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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 pauci-cellular nature of cerebrospinal (CSF), particularly ventricular CSF, and the rapid cell death following sampling, incumbers the use of flow cytometric analysis of these samples in the investigation of central nervous system (CNS) pathologies. Developing a method that allows long-term storage and batched analysis of CSF samples without compromising cell integrity is highly desirable in clinical research, given that CSF is often sampled after hours creating logistical difficulties for fresh processing. We examined percentages and relative proportion of peripheral and brain-derived immune cells in Cryopreserved and Transfix-treated CSF, compared to freshly processed CSF. Cell proportions were more comparable between Fresh and Cryopreserved CSF (mean of differences = 3.19), than between Fresh and Transfix-treated CSF (mean of differences = 14.82). No significant differences in cell percentages were observed in Fresh versus Cryopreserved CSF; however significantly lower cell percentages were observed in Transfix-treated CSF compared to Fresh CSF [(CD11b++ (p = 0.01), CD4+ (p = 0.001), CD8+ (p = 0.007), NK cells (p = 0.04), as well as CD69+ activation marker (p = 0.001)]. Furthermore, loss of marker expression of various lymphocyte sub-populations were observed in Transfix-treated CSF. Cryopreservation is a feasible option for long-term storage of ventricular CSF and allows accurate immunophenotyping of peripheral and brain-derived cell populations by flow cytometry.

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.

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 (

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.

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.

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.

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.

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.

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.

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.

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.

The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.

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.

Herein, a systematic study of the morphology and local defect distribution in epitaxially laterally overgrown (ELO) AlN on c‐plane sapphire substrates with different off‐cut angles ranging from 0.08° to 0.23° is presented. Precise measurements of the off‐cut angle α, using a combination of optical alignment and X‐ray diffraction with an accuracy of ±5° for the off‐cut direction and ±0.015° for the off‐cut angle, are carried out. For ELO AlN growth, a transition from step flow growth at α 0.14° is observed. Furthermore, the terraces of the step‐bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveal a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction in the threshold excitation power density for optically pumped ultraviolet‐C (UVC) lasers with smooth surface morphologies is observed.

The influence of the sapphire substrate off-cut angle from c-plane orientation on the surface morphology of AlN/AlGaN layers and the performance of 310 nm light emitting diode structures has been investigated. Increasing the off-cut angle from 0.1° to 10° towards the m-plane leads to a change of the (Al,Ga)N growth mode, threading dislocation density, and emission characteristics of ultraviolet light emitting diodes (LEDs). The growth mode changed from step-flow with bilayer steps to step-bunching and finally to rough 3D growth. The lowest threading dislocation density has been observed for structures deposited on sapphire with an off-cut angle of 0.65°, whereas the emission power is the highest for structures grown on substrates with an off-cut of 0.48°. Electroluminescence measurements display single peaks for structures on substrates with off-cut angles of up to 0.65°. The emission peak full width at half maximum remains nearly constant (~9.5 nm) for structures with off-cut angels up to 0.21° and increases to 11.7 nm and 21.0 nm for 0.48° and 0.65°, respectively. The emission wavelength is almost unchanged by the use of off-cut angles up to 0.48° (311 nm) and increases to 319 nm for 0.65°. Larger off-cut angles cause multiple emission peaks indicating compositional inhomogeneity of the InAlGaN quantum wells.

In this paper we describe the scanning electron microscopy techniques of electron backscatter diffraction, electron channeling contrast imaging, wavelength dispersive X-ray spectroscopy, and cathodoluminescence hyperspectral imaging. We present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping, and light emission from a range of UV-emitting nitride semiconductor structures. We aim to illustrate the developing capability of each of these techniques for understanding the properties of UV-emitting nitride semiconductors, and the benefits were appropriate, in combining the techniques.

Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 µm x 1 µm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.

The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at an aluminum mole fraction of 84%. To allow the growth of thick LED heterostructures we demonstrated that such a buffer can be overgrown with higher growth rate AlGaN, yielding a low grain density of 1.0 × 106 cm−2 and a smooth morphology with a rms roughness of 2.5 nm by avoiding misoriented crystal propagation during nucleation.

Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance-voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 +/- 0.7 MV cm(-1)), Al0.26Ga0.74N (2.3 +/- 0.6 MV cm(-1)), Al0.34Ga0.66N (3.1 +/- 0.6 MV cm(-1)), Al0.41Ga0.59N (4.0 +/- 0.7 MV cm(-1)) and Al0.47Ga0.53N (5.0 +/- 0.8 MV cm(-1)) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance-voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration. (C) 2019 The Japan Society of Applied Physics

Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1−xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1−xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3×1018 cm−3 and 2.8×1019 cm−3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.

In this paper the internal electric fields of nearly lattice matched InAlN/GaN heterostructures were determined. Pin-diodes containing InAlN/GaN heterostructures grown on sapphire substrates by metalorganic vapour phase epitaxy were fabricated by standard lithography and metallization techniques. To determine the polarization fields in the InAlN quantum wells capacitance-voltage-measurements were performed on the pin-diodes. To reduce the measurement error, the heterostructure thicknesses were accurately determined by transmission electron microscopy. Large polarization fields, which correspond mainly to the spontaneous polarizations, for In0.15Al0.85N ( MV cm−1), In0.18Al0.82N ( MV cm−1) and In0.21Al0.79N ( MV cm−1) quantum wells were observed. The results of the internal field strength and field direction are in excellent agreement with values predicted by theory and a CVM-based coupled Poisson/carrier transport simulation approach.

The waveguide strain and the surface morphologies of AlGaN-based laser heterostructures emitting in the deep UV spectral range have been investigated. In particular, the impact of the AlGaN heterostructure design on the strain relaxation as well as the effect of the growth temperature on the surface morphology were explored. We found strain-induced plastic relaxation for laser heterostructures with 130 nm thick Al0.45Ga0.55N waveguide layers, whereas pseudomorphic growth was obtained for laser heterostructures with Al0.70Ga0.30N waveguide layers. Optically pumped lasing near 280 nm was demonstrated for the coherently grown laser heterostructures. A strong correlation of the surface morphology with the waveguide growth conditions was also observed. Low growth temperatures of 900 °C lead to a high density of V-pits originating from dislocations. By increasing the growth temperatures to 1070 °C the V-pit density significantly decreases, resulting in a more than two-fold reduction of the threshold power density of optically pumped lasers.

The degradation behavior of ultraviolet-B light emitting diodes (UV-B LEDs) emitting near 310 nm has been investigated and a method to localize the degradation effects is presented. Measurements of the electro-optical characteristics of UV-B LEDs, during a 200 h constant-current degradation study, showed an initial fast decrease in the optical power accompanied by a decrease in the drive voltage and an increase in the capacitance. Furthermore, by using a specially designed contact geometry, it was possible to separate the degradation of the electrical properties of the p-layers and p-contacts from the degradation of the active region and n-side of the LED heterostructure. Our investigations show that the initial changes in capacitance and voltage can be attributed to changes in the p-side and at the p-contact of the LED, which can be explained by an activation of Mg dopants.The degradation behavior of ultraviolet-B light emitting diodes (UV-B LEDs) emitting near 310 nm has been investigated and a method to localize the degradation effects is presented. Measurements of the electro-optical characteristics of UV-B LEDs, during a 200 h constant-current degradation study, showed an initial fast decrease in the optical power accompanied by a decrease in the drive voltage and an increase in the capacitance. Furthermore, by using a specially designed contact geometry, it was possible to separate the degradation of the electrical properties of the p-layers and p-contacts from the degradation of the active region and n-side of the LED heterostructure. Our investigations show that the initial changes in capacitance and voltage can be attributed to changes in the p-side and at the p-contact of the LED, which can be explained by an activation of Mg dopants.

An extensive analysis of the degradation characteristics of AlGaN-based ultraviolet light-emitting diodes emitting around 265 nm is presented. The optical power of LEDs stressed at a constant dc current of 100 mA (current density = 67 A/cm2 and heatsink temperature = 20 °C) decreased to about 58% of its initial value after 250 h of operation. The origin of this degradation effect has been studied using capacitance-voltage and photocurrent spectroscopy measurements conducted before and after aging. The overall device capacitance decreased, which indicates a reduction of the net charges within the space-charge region of the pn-junction during operation. In parallel, the photocurrent at excitation energies between 3.8 eV and 4.5 eV and the photocurrent induced by band-to-band absorption in the quantum barriers at 5.25 eV increased during operation. The latter effect can be explained by a reduction of the donor concentration in the active region of the device. This effect could be attributed to the compensation of donors by the activation or diffusion of acceptors, such as magnesium dopants or group-III vacancies, in the pn-junction space-charge region. The results are consistent with the observed reduction in optical power since deep level acceptors can also act as non-radiative recombination centers.

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.

Recent progress in the development of AlGaN-based materials and high-efficiency ultraviolet light emitting diodes (UV-LEDs) will be reviewed. The impact of UV-LEDs on applications in environmental sensing, life sciences, and medical diagnostics will be illustrated.

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.

AlGaN multiple quantum well (MQW) laser heterostructures for emission around 240 nm have been grown by metalorganic vapor phase epitaxy on epitaxially laterally overgrown AlN/sapphire templates. The edge emitting laser structures showed optically pumped lasing with threshold power densities in the range of 2 MW/cm². The offcut angle of the sapphire substrates as well as the number and the width of the quantum wells were varied while keeping the total thickness of the gain region constant. A larger offcut angle of 0.2° leads to step bunching on the surface as well as Ga accumulation at the steps, but also to an increased inclination of threading dislocations and coalescence boundaries resulting in a reduced dislocation density and thus a reduced laser threshold in comparison to lasers grown on ELO with an offcut of 0.1°. For low losses, samples with fewer QWs exhibited a lower lasing threshold due to a reduced transparency pump power density while for high losses, caused by a higher threading dislocation density, the quadruple quantum well was favorable due to its higher maximum gain.

This paper will discuss the progress towards UV-laser diodes with emission wavelengths

This paper reports a comprehensive analysis of the origin of the electroluminescence (EL) peaks and of the thermal droop in UV-B AlGaN-based LEDs. By carrying out spectral measurements at several temperatures and currents, (i) we extract information on the physical origin of the various spectral bands, and (ii) we develop a novel closed-form model based on the Shockley–Read–Hall theory and on the ABC rate equation that is able to reproduce the experimental data on thermal droop caused by non-radiative recombination through deep levels. In the samples under test, the three EL bands are ascribed to the following processes: band-to-band recombination in the quantum wells (main EL peak), a parasitic intra-bandgap radiative transition in the quantum well barriers, and a second defect-related radiative process in the p-AlGaN superlattice.

The uniformity of emission from deep ultraviolet light emitting diodes (UV LEDs) is investigated. The AlGaN-based heterostructures of the UV LEDs emitting around 235 nm were grown by metalorganic vapor phase epitaxy on epitaxially laterally overgrown AlN/sapphire substrates. The impact of different device designs on the spatial distribution of the electroluminescence for a series of UV LEDs is studied. Due to the relatively high resistivities of n-AlGaN and p-AlGaN layers, ranging from 10 to 0.1 Ω cm as well as specific contact resistances approaching 1 Ω cm2, the emission patterns revealed heavy current crowding at the mesa edges causing a drop of power in the center of the emitting area and an asymmetry towards the side of the bonding pad of the n-contact. Simple analytical models considering the transfer and the current spreading length could only qualitatively explain the observed emission pattern. Using a 3D electro-thermal simulation of the current spreading in the LEDs the experimentally observed emission pattern could also be quantitatively reproduced. Based on these findings the 3D electro-thermal simulation was employed to optimize the contact geometry of the deep UV LEDs in order to achieve a more uniform power distribution.

The formation of three-dimensional truncated pyramids after the deposition of AlN/GaN superlattices onto (0001) AlN/sapphire templates has been analysed by atomic force microscopy as well as transmission electron microscopy. V-pits in AlN layers and the formation of nano-mounds around the v-pit edges are suggested to be responsible for the pyramid formation. Keeping the individual AlN layer thickness at 2.5 nm in the 80xAlN/GaN superlattice, the transformation to the three-dimensional pyramids is observed when the individual GaN layer thickness exceeds 1.5 nm. A subsequent overgrowth of the pyramidal structures by AlGaN results in inhomogeneous Ga distribution in the layers and laterally inhomogeneous strain states. Nevertheless, compared to the growth on planar layers, the overgrowth of the truncated pyramids leads to a slight reduction in dislocation density from 1 1010 cm−2 (for GaN thickness of 1 nm in SL) to 7 109 cm−2 (for GaN thickness of 2 nm in SL). The non-planar growth front and thus the compositional inhomogeneity in AlGaN vanish gradually with increasing AlGaN thickness. As a result, homogeneous 4 μm thick Al0.5Ga0.5N buffer layers suitable for the fabrication of UV-B LED structures can be obtained.

Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency.