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A concept for vertical-cavity surface-emitting lasers (VCSELs) is proposed and demonstrated to obtain a lasing wavelength with unprecedented temperature stability. The concept is based on incorporating a dielectric material with a negative thermo-optic coefficient, dn/dT, in the distributed Bragg reflectors (DBRs) to compensate the positive dn/dT of the semiconductor cavity. In a short cavity, the optical field has a significant overlap with the DBRs, and the redshift of the lasing wavelength caused by the semiconductor cavity can be compensated by the negative dn/dT of the DBRs. Here, proof of this concept is presented for optically-pumped VCSELs emitting at 310 nm, demonstrating a lasing wavelength that even blueshifts by less than 0.1 nm over an 80 °C range with a maximum slope of –3.4 pm K−1. This is to be compared with a redshift of 1–1.5 nm over the same temperature range reported for III-nitride blue-emitting VCSELs. Furthermore, this method can also be implemented in VCSELs with longer cavity lengths by including a dielectric layer between the semiconductor and the DBR. The approach used here to obtain a temperature-stable lasing wavelength is generic and can therefore be applied to VCSELs in all material systems and lasing wavelengths.

The long-term stability of ultraviolet (UV)-C light-emitting diodes (LEDs) is of major importance for many applications. To improve the understanding in this field, we analyzed the degradation of AlGaN-based UVC LEDs and modeled the variation of electrical characteristics by 2D simulations based on the results of deep-level optical spectroscopy (DLOS). The increase in the forward leakage current observed during ageing was ascribed an increase in trap-assisted tunneling. The analysis of the degradation kinetics suggests the role of a defect diffusion process, possibly involving impurities coming from the p-type layers.

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.

Multiresistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) cause serious postoperative infections. A skin tolerant far-UVC (2. 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.

Ultraviolet 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 (

The electroluminescence of UVB light-emitting diodes emitting at 310 nm before and after 1000 h of operation is studied in the temperature range from 20 to 340 K. Before operation, the external quantum efficiency (EQE) at 10 mA gradually increases with decreasing temperature from 0.8% at 340 K to 1.8% at 150 K and then levels off. This trend is attributed to a reduction of non-radiative recombination and finally the domination of radiative recombination at low temperatures. After 1000 h of operation, the EQE has dropped to 0.45% at 340 K with a maximum EQE of 1.4% at 80 K, followed by a drop for temperatures below 80 K. These findings suggest a stress-induced reduction of both the radiative recombination efficiency and the carrier injection efficiency.

AlGaN-based far ultraviolet-C (UVC) light emitting diodes (LEDs) with a peak emission wavelength below 240 nm typically show a long-wavelength tail at >240 nm that is detrimental to the use of the devices for skin-friendly antisepsis. We present the development of far-UVC LEDs with reduced long-wavelength emission using a HfO2/SiO2-based distributed Bragg reflector (DBR) filter. When the DBR filter is directly mounted on an LED package, the long-wavelength emission around 250 nm is reduced by two orders of magnitude while the transmitted output power is reduced down to 18%–27% of the initial value for DBR filters with cut-off wavelengths at 237–243 nm. As the transmission through the DBR filter depends strongly on the angle of incidence of the radiation, the transmitted output power of the spectrally pure far-UVC radiation can be doubled when an additional collimating lens is used on top of the LED package before passing through the filter.

In the past years, light-emitting diodes (LED) made of GaN and its related ternary compounds with indium and aluminium have become an enabling technology in all areas of lighting. Visible LEDs have yet matured, but research on deep ultraviolet (UV) LEDs is still in progress. The polarisation in the anisotropic wurtzite lattice and the low free hole density in p-doped III-nitride compounds with high aluminium content make the design for high efficiency a critical step. The growth kinetics of the rather thin active quantum wells in III-nitride LEDs makes them prone to inhomogeneous broadening (IHB). Physical modelling of the active region of III-nitride LEDs supports the optimisation by revealing the opaque active region physics. In this work, we analyse the impact of the IHB on the luminescence and carrier transport III-nitride LEDs with multi-quantum well (MQW) active regions by numerical simulations comparing them to experimental results. The IHB is modelled with a statistical model that enables efficient and deterministic simulations. We analyse how the lumped electronic characteristics including the quantum efficiency and the diode ideality factor are related to the IHB and discuss how they can be used in the optimisation process.

The radiative recombination efficiency (RRE) of AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates was investigated by picosecond-laser–excited photoluminescence (PL) spectroscopy under selective and non-selective excitation conditions. The PL efficiency, which was deduced by excitation-power-dependent PL measurements at low temperature (LT) and room temperature (RT), was unity at LT under both excitation conditions; However, at RT, the PL efficiency under non-selective excitation conditions was lower than that under selective excitation conditions. Time-resolved PL measurements revealed that, under non-selective excitation conditions, additional carriers are provided from the surrounding layers to the quantum-well layers, especially at LT. Therefore, at RT, the PL efficiency does not correspond to the RRE under non-selective excitation conditions. We propose a model to explain carrier dynamics under the two excitation conditions showing that the PL efficiency equals the RRE under selective excitation conditions.

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.

Driven by a wide range of applications, as shown in Fig. 1(a) , the development of AlGaN-based light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) has greatly intensified. In contrast to conventional ultraviolet sources UV-LEDs exhibit small form factors, operate at moderate dc voltages, show long lifetimes, and their emission covers the entire UVB and large parts of the UVC wavelength range. Currently, the main driving force are high volume applications like water purification, disinfection of surfaces and appliances, inactivation of germs in air treatment systems as well as sterilization of medical equipment with a focus on UVC-LEDs emitting near the germicidal effectiveness peak around 270 nm [1] . This presentation will provide an overview of state-of-the art in DUV-LED device technologies and present recent advances in the development of high quality AlGaN materials by metalorganic vapour phase epitaxy (MOVPE). We will discuss different approaches to improve the internal quantum efficiency of UV light emitters, including the growth of low defect density AlN layers on sapphire substrates by epitaxial lateral overgrowth (ELO) as well as sputtered and high temperature annealed AlN on sapphire [2] . We will demonstrate AlGaN quantum well (QW) LEDs with output powers of more than 50 mW for single-chip emitters near 265 nm and explore the wavelength limits of deep UV-LEDs with emission as short as 217 nm. These far UV-LEDs are ideally suited for sensing applications like the monitoring of combustion engines, toxic gases, nitrates in water, and may also be utilized for the in-vivo inactivation of multi-drug-resistant germs or viruses without damaging the human skin. Although a strong decline can be observed in the external quantum efficiency (EQE) of UV-LEDs emitting below 250 nm, as shown in Fig 1(b) , considerable advances have been made in the development of 233 nm LEDs with output powers of nearly 1.9 mW at 100 mA corresponding to an EQE of 0.35% [4] . We will discuss the root causes for the EQE decline, including changes in the optical polarization of light emission from deep UV AlGaN QWs and their effects on light extraction as well as changes in internal quantum efficiency and carrier confinement. We will provide an outlook of future progress in DUV-LED performance and demonstrate first applications including a spectrally pure 233 nm irradiation system for the in-vivo inactivation of germs.

Light emitting diode structures emitting in the ultraviolet spectral range are investigated. The samples exhibit defect luminescence bands. Synchrotron-based photoluminescence excitation spectroscopy of the complicated multi-layer stacks is employed to assign the origin of the observed defect luminescence to certain layers. In the case of quantum well structures emitting at 320 and 290 nm, the n-type contact AlGaN:Si layer is found to be the origin of defect luminescence bands between 2.65 and 2.8 eV. For 230 nm emitters without such n-type contact layer, the origin of a defect double structure at 2.8 and 3.6 eV can be assigned to the quantum wells. A part of this research was carried out at the light source DORIS III at DESY. DESY is a member of the Helmholtz Association (HGF). We would like to thank A. Kotlov for excellent assistance in using beamline I at DESY. This work was partially supported by the Federal Ministry of Education and Research (BMBF), under Contract No. 13N9933 and Berlin WideBase initiative under Contract No. 03WKBT01D and the German Research Council within the Collaborative Research Center 787. pre-print 465 KB

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.

In this work, the growth and conductivity of semipolar AlxGa1−xN:Si with (11-22) orientation are investigated. AlxGa1−xN:Si (x = 0.60 ± 0.03 and x = 0.80 ± 0.02) layers were grown with different SiH4 partial pressures, and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive x-ray spectroscopy and x-ray diffraction, and the Si-concentration was measured by wavelength dispersive x-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 Ω cm for x = 0.6 and 0.042 Ω cm for x = 0.8 were achieved. For both aluminum mole fractions, the resistivity exhibits a minimum with the increasing Si concentration, which can be explained by compensation due to the formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.

We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1 x 10 16 cm(-3) to 2 x 10 18 cm(-3) in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1 x 10 17 cm(-3) to 7 x 10 18 cm(-3). On the other hand, we find predominantly neutral cation vacancies with concentrations above 5 x 10 18 cm(-3) in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.

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., publishedVersion

Increase of light extraction efficiency (LEE) and total output power of UV light emitting diodes (LEDs) emitting at 265 and 310 nm, respectively, after encapsulation with a UV-transparent silicone are studied. Raytracing simulations suggest that a properly placed hemispherical encapsulation with a refractive index in the range from 1.4 to 1.8 enhances the LEE from 8% to up to 16% for flip-chip mounted UV LEDs with non-reflective metal contacts. The simulations also show that the absorption coefficient of the encapsulant determines the maximum LEE and optimum dome diameter and that it should be below 3 cm−1. The silicone encapsulant exhibits a refractive index of 1.47 (1.45) and an absorption coefficient of 1.3 cm−1 (0.47 cm−1) at 265 nm (310 nm). AlGaN/sapphire-based UVC and UVB LED chips were flip-chip mounted on planar AlN ceramic packages and encapsulated with a 1.5 mm-radius hemispherical silicone dome. The total output power at an operation current of 350 mA increased from 27 to 46 mW for 265 nm LEDs and from 45 to 78 mW for 310 nm LEDs. This corresponds to an enhancement of about 70%, which agrees with the simulations. Moreover, far-field measurements of encapsulated LEDs showed a narrowing of the emission cone.

During the epitaxy of AlGaN on sapphire for deep UV emitters, significant lattice mismatch leads to highly strained heterojunctions and the formation of threading dislocations. Combining cathodoluminescence, electron beam induced current and x-ray microanalysis reveal that dislocations with a screw component permeate through a state-of-the-art UVC LED heterostructure into the active region and perturb their local environment in each layer as growth progresses. In addition to acting as non-radiative recombination centers, these dislocations encourage high point defect densities and three-dimensional growth within their vicinity. We find that these point defects can add parasitic recombination pathways and compensate intentional dopants.

Single longitudinal mode emission of laterally coupled distributed-feedback (LC-DFB) laser diodes (LDs) based on InGaN/GaN multiquantum-well structures containing 10th-order surface Bragg gratings with V-shaped grooves is demonstrated. The gratings were fabricated alongside a 2-μm-wide contact stripe by i-line stepper lithography and inductively coupled plasma etching. A single peak emission at 404.6 nm with a fullwidth at half-maximum of 0.04 nm was achieved at an output power of about 46 mW under pulsed laser operation. The shift of the lasing wavelength of LC-DFB LDs in the temperature range from 22 °C to 45 °C was around three times smaller than that of comparable ridge waveguide Fabry-Perot LDs.

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.

Homoepitaxy on vicinal surfaces may proceed by either step-flow or step-bunching growth. This surface morphology transition is correlated with the inverse Ehrlich-Schwoebel barrier and the terrace width on the vicinal surface. We discuss a growth model based on the surface diffusion theory of Burton, Cabrera, Frank, and Schwoebel to account for conditions inducing the morphology transition. Based on a Monte-Carlo method, the transition conditions of the diffusion model are investigated regarding the inverse Ehrlich-Schwoebel barrier, the diffusion length, the vicinal terrace width, and the adatom mean resident time. This provides insights into crucial parameters and allows to predict the growth morphology by correlating theory and growth parameters. The theoretical results are applied to control the homoepitaxial growth of AlN.

In this paper we investigate the growth of metamorphic Al 0.5 Ga 0.5 N :Si on c-plane AlN/sapphire. The structural properties of the AlGaN:Si pseudo substrates and the electro-optical characteristics of subsequently grown UVB LEDs are being examined. We demonstrate, that superlattices allow the controlled strain relaxation of Al 0.5 Ga 0.5 N by rearrangement of threading dislocations, thus preventing the formation of cracks. This study investigates AlN/GaN superlattices with a nominal GaN layer thickness between 1.0 nm and 2.5 nm at a fixed AlN layer thickness of 2.5 nm. The number of superlattice-periods was also varied between 20 and 120. It was found that beyond a GaN layer thickness of 1.5 nm three-dimensional structures are formed. Additionally, these three-dimensional structures reduce the local defect density of the subsequently grown Al 0.5 Ga 0.5 N layer. Although the Al 0.5 Ga 0.5 N layer appears to be almost fully relaxed, the relaxation state of this pseudo substrate, was found to be dependent on the GaN layer thickness in the superlattice. After optimizing the superlattice structure we were able to grow crack free 4 μm thick Si-doped Al 0.5 Ga 0.5 N layers and on top UVB LEDs with a fully strained active region emitting at 310 nm with output powers of more than 18 mW at 500 mA.

In this paper, we will present the development of a compact LED-based optical gas sensing system in the ultraviolet-C spectral region. This includes the design of the LED heterostructure emitting near 226 nm, the development of an LED chip, and the implementation into a gas sensing system capable of detecting nitrogen oxide concentrations in the ppm range.

Recent progress in the development of deep ultraviolet lasers is reviewed as well as challenges for the heterostructure design and epitaxial growth for AlGaN-based laser diodes are discussed. The growth of AlN on sapphire and AlGaN heterostructures is reviewed and its impact on the performance characteristics of lasers in the UVC spectral range is presented. We achieve optically pumped AlGaN multiple quantum well (MQW) lasers near 270 nm with threshold power densities of less than 800 kW/cm2 and optically pumped AlGaN MQW lasers emitting at record shortest wavelength of 237 nm. We discuss critical stepping stones towards the development of current injection deep UV laser diodes including studies on Si- and Mg-doping of AlGaN with high aluminum mole fractions. n-Al0.8Ga0.2N cladding layers with resistances as low as 0.026 Ω cm are realized and UVC-transparent p-AlGaN cladding layers are developed. Finally, electroluminescence from current injection broad area UV laser diodes is demonstrated with maximum current densities of 4.7 kA/cm2. At the end, we provide an outlook of future prospect for deep UV laser diodes and discuss alternative approaches, e.g., electron beam pumping.

Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic properties, enabling novel functionalities and applications. Although a variety of lithography techniques are currently employed to nano-engineer these materials, the scalability and cost of the fabrication process can be an obstacle for large-scale manufacturing. In this paper, we report on the use of a fast, robust and flexible emerging patterning technique called Displacement Talbot lithography (DTL), to successfully nano-engineer III-nitride materials. DTL, along with its novel and unique combination with a lateral planar displacement (D2TL), allow the fabrication of a variety of periodic nanopatterns with a broad range of filling factors such as nanoholes, nanodots, nanorings and nanolines; all these features being achievable from one single mask. To illustrate the enormous possibilities opened by DTL/D2TL, dielectric and metal masks with a number of nanopatterns have been generated, allowing for the selective area growth of InGaN/GaN core-shell nanorods, the top-down plasma etching of III-nitride nanostructures, the top-down sublimation of GaN nanostructures, the hybrid top-down/bottom-up growth of AlN nanorods and GaN nanotubes, and the fabrication of nanopatterned sapphire substrates for AlN growth. Compared with their planar counterparts, these 3D nanostructures enable the reduction or filtering of structural defects and/or the enhancement of the light extraction, therefore improving the efficiency of the final device. These results, achieved on a wafer scale via DTL and upscalable to larger surfaces, have the potential to unlock the manufacturing of nano-engineered III-nitride materials., Nanolithography: III-nitride semiconductors Displacement Talbot lithography enables the low-cost fabrication of a range of nanostructures from III-nitride materials. III-nitride semiconductors provide interesting and useful optical properties when formed into nanostructures, with numerous device applications. However, several key performance parameters are sometimes limited due to crystal defects, or for devices larger than the nanoscale. Now, an international group led by Philip Shields from University of Bath, UK, apply Displacement Talbot lithography, and a lateral planar displacement variation, to generate low-cost 3D nanostructures. Fabricated devices include AlN nanorods, GaN nanotubes and InGaN/GaN core-shell nanorods. This technology might enable the large-scale, low-cost fabrication of nanostructures for optoelectronic applications with enhanced light extraction, such as for light emitting diodes, and is also appropriate for other semiconductor materials.

The application of quaternary In x Al y Ga1−x− y N active regions is a promising path towards high efficiency UVB-light emitting diodes (LEDs). For the utilization of In x Al y Ga1−x−y N, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the trimethylaluminium (TMAl) and trimethylindium (TMIn) flux on the composition and luminescence properties of In x Al y Ga1−x−y N layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 μmol min−1 led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 μmol min−1 resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analyzing the emission properties of these InN-rich regions showed that their emission energy is inhomogeneous and varies by ≈150 meV. We provide evidence that the formation of these InN-rich regions is highly dependent on the AlN and InN composition of the layer and that their formation will strongly affect the performance of In x Al y Ga1−x−y N LEDs.

Consistent studies of the quaternary composition are rare as it is impossible to fully determine the quaternary composition by X-ray diffraction or deduce it from that of ternary alloys. In this paper we determined the quaternary composition by wavelength dispersive X-ray spectroscopy of Inx Aly layers grown by metal organic vapor phase epitaxy. Further insights explaining the peculiarities of Inx Aly Ga1-x-yN growth in a showerhead reactor were gained by simulations of the precursor decomposition, gas phase adduct formation and indium incorporation including desorption. The measurements and simulations agree very well showing that the indium incorporation in a range from 0% to 2% is limited by desorption which is enhanced by the compressive strain to the relaxed Al0.5Ga0.5N buffer layer as well as indium incorporation into AlN particles forming in the gas phase. Utilizing Inx Aly Ga1-x-yN layers containing 2% of indium for multiple quantum wells (MQWs), it was possible to show an almost five times higher photoluminescence intensity of InAlGaN MQWs in comparison to AlGaN MQWs.

The light extraction efficiency (LEE), external quantum efficiency (EQE), and current–voltage characteristics of deep ultraviolet light emitting diodes (DUV-LEDs) with different aluminum mole fractions in the p-AlGaN layers have been investigated. Optimizing the p-AlGaN layer composition requires a tradeoff between reducing the absorption losses and limiting the increases in the p-contact resistance and operation voltage. AlGaN multiple quantum well LEDs emitting around 263 nm with different AlGaN:Mg short period super lattices (p-SPSL) ranging from x = 33% (UV-absorbing) to x = 68% (UV-transparent) average aluminum mole fraction have been explored. DUV-LEDs with different p-contact metals and UV-reflectivities have been characterized by electroluminescence measurements and analyzed by ray-tracing simulations. The comparison shows an increased operating voltage and a five-fold increase of the on-wafer EQE with a maximum value of 3.0% for DUV-LEDs with UV-transparent p-SPSL (x = 68%) and UV-reflective indium contacts in comparison to LEDs with a UV-absorbing p-SPSL (x = 33%). Ray-tracing simulations show that the increase in EQE can be partially ascribed to a 2.5-fold improved LEE in combination with a two-fold increase in internal quantum efficiency.

We present the growth and electro-optical characteristics of highly transparent AlGaN-based tunnel heterojunction light-emitting diodes (LEDs) emitting at 232 nm entirely grown by metalorganic vapor phase epitaxy (MOVPE). A GaN:Si interlayer was embedded into a highly Mg- and Si-doped Al 0.87 Ga 0.13 N tunnel junction to enable polarization field enhanced tunneling. The LEDs exhibit an on-wafer integrated emission power of 77 μW at 5 mA, which correlates to an external quantum efficiency (EQE) of 0.29% with 45 μW emitted through the bottom sapphire substrate and 32 μW emitted through the transparent top surface. After depositing a highly reflective aluminum reflector, a maximum emission power of 1.73 mW was achieved at 100 mA under pulsed mode operation with a maximum EQE of 0.35% as collected through the bottom substrate.

An optically pumped GaN-based laser structure with 10th-order laterally coupled surface grating is demonstrated. The fabrication involved i-line photolithography and dry etching, avoiding more complex technologies such as multiple epitaxy steps. The lasing threshold of the laterally coupled distributed-feedback (LC-DFB) laser was similar to that of a ridge waveguide Fabry–Perot (RW-FP) laser. Single-peak emission with a full width at half maximum of 0.06 nm at 404.2 nm was achieved for LC-DFB lasers. In contrast to the RW-FP lasers, the LC-DFB laser is shown to exhibit a smaller shift of the emission wavelength with temperature.

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.

Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1–10 nm thickness, covering a 3 nm thick In0.11Ga0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760 °C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification.

X-ray diffraction and TEM investigations of MOVPE grown AlN on sapphire with small off-cuts to a- and m-plane reveal the influence of the off-cut direction and angle on the reduction of threading dislocation density by annihilation during growth. Higher off-cut angles as well as off-cut to a-plane seem to facilitate the annihilation, with the main reduction taking place within the first 300 nm layer thickness. On planar substrate the thickness is limited by cracking to below 2 μm which also limits the ability to further reduce the defect density. By epitaxial lateral overgrowth on stripe patterned substrates the crack-free thickness is increased and further reduction of the defect density is possible. This process is effective up to 3–5 μm layer thickness. Using templates with off-cuts ≥0.2° to m-plane, step bunching perpendicular to the stripe direction occurs and bends the vertically directed threading dislocations into inclined grain boundaries starting from the point of coalescence. These partially block/incline threading dislocations over the ridge areas and thus further reduce the dislocation density. The dislocations are concentrated in stripes over the ridges and the coalescence areas. For smaller off-cut to m or especially for off-cut to a-plane, the dislocation distribution is more homogeneous but nevertheless stripe-like with alternating densities of low 108 cm−2 in the laterally overgrown areas and low 109 cm−2 in the areas over the ridges and the coalescence lines.