This PDF file contains the front matter associated with SPIE Proceedings Volume 11706, including the Title Page, Copyright information and Table of Contents

Welcome and Introduction to SPIE Photonics West OPTO conference 11706: Light-Emitting Devices, Materials, and Applications XXV

The light-emitting layer was formed on the side of the nano-sized rod that had been formed by the nanomold process, and the InGaN/GaN MQWs and barrier layers were also fabricated. Through the design and growth of such an emission structure, it was possible to form a light-emitting layer on the side of the nano-sized rod. The change of the light emission wavelength was measured for each pitch of the nano-sized rods. A change of the emission wavelength with the pitch of the nano-sized rods suggests that two or more emission wavelengths can be realized in a single device. This wavelength variability, based on the arrangement of the nano-sized rods, shows that a white LED can be fabricated that does not require phosphor material to be applied. Based on these results, the semiconductor light source using a nanosized rod structure can be applied to various fields, such as flat or flexible displays and smart lighting.

Controlled growth and increased radiative recombination rates of InGaN quantum dots (QDs) are demonstrated. The InGaN QDs are grown by a self-assembly (SA) method using metal-organic chemical vapor deposition on planar GaN and photoelectrochemically (PEC) etched InGaN QD templates. The PEC QD templates are formed from InGaN layers with a coherent source, which results in controlled QD sizes, and are capped and planarized with AlGaN/GaN layers. The PEC QDs behave as seeds via localizing strain near the QDs, which provide improved control of the SA QD growth. The SA QDs grown on PEC QD templates are smaller and have controlled sizes. Multiple quantum dots, consisting of 4 periods of SA QDs and AlGaN/GaN barrier layers, grown on PEC QD templates have higher radiative recombination rates. The higher rates are a result of the controlled and smaller-sized SA QDs. Forming SA QDs on PEC QDs templates can enhance the performance of InGaN-based QD emitters.

A fluidic assembly of III-nitride micro-light-emitting diodes (μLEDs) is demonstrated by applying external forces. This work introduces a solution-based mass transfer approach for inorganic semiconductor devices with the advantages in being material- and dimensional-agnostic. This method offers precise control in device alignment and orientation. The alignment of μLEDs in solution is controlled by acoustic focusing and the induced pressure forces, and the device orientation is modulated using magnetic field. The detailed mechanisms of the use of acoustic focusing and magnetic field are discussed. This fluidic assembly technique allows high throughput rate and roll-to-roll manufacturing, which is critical to the mass transfer of μLEDs for display and functional fabric applications.

MicroLED arrays are arrangements of small, individually addressable LED pixels, which offer the possibility to obtain any desired illumination pattern with a high spatial and temporal control. In this work, we report on the advanced fabrication of micro- and nanoLED arrays and their system integration including electronics and graphical user interface. The combination of the chip-based LED array, electronics and software is called a MicroLED platform. The light density output, homogeneity and switching behavior of the arrays are investigated. The system allows the modulation of the single light sources with a spatial resolution in the micron range and enables the programming of different light patterns at modulation frequencies up to the MHz range. The fully integrated system with microLED array and electronics is shown during presentation are now available as Structured Micro Illumination Platforms (SMILE) for applications from sensing to microscopy to quantum computing.

The success story of blue light-emitting diodes over the past decade is underpinned by the high internal quantum efficiency (IQE) of the InGaN/GaN quantum wells (QWs) within the active region. Yet this extraordinary IQE can only be achieved by adding an indium-containing layer before the QWs—the “underlayer” (UL). In this work, we evidence that any indium-containing III-nitride layer acts to incorporate surface defects; the UL hence acts as a getter for surface defects to avoid their incorporation into the QWs as IQE-reducing point defects (PDs). We directly observe the reduction in PD-density caused by increasing the UL thickness.

We will discuss recent insights in the efficiency of III-Nitride emitters, and consequences for their use in novel applications, including micro-displays.

The reduction in efficiency by shrinking device dimensions of micro-light-emitting diodes (μLEDs) has been identified as one of the main drawbacks in the literature. The decrease in efficiency is attributed to surface recombination and sidewall damage due to dry etching, where the efficiency drop is more severe in the AlGaInP material system because of the higher surface recombination velocity and the greater minority carrier diffusion length, compared to the III -nitride system. In this work, the device performance with and without dielectric sidewall passivation using plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD) are first compared, since PECVD is the common method for dielectric sidewall passivation. It is shown that ALD is more effective in terms of suppressing leakage current and enhancing light output power. Moreover, the efficiency of devices with ALD sidewall passivation is partially recovered, indicating that the efficiency drop in μLEDs can be lessened or mitigated by post-etch fabrication techniques. To further improve the efficiency characteristic, the combination of chemical treatments and ALD sidewall passivation are employed to demonstrate size-independent efficiency performance of III-nitride and AlGaInP μLEDs.

InGaN based blue and green LEDs are quite attractive owing to their high efficiency, low power consumption, and high performance even at scaled dimensions. However, for red emission higher indium content in the QWs leads to poor performance due to increased strain in the structure. We will discuss a novel elastic strain relaxation technique using porous GaN pseudo-substrates which will allow enhanced indium uptake in the quantum wells for high-efficiency red InGaN micro-LEDs outperforming AlInGaP based micro-LEDs. Results including green and orange luminescence from micro-LEDs (<5 µm) fabricated on these pseudo-substrates will be discussed.

Micro-displays with pixel pitch smaller than 10 µm are required for augmented reality. The three primary colors can then be achieved with the same material: the InxGa1-xN alloy. However, when strained on GaN, its InN mole fraction x is limited to 25%. By using a full InGaN structure grown on a relaxed InGaN pseudo-substrate, such as the InGaNOS substrate from Soitec, the In incorporation rate is enhanced. An internal quantum efficiency higher than 10% at 640 nm an In content higher than 25% in InxGa1-xN/InyGa1-yN quantum wells will be shown. 10x10 µm² red micro-light emitting diodes will be also presented.

Multi-color red, green, and blue (RGB) micro-light emitting diode (LED) displays often require the use different material systems to obtain high-efficiency RGB pixels. While blue and green micro-LEDs can be fabricated with sufficiently high external quantum efficiencies (ηEQE) using the InGaN materials system, red InGaN emitters currently exhibit much lower ηEQE. The reduction of InGaN LED efficiency at longer wavelengths can be attributed to the Quantum Confined Stark Effect (QCSE), which reduces the electron-hole wavefunction overlap (Γe_hh) and radiative recombination rate (𝑅_𝑠𝑝), and worsens at longer wavelengths with increasing QW In-content. Consequently, higher efficiency AlInGaP LEDs are usually used for the red pixels in micro-LEDs, complicating the fabrication process. In this work, InGaN-delta-InN quantum well (QW) LEDs with InGaN quantum barriers on InGaN substrates are shown to produce significant enhancement in electronhole wavefunction overlap and 𝑅_𝑠𝑝 over the entire red emission regime and into the near-infrared. Analysis and comparison of various InGaN-delta-InN QWs with InGaN barriers and InGaN/InGaN QWs emitting at 630 nm was performed using self-consistent six-band 𝑘 ∙ 𝑝 formalism. Wavefunction overlap from InGaN-delta-InN QWs was shown to increase by 3.5x when compared to an InGaN/InGaN QW at 630 nm. These improvements in wavefunction overlap were shown to lead to ~5 - 7x and ~3 - 10x enhancements in 𝑅_𝑠𝑝 and IQE, respectively. With growth of InN monolayers on InGaN now readily achievable, this novel delta-InN active region on InGaN substrate design could pave the way for high efficiency, native red-emitting InGaN LEDs and allow for monolithic fabrication of InGaN micro-LED displays.

The physics of the bottom tunnel junction (BTJ) and its improvement over standard p-up geometry in InGaN blue LEDs is quantified through pulsed power measurements. It is found that the peak external quantum efficiency (EQE) and wall-plug efficiency (WPE) for a p-down BTJ LED is about threefold that of its counterpart, the p-up top tunnel junction (TTJ) LED. This is contributed to increased radiative recombination and reduced electron overflow. Further, the peaks occur at lower current densities for the BTJ device, suggesting earlier saturation of Shockley-Read-Hall traps. In the droop regime, where electron overflow, device heating, and 3-particle interactions are significant, the performance of the BTJ is found to be consistently better than that of the TTJ, converging at large current densities where the polarization fields are screened.

We demonstrated an enhancement of light-extraction efficiency (LEE) in an AlGaN UVC light-emitting diode (LED) using photonic-crystal (PhC)-reflector fabricated on a p-GaN contact layer, which was introduced to realize both of low resistivity and high-reflectivity in p-contact layer. We fabricated an AlGaN UVC-LED with PhC-reflector on p-GaN contact layer, and confirmed that the external quantum efficiency (EQE) was increased by 1.7 times by introducing PhC-reflector. We also fabricated flip-chip UVC LED with PhC-reflector on p-contact layer and obtained more than 50 mW output power.

UV LEDs are promising. While germicidal ultraviolet irradiation (wavelength range 240-280 nm) is an effective disinfectant but hazardous, far UV-C radiation (200-225 nm) is claimed to be both effective for disinfection and safe for human skin and eyes under direct exposure. However, deep UV AlGaN-based LEDs are currently too costly for widespread use as they suffer from poor efficiency and low light output power performance, especially for the far UV regime. When lowering the wavelength by increasing the Al content, the in-plane intensity of transverse-electric (TE) polarized light decreases relative to that of the transverse-magnetic (TM) polarized light and thus reduces the light extraction efficiency. We present our recent progress towards developing a self-consistent multiband k.p Schrödinger-Poisson-Current solver for optimization of the epitaxial layout. It considers the strain-induced effects on the valence band structure and on the related optical transition matrix elements.

Planar deep-ultraviolet (DUV) light emitting diodes (LEDs) suffer from extremely low external quantum efficiencies (EQEs) due to poor light extraction efficiencies (LEE) which are often less than 1%, hindering their widespread use. In AlGaN DUV LEDs with high Al-content, the positioning of the valence subbands leads to dominant transverse magnetic (TM)-polarized emission which is difficult to extract from planar devices. To improve the LEE of DUV LEDs, techniques such as surface roughening and nanowire formation have been used. Nanowires are especially promising for DUV LEDs because they allow for very efficient extraction of TM-polarized light through their sidewalls. In this work, we demonstrate a novel “inverse taper” profile in AlGaN nanowires, in which the base of the nanowire can be narrowed to have a smaller diameter than the top through a KOH-based wet etch process. Hydroxyl-based chemistries are known to have a lower etch rate against the c-plane of wurtzite AlGaN alloys. Here, we report on observations of 0.8% KOH at 80℃ exhibiting a unique selectivity to a different wurtzite crystal plane, believed to be the (202̅ 1) plane, allowing for formation of an inverse taper structure. Finite difference time domain (FDTD) simulations at 280 nm reveal that AlGaN nanowire LEDs with high sidewall inverse taper angles can have greater than 75% and 90% LEE for TE and TM-polarized light respectively, ~2.5x higher than the LEE of vertical sidewall nanowires. This novel phenomenon may allow for significant improvements in the LEE of DUV nanowire LEDs.

Deep ultraviolet (UV) light emitting diodes (LEDs) have a wide range of applications such as water treatment, medical diagnostics, sterilization of medical devices and gas measurement. Depending on the desired application, the emission characteristics of UV light must be optimized. In addition to improving the external quantum efficiency, the radiation properties of the UV LED chip can be influenced and supported by the performance of the housing and the assembly technology. We will present some results on how to improve the optical performance of UV LED chips through optically UV-transparent materials, which are complemented by some basic aging studies. Furthermore, the influence of differently shaped lenses in combination with and without an integrated aluminum reflector is presented. We want to show the influence of the different optical and thermal elements individually and in combination to show the possibilities to design the optimal radiator module with the desired properties.

The usage of UV-LEDs is growing for a variety of applications such as the disinfection of water, the sterilization of medical equipment and the industrial UV curing. It is important that the wavelength and the intensity of radiation needed for the specific application is reliably provided and covers the whole target area evenly distributed. The individual high precision power supply and control of UV-LEDs is the main application of the eyIC4UV-01 chip. The eyIC4UV-01 is an integrated circuit that can measure the light intensity, temperature, current and forward voltage of a connected UV-LED.

Through development effort of MOCVD for LED application, we attempt to find new application towards post COVID-19. UVLED has feature of small size and high speed response which is different from mercury lamps while output power and/or wall plug efficiency is not matured. Development status of MOCVD and its process for UVLED process is presented. Ex-situ high temperature furnace for AlN template exhibit excellent crystalline quality. Ex-situ dry cleaning tool is utilized for cleaning MOCVD components without Cl contamination.

The development of high-power and reliable deep-UV optical devices requires low dislocation density AlN substrates. In this study, thick AlN layers were homoepitaxially grown on PVT-AlN(0001) substrates by HVPE method at a growth rate above 100 μm/h. The grown layer showed low dislocation densities of less than 1E4 cm^-2 and a high deep-UV optical transparency. Recent expansions in both diameter of the substrate and size of the growth reactor have enabled mass-production of 2-inch-diameter HVPE-AlN substrates for deep-UV optical devices. This work was partially supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan.

Visible wavelength data communication is of interest for short distance chip-to-chip interconnects and free-space links such as Li-Fi, where modulated sources are incorporated in lighting systems. For the former, both high modulation bandwidth and low power consumption are critical. At Avicena, we have developed efficient high-speed light-emitting structures capable of multi-Gb/s NRZ modulation, which operate down to a few microamps of drive current. We have demonstrated a high speed and low energy optical communication links using these novel devices.

This presentation describes recent activities on ultra-high speed Optical Wireless Communications (OWC) using Gallium-Nitride micro-LEDs designed and fabricated at CEA-Leti. Micro-LEDs are one of the most promising OWC optical sources due to their high illumination efficiency and their large modulation bandwidths. Pre-liminary work focused on the implementation of a 10-μm single blue micro-LED on sapphire wafer within an experimental OWC setup, mixing software generation of direct-current optical orthogonal frequency division multiplexing (DCO-OFDM) patterns and hardware optical components for light collection, high speed photo-detection and digital acquisitions. Intensity modulation conveys DCO-OFDM waveform and direct detection is used at reception. A high current density of 25.5 kA/cm² provided a modulation bandwidth of 1.8 GHz. Associated to bit and power loading with up to a 256-QAM subcarrier modulation, it enabled a new data rate of 7.7 Gb/s, compared to the previous record of 5.37 Gb/s reached with a blue 21-μm microLED in 2016. Towards a better understanding of the micro-LED design impact on OWC performance, next investigations will study the electrical modelling of such micro-LEDs in the high frequency regime. Future works will cover the use of large arrays of more than 10 thousands micro-LEDs. The first objective is to open the way to new digital-to-optical modulations by independently driving each pixel, to remove digital-to-analogue converter and target highly integrated system-on-chips for ultra-high speed OWC transmitters. Secondly, higher emitted optical power is expected to open such technology to indoor multiple access applications where light collection and emitter-receiver alignment may not be possible anymore.

To support people’s wayfinding activities in unfamiliar indoor environments, a method able to generate ceiling landmark route instructions using Visible Light Communication (VLC) is proposed. The system is composed of several transmitters (ceiling luminaries) which send the map information and path messages required to wayfinding. Mobile optical receivers, using joint transmission, extracts theirs location to perform positioning and, concomitantly, the transmitted data from each transmitter. Bidirectional communication between the emitters and the receivers is available in strategic optical access point. Typical scenarios are simulated and include finding places and guiding users across different floors. Data from the sender is encoded, modulated and converted into light signals emitted by the transmitters. Tetra-chromatic white sources are used, providing a different data channel for each chip. At the receiver, the modulated light signal, containing the ID and the 3D geographical position of the transmitter and wayfinding information, is received by SiC photodetectors with light filtering and demultiplexing properties. Since lighting and wireless data communication is combined, each luminaire for downlink transmission become a single cell, in which the optical access point (AP) is located. The coded light signals are interpreted directly by the receivers of the users positioned underneath. The effect of the location of the APs is evaluated and a model for the different cellular networks is analyzed. Orthogonal and hexagonal topologies are tested, and a 3D localization design, demonstrated by a prototype is presented. Uplink transmission is implemented and the 3D best route to navigate through venue calculated. The results show that the system make possible to determine the position of a mobile target inside the network, to infer the travel direction along the time and to interact with information received and optimize the route towards a static or dynamic destination.

We investigated the behavior of a Raman silicon laser based on a photonic-crystal nanocavity while it was irradiated with electrostatic charges from a static-electricity generator. Laser oscillation stopped after a few seconds of a weak irradiation and the time needed for the recovery of laser oscillation increased with irradiation time. We measured the resonance spectra of the nanocavity modes during irradiation. It indicates that such a lasing interruption can be caused by wavelength shifts of the nanocavity modes and absorption losses due to the electrostatic charges on the nanocavity surface. These results are useful for the sensing of electrostatic charges.

In recent years there has been a rapidly increasing demand for CO2 sensors for applications in health monitoring, control of air quality and horticulture. Amongst the various approaches reported so far, the AlGaInSb quaternary alloy shows great promise for the development of compact Light Emitting Diodes (LEDs) as it offers bandgap Type-I alignments, which enable the design of effective multi-quantum well (MQW) active regions. In this paper we show a more than fourfold improvement in wall-plug efficiency by optimising both the strain in AlGaInSb MQW active regions and the fabrication process flow of LEDs emitting at 4.26 um.

Mid-infrared, interband-cascade, light-emitting devices (ICLEDs) have the potential to improve the performance of trace-gas sensors for air quality and greenhouse gas measurements. ICLEDs are broadband, incoherent, high-optical-power devices (up to ~5 mW continuous-wave at room temperature). We present an ICLED-based, methane sensor using a hollow-core fiber and direct absorption spectroscopy. A 1σ noise equivalent absorption of 0.17 ppmv CH4 at 1 Hz was achieved (1.13e-5 absorbance). Sub-ppmv methane detection is relevant for monitoring emissions near sources such as petrochemical infrastructure, agricultural activities, and wastewater treatment plants.

iBeam Materials is developing a revolutionary new technology to make paper-thin and ultra-flexible monolithic microLED displays. This new technology is based on using ion-beam crystal alignment to fabricate epitaxial GaN films on flexible metal foil. This approach will eliminate the traditional restrictions for the form factor of displays, redefining what devices look like and opening up new classes of mobile, wearable, lighting, signage and instrumentation products. These products will be enabled by the ability to make flexible sheets of LEDs at low cost. The sheets of LEDs, produced by roll-to-roll manufacturing, will allow large-area monolithic integration of LEDs and active transistor devices. MicroLEDs made on robust sheets of thin metal foil will in this way deliver the super-bright and power efficient displays of the future.

Beta-gallium oxide (β-Ga2O3) is an emerging ultra-wide bandgap (4.8 eV) [1] semiconductor material. The high band gap leads to a theoretical breakdown field strength of 8 MV/cm [2], which enables the potential for β-Ga2O3 to outperform SiC and GaN and to become the next generation of high-performance material with respect to power electronic applications. Since transistors based on β-Ga2O3 benefit from a low on-resistance at a given breakdown voltage less power losses within a transistor switching operation can be achieved [3]. Thereby the electrically active part of the device is made up of a homoepitaxial thin layer. To fulfil the requirements from the device side, the epitaxial β-Ga2O3 layer has to have high crystalline perfections. To achieve such high quality layers was the aim of this work. β-Ga2O3 thin layer were grown homoepitaxially by MOVPE [4, 5] on Mg-doped β-Ga2O3 (100) substrates [6] with 2°, 4° and 6° miscut [7]. A development of the growth process with respect to growth modes and growth rate has been done. The Investigations were focusing on the interplay of the technical parameters i.e.: chamber pressure, Ar- push gas, precursor flux and their ratios. A transition from 2D island to step flow to step bunching growth modes has been achieved by varying the chamber pressure and/or the Ga/O2 ratio. This work shows that an adjusting of the effective diffusion length to the substrate step width is crucial to obtain step flow growth and with that high quality β-Ga2O3 (100) layers with electron mobilities above 150 cm²/Vs.

As a potential material for white light source, high luminous efficiency of fluorescent SiC requires a concentration of higher than 1.0×10¹⁹ cm^-3 for both the donor (Nd) and acceptor (Na) and a film thickness of about 120 μm. In this study, we focus on the epitaxial growth of fluorescent 4H-SiC by closed sublimation growth in terms of different growth temperatures to increase the doping concentration of boron and nitrogen. In addition, the effect of growth pressure on the surface morphology was investigated. The photoluminescence (PL) intensity of the sample grown at 1900 °C is higher than that grown at 1800 °C. This result indicates that a higher temperature of 1900 °C can increase the boron (B) and nitrogen (N) doping concentration. However, it was confirmed that the surface of the sample grown at 1900 °C was rough. The surface morphology was significantly improved with an increased pressure to 9000 Pa. Meanwhile, the growth time was extended to increase the film thickness, and the amount of BN, which is the doping source of B acceptor, was increased accordingly. As a result, strong PL emission intensity with high internal quantum efficiency (IQE) was successfully demonstrated in 4H-SiC sample, which is comparable to that in fluorescent 6H-SiC.

We report on the fabrication, structure and characteristic of porous fluorescent SiC using voltage-controlled anodic oxidation method. In this experiment, the fluorescent SiC substrates were immersed in hydrofluoric acid for anodic oxidation etching. The porous structures changed significantly as the voltage increased to 10 V, 20 V and 30 V. Under a high voltage, it is considered that a large amount of electrons were injected into the fluorescent SiC and increased the anodic reaction rate. In the photoluminescence (PL) spectra, a emission peak in the short wavelength region was observed. And these emission intensities increase as the voltage increase.

Recently, quality of free-standing gallium nitride (GaN) substrate has been improved. For example, single crystal GaN substrate with threading dislocation density (TDD) of 104 cm-2 is available. Meanwhile, there is a progress in observation method of dislocation in GaN. In addition to conventional etch pits method, X-ray topography, and CL observation, recently, multiphoton PL microscope which can observe dislocation three-dimensionally is actively being carried out. However, these methods are not simple observation because these are destructive observation or require a lot of labor and time. In this study, we attempted crystal evaluation of GaN with a newly developed birefringence microscope, which can be used with the same convenience as the conventional optical microscope. GaN is a material having birefringence, so that strain around dislocation can be seen with birefringence microscope as retardation[1]. Therefore, we tried observing commercially available GaN substrate with TDD of 106, 105, 104 cm-2. As results, an useless random contrast image was obtained with the substrate with TDD of 106 cm-2. With the substrate with TDD of 105 cm-2, distinctive contrasts that seem to coincide with the dislocation cores were obtained, but it was an unclear image. On the other hand, with the substrate with TDD of 104 cm-2, we could obtain contrasts that seems to be independent dislocation cores and surrounding strain field. It seems that the strain field of each dislocation overlapped because the distance between dislocations was too close in substrate with TDD of 106 and 105 cm-2. For the substrate with TDD of 104 cm-2, comparison with the X-ray topography image taken at BL8S2 of Aichi SR was carried out, and it was confirmed that the dislocation was exist at the position of the contrast in the image taken by birefringence microscope. However, we also found that there is no contrast at some dislocation positions. This is considered to be caused by that no strain occurs in the planar direction when the threading dislocation is pure screw dislocation. In some of the dislocations, butterfly type contrast which is theoretically expected to appear around edge dislocation was observed. We consider that this novel birefringent method is very useful method to discriminate edge component and its direction of Burgers vector, and also to discriminate screw dislocation in combination with other methods.

To enable LED technology we developed the MOCVD technology for III-As/P/N based structures in reactors ranging from small scale to large production scale Planetary Reactors. The closed coupled showerhead reactor series (CCS reactor) and the Planetary Reactor® developed in parallel and both gave production proven results. The focus of epitaxy production technology developments has been put on improved process control, productivity enhancements, in-situ metrology for smart system control and predictive maintenance. A comprehensive summary and recent progress will be reported here. The application of the underlying physical, technical and chemical fundamentals explain the engineering concept.

Optical semiconductor devices such as light emitting diodes (LED) and semiconductor lasers are widely used today in a constantly growing variety of applications. Epitaxial growth by MOCVD is one of the first and even most complex manufacturing step in the production of these devices. Tightening industry requirements in terms of cost reduction and yield improvement have led to an increased usage of in-situ metrology for advanced process control in MOCVD. Consequently, optical in-situ metrology has become an integral part of process control in the production of optical semiconductor devices. In this talk we will present recent improvements of the optical in-situ metrology equipment. This will include real wafer temperature measurements, UV-based reflectometry and full spectroscopic reflectance measurements, which are perfectly suited for studying complex heterostructures. We will also show examples for close-loop control concepts that provide direct benefit to manufacturing.

With the aim of identifying and quantifying the principal sources of performance improvements and cost reductions in white light emitting diode manufacturing, we collect historical data on device cost and performance, technological breakthroughs and manufacturing innovation for phosphor-converted white light-emitting diodes for the past 20 years. We find that technological breakthroughs and process innovation contributed to performance improvements across the entire range of LED device sub-efficiencies, resulting in the overall increase in the lamp efficiency for the highest performing devices at a test current of 350mA from 5.8% to 38.7% between 2002 and 2020. We further develop a bottom-up manufacturing cost model with process-step resolution that captures improvements in throughput, yield and related costs of all relevant manufacturing steps, as well as economies of scale, to analyse progress in LED manufacturing cost structure between early manufacturing in 2002 and mature industry in 2020. We estimate that the cost of manufacturing low-power and mid-power light-emitting diode packages at a US location using state-of-the-art equipment has dropped from 1.11$(2020) in 2002 to 0.05$(2020) in 2020, a 95.5% decrease. We also find that the largest contribution to overall cost reduction has come from increase in wafer size, and that in 2020, LED chip packaging is the largest contributor to cost at 60%.

Veeco MOCVD solutions are capable of supporting multiple substrates (GaAs, InP, sapphire, Si), and offer seamless transition to larger substrate sizes. For 6” GaAs red micro LED, Lumina® has demonstrated total population wavelength yield of >95% in 3 nm bin with defectivity <0.5 / cm2 @ >2um and 25% higher throughput than other platforms. For 6” sapphire miniLED, EPIK® has demonstrated within wafer wavelength 1sigma uniformity of 0.68nm (blue) / 1.24nm (green). For microLED on 200mm and 300mm silicon, Veeco has developed Propel® single wafer reactor for best in-class uniformity. Details of the technology and current data will be discussed.

Quantum dots (QDs) are ideal light emitters for display applications and currently employed as color converters in large-panel LCD displays with backlights. However, quantum dot light-emitting diodes (QD-LEDs) are considered as next generation display devices that can fully harness excellent optical properties of QDs. Through a few decades of research, there has been much progress in QD-LEDs for Cd-based QDs, but the development of environmentally friendly QD-LEDs has remained a challenge. Here, we present progress in the development of Cd-free QD-LEDs based on InP and ZnSe QDs.

We investigated the effect of the partial replacement of Pb2+ with Mg2+ in CsPbX3 (X-halides) on photophysical properties and their potential applications in light-emitting diodes (LEDs). The new composites exhibited a slight variation in the lattice dimension without altering their structure and morphology. However, the replacement of Pb2+ with Mg2+ imparted enhancement in photoluminescence and bandgap energies with maintaining the high luminous property. As a result, the application of these nanocrystals as the color conversion layers (3D printing-based design) in the fabrication of white LEDs generated white light with excellent color characteristics.

High luminance light modules have been developed during the past few years based on luminescent concentrators. By using wavelength conversion, the etendue limitation that prohibits brightness increase of light that has been generated is removed, which is the principle of e.g. solar luminescent concentrators and the High Lumen Density (HLD) light engines developed for projection systems. While high-power LEDs are generally limited to a brightness of ca 200 Mnit (2 x 108 lm/m2sr), LED-pumped HLD modules have demonstrated brightness values of more than 1 Gnit. With these light sources requirements can be met for high flux applications with limited source size that are out of reach of LEDs. Such cases are found in e.g. stage and entertainment lighting or in front projection. So far, the luminescent concentrator light sources were based on single-crystalline (SC) converters. In this paper we report on the development of HLD light engines based on LED-pumped polycrystalline (PC) luminescent concentrators and simplified light source architectures. We demonstrate that with LuAG:Ce and LuYAG:Ce PC luminescent concentrators, emitting in the green-yellow spectral range, identical emission spectra and module performance characteristics can be achieved as with SC luminescent concentrators. This is successfully combined with improvements that were made with respect to HLD luminance and module efficacy. Key challenges for the PC luminescent converters are found in the minimization of light scattering. We conclude that with HLD modules based on PC concentrators, over 15 klm @ <70 lm/W can be delivered with a source brightness well over 1 Gnit. By design, the preferred trade-offs can be made between efficiency, luminance, luminous flux, module size, and cost, by which flux values over 20 klm are feasible as well. Thanks to this simplified concept, further optimization for specific applications is enabled, including the application of more temperature-sensitive converter materials for e.g. longer wavelengths.

Laser pumped phosphor (LPP) light sources overcome the luminance limits of LEDs and enable luminance levels > 1000 cd/mm² with solid state lighting (SSL). While laser pumped phosphor wheels are already widely used in digital projectors, static luminescent converters accelerate the transition to LPP for other etendue limited lighting applications as well, as they enable compact and reliable light engines. We present performance data of static converters under different irradiance and cooling conditions. Supported by numerical simulation, we derive comprehensive design rules for the most efficient use of static ceramic converter assemblies with respect to typical requirement profiles of light engines.

SiO₂,MgO-codoped Al₂O₃−Ce:YAG composite ceramics with different Ce³⁺ doping concentrations (0.05-0.3 at%) have been fabricated via reactive vacuum sintering, for the purpose of high power white light-emitting diodes (WLEDs). Optical and luminescence characteristics of color converters were investigated by transmittance spectra and photoluminescence examinations. It is found that WLEDs based on synthesized phosphors with 0.05-0.1 at% Ce³⁺ doping have an optimal balance between high color-rendering index, luminous efficacy and appropriate correlated color temperature values.

Single crystal phosphor, which is a transparent material, absorbs blue light and emits yellow light. As a result, it is the ideal material for conversion of blue laser light into yellow visible light allowing high efficiency, and high temperature operations. When fabricated into a waveguide, it provides a large volume and surface area for ease of heat sinking. The transparent property of the crystal phosphor material provides features are not available in other phosphor materials such as glass phosphor and ceramic phosphor in which the laser energy is all concentrated in a small spot with small thickness making heat removal very challenging. This paper presents an end-pumped structured crystal phosphor waveguide design such that the volume of laser absorption and heat dissipating surfaces are both larger with a small output cross-section for small etendue operations. Preliminary prototypes have been fabricated and tested with expected results. Several other structures and measurements will be presented.

Zero – dimensional (0D) hybrid organic – inorganic lead halides are receiving global attention because of their outstanding photophysical properties, tunable light emission properties, and relatively high photoluminescence quantum yields (PLQY). Despite these advances, the innate toxicity of lead and poor stability are obstacles for their large-scale commercialization. Here, we report a new, lead-free hybrid organic inorganic compound (C18H22N)InBr4. In this study, the organic precursor salt (C18H22NBr) was studied and compared to the hybrid compound (C18H22N)InBr4. We see a two-fold increase in the PLQY, and improved stability in the hybrid compound compared to the organic salt on its own.

Circadian metrics quantify the non-image forming effects of light. Circadian metrics are based on the spectral power distribution (SPD) and intensity of a light source and human visual and non-image forming response to optical radiation. However, circadian metrics can only estimate static SPDs. Multi-primary LED systems generate a vast number of SPD combinations. A single metric cannot characterize the metric variation enabled by tunable solid-state lighting devices. There are no known metrics to quantify the capabilities of tunable lighting systems in terms of circadian metrics. A three-dimensional metric is proposed to quantify the variability in circadian metrics. The dimensions of the proposed circadian metric variability (CMV) are (a) a circadian metric, (b) energy efficiency metric, and (c) a lighting quality metric. Sample CMV measures are provided by generating SPDs in a seven-channel tunable lighting system at 20% dimming steps. Resulting in 279,936 test SPD combinations are filtered for nominal “whiteness” according to ANSI C78.377-2017 specifications. Each dimension of CMV is discretized by rounding the metric to significant figures. Each discretized unit of the CMV space can be considered significant or detectable different. The total volume of CMV represents the number of unique SPDs that can be created within a multi-primary LED system. A graphical representation of CMV shows the interrelation between different measures. CMV quantifies variation at the product level rather than in-situ variability of biological stimuli.

How to accelerate the transition to Industry4.E [1] by reducing development time and cost while maintaining product quality? The Delphi4LED European project ECSEL-JU grant #692465 developed a state-of-the-art methodology to establish a standardized approach for generating and integrating multi-domain LED digital twins from measurement data. An LED multi-domain compact model (MDCM), also called digital twin (DT), simulates the working space of an LED with the advantage that it does not include LED vendor proprietary information. This allows LED vendors to share them with end-users without disclosing their sensitive intellectual property. Major steps consist of 1) characterizing the physical LED in a standardized way that combines thermal/electrical/optical tests in accordance with combined testing standards (JESD 51-51, JESD 51-52, JESD 51-14, CIE 225:2017); 2) reporting the characterized sets of data (Current- Voltage-luminous Flux (IVL) and thermal impedance characteristics) in a standardized way; 3) generating the MDCM LED from the set of data; 4) coupling the MDCM LED to a 3D luminaire CFD (computational fluid dynamics) model. The LED package MDCM consists of 1) a boundary condition-independent compact thermal model (CTM) of the physical LED package and 2) a multi-domain opto-electrical model that describes the LED operation close to the chip level. The former can be created with the help of measured transient thermal impedance, Zth(t), of the LED package, while the parameters of a opto-electrical model can be identified from the isothermal IVL characteristics of the LED device. The paper presents the state-of-the-art LED digital twin creation approach, the importance of standardization and the latest results in terms of accuracy and calculation time gains for a LED luminaire level simulation.

Luminaire luminance uniformity is an important aspect that can affect perceived lighting quality, discomfort glare, and efficacy. While several metrics have been proposed to characterize luminance uniformity, previous studies have shown that current metrics such as Max:Min or Avg:Min luminance ratios can be insensitive to important differences in luminance gradient that may affect perceived uniformity. In an attempt to resolve this issue, previous studies incorporated a contrast sensitivity function for the human eye based on spatial frequency, such as in the recently proposed UHVS metric; however, this metric has not been comprehensively studied in relation to perceived uniformity ratings. The study presented in this paper aimed to examine the relationship between UHVS and perceived uniformity ratings. Specifically, simulated luminance patterns were presented, and participants were asked to assess uniformity using a twoalternative forced-choice procedure. The results of 94 participants’ evaluations showed a significant correlation between UHVS and perceived uniformity. However, comparisons between patterns that had similar UHVS sometimes resulted in statistically different ratings and comparisons between patterns that had larger differences in UHVS sometimes did not result in a statistically significant difference in ratings. These results suggest that UHVS might be used for general guidance but may warrant further studies to better understand its sensitivity and improve its alignment with perceived uniformity ratings.

Due to their unique properties, III-nitride vertical -cavity surface-emitting lasers (VCSELs) are expected to enable several applications ranging across communication, sensing and ranging and spectroscopy. Their technological feasibility however has been hindered by the lack of an electrically conductive, easily manufacturable, wide reflection stopband distributed Bragg reflector (DBR) to serve as the bottom reflector. Here we present the first electrically injected III-nitride VCSEL on an electrically conductive DBR using nanoporous (NP) GaN operating at roomtemperature. The measured threshold current density and the maximum light output power were 42 kA/cm² and 0.17 mW, respectively, at 434 nm. Due to the electrical conductivity of the bottom NP GaN DBR, vertical injection was demonstrated and compared with intracavity injection. No change in the threshold current density, the slope efficiency or nearfield pattern was observed demonstrating the feasibility of efficient vertical injection in NP GaN VCSELs. Filamentary lasing was observed within the VCSEL aperture and its correlation with the lasing linewidth was studied.

Current VCSEL packages designs are shrinking further, while simultaneously adding more features into the package. Moreover, to fulfill the steadily rising requirements UV curable materials with additional curing mechanism offer both high reliability and cost-efficient mass production. In parallel these functional materials enable engineers to optimize current packages due to the versatility of dispensing and curing. For example, high units per hour necessitates UV curing, whereas shadow zones require heat, humidity or anaerobe curing solely or additionally to UV fixation. Local optical coatings, dam stacking with high aspect ratio, reflow stable lens bonding, optical bandpass filters for different wavelength are feasible today and even more, supporting the next generation of VCSEL packages in both automotive and consumer applications.

In this study we present the novel approach of GaN-based high-contrast grating (HCG) mirrors as highly reflective top mirrors in GaN vertical-(external)-cavity surface-emitting lasers [V(E)CSELs]. These mirrors can well be integrated into a conventional process flow for GaN-based VECSEL devices – in contrast to conventional Bragg mirrors. Results of thorough growth experiments performed to reliably fabricate high quality cavity structures with well-defined and fine-tuned optical properties will be reported. The properties of HCG mirrors have been calculated by extensive optical device simulations. Properties of HCG structures have been analyzed experimentally by reflection measurements and results will be discussed with a focus on HCG design, potential material combinations, compatibility with existing process steps in GaN technology and the various ways of integration into VECSEL structures.

The spectral power distribution of light sources can be optimized to achieve optimal target goals, such as energy efficiency, color quality, and circadian synchronization. Although several researchers investigated the optimization of light sources for circadian and color rendition metrics, the complex relationship and trade-offs between three distinct parameters (circadian, color rendition, and energy efficiency metrics) have not been widely investigated. Here, multilevel analyses of four clusters of metrics (radiometric, photometric, colorimetric, and circadian) have been conducted. Metric correlation analysis was conducted for 308 theoretical and commercially available light sources, and results suggest that melanopic irradiance correlates highly with irradiance but does not correlate well with correlated color temperature. There was a negative but weak correlation between circadian metrics and energy efficiency measures. Energy efficiency and color quality were inversely correlated. In the following analysis, the spectra of an 18-channel LED system were optimized using a multi-objective genetic algorithm in several combinations, and the effect of the number and the order of the optimization parameters on metric relationships were analyzed. The number of optimization parameters significantly affected the optimal results. The order of optimization parameters only changed the relationship between color fidelity index and luminous efficacy of radiation but did not have an impact on the color fidelity index and melanopic radiance relationship. There was an inverse relationship between melanopic irradiance and the luminous efficacy of radiation. Trade-off analysis highlights the importance of the order and number of target parameters in the optimization process.

Microscopy in the life sciences is moving from traditional Mercury and Xenon lamp to LED technology. LEDs provide long lifetimes, increased stability and no need for replacing or disposing of toxic bulbs. Applications that require uniform spectral stability across the near-UV to visible spectrum have relied on Xenon lamps. Fura-2 applications have relied on Xenon lamps for decades despite its low optical power. We present a new LED illumination system that can cover applications from Fura-2 to Cy7.5. A single device that can excite common fluorophores between 340nm and 800nm, with no compromise while the user benefits from LED technology.

There is a growing interest in including light source spectrum in advanced lighting software tools and simulations. Given that traditional lighting software tools have been used primarily for calculating photometric quantities, their simplifying assumptions may not be suitable for calculating other α-opic quantities. Commonly used simulation tools use three values to represent the three primary colors (red, green, and blue), but others have expanded the number of bands (i.e., the spectral resolution) to 9, 27, or 81 in an attempt to more accurately represent variations in light source spectrum and downstream spectrally-derived quantities. It remains unclear, however, to what extent spectral resolution affects calculated quantities. To address this gap, a numerical analysis was completed using a large spectral power distribution database (n = 1,302 light sources). Calculated illuminance, α-opic irradiance, luminous efficacy of radiation (LER), melanopic efficacy of radiation (MER), and melanopic to photopic (M/P) ratio were compared for spectral resolutions of 3 and 9 bands compared to a baseline of 81 bands. Across all examined lighting quantities, considerable errors—a mean absolute percent error of 19%— were found when using 3 band calculations. These were reduced to 4% for 9 band calculations. The errors varied by metric and light source type. The results suggest that the use of 9 bands can more accurately characterize performance of different light source types across a range of metrics compared to using 3 bands.

OLEDs based on ITO as anode, TPD/Alq3 as organic PN junction, and Al as cathode, represent an important standard model. Since TPD and Alq3 are thin layers (< 100 nm), even at low bias voltage, V (~1 V), the electric field is relatively high (~10⁸ V/m). During the very first bias voltage application to ITO/TPD/Alq3/Al OLEDs, some unexpected currents add to the injection of electrons and holes. These “primary processes” induce irreversible structural alterations in OLEDs. This conclusion is based on the fact that the forward bias current-voltage characteristic curves, I-V, systematically show a first scan that is very different from all subsequent ones. Below 4 V, the first scan shows two current “bumps” (near 1 V and 2.5 V). The current density is ~1 mA/cm². For all subsequent scans, the bumps disappear, and the current density decreases to ~10 μA/cm². Above 3 V, the I-V curves show current “spikes”, up to ~100 μA/cm². Since no light is emitted when the bumps and spikes occur, these current surges are not due to electron-hole recombination. We suggest that four processes occur nearly simultaneously: (1) ionic transport due to ions uniformly distributed inside the organic layers, (2) ionic transport similar to the propagation of a Haynes-Shockley current pulse, (3) electrochemical cathode oxidation via the Cabrera-Mott mechanism, (4) sporadic rotations of organic molecules, similarly to liquid crystals and responsible for current spikes. Processes (1) and (2) are both responsible for the first bump, while (3) and (4) are both linked to the second bump. The theoretical model developed for the I-V characteristic curves agrees well with measurements. These irreversible primary processes could be partly responsible for OLEDs aging.

Ternary group 11 halides, especially those utilizing copper (I), have recently been explored as relatively non-toxic alternatives to the popular luminescent lead halide perovskites. This talk will focus on one such family, A2CuX3 (A=Rb+, K+; X= Cl-, Br-), which exhibit outstanding light emission properties, with up to unity photoluminescent quantum yields. These samples, along with other ternary group 11 halides, have been investigated through an in-depth optical characterization. The ultrabright photoluminescence in the studied group 11 halides is ascribed to self-trapped excitons localized on the [CuX3]2- anionic substructure based on density functional theory (DFT) calculations and optical spectroscopy results.

Photoluminescence and light reflectance properties of silica and titania nanoparticles (NPs) in two bilayer configurations were studied. In the first configuration, Titania and Silica NPs monolayers were deposited on a silicon wafer, the Titania NPs monolayer was the one in contact with the wafer and silica NPs was the top monolayer. The second configuration consisted in Silica NPs bilayer. The periodic monolayers were deposited by the spin coating technique. The concentration of colloidal suspension of NPs and their distribution over the surface were controlled by the revolutions per minute (rpm) during spin coating deposition. A set of samples were annealed with increments of 10 minutes, ranging from 0 min (i.e. not annealed) to 60 min. As well other samples were annealed during 2 and 3 hours, at 1,100 °C. The bilayers were characterized using several techniques as Fourier Transform Infrared Spectroscopy (FTIR), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), reflectance and photoluminescence (PL) measurements. For the first configuration, our results shown that for short annealing time the PL emission increased 6 times with respect to the non annealed sample. However, the light reflected by the first configuration is lower than the one reflected by the second configuration. These results can be attractive for light emitting devices as well as for absorbing layers on crystalline silicon solar cells.

Laser excited phosphor wheel light source has an upper output power limit due to the issues in heat dissipation and the high-power density at the focus on the phosphor. This paper presents a stationary phosphor plate with a compact rotating optical system such that the focused laser spot on the phosphor plate follows a circular path providing a larger effective excitation area while maintain the small etendue of a single stationary focused spot. With an 8 mm diameter circular path, a theoretical total of 1,870 W of laser power focused into a 2 mm spot can produce a total 500,000 lumens of visible light. This will make a 120,000 screen-lumen projector possible.

The introduction of non-toxic guest cations (zinc, Zn2+) in the synthesis of CsPbX3 NCs leads to the realization of lead-reduced CsPb1-xZnxX3 NCs, confirming the substitution of Pb2+ up to 15%. As-synthesized NCs exhibits lattice contraction accompanied by increased bandgap, blue-shifted photoluminescence emissions, and high quantum yield. Besides, the blue, green, yellow, and red-emitting CsPb1-xZnxX3 NCs (at x=15%) obtained by adjusting the composition of different halides when used as color conversion layers emits warm to cool white light with remarkable color qualities. Notably, this development will promote the halide perovskite NCs one step closer to their practical applications.

Colloidal quantum dots (QDs) have attracted considerable interest because of their unique properties, like high quantum efficiencies, size dependent emission wavelength, high color purity and a low-cost solution processability. These properties make them one of the most promising emitting materials for thin film light-emitting diodes (LEDs) of the future. Although QD-LEDs have been in the focus of science for quite some time, many topics are still neglected, like the dynamics, that show the speed of switching for these LEDs. However, this knowledge could be decisive for the use of such LEDs in communication or sensor technology. This work is intended to provide an overview of the influence of the organic hole transport layer (HTL) on the dynamics of QD LEDs, since the organic semiconductors used represent a limiting factor due to their low mobility. The substances 4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP), N,N-Bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) and N,N,N,N-Tetrakis(3-methylphenyl)-3,3-dimethylbenzidine (HMTPD) are used as HTLs. The examined LEDs are constructed in the same way and differ only in the material used for the HTL as well as their thicknesses. The dynamics of these samples will be investigated by electro-optical measurements. To get a better insight, hole only devices were also produced with the HTLs to compare their properties. We assume that these results mark the progress in the development and realization of faster QD LEDs

Conventional RGB sub-pixel layout for displays has low geometric fill factor, which show limitations in application for high resolution and efficiency improvement. Color-tunable pixel, which can reproduce various colors with a single pixel, can be an innovative approach to improve the fill factor issue. The bilayered emission layer (EML) structure is a method to implement the color-tunable pixels. However, a simple bilayer structure exhibit limited color-tuning range. Thus, color-modulation layer (CML) insertion between EMLs is essential to control the carrier distribution. CML materials are usually dissolved in organic solvent for solution process. Here, a process issue occurs as organic solvents can damage the previous QD layer. In this research, we analyze the effects of various organic solvents on the QD layer and suggest the most suitable solvent that minimizes the damage. The bilayer QD-LED structure is ITO/PEDOT:PSS/PVK/green QD(CdSe@ZnS)/organic solvents/red QD(CdSe@ZnS)/ZnO/Al. Toluene, chlorobenzene, 1,4-dioxane, and n-octane are used for organic solvent process and deposited on the QD layer. The current density and the luminance of the device without solvent process and with toluene, 1,4-dioxane, and n-octane process are (99.5 mA/cm² and 908 cd/m²), (63.7 mA/cm² and 490 cd/m²), (63.1 mA/cm² and 304 cd/m²), and (43 mA/cm² and 200 cd/m²), respectively, at 9.5V. These results indicate that toluene, 1,4-dioxane, and n-octane process caused severe damage on the green QD layer. However, current density and luminance of chlorobenzene process are 208.8 mA/cm², 934 cd/m², respectively, at 8V. Moreover, chlorobenzene treated device showed even wider color-tuning range than the one without solvent process. Therefore, chlorobenzene is the most suitable solvent for the CML..

In this work we theoretically propose a novel method towards obtaining a two-dimensional beam steering system which could be used for LIDAR applications. The method utilizes a hybrid plasmonic waveguide in a cylindrical topology creating a nanoscale vertical cavity surface emitting phaseshifter (VCSEP). This approach enables a sub-wavelength spacing between each phase shifter and therefore provides a larger field of view. Each VCSEP consists of a highly doped sub-micron silicon pillar coated with a thin layer of non-linear material then another layer of conductive metal. The input optical carrier is inserted from the polished back side of the silicon substrate, which is then modulated by the phase shifter and re-emitted in the vertical direction to form a beam in the far field. The hybrid plasmonic localization increases the effective interaction length of the light with the non-linear material within a silicon core resonant cavity. Increased interaction length due to plasmonic effects in the vertical architecture leads to an enhanced phase shift. A lower loss is possible while retaining high localization due to an overlap of photonic mode and plasmonic modes. The impedance mismatch between the vertical hybrid plasmonic waveguide and free space will create a low finesse, low-Q cavity resonator. The single VCSEP device can be considered as a low finesse Fabry-Perot resonator that can be used for phase modulation with less than 3dB amplitude modulation. We characterized the optical response of a single VCSEP, with an aspect ratio of 12.5 which has an FSR of 47.25±2.5nm and transmission variation of 3dB.