We investigate the activation yield and optical properties of the negatively charged nitrogen-vacancy (NV (-)) and silicon-vacancy (SiV (-)) centers in single-crystal diamonds, focusing on the effect of proton (p) and carbon-ion (C) irradiation on their formation. The samples, either nitrogen-rich or silicon-implanted, are grown by chemical vapor deposition or high pressure-high temperature synthesis. They are irradiated over three orders of magnitude in fluence, up to similar to 10(14)C/cm(2) or similar to 10(16) p/cm(2), generating up to similar to 10(4) ppm of extra vacancies at the end-of-range. Following thermal annealing at 1150 degrees C for 1 h, we characterize the samples using time-resolved spectroscopy, optical spectroscopy, and optically detected magnetic resonance. The optical properties of the NV (-) and SiV- centers remain stable even at vacancy concentrations of similar to 10(3)-10(4) ppm. At the same time, the activation yield of substitutional nitrogen and (primarily) interstitial silicon increases significantly with vacancy density, from below 2 % to approximately 15-20 % for both centers. A statistical model of defect dynamics during annealing accounts for these results, showing that the activation yield follows a logarithmic dependence on local vacancy concentration-extending over three decades for NV (-) and two for SiV (-).

Multiply twinned particles, MTPs, are fascinating crystallographic entities with a number of controllable properties originating from their symmetry and cyclic structure. In the focus of our studies are diamond MTPs hosting optically active defects, objects demonstrating a high application potential for emerging optoelectronic and quantum devices. In this work, we discuss the growth mechanisms along with the microstructural and optical properties of the MTPs aggregating high-density of silicon-vacancy complexes on the specific crystal irregularities. It is demonstrated that the silicon impurities incite a rapid growth of MTPs via intensive formation of penetration twins on 100 facets of regular octahedral grains. We also show that the zero-phonon-line emission from the Si color centers embedded in the twin boundaries dominates in photo- and electroluminescence spectra of the MTP-based light-emitting devices defining their steady-state optical properties.

Optik & PhotonikVolume 13, Issue 3 p. 43-45 Materials ProcessingFree Access Laser Repair of Defects in GaN LEDs Excimer laser defect ablation is the optimum method for repairing the two major LED types Ralph Delmdahl, Ralph Delmdahl ralph.delmdahl@coherent.com +49 551 6938 397 | Fax: +49 551 6869 1 Coherent, Product Marketing Manager, Hans-Böckler-Str. 12, 37079 Göttingen GermanySearch for more papers by this authorThorsten Passow, Thorsten Passow thorsten.passow@iaf.fraunhofer.de +49 761 5159 503 | Fax: +49 761 5159 71503 Fraunhofer IAF, Tullastrasse 72, 79108 Freiburg GermanySearch for more papers by this authorMichael Kunzer, Michael Kunzer +49 761 5159 503 | Fax: +49 761 5159 71503 Fraunhofer IAF, Tullastrasse 72, 79108 Freiburg GermanySearch for more papers by this authorMichael Binder, Michael Binder michael.binder@osram-os.com +49 941 850 50 | Fax: +49 941 850 3305 Osram Opto Semiconductors GmbH, Leibnizstr. 4, 93055 Regensburg GermanySearch for more papers by this author Ralph Delmdahl, Ralph Delmdahl ralph.delmdahl@coherent.com +49 551 6938 397 | Fax: +49 551 6869 1 Coherent, Product Marketing Manager, Hans-Böckler-Str. 12, 37079 Göttingen GermanySearch for more papers by this authorThorsten Passow, Thorsten Passow thorsten.passow@iaf.fraunhofer.de +49 761 5159 503 | Fax: +49 761 5159 71503 Fraunhofer IAF, Tullastrasse 72, 79108 Freiburg GermanySearch for more papers by this authorMichael Kunzer, Michael Kunzer +49 761 5159 503 | Fax: +49 761 5159 71503 Fraunhofer IAF, Tullastrasse 72, 79108 Freiburg GermanySearch for more papers by this authorMichael Binder, Michael Binder michael.binder@osram-os.com +49 941 850 50 | Fax: +49 941 850 3305 Osram Opto Semiconductors GmbH, Leibnizstr. 4, 93055 Regensburg GermanySearch for more papers by this author First published: 12 June 2018 https://doi.org/10.1002/opph.201800020Citations: 1 Ralph Delmdahl: is product marketing manager at Coherent in Göttingen. He holds a PhD in laser physics from Braunschweig Technical University and master degrees in economics and business administration from the Open University in Hagen. Thorsten Passow: joined the Fraunhofer Institute for Applied Solid-State Physics in 2007. He is responsible for technology development and device characterisation of III-nitride-based LEDs and photodetectors. He holds a PhD in physics from the University of Bremen. Michael Kunzer: heads the nitride optoelectronics group at the Fraunhofer Institute for Applied Solid-State Physics (IAF) in Freiburg. He holds a PhD in physics from the University of Freiburg and is author or co-author of more than eigthy scientific publications and contributions to international conferences. Further author: Michael Binder (Osram Opto Semiconductors GmbH) AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume13, Issue3Special Issue: Best of ApplicationsJune 2018Pages 43-45 RelatedInformation

In this work an adaptive panel in the visible spectral range is presented. Principal possibilities and basic aspects of adaptive camouflage in the VIS are considered and some details are discussed. The panel consists of modular tiles, each containing several high power four-color-LEDs controlled by a microcontroller and high current power supply and each tile designed to operate autonomously. To control the color and the intensity several color sensors were integrated into the system. The purpose of the panel is to take on a uniform color to best match its appearance to a given reference color, where both the panel and the reference color are subject to the same environmental conditions. The panel was not designed, however, to produce different camouflage patterns. The tiles on the surface were covered by a dark plastic plate in order to provide dark and saturated colors and to guarantee a dark appearance in the passive state of the system. As was to be expected, extreme situations like high ambient brightness and direct solar illumination turned out to be particularly challenging. Substantial tests and some modifications were performed to achieve a satisfactorily uniform color reproduction of a given reference color. Physical measurements as well as observer tests have been performed to demonstrate the capability of the adaptive system.

Defect repair of GaN-based light-emitting diodes (LEDs) by ultraviolet laser micromachining is reported. Percussion and helical drilling in GaN by laser ablation were investigated using 248 nm nanosecond and 355 nm picosecond pulses. The influence of laser ablation including different laser parameters on electrical and optical properties of GaN-based LED chips was evaluated. The results for LEDs on sapphire with transparent conductive oxide p-type contact on top as well as for thin-film LEDs are reported. A reduction of leakage current by up to six orders in magnitude and homogeneous luminance distribution after proper laser defect treatment were achieved.

This work demonstrates the suitability of AlGaN/GaN-on-Si Schottky diodes in a non-isolated buck converter for LED applications. To the authors' knowledge, this is the first time that AlGaN/GaN-on-Si Schottky diodes have been employed as flyback diodes in a non-isolated buck converter. First the diodes are specifically developed and characterized. Afterwards the diodes are tested in an advanced non-isolated buck converter for LED modules. The circuit is characterized and the approach is discussed and compared to state-of-the-art solutions.

Resonant-cavity light-emitting diodes emitting at around 400 nm based on an undoped bottom AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric SiO2/ZrO2 DBR with circular emitting apertures of diameters ranging from 5 to 200 mu m are demonstrated. The current distribution is investigated by luminance distribution imaging and three-dimensional device simulations for different current densities. The current distribution exhibits a maximum in the aperture centre or is homogeneous up to an aperture diameter of 50 mu m independent of the current density. A minimum occurs in the aperture centre for larger diameters increasing with increasing diameter and current density. The current distribution improves with larger n-GaN thickness and higher contact resistance between the transparent In2O3: Sn (ITO) electrode and the p-GaN contact layer. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Translucent, polycrystalline YAG:Ce disks with 0.1, 0.5, and 1 mol% concentrations of cerium as well as different resulting thicknesses of 0.5 and 0.8 mm were prepared by reaction sintering of yttria, alumina, and cerium oxide under vacuum at a temperature of 1800 °C. The obtained samples displayed a microstructure with a Y_3Al_5O_12 main phase interrupted by an intended scattering phase of alumina. Furthermore, the total forward transmittance was between 75 and 78% with the typical absorption bands of cerium at wavelengths of 330 and 460 nm. Phosphor conversion light-emitting diodes (LEDs) with 0.88 W power dissipation have been prepared from these YAG:Ce ceramic disks as converters and blue 455 nm LED chips for excitation. Their color coordinates in the CIE (Commission Internationale de l’Eclairage) diagram determined by spectral photometry depend on the concentration of the Ce dopant and the ceramic converter thickness. The highest luminous flux of 276 lm with an efficiency of 76.6 lm/W was measured for a converter with 0.5 mol% dopant concentration and 0.8 mm thickness emitting a yellowish white light.

Improved aluminum-gallium-nitride (AlxGa1-xN) p-i-n photodetectors with different active regions are reported, designed for the measurement of UV-A (315 to 380 nm), UV-B (280 to 315 nm), and UV-C (< 280 nm) radiation. The spectral responsivity of AlxGa1-xN photodetectors can be tailored by bandgap engineering of the AlxGa1-xN layers and integration of filter layers. Intrinsically visible-blind p-i-n photodetectors are measured on-wafer and packaged in TO-18 headers. Photocurrent measurements in photovoltaic mode result in responsivity values of up to 0.21 A/W for UV-A (EQE = 70%), 0.14 A/W for UV-B (EQE = 56%), and 0.11 A/W for UV-C (EQE = 57%), respectively. The room temperature dark current density values as low as 30 pA/cm(2) at a reverse bias of -3 V yield a specific detectivity of more than 4 x 10(14) cm Hz(0.5)/W. Response time data of the p-i-n photodiodes indicate a rise time of 1.7 ns and a fall time (1/e) of 4.5 ns. Long-term stability tests over 1000 h at an irradiance of 5W/cm(2) demonstrate the potential of these photodetectors for demanding applications such as the continuous monitoring of high irradiance ultraviolet light sources.

Currently available cochlear implants are based on electrical stimulation of the spiral ganglion neurons. Optical stimulation with arrays of micro-sized light-emitting diodes (µLEDs) promises to increase the number of distinguishable frequencies. Here, the development of a flexible GaN-based micro-LED array as an optical cochlear implant is reported for application in a mouse model. The fabrication of 15 µm thin and highly flexible devices is enabled by a laser-based layer transfer process of the GaN-LEDs from sapphire to a polyimide-on-silicon carrier wafer. The fabricated 50 × 50 µm2 LEDs are contacted via conducting paths on both p- and n-sides of the LEDs. Up to three separate channels could be addressed. The probes, composed of a linear array of the said µLEDs bonded to the flexible polyimide substrate, are peeled off the carrier wafer and attached to flexible printed circuit boards. Probes with four µLEDs and a width of 230 µm are successfully implanted in the mouse cochlea both in vitro and in vivo. The LEDs emit 60 µW at 1 mA after peel-off, corresponding to a radiant emittance of 6 mW mm−2.

High-efficiency AlGaN-based 355 nm UV light-emitting diodes (LEDs) grown on low-dislocation-density AlGaN/sapphire templates with an output power of 9.8 mW (22.7 mW) at a DC current of 40 mA (100 mA) are reported. The corresponding maximum external quantum efficiency and maximum power efficiency are 7.2 and 6.5%, respectively. Based on a rate equation model, a method is presented to derive the extraction as well as the injection and internal quantum efficiency as a function of the driving current. The thus obtained injection and internal quantum efficiencies amount to 51 and 47% at 40 mA, the extraction efficiency to 29%.

Charge-states modulation of nitrogen-vacancy (NV) centers incorporated into single crystal diamond films attracts increasing attention for solid-state qubits applications. Here, we discuss the electro- and photoluminescence emission properties of NV centers incorporated by gas phase nitrogen delta-doping of the intrinsic diamond layer of a positive-intrinsic-negative (PIN) junction diode. The experiments show that the charge state of NV centers can be intentionally controlled by applying well-defined external bias voltages. It can be switched from the negatively charged state NV− to the neutral charged state NV0 when a strong forward bias potential is applied. This can be switched back by application of reverse potentials. These results will be discussed assuming basic electronic properties of diamond PIN diodes, including the variation of spectral properties as well as the dynamics of charge state transitions.

The development of a process chain allowing for rapid prototyping of GaN-based light-emitting diodes (LEDs) is presented, which does not rely on photolithography. Structuring of the epitaxial layers is realized by direct-writing laser ablation, allowing a flexible chip layout that can be changed rapidly and at low cost. Besides contact metallization and trench formation, mesa definition is the most critical processing step. For mesa formation and to expose the n-GaN contact layer, the epitaxial grown p-GaN layer together with the active region has to be removed completely without forming cracks or crystal defects in the n-GaN layer or the mesa sidewalls, which would cause sidewall leakage currents. In developing an appropriate laser ablation process that meets these requirements, three different laser systems have been employed in a comparative study. These are a frequency-tripled picosecond (ps) Nd:YVO4 laser emitting at a wavelength of 355 nm and a pulse length of 10 ps and two 20 nanosecond (ns) pulse length laser systems, operating at a wavelength of 248 nm (Excimer laser) and 355 nm (frequency-tripled Nd:YVO4 laser), respectively. First, the laser sources are compared regarding the morphological properties of the resulting laser trenches. Due to band filling effects resulting in optical bleaching of the GaN material when irradiating with ps-laser pulses at 355 nm, the resulting ablation process suffers from cracking. Laser ablation using ns-pulses at both 355 nm and 248 nm leads to crack-free material removal up to a well-defined depth. To keep reverse-bias leakage currents at a level comparable to that of conventional dry-etched mesa-LEDs, subsequent wet etching is essential to remove residues in the mesa-trenches irrespective of the laser source used. Besides wet etching, an additional annealing step has to be applied to mesa-trenches fabricated using ns- and ps-laser pulses at a wavelength of 355 nm. Due to the larger penetration depth at 355 nm, defects causing an increased leakage current are generated in the quantum well region during laser irradiation, which manifest themselves by a spectrally broad defect luminescence. To separate and quantify the contributions from the mesa sidewall leakage and from the areal leakage through the p-n-junction, a series of LEDs have been fabricated and analyzed for which the mesa perimeter length was varied while keeping the mesa area constant. In this way, it is shown that laser ablation with ns-pulses at 248 nm results in the lowest sidewall leakage current as well as forward bias voltage-current and output power-current characteristics very similar to those of conventionally dry etched reference LEDs.

So far, most of the studies on GaN doped with beryllium have mainly concentrated on possible p-type doping. Unfortunately, realization of p-type conductivity in such a way appeared to be very difficult. It seems, however, that bulk crystals doped with beryllium can be used as white light converters in the monolithic white light emitting diodes. To realize monolithic white light emitting diode, we used blue light emitting diodes and a single GaN:Be crystal as converter. High value of the Color Rendering Index gives hope for obtaining an effective light converter based on gallium nitride doped with beryllium.

Eutectic aluminum–germanium wafer bonding was used to fabricate (AlGaIn)N thin-film light-emitting diodes (LEDs). Wafer bonding was carried out on 2″ wafer level at a bond temperature of 470 °C using patterned Al bond pads on the GaN-on-sapphire LED epiwafer and plain Ge substrates. The microstructure of the joint formation was characterized via cross-section analysis using scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy was used to investigate the bond interface. The shear strength was determined to be 1–2 kN/cm2. The formation of a liquid Al–Ge phase is evident from cross-section analysis and optical microscopy. During solidification, Al and Ge are separated into distinct phases again, which is revealed by EDX. The obtained bond is not free of micro-voids, yet it is mechanically stable and suited for the fabrication of thin-film LEDs by removing the sapphire substrate via laser lift-off, which is also demonstrated.

High-efficiency AlGaN-based 355 nm UV light-emitting diodes (LEDs) grown on low-dislocation-density AlGaN/sapphire templates are reported. Flip-chip-mounted mesa LEDs exhibit an output power of 9.8 mW (22.7 mW) at a DC current of 40 mA (100 mA), corresponding to an external quantum efficiency of 7% at 40 mA. Due to the low forward voltage of only 3.8 V at 40 mA, this translates into a power efficiency of 6.5% at 40 mA. These performance data have been achieved under optimized growth conditions for the low-In-content (AlGaIn)N single quantum well and a reduced thickness of the absorbing p-GaN top layer.

Gallium nitride (GaN) layers grown on sapphire substrate wafers have been successfully separated using a novel line beam laser lift-off (LLO) approach. The absorption of the 248nm excimer laser radiation by the GaN through the sapphire wafer results in the formation of metallic gallium and nitrogen gas. The sapphire wafer was easily removable by heating above the Gallium melting point. The metallic Gallium phase has been inspected via diverse microscopic surface analysis techniques after line beam laser lift-off processing. The measurements indicate that the sapphire separation process using line beam laser scanning has only a marginal impact on the structural quality of the GaN layer. The line beam LLO results are compared with conventional square field LLO processing results and are evaluated in view of aptness for mass production of high brightness light emitting diodes (HB-LEDs).

Picosecond (ps) lasers provide a universal tool for material processing. Due to the short pulse length material is removed by a process called "cold ablation", with minimal thermal damage to neighbouring regions. As a result, better defined structures with smother and cleaner side walls can be fabricated than with nanosecond (ns) laser pulses. This offers new possibilities for laser processing in semiconductor technology for both semiconductor materials as well as contact and bond metallizations. The fabrication of optoelectronic devices such as light-emitting diodes (LEDs) typically involves photolithography steps, requiring specific lithography masks be fabricated which, in particular for prototyping, is expensive and time consuming. Therefore it would be attractive for a range of applications to replace these steps by direct writing techniques such as laser processing, which will speed up e. g. the development and prototyping of new devices. We report on fully laser processed planar GaN-based LEDs fabricated without any photolithography steps. On the bare semiconductor wafer, isolation trenches and mesa structures are formed directly by ultraviolet ps laser pulses. For the direct deposition of patterned ohmic contact metallizations, the ps laser fabrication and subsequent use of high resolution shadow masks is presented, which exhibit a significantly reduced sidewall roughness compared to masks produced by ns laser pulses. Due to the higher precision of the laser defined masks it becomes possible to deposit multiple layers, through the use of alignment marks, similar to multiple mask level photolithography. Finally, the ps laser processed LEDs are electrically and optically characterized and their characteristic compared with that of conventionally fabricated mesa LEDs.

Improving the internal quantum efficiency is still a major challenge for realizing efficient III-nitride-based light-emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on the influence of a very low In amount of less than 1% on the luminescence properties of single Ga(In) N/AlGaN quantum well (QW) structures and the performance of AlGaN-based LEDs with an emission wavelength of about 350 nm. The samples were grown by metalorganic vapor phase epitaxy on low defect density AlGaN buffer layers on (0001) sapphire substrates. The temperature dependence of the QW emission energy reveals a clear "S-shape" behaviour indicating a significant carrier localization for In containing QWs. The electroluminescence efficiency of LEDs with a GaInN QW active region could be improved by a factor of 4 compared to LEDs with GaN QW, resulting in a continuous wave (cw) output power of 8.2 mW at 40 mA. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The fabrication of optoelectronic devices such as light-emitting diodes (LEDs) typically involves photolithography steps, requiring specific lithography masks. This approach is expensive, inflexible and time consuming, in particular for prototyping. Therefore it would be attractive to replace these steps by direct writing techniques such as laser processing, which would speed up, for example the development and prototyping of new devices. Picosecond lasers provide a universal tool for material processing. Due to the short pulse length, material is removed by a process called "cold ablation," with minimal thermal damage to neighboring regions. As a result, better-defined structures with smoother and cleaner side walls can be fabricated compared to nanosecond-pulsed laser-based processing. We report on fully laser-processed planar gallium nitride-based LEDs fabricated using only ps laser processing for pattern definition and material removal. On the bare semiconductor wafer, isolation trenches and mesa structures are formed directly by ultraviolet ps laser pulse writing. For the direct deposition of patterned ohmic contact metallizations, the ps laser fabrication and subsequent use of high-resolution shadow masks is presented. Finally, the ps laser-processed LEDs are electrically and optically characterized and their characteristics compared with those of conventionally fabricated mesa LEDs. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.OE.51.11.114301]