This PDF file contains the front matter associated with SPIE Proceedings Volume 12441, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The carrier recombination ABC model is a powerful tool to understand the performance of Light-Emitting Diodes (LEDs) such as Light Extraction Efficiency (LEE), Internal Quantum Efficiency (IQE), and lifetime. Starting from the ABC model we demonstrate that the optical-output-power junction-temperature dependence can be used to obtain an LED’s LEE and IQE. Using this analysis to a high power Deep Ultraviolet (DUV) LED and a state-of-the-art blue LED we obtain LEE values of 16% and 80% for the DUV and blue LEDs, respectively. This points LEE to be the main factor for efficiency improvement of DUV LEDs. Also, if IQE (not LEE) is responsible for the optical power decay, an LED lifetime model based on the ABC carrier recombination can be established to predict LED’s lifetime, and the model shows that the LED lifetime is mainly determined by the initial defect density and the defect generation interest rate. We present room temperature lifetime measurement data up to 3,000 hours for some 267 nm DUV LEDs, and the lifetime to maintain 70% of the initial optical power (L70) according to the lifetime model can be extrapolated to be of 20,000 to 120,000 hours. In view of the data presented in this paper, high-efficiency, long-lifetime DUV LEDs can be realized if the light extraction efficiency, initial defect density and uniform current spreading can be optimized.

UVC LED emitters have found important scope in the disinfection of bacteria and viruses. With this work we report on the reliability of commercial UVC LEDs with different wavelengths as a function of the driving current and the effect of lifetime on the system design. The reliability of commercial UVC LEDs by means of electrical, optical and spectral resolved measurements has been evaluated in our work. We then carried out stress tests at increasing driving current in order to define the acceleration effect of the current on the device reliability. By defining a current dependent degradation model, we identified the optimal operating condition to balance the number of disinfection cycles and their duration.

Until recently, lack of stable p-type doping source in HVPE hindered its use to the application of III-nitride light emitting devices. Recently, K. Ohnishi discovered the stable MgO source for p-type doping in GaN HVPE. This has enabled the use of HVPE for light emitting devices as well as electron devices such as vertical MOSFET. In this article, we will review the current HVPE technology of p-GaN HVPE, multi-junction AlInGaP/InGaP/GaAs solar cell, InGaN HVPE by trichlorides, and discuss the challenge and opportunities of III-nitride HVPE in terms of epitaxial layer design and the remaining issues of the growth of the low temperature buffer, MQWs as well as the source supply design to grow multilayer structures.

To reduce the operating voltage, we analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction Tunnel Junction (TJ) deep-ultraviolet light-emitting diode using phase-shifting electron holography. We obtained a phase image reflecting the band alignment of the p–n homojunction and derived a depletion layer width of approximately 10 nm. We found the AlGaN homojunction TJ forms a p-n junction. Furthermore, the operating voltage reached 8.8 V at 63 A cm^-2 by optimizing the structural characteristics of the AlGaN TJ, such as the thickness and impurity concentration, where the thickness of the TJ was 23 nm. We found that the TJ thickness should be at least the same as the depletion layer width at the AlGaN TJ.

We demonstrate low energy optical interconnects (0.6pJ/bit at 1E-4 error rate and 1.3pJ at 1E-12) using a custom 32-channel microLED-based optical link with monolithically integrated drivers, photodetectors, and TIAs. Each channel is modulated at 2Gb/s and the signal is carried either via multicore fiber or through free space.

High-performance LiDAR systems play an important role in autonomous driving by providing a high-resolution 3D representation of the driving environment. Complementing version-based object detection, LiDAR systems must furnish reliable and appropriate information for the vehicle. Current LiDAR systems for vehicle applications prefer to be solid-state to achieve high system robustness. However, the lack of beam steering limits the beam flux density and thus the detection range of a solid-state LiDAR system. One of the main reasons is that the emitted irradiant flux must be distributed to a large number of pixels of a focal plane array detector, which results in a single pixel receiving only a small optical power. To increase the optical power reaching at the detector, this paper investigates the influence of different beam shaping methods on the detection range. Subsequently, an irradiation pattern to maximize the detection range for solid-state LiDAR systems is determined. Based on the determined irradiation pattern, we propose an optical concept for both the emitter and detector sides of the solid-state LiDAR system.

A full InGaN structure is designed on the partially relaxed InGaN pseudo-substrate fabricated by Soitec (InGaNOS). By combining its in-plane lattice parameter and the growth conditions of the active zone, it permits to cover the whole visible range with thin quantum well width, thus by increasing the InN mole fraction in the quantum wells. An InGaN/GaN superlattice based buffer layer and a new InGaNOS substrate with a low V shaped defect density helps to get a better crystalline quality. It leads to an internal quantum efficiency higher than 10% at 640 nm. 10 µm circular micro-light emitting diodes show a red electroluminescence with a central emission wavelength of 625 nm. An external quantum efficiency of 0.14% at 8 A/cm² at 625 nm is also demonstrated. The light extraction efficiency is estimated to be below 4%, mainly due to the emission from backside through the buried oxide and the sapphire substrate.

GaN-on-Silicon nanowire technology is promising for several display applications. Since one of the microLED display major challenges is to drastically reduce the cost, the possibility to downsize the microLED is paramount. Aledia has developed a nanowire microLED technology on large Si wafers which keeps the same blue emission efficiency for ⪅2μm size devices containing only 1 NW as for larger devices containing up to few hundreds of NWs. A RGB integrated pixel relying on this technology is presented where quantum dots are used for green and red color conversion. Another nanowire microLED concept for AR/MR applications is also presented where light emission directivity and pixel size downsizing are mandatory.

Light sources with digitally addressable elements have many potential uses in the lighting and display space. At the lighting end of the application range, recent developments have resulted in phosphor-converted segmented LEDs and mini-LEDs with single element dimensions of the order of 100 µm. When combined with suitable directional optics and controls this device architecture can generate dynamic light distributions for a wide variety of scenarios. Initially developed for Adaptive Driving Beam (ADB) technology for vehicle headlights, the basic concepts translate to many general lighting applications where they provide a completely new approach to reduced power consumption, improved illuminance uniformity and digitized design and commissioning. At the display end of the range, micro-LED technology with typical pixel sizes of the order of 1 µm to 10 µm promises to reduce power consumption of both direct-view and projection displays, enabling practical wearable AR/VR devices. In this paper, we will present our integral approach to micro- and mini-LED device technology and backplane integration across this application range and describe a prototype light engine designed to evaluate the benefits of this technology in lighting applications.

Controlling and manipulating individual quantum systems underpins the development of scalable quantum technologies. Hexagonal boron nitride (hBN) is emerging as an exceptional platform for applications in quantum photonics. The two-dimensional van der Waals (vdW) crystal hosts single photon emitting defects (quantum emitters) opening new functionality currently inaccessibly with other 3D quantum sources. Due to the two-dimensional nature of the crystal, hBN is an ideal material to integrate into vdW heterostructure devices. These devices have recently been shown to enable electrical modulation of single photon emission. Additionally, the study of the dephasing mechanisms of these sources helps assess the future utilization of hBN emitters in quantum interference experiments.

Structured LED materials such as micro/nanowires are highly promising as they enable superior light outcoupling, tunable directionality and have a small footprint for demanding display applications. In such nanostructured devices, individual electrical contacting and analysis is a tedious and complicated process. Spatially-resolved cathodoluminescence (CL) spectroscopy, in which the electron-beam-induced radiation is collected inside an electron microscope, holds great potential for contactless nanoscale optoelectronic material inspection of semiconductor materials. Conventionally, CL experiments focused on measuring the intensity and spectral content of the light but recently there have been several relevant advancements in the CL technique. Here, we will describe some of those developments.

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

This paper investigates the factors that influence the efficiency and reliability of InGaN based visible light emitting diodes with non-optimized carrier injection. The devices under test are LEDs with a single quantum well with a nominal emission wavelength of 495 nm at I = 100 mA. We stressed the devices with a constant current density of J = 80 A/cm², at room temperature, for 25000 minutes. We monitored the optical performance of the devices before and during stress. From the preliminary characterization we observed an increment in the optical power followed by a blue shift as a function of current. Simulation results highlight an asymmetric carrier injection, in particular a lack of electrons in the low bias regime. The low injection efficiency is also confirmed by temperature-dependent measurements, where we observed an increment in the OP with increasing temperature. During an ageing experiment, we observed an increment in OP for high injection level, accompanied with a blue shift in the peak emission wavelength. This result suggests an improvement in the injection efficiency and or a better carrier confinement. In this regard, we performed photoluminescence measurements during stress, which confirm the hypothesis of a better carrier confinement. In particular, PL signal show an increasing trend during the ageing process, which can be ascribed to the generation of negatively charged defects in the quantum barrier, with consequent impact on carrier confinement.

Metal halide Perovskite Light-Emitting Diodes (PeLEDs) rapidly advance in their External Quantum Efficiency (EQE), highest brightness, and operation lifetime in recent years. However, solution-processed PeLEDs usually encounter uniformity issues of crystal growth, which make them difficult to realize large-area fabrication, despite their outstanding device performance. With the intrinsic advantage of high reproducibility and uniformity in thin-film quality, vacuum-deposited PeLEDs possess a great potential in industrial mass production. Although breakthroughs have been observed in vacuum-deposited PeLEDs recently, the strategy of choosing their Hole-Transport Materials (HTMs) still follows the experience of solution-processed PeLEDs. In this work, we demonstrated a simple approach to significantly improve vacuum-deposited perovskite PeLEDs by inserting a thin vacuum-deposited interfacial organic layer between 1,1-Bis[(di-4-tolylamino) phenyl] cyclohexane (TAPC) HTM and perovskite emission layer (EML). With the evidence of X-ray Photoelectron Spectroscopy (XPS), we showed that the interfacial layer successfully inhibited the formation of metallic Pb0 caused by the TAPC/perovskite chemical degradation. The device with the interfacial layer exhibited a luminance of 55968 cd m-2, a current efficiency of 33.2 cd A-1, and an EQE of 9.40%, which was a 4-fold enhancement compared to that of the device without the interfacial layer. The results of EQE and brightness are among the highest reported values in vacuum deposited PeLEDs.

An integrated approach that optimizes traffic signals and vehicle trajectory at urban intersections using Visible Light Communication (VLC) is proposed. A Connected Vehicle (CV) platoon approaches a signalized intersection, and downstream CVs queue before the stop lines. Light is used to communicate information between CVs and the infrastructure using streetlamps, intersection signals, and headlights. Interaction with traffic is coordinated by an intersection manager. Integrated control is flexible and adaptive to traffic demands since different traffic movements are incorporated during multiple signal phases. As part of the simulation process, an open-source urban mobility simulator SUMO creates the desired scenarios and generates different urban traffic flows. VLC queue/request/response mechanisms and temporal/space relative pose concepts are used. Using sequence state durations, phase diagrams, and average speed measurements, the system dynamically controls traffic flows at intersections using a Deep Reinforcement Learning (DRL) algorithm, minimizing rush hour bottlenecks, through joint Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications. Comparisons with trajectory optimization and signal optimization demonstrate the benefits on throughput, delay, and vehicle stops, and reveal the optimal patterns for signals and trajectory.

We fabricated a Short Resonant-Cavity Light Emitting Diodes (SRC-LEDs) with emission wavelength at 3.4 µm using a metal ground plane positioned quarter-wavelength from the active region and the total semiconductor optical cavity thickness of five quarter-wavelengths. Devices were fabricated using wafer bonding and substrate removal process. Experimental testing in continuous wave operation at room temperature demonstrates six times enhancement of the optical power output of short resonant-cavity LEDs compared to a reference conventional “bulk” LEDs with an identical active region and optical radiation extraction through a thinned-down doped substrate.

Light-Emitting Diode (LED) based sensor system is developed for ex-situ placements at fluid-tanks to monitor the level of fluids through level gauge displays. The sensor system is designed for remote monitoring of operational status of industrial heavy equipment through a low-footprint internet of things (IoT) approach without involving extensive mechanical or electrical overhauling for sensor deployment. Sequential and non-sequential ray tracing simulation over visible and near infrared spectrum were conducted for optical beam propagation through air-fluid, air-glass, and glass-fluid interfaces. Fluid specific refractive index and propagation parameters were used in ray tracing simulation to design the optical path and wavelength of operation that are specific to fluids and the fluid tank configurations. Our low-footprint non-invasive approach of fluid level monitoring allows their deployment on industry installations with minimum interruption of services.

Light-Emitting Diodes (LEDs) are commonly used in various applications in the field of optical metrology and remote sensing, typically as light or signal sources. They stand out with their high energy efficiency and long lifespan, provide high brightness and low coherence and at the same time, they are inexpensive optical components. Apart from the field of optical metrology, low coherent short-pulsed light sources with high repetition stability and tunable amplitude are widely used for calibration and referencing of optical systems. This work focuses on the generation of short optical pulses (down to 10 ns FWHM) with conventional high-power LEDs for metrology purposes. A pulse generation technique, which allows tuning the pulse width and amplitude through the temporal management of three independent edges of TTL-like signals, is presented. The introduced technique considers the LED to be a first-order system and exploits this property to turn the task of electric current management into the task of temporal edge management. The proposed modulation technique is demonstrated and evaluated on the discrete component-based implementation. The achieved relation between the pulse width and the output optical intensity was demonstrated. The pulse-pulse stability (deviations in range of −60 dB) and the long-time stability of the realized modulation system were investigated in a long-time test series by different pulse amplitudes. The spectral properties of the output optical signals of the applied LEDs when excited by a short intensive current pulse were measured with a time-shifting integration method and are presented temporally resolved.

Micro-Algae production has seen growth of interest as a renewable source of food, nutraceutical, and biofuels. In this work we studied the integration of Light Emitting Diodes (LED) in Photobioreactors (PBRs) to increase the algal production efficiency. A multiwavelength LED light source with selectable intensity able to operate under Continuous Wave irradiation (CW) or with high intensity, short pulses, has been developed. The efficiency of biomass production has been examined both for CW and pulsed operations. The efficiency obtained with the Photovoltaic (PV) + LED light source is then compared to that of the solar radiation to analyze the performances of an indoor and outdoor system.

Optical Wireless Communication (OWC) systems are limited in bandwidth by their electro-optical components. We study the receiving photo diode, its surface area and the associated capacitance. A large–area detector improves the signal–to–noise ratio, but a small size allows a high bandwidth. An optimum detector size exists. It depends on photodiode properties, the trans-impedance amplifier noise performance and the received signal strength. Results are relevant to the design of indoor LiFi systems with wide coverage as well as for tracking acquisition in long–range free space optical applications.

GaN/(In,Ga)N heterostructure based visible Light Emitting Diodes (LED) have enabled a wide range of solid-state lighting applications through excellent efficiency and power output in the shorter wavelengths (≤ 475nm) of violet/blue emission. However, the efficiency of emitters in the longer wavelength range (≥ 500nm) drops drastically due to the need to include higher Indium-content in the InGaN quantum wells. Large average polarization fields for high Indium-content quantum wells for conventional P-up structure, opposes the depletion field leading to large electrostatic barriers for both electrons and holes injection. LEDs fabricated along the N-polar direction with a p-up orientation or Ga-polar direction with p-down orientation lower such electrostatic barriers to carrier injection due to alignment of the polarization dipole field and depletion region field. This can therefore theoretically improve the electrical injection efficiency and reduce the forward voltage of operation. Such a Ga-polar p-down LED requires a bottom buried tunnel junction to avoid current spreading issues for a buried p-GaN layer. In this report, we demonstrate for the first time Ga-polar p-down green emitting LEDs using bottom tunnel junctions and having external quantum efficiencies comparable to those of equivalent p-up LEDs grown by Metal Organic Chemical Vapor Deposition (MOCVD).

Automotive headlamps equipped with high-resolution modulator technology can provide comfort and safety features like a glare-free high beam and road projections. Previous research has shown the importance of beam shaping for automotive headlamps with a peak of the illuminance in the center of the FoV for longer viewing range. The peak of the illuminance can be achieved through the use of a distorting optical system, which is proved for a homogenously illuminated DMD. However, DMD-based headlamps are limited in their use due to systemic disadvantages. In particular, luminous efficacy and contrast are advantages of µLED-Array over illuminated DMDs, which can be further exploited for improved automotive headlamps. In this paper, we present the optical design of a distorting µLED-Array-based headlamp. In particular, we address the challenges of optical design for lambertian emitters and compare the µLED-Array-based system with a DMD-based system. The use of distorting µLED headlamps is evaluated in conclusion.