The 2020 UV emitter roadmap

Solid 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

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well

Inactivating SARS-CoV-2 Using 275 nm UV-C LEDs through a Spherical Irradiation Box: Design, Characterization and Validation

We report on the design, characterization and validation of a spherical irradiation system for inactivating SARS-CoV-2, based on UV-C 275 nm LEDs. The system is designed to maximize irradiation intensity and uniformity and can be used for irradiating a volume of 18 L. To this aim: (i) several commercially available LEDs have been acquired and analyzed; (ii) a complete optical study has been carried out in order to optimize the efficacy of the system; (iii) the resulting prototype has been characterized optically and tested for the inactivation of SARS-CoV-2 for different exposure times, doses and surface types; (iv) the result achieved and the efficacy of the prototype have been compared with similar devices based on different technologies. Results indicate that a 99.9% inactivation can be reached after 1 min of treatment with a dose of 83.1 J/m2

Gruppe III-Nitrid basierte UVC LEDs und Laser mit transparenten AlGaN:Mg Schichten und Tunneldioden, hergestellt mittels MOVPE

In this work, AlGaN-based light-emitting diodes (LEDs) and lasers with emission wavelengths in the deep ultraviolet (UVC) spectral range are produced, analyzed, and optimized. Here, the focus is on the UV transparency of the structures, enabling high light extraction efficiency for UVC LEDs and being a necessary condition for UVC laser diodes, however at the same time challenging due to low electrical conductivity. AlN and AlGaN layers as well as heterostructures for devices are grown by metalorganic vapor phase epitaxy. A systematic analysis of the influence of individual layer properties on the emission properties of LEDs and lasers is provided. Defect reduced (ELO) AlN layers on sapphire and AlN substrates serve as basis for the epitaxial growth of AlN and AlGaN layers. By analyzing the influence of substrate offcut on surface morphology, atomically smooth AlN layers are reproducibly obtained on both types of substrates for offcut angles < 0.17°. For the realization of n-type AlGaN:Si cladding layers, the influence of growth parameters such as temperature, gas phase composition and growth rate was separately analyzed. Highly conductive, uniform and smooth AlGaN:Si layers were obtained by the implementation of a superlattice concept with 10 s growth interruptions to increase the diffusion length of metal adatoms. Despite high compressive strain, pseudomorphic laser structures with three-fold quantum wells were obtained with emission wavelength at 270 nm by the choice of Al0.7Ga0.3N waveguide composition, whereas lower aluminum contents lead to partial strain relaxation. In addition, the formation of V-pits acting as scattering centers in the waveguide was successfully reduced by increasing the growth temperature from 900 ℃ to 1080 ℃. Finally, the influence of these individual optimization steps on laser properties was analyzed. Optically pumped UVC lasers with laser threshold, spectral linewidth reduction, and TE polarized emission above threshold were shown near 270 nm. By reducing the surface roughness, the laser thresholds were reduced by a factor of seven. Electrical injection mechanisms were experimentally analyzed by electroluminescence measurements on transparent UVC LEDs with waveguide system, and combined with simulations of optical modes and the corresponding losses. By the variation of composition and layer thickness of waveguide and cladding layers an optimized heterostructure design for UVC laser diodes with 200 nm thick Al0.76Ga0.24N:Mg cladding layers was found. This design simultaneously enables efficient carrier injection and sufficient mode confinement with low optical losses of 40 cm-1. As an unconventional alternative to resistive AlGaN:Mg layers, tunnel junctions (TJ) in reverse bias configuration were implemented into the UVC LED heterostructure for efficient injection of holes. By the initial optimization of individual TJ components, such as doping concentrations at the TJ interface or the composition of an interlayer, the first demonstration of functional TJ-LEDs with AlGaN tunnel homojunction was achieved, as well as the first demonstration of AlGaN-based TJ-LEDs grown by metalorganic vapor phase epitaxy. Based on these devices, the interlayer thickness was varied to exploit polarization charges at the interface in order to reduce the space charge region width and enhance tunneling probabilities. Using 8 nm thick GaN interlayers, a reduction of the operation voltage by 20 V was achieved, as well as TJ-LEDs with external quantum efficiencies of 2.3% and emission powers of 6.6 mW at 268 nm and 0.26 mW at 232 nm.In dieser Arbeit werden AlGaN-basierte Leuchtdioden (LEDs) und Laser mit Emissionswellenlängen im tiefen ultravioletten (UVC) Spektralbereich hergestellt, charakterisiert und optimiert. Dabei liegt die UV-Transparenz der Strukturen im Fokus, die hohe Lichtextraktionseffizienz für UVC LEDs ermöglicht und eine notwendige Bedingung für UVC Laserdioden darstellt, gleichzeitig aber aufgrund geringer elektrischer Leitfähigkeit herausfordernd ist. AlN und AlGaN Schichten sowie Heterostrukturen für Bauelemente werden mittels metallorganischer Gasphasenepitaxie hergestellt und der Einfluss einzelner Schichteigenschaften auf die Emissionseigenschaften von LEDs und Lasern systematisch analysiert. Defektreduzierte (ELO) AlN Schichten auf Saphirsubstraten sowie AlN Substrate dienen als Basis für das epitaktische Wachstum von AlN und AlGaN Schichten. Durch die Analyse des Einflusses des Substratfehlschnittes auf die Oberflächenmorphologie konnten atomar glatte AlN Schichten auf beiden Substrattypen für Fehlschnittwinkel < 0.17° reproduzierbar hergestellt werden. Die AlGaN:Si Wachstumsparameter Temperatur, Gasphasenzusammensetzung und Wachstumsrate wurden separat variiert. Leitfähige, homogene und glatte AlGaN:Si Schichten konnten durch die Umsetzung eines Übergitterkonzeptes mit je 10 s Wachstumsunterbrechung zur Erhöhung der Diffusionslänge von Metalladatomen realisiert werden. Pseudomorphe Laserstrukturen mit Dreifach-Quantenfilmen und Emissionswellenlängen von 270 nm wurden trotz stark kompressiver Verspannung mittels Al0.7Ga0.3N Wellenleitern realisiert, wogegen geringere Aluminiumgehalte zu Teilrelaxation der Verspannung führen. Zudem konnte die Ausbildung von V-Pits als Streuzentren im Wellenleiter durch Erhöhung der Wachstumstemperatur von 900 ℃ auf 1080 ℃ erfolgreich reduziert werden. Schließlich wurde der Einfluss dieser einzelnen Optimierungsschritte auf die Lasereigenschaften analysiert. Optisch gepumpte UVC Laser mit spektraler Einschnürung, Laserschwelle sowie TE polarisierter Emission nahe 270 nm wurden gezeigt. Durch Reduktion der Oberflächenrauheit konnte die Laserschwelle schrittweise um den Faktor sieben reduziert werden. Elektrische Injektion wurde mittels Elektrolumineszenz an transparenten UVC LEDs mit Wellenleitersystem experimentell analysiert und mit Simulationen optischer Moden und deren Verluste kombiniert. Durch die Variation von Zusammensetzung und Schichtdicke von Wellenleiter- bzw. Mantelschichten konnte ein optimiertes Heterostrukturdesign für UVC Laserdioden mit 200 nm dicken Al0.76Ga0.24N:Mg Mantelschichten gefunden werden, welches gleichzeitig effiziente Ladungsträgerinjektion und ausreichenden Modeneinschluss mit geringen optischen Verlusten von 40 cm-1 ermöglicht. Als unkonventionelle Alternative zu resistiven AlGaN:Mg Schichten wurden Tunneldioden (TJ) zur Löcherinjektion implementiert. Durch die anfängliche Optimierung individueller Komponenten wie der Zusammensetzung einer Zwischenschicht oder der Dotierlevel an der Grenzfläche, wurde die erste Demonstration AlGaN-basierter TJLEDs ermöglicht, die mit metallorganischer Gasphasenepitaxie gewachsen wurden. Auf dieser Basis wurde die Zwischenschichtdicke gezielt variiert, um Polarisationsladungen an der Grenzfläche zur Reduktion der Raumladungszonenbreite auszunutzen und die Tunnelwahrscheinlichkeit zu erhöhen. Mit 8 nm GaN Zwischenschichten wurde eine Spannungsreduktion um 20 V erreicht, sowie TJ-LEDs mit externer Quanteneffizienz von 2,3% und Emissionsleistung von 6,6 mW bei 268 nm und 0,26 mW bei 232 nm

A practical degradation based method to predict long-term moisture incursion and colour change in high power LEDs

The effect of relative humidity on LEDs and how the moisture incursion is associated to the color shift is studied. This paper proposes a different approach to describe the lumen degradation of LEDs due to the long-term effects of humidity. Using the lumen degradation data of different types of LEDs under varying conditions of relative humidity, a humidity based degradation model (HBDM) is developed. A practical estimation method from the degradation behaviour is proposed to quantitatively gauge the effect of moisture incursion by means of a humidity index. This index demonstrates a high correlation with the color shift indicated by the LED's yellow to blue output intensity ratio. Physical analyses of the LEDs provide a qualitative validation of the model, which provides good accuracy with longer periods of moisture exposure. The results demonstrate that the HBDM is an effective indicator to predict the extent of the long-term impact of humidity and associated relative color shift

III-N optoelectronic devices: understanding the physics of electro-optical degradation

III-N optoelectronic devices are of great interest for many applications. Visible emitters (based on InGaN) are widely used in the lighting, display and automotive fields. Ultraviolet LEDs (based on AlGaN) are expected to be widely used for disinfection, medical treatments, surface curing and sensing. Photodetectors and solar cells based on InGaN are also of interest, thanks to their great robustness and wavelength tunability. III-N semiconductors are expected to be robust, thanks to the wide bandgap (allowing high temperature operation) and to the high breakdown field (favoring the robustness against electrostatic discharges and electrical overstress). However, InGaN- and AlGaN-based devices can show a significant degradation when submitted to long-term ageing. Several driving forces can contribute to the worsening of the electrical and optical characteristics, including the operating temperature, the current, and the rate of non-radiative recombination in the quantum wells. The goal of this paper is to discuss the physics of degradation of III-V devices, by presenting a set of recent case studies, evaluated in our laboratories