This PDF file contains the front matter associated with SPIE Proceedings Volume 12001, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

ISAMU AKASAKI, crystal grower, a pioneer of GaN-based blue light-emitting diodes (LEDs), and a Nobel Laureate in Physics, passed away because of pneumonia on April 1, 2021 at the age of 92. According to the Nobel Foundation, the LED lamp holds considerable promise for improving the quality of life of over 1.5 billion people worldwide who lack access to electricity grids. Owing to its low power requirements, it can be powered by cheap local solar energy. In this article, his pursuit of blue LEDs for 20 years is described.

Micro-photoluminescence (μPL) pinhole scans of an InGaN multi-quantum well (MQW) and single-quantum well (SQW) LED structure show large PL spots compared to the relatively small excitation area. We observe a lateral diffusion over a wide range in the order of 10 µm in both samples. The excitation density is varied to identify a dependence of the diffusion length on the amount of injected charge carriers. Overall the MQW sample shows a smaller diffusion length compared to the SQW sample, which is attributed to the different sample quality. In the MQW sample no significant change in diffusion length with varying excitation power is observed, whereas in the SQW sample the charge carrier diffusion increases with smaller excitation densities. Our spectral resolved μPL setup allows to measure the spectra as a function of the distance, which gives the possibility to investigate the energetic shift. As a result the PL spectra of both samples show a pronounced blue-shift at the center of the PL spots which is assigned to the screening of the quantum confined Stark effect (QCSE) and the band-filling.

Efficient high-power operation of light emitting diodes based on InGaN quantum wells (QWs) requires rapid interwell hole transport and low nonradiative recombination. The transport rate can be increased by replacing GaN barriers with that of InGaN. Introduction of InGaN barriers, however, increases the rate of the nonradiative recombination. In this work, we have attempted to reduce the negative impact of the nonradiative recombination by introducing thin GaN or AlGaN interlayers at the QW/barrier interfaces. The interlayers, indeed, reduce the nonradiative recombination rate and increase the internal quantum efficiency by about 10%. Furthermore, the interlayers do not substantially slow down the interwell hole transport; for 0.5 nm Al_0.10Ga_0.90N interlayers the transport rate has even been found to increase. Another positive feature of the interlayers is narrowing of the QW PL linewidth, which is attributed to smoother QW interfaces and reduced fluctuations of the QW width.

Deep defects have a fundamental role in determining the electro-optical characteristics and in the efficiency of InGaN light-emitting diodes (LEDs). However, modeling their effect on the electrical characteristics of the LED is not straightforward.

In this paper we analyze the impact of the defects on the electrical characteristics of LEDs: we analyze three single-quantum-well (SQW) InGaN/GaN LED wafers, which differ in the density of defects. Through steady-state photocapacitance (SSPC) and light-capacitance-voltage measurements, the energy levels of these deep defects and their concentrations have been estimated.

By means of a simulation campaign, we show that these defects have a fundamental impact on the current voltage characteristic of LEDs, especially in the sub turn-on region. The model adopted takes into consideration trap assisted tunneling as the main mechanism responsible for current leakage in forward bias.

For the first time, we use in simulations the defect parameters (concentration, energy) extracted from SSPC. In this way, we can reproduce with great accuracy the current-voltage characteristics of InGaN LEDs in a wide current range (from pA to mA).

In addition, based on SSPC measurements, we demonstrate that the defect density in the active region scales with the QW thickness. This supports the hypothesis that defects are incorporated in In-containing layers, consistently with recent publications.

Degradation of GaN-based laser diodes after some time of operation appears usually as raised threshold current, lower slope efficiency, or increased voltage. We investigate stressed and non-stressed laser diodes using micro-electroluminescence (μEL) and micro-photoluminescence (μPL). In μEL, the stressed device exhibits darker regions, which are correlated with a red-shifted emission. Both observations indicate a lower carrier density in these darker areas. Our μPL measurements do not suggest a corresponding increased defect density in these regions. This study shows inhomogeneous pumping of the active region that can be explained by current path formation from non-uniform hydrogen distribution. Additionally a background of increased nonradiative recombination is found in the stressed device, which is unrelated to the conductivity degradation.

GaN-based solar cells are promising devices for application in space environment, concentrator solar systems and wireless power transmission. Thus, it is essential to understand their degradation kinetics when submitted to high-temperature, high-intensity stress. We submitted GaN-based multiple-quantum-well solar cells with AlGaN electron-blocking-layer to two step-stress experiments at 35 °C and 175 °C in short-circuit condition under 405 nm monochromatic excitation by increasing optical power from 47 W/cm² to 375 W/cm². We found almost no degradation in the dark-IV, light-IV, electroluminescence and photocurrent characteristics after low-temperature stress, whereas the degradation after hightemperature stress was significant: we observed a lowering in power-conversion efficiency, a decrease in open-circuit voltage and an increase in low forward bias current. We then submitted the device to several constant power stress at 180 W/cm² for 100 hours at 95 °C, 135 °C and 175 °C. We found that, by increasing the temperature, the short-circuit current during the stress decreased of 7%, 9% and 12.5% respectively. Dark IV characteristics showed an increase in low-forward bias current stronger at 175 °C. We also found a higher decrease in open-circuit voltage, external quantum efficiency, power conversion efficiency and electroluminescence with higher stress temperature. The causes of degradation are possibly diffusion mechanisms, which increase defect density in the p-GaN bulk region and/or in the GaN barrier region, promoting trap-assisted tunneling mechanisms, leading to the decrease in open-circuit voltage, and non-radiative recombination mechanisms, that cause the drop in quantum efficiency.

Many applications benefit from single mode operation of vertical-cavity surface-emitting lasers (VCSELs) due to the need for a consistent beam profile and wavelength. While long cavity VCSELs have demonstrated good results in recent years, with examples of sub 1mA thresholds for devices with blue and green emission, the mode has not been fully controlled. Traditionally, VCSELs, with their short cavities, inherently maintain single longitudinal mode operation while struggling with multi-lateral modes. However, the use of a long cavity device with a curved mirror provides the opposite challenges for mode control, as the lateral mode can be controlled with the lens curvature, but the long cavity length allows for multiple longitudinal modes. In this paper, we will present the use of a filtering mirror with a highly varying reflectivity spectrum for obtaining single longitudinal mode operation up to 2mW and 50 kA/cm². The filtering mirror reflectivity leads to significant mirror loss for adjacent longitudinal modes. Thus, by engineering the beam profile with the lens and filtering the longitudinal modes with the mirror structure, we are able to control the output shape, divergence angle, and wavelength stability of the device. This mode control, along with the low thresholds and lifetimes greater than 2000 hours, shows the potential of the long cavity structure for a variety of applications.

This paper reports the latest device performance of high-power blue laser diodes (LDs). The epitaxial structures of LDs were grown by metal organic chemical vapor deposition (MOCVD) on C-plane free-standing GaN substrates. And a ridge type structure was formed at the top of p-type layers. The ridge width of the LD was 45 μm. Electrodes of the n-type and p-type were formed at the substrate and the ridge respectively. And the front and rear sides were obtained by cleavage at the m-plane surface. These faces were covered by dielectric mirrors. Every LD chip was mounted on a heat sink using a junction down method in a TO-Φ9 mm package for suppressing thermal resistance. We optimized the epitaxial and the device structures for high efficiency and high optical output power. A New developed 455 nm blue LD showed the optical output power and the voltage of 5.73 W and 3.82 V at the forward current of 3 A under Continuous Wave (CW) operation. The wall-plug efficiency (WPE) of the 455 nm blue LD was 50.0 % at 3 A. This is the highest WPE reported so far. The peak WPE of the 455 nm LD was 51.2 % at the forward current of 2 A.

We investigate the lateral near field of blue laser diodes with 10 μm broad ridges in pulsed conditions. While scanning the near field step by step with the help of a streak camera, we observe complex dynamics due to lateral-longitudinal mode competition, wavelength shifts and lateral mode switching. Additionally, a high-resolution spectrometer enables us to differentiate between the different mode combs which form the longitudinal mode spectrum. We observe filling of the gain volume and a spectral broadening with increasing current, as well as a slight asymmetry and an inhomogeneous lateral mode pattern.

The market for UV LEDs is experiencing a rapid growth, also driven by the need for effective and efficient disinfection systems. Before UV LEDs can be widely accepted by the market, they need to demonstrate a high reliability, with lifetimes of several thousands of hours. Several physical processes may limit the reliability of UVB and UVC LEDs, resulting in a loss in efficiency during long term operation.

This paper aims at discussing the most relevant processes that can lead to the degradation of UVB and UVC LEDs, with focus on: (i) instability of the electrical properties, which may result in gradual changes in the turn-on voltage of the devices during long-term operation. (ii) The generation of defects within the active region of the devices, with consequent increase in the Shockley-Read-Hall non-radiative recombination rate. Optical spectroscopy is found to be very effective for the identification of deep (midgap) traps during operation of the devices. (iii) trap states near the junction, with consequent impact on trap-assisted-tunneling of the current-voltage characteristics. (iv) the propagation of point defects, especially impurities, and accumulation of charges at heterointerfaces, that can reduce the carrier injection efficiency, thus leading to a decrease in the emitted optical power.

In the last year the market of ultraviolet (UV) light-emitting diodes (LEDs) had a huge increase due to the higher demand of devices caused by the pandemic crisis. In fact, it was demonstrated that UV LEDs, and in particular UV-B and UV-C LEDs, could be used as efficient sources for the disinfection of surfaces against Sars-CoV-2. In this work, we investigated electrical, optical and spectral degradation mechanisms in a series of commercial UV-C LEDs (275 nm – 280 nm) available on market. We present an exhaustive comparison of the main sample characteristics, studying their evolution when the LEDs are submitted to constant current stress tests (for about 350 h) at the absolute maximum current indicated in the respective datasheets. In particular, we carried out an extensive set of measurements, including current-voltage (I-V), optical power-current (L-I) and power spectral density (PSD) characteristics at various steps of stress, combined with the analysis of the drive voltage during the tests and of the thermal droop of the devices. We found: (i) a set of LEDs with a promising L80 of 10000 min (about 170 h), (ii) the presence of parasitic peaks and bands in all devices, ascribed to radiative recombination outside the QWs, and (iii) a substantial increase in thermal droop in all LEDs due to a decrease in injection efficiency and an increase in SRH recombination events during the stress tests.

This paper presents results concerning the dynamic performance and reliability of Fe-doped and C-doped 0.15-μm gate AlGaN/GaN HEMTs. Step-stress tests at increasing drain-source voltage and different gate-source voltages are specifically reported. Fe-doped HEMTs exhibit, under both off- and on-state conditions, excellent parametric stability up to breakdown. C-doped devices are instead affected by enhanced degradation effects during the step stress experiments compared to Fe-doped ones, consisting of R_ON increase during off-state stress and both threshold-voltage and RON increase under on-state conditions. 2D hydrodynamic device simulations are used to validate hypotheses on the physical mechanisms underlying the observed, distinctive degradation effects. The role of C doping in causing additional degradation compared to Fe-doped device is explained with the aid of device simulations as follows: 1) under off-state conditions, hole emission from the C_N acceptor traps in the gate-drain region of the buffer leads to an R_ON increase which is not completely recovered during the typical recovery time interval following each stress phase and therefore accumulates during the step stress experiment; 2) under on-state conditions, channel hot electrons are injected (besides towards the surface) into the buffer where they can be captured by C_N traps under the gate and in the gate-drain region, inducing semi-permanent threshold-voltage and R_ON increases.

GaN power amplifiers based on AlGaN-GaN high electron mobility transistor (HEMT) technology are produced for defense applications that require high voltage, high power, and high efficiency operation. Many groups have reported various failure mechanisms in GaN HEMTs over the last two decades. However, the fact that no dominant failure mode and mechanism has been identified is a major concern for space applications. Earlier our group reported radiation effects of commercial high-power GaN HEMTs exposed to protons and heavy ions as well as failure mechanisms in RF GaN HEMTs fabricated per our design. For the present study, we continued to investigate RF AlGaN-GaN HEMTs grown on SiC substrate with Ni-Pd-Au Schottky contacts. Our GaN HEMTs had a Schottky gate length of 250 nm, a total gate width of 6 × 150 μm periphery, and a field plate. Our HEMTs were irradiated with 50 MeV protons at the Lawrence Berkeley National Laboratory, with three different doses of 42, 160, and 889 krad(Si) for this experiment. All our irradiated devices were exposed to protons while they were shorted. First, pre- and post-irradiation DC I-V characteristics were compared, and DC bias-temperature stress tests were performed on unirradiated HEMTs. Some HEMTs were thermally stressed as monitor samples for comparison. Second, we employed focused ion beam (FIB) techniques to prepare TEM specimens from degraded devices for defect and chemical analysis using a high resolution TEM. AFM techniques were also employed to study gate pitting.

Aluminum gallium-nitride (Al_xGa_1–xN)-based metal−semiconductor−metal (MSM) ultraviolet photodetectors photodetectors (PD’s) have been successfully designed and fabricated using conventional photolithography techniques and tested experimentally to study their spectral sensitivity across different Al content, x, with x varying from 0 to 0.3. (Al)GaN-based UV PD’s have wide and tunable direct band gaps. The ability to easily select the photodetected wavelength by simply varying the aluminum content of GaN thin film (Al_xGa_1–xN) is a significant advantage of these group III–V compounds. Typically, MSM PD’s grown on (Al)GaN thin films result in ultrafast photodetection because of their highly mobile carriers. These devices are limited by the carrier transit time due to the negligible capacitance presented by the interdigitated fingers. Various electrode geometries were fabricated to investigate the influence of metal contact shapes on the devices’ performance indices with emphasis on the response speed and bias-voltage–independent efficiency. Coupled with the independently measured Hall mobility and electric field, we computed the carrier transit time of the devices to be as short as 1.31 ps and the bias-voltage−independent external quantum efficiencies were as high as 70% at 60 V for n-doped and intrinsic devices when operated in the reverse bias regime. Here, (Al_x)Ga_1–xN photodetectors were designed to explore spectral sensitivity by altering x from 0 to 0.3.

LEDs on a GaN epitaxial wafer grown on a Si substrate were directly bonded on a Si-based circuit board by utilizing via holes. The via holes were etched by deep reactive ion etching (DRIE) on the Si side of the LED wafer. The un-doped GaN in the via holes was further removed by inductively-coupled-plasma reactive ion etching (ICP-RIE). Metal layers were then deposited on the via holes and annealed. We used lead-free solder to bond the n-GaN of LEDs to the n-electrodes on a metal-wire-patterned Si wafer. Characterization in terms of I-V, L-I curves, emission images and spectrum is reported.