; Martin, Robert; Mehnke, Frank; Meneghini, Matteo; Ougazzaden, Abdallah; Parbrook, Peter J.; Rajan, Siddharth; Reddy, Pramod; Römer, Friedhard; Friedhard, Jan; Sarkar, Biplab; Scholz, Ferdinand; Schowalter, Leo J; Shields, Philip; Sitar, Zlatko; Sulmoni, Luca; Wang, Tao; Wernicke, Tim; Weyers, Markus; Witzigmann, Bernd; Wu, Yuh-Renn; Wunderer, Thomas; Zhang, YueweiSolid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm - due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments. © 2020 IOP Publishing Ltd.
- ItemQuantification of Trace-Level Silicon Doping in Al x Ga1-xN Films Using Wavelength-Dispersive X-Ray Microanalysis(New York, NY : Cambridge University Press, 2021) Spasevski, Lucia; Buse, Ben; Edwards, Paul R.; Hunter, Daniel A.; Enslin, Johannes; Foronda, Humberto M.; Wernicke, Tim; Mehnke, Frank; Parbrook, Peter J.; Kneissl, Michael; Martin, Robert W.Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35–100 ppm [corresponding to (3–9) × 1018 cm−3] in doped AlxGa1–xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1–xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1–xN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost-effective and time-saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.
- ItemSkin tolerant inactivation of multiresistant pathogens using far-UVC LEDs([London] : Macmillan Publishers Limited, part of Springer Nature, 2021) Glaab, Johannes; Lobo-Ploch, Neysha; Cho, Hyun Kyong; Filler, Thomas; Gundlach, Heiko; Guttmann, Martin; Hagedorn, Sylvia; Lohan, Silke B.; Mehnke, Frank; Schleusener, Johannes; Sicher, Claudia; Sulmoni, Luca; Wernicke, Tim; Wittenbecher, Lucas; Woggon, Ulrike; Zwicker, Paula; Kramer, Axel; Meinke, Martina C.; Kneissl, Michael; Weyers, Markus; Winterwerber, Ulrike; Einfeldt, SvenMultiresistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) cause serious postoperative infections. A skin tolerant far-UVC (< 240 nm) irradiation system for their inactivation is presented here. It uses UVC LEDs in combination with a spectral filter and provides a peak wavelength of 233 nm, with a full width at half maximum of 12 nm, and an irradiance of 44 µW/cm2. MRSA bacteria in different concentrations on blood agar plates were inactivated with irradiation doses in the range of 15–40 mJ/cm2. Porcine skin irradiated with a dose of 40 mJ/cm2 at 233 nm showed only 3.7% CPD and 2.3% 6-4PP DNA damage. Corresponding irradiation at 254 nm caused 11–14 times higher damage. Thus, the skin damage caused by the disinfectant doses is so small that it can be expected to be compensated by the skin's natural repair mechanisms. LED-based far-UVC lamps could therefore soon be used in everyday clinical practice to eradicate multiresistant pathogens directly on humans.