This PDF file contains the front matter associated with SPIE Proceedings Volume 12886, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Gallium nitride (GaN) crystals of the best structural quality are grown by ammonothermal method in a supercritical ammonia solution inside high pressure autoclaves. This lecture will focus on the basic ammonothermal growth. The growth mechanism in different crystallographic directions, growth morphology and structural quality of GaN crystals, will be discussed. Structural properties and shape of the seeds will be shown. The influence of the crystallization run parameters, the internal configuration of the autoclave and the structural quality of the obtained GaN will be discussed. The path from bulk crystal to finished substrate of GaN will be presented. All challenges and difficulties to grown bulk GaN from ammonothermal method will be demonstrated. Scenarios for the further development of bulk GaN crystallization will be presented.

Surface properties are important for structures such as micropillars and nanowires, which are critical for emerging devices including μLEDs, nano-lasers, and vertical power transistors due to increased surface to volume ratios. Fabrication of III-Nitride micropillars can be realized through a top-down approach, where structures are defined through lithography and reactive ion etching (RIE). While effective at forming these micropillar structures, RIE etching leaves behind roughened, non-vertical sidewalls. This surface damage increases non-radiative recombination, forms current leakage paths, and can severely degrade device performance. However, damage can be removed through a follow-up wet etch in potassium hydroxide (KOH) solution. KOH acts as a crystallographic etchant, preferentially exposing vertical <1-100> m-planes, producing smooth, vertical sidewalls. Here, we investigate KOH wet etch passivation for 2.5 μm diameter top-down fabricated GaN micropillars utilizing different temperatures and solution concentrations, and the effects of a Ni etch mask present during wet etching. We observed an average etch rate of 11.67 nm/min for micropillars etched in 60% AZ400k solution compared to 9.44 nm/min for micropillars etched in 20% AZ400k solution, both at a temperature of 80°C. At a constant 40% AZ400k concentration, an average etch rate of 14.39 nm/min for micropillars etched at 90°C are observed compared to 9.89 nm/min for micropillars etched at 70°C. Micropillars with a Ni etch mask present during KOH etching have an average etch rate of 9.46 nm/min compared to 12.83 nm/min for those without a Ni mask. The effects of KOH etching work to further optimize the performance of GaN-based micropillar and nanowire devices.

Positron annihilation spectroscopy is a powerful set of methods for the detection, identification and quantification of vacancy-type defects in semiconductors. In the past decades, it has been used to reveal the relationship between the (opto-)electronic properties and specific defects in a wide variety of elemental and compound semiconductors, including nitrides. In binary compounds, the selective sensitivity of the technique is rather strongly limited to cation vacancies that possess significant open volume and suitable charge: negative of neutral. The focus of this paper is in the recent advances in combining state-of-the-art positron annihilation experiments and ab initio computational approaches in identifying and quantifying defects in AlGaN alloys and GaN/AlGaN device structures.

Longitudinal optical-like phonon resonant mid-infrared emission (LORE) and absorption peaks of micro-surface line and space structures of metallic material plates on GaN wafers are observed at 500 – 630 K. The emission peak energy is located in a range of 670 – 730 cm^-1 for GaN, which is lower than the LO-phonon energy of approximately 730 cm^-1 at 630 K. This feature is contrasted with that of the LO-phonon resonant emission from the Au-GaAs microstructures resonating with the LO phonon. The emission mechanism is clarified using a structure of n++-GaN with an electron density of 1×10²⁰ cm^-3 as a metallic material. These emission lines have another notable feature, i.e., the observed peak energies are independent of the polar emission angle, unlike the emissions by surface phonon polaritons showing a significant directive nature of peak energies. The present results show that each peak energy in the emission spectrum is positioned at the zero-point of the real part of the dielectric function comprising the components of the transverse optical phonon and other electric dipoles induced by the LO-like modes, excluding the target mode. This type of emission is also obtained for the structures of Al_xGa_1-xN films. These results suggest the feasibility of high-efficiency emission in a range of 670 – 900 cm^-1 when the devices are positioned in a cavity with high reflectivity of blackbody-like radiation.

Visible light communication (VLC), which utilizes LEDs, promises superior privacy and security and reduced impact on surrounding electronics compared to traditional Wi-Fi. This paper explores the potential of InGaN-based micro-light-emitting diodes (micro-LEDs) in high-speed VLC applications, focusing on yellow-green micro-LEDs with nanoporous distributed Bragg reflector (NP-DBR) and red InGaN micro-LEDs. Yellow-green micro-LEDs achieved a maximum external quantum efficiency (EQE) of 8.7%, bandwidth of 442 MHz, and data rate of 800 Mbit/s, while red micro-LEDs demonstrated an EQE of 5.95%, maximum bandwidth of 424 MHz, and data rate of 800 Mbit/s. The application of four core technologies, including circular devices and electrodes, reduced contact electrode area, atomic layer deposition (ALD) for passivation protection, and multi-chip parallel arrays, enhanced optoelectronic characteristics. This paper also highlights the superior performance of InGaN-based red micro-LEDs with a single quantum well (SQW) structure over double quantum wells (DQWs) for VLC applications. The SQW structure yielded higher maximum EQE, modulation bandwidth, and faster transmission rates, paving the way for the potential of full-color micro-display and high-speed VLC applications.

We tested the optical wireless power transmission (OWPT) using InGaN photovoltaic cells and lasers below 400 nm. As a result, 24.5% and 20.8% photoelectric conversion efficiencies were obtained at laser power densities of 16.5 mW/cm² and 398 mW/cm², respectively. The conversion efficiency decreased as incident laser power increased. On the other hand, the conversion efficiencies increased with increasing the temperature of the InGaN photovoltaic cells. We investigated the causes of these phenomena and found that lower resistance and improved crystal quality of InGaN cells significantly improve conversion efficiency further.

Gallium nitride (GaN) ultraviolet (UV) laser diodes (LDs) show tremendous promise for optical communications, data storage, and medical applications due to their compact size and higher efficiency compared to gas lasers. Typically, GaN UV LDs utilize a symmetric waveguide structure surrounding a multiple quantum well (MQW) active region for optical confinement. By increasing the thickness of these waveguides, device performance can be enhanced by reducing absorption losses. However, thin waveguides offer decreased carrier losses and improved electrical performance. These two competing effects can be balanced through the use of an asymmetric waveguide structure, composed of a thin upper waveguide and thick lower waveguide, in order to minimize both carrier (hole) losses as well as optical losses. Here, we demonstrated an edge-emitting ridge waveguide UV GaN LD emitting at ~392 nm. Mirror facets were fabricated through reactive ion etch and potassium hydroxide wet etch. These LD structures with InGaN/GaN MQWs and AlGaN cladding layers were grown via metalorganic chemical vapor deposition on a patterned sapphire substrate and utilize an asymmetric 100 nm thick upper unintentionally doped GaN (uGaN) waveguide and 500 nm thick lower uGaN waveguide structure. We have successfully demonstrated a LD device with 1000 μm cavity length with a lasing threshold of 2.2 A, and 111.8 mW per facet peak optical output power with a differential efficiency of 3%. This demonstration paves the way for GaN LDs with improved differential efficiency at high current densities through the use of optimized asymmetric waveguide structures.

This report shows the latest developments of Gallium nitride (GaN)-based blue (455nm) and green (525nm) edge-emitting laser diodes (LDs). The epitaxial layers were grown on c-plane free-standing GaN substrates by metal-organic chemical vapor deposition (MOCVD), and a ridge-type structure for refractive-index waveguide was fabricated. Each LD chip was mounted on a heat sink in a TO-Φ9 mm CAN package by a junction down method for improving thermal dissipation. Optimization of epitaxial layers and device structures has led to improve the wall-plug efficiency (WPE) of LDs. The WPE and the optical output power of the blue LD have reached to 52.4% and 5.99 W at the current of 3.0 A under continuous wave (CW) operation, respectively. We also confirmed that the WPE and the optical output power of the green LD were 24.2 % and 1.90 W at the CW current of 1.9 A, respectively. Each WPE is the highest value ever reported of blue and green LDs. On top of this, the lifetime tests of both LDs over 1000 hours indicate the long lifetime more than 30,000 hours defined by the time when an optical output power is expected to be lower than the half of initial value.

An n-type InAlN cladding design based on multiple GaN/InAlN pairs is successfully implemented in edge-emitting laser diodes (LDs) and superluminescent light emitting diodes (SLEDs) emitting in the blue and green spectral range. Thanks to the stronger refractive index contrast with respect to waveguiding layers enabled by this approach, larger optical confinement factors are obtained. The resulting larger modal gains translate into remarkable performance improvements for LD and SLEDs with respect to conventional AlGaN based claddings. LDs with threshold currents as low as 3 mA in the blue and 12 mA in the green spectral range are demonstrated. Similarly, an operating current decrease of >100 mA is reported for state-of-the-art green SLEDs.

In this study, growth of crack-free AlGaN-based UVC LEDs using face-to-face high-temperature annealed AlN templates on 6-inch sapphire substrates was investigated by production scale metal-organic chemical vapor deposition (MOCVD). The utilization of face-to-face annealed sputtered-deposited thin AlN films templates successfully mitigated wafer bowing, leading to crack-free growth. Additionally, single-peak UVC emissions were obtained in the growth of 6-inch × 7-wafers, and the in-plane uniformity of (Max – Min) / Average was approximately 1.6%. Face-to-face high-temperature annealed AlN proves to be suitable for growing large-diameter UVC LEDs wafers, paving the way for mass production.

We investigated the influence of V-pits on the turn-on voltage of GaN-based high periodicity multiple quantum wells (MQWs) solar cells with different thickness of the p-GaN layer. Experimental current-voltage characteristics indicate that the sample with the thinnest p-GaN layer presents an early turn-on, which is not present for samples with a thicker p-GaN layer. Focusing on the V-defects analysis, through scanning electron microscopy (SEM) we found no difference in the density and dimensions of V-pits between sample with different p-GaN thickness. Through TCAD Synopsys Sentaurus simulations, the main non-illuminated current-voltage characteristics are reproduced considering V-defects. The results indicate that V-pits play a dominant role in current conduction, especially for the devices with the thinnest p-GaN layer due to the insufficient V-pit planarization. For such devices V-pits penetrate the junctions, and locally put the MQWs region in closer connection to the p-side contact, resulting in the formation of localized short circuit paths. Finally, a Gaussian distribution of V-pits dimensions and depth is considered to reach a good matching of experimental data. Based on combined electrical analysis, microscopy investigation and 2D simulations, results provide insight on the role of V-pits on the electrical performance of GaN-based MQWs solar cells. The outcome of this work will be useful for the design of future high-periodicity quantum wells devices, ensuring the desired turn-on voltage and showing the existence of a trade-off between the need of a thin p-GaN (to increase short-wavelength efficiency) and a thicker p-GaN, to avoid insufficient V-pit planarization.

We investigated the carrier localization effect in μLEDs with low (blue) and high (green) indium composition quantum wells through the intensive analysis of the size-dependent forward bias current and electroluminescence mapping. We have observed a negative surface current and high electroluminescence intensity in random spots in green μLEDs, indicating the existence of a converging current to the deep well. On the other hand, in blue μLEDs, we observed a diverging current towards the mesa sidewall. We experimentally confirmed through EQE and J_{max EQE} that the converging current caused by carrier localization in green μLEDs is responsible for the size-immunity properties in green μLEDs.

Defects in the active region of light-emitting diodes (LEDs) have a strong correlation with low-frequency electrical and, consequently, optical fluctuations of the light source. We investigate these two types of noise in red, blue, and green commercially available LEDs using a superposition model of stationary (thermal and shot noise) and excess noise (flicker and generation-recombination noise). The dependence of the degree of the low-frequency component on the supply current and the LED material was obtained. The occurrence of Lorentzinana-type spectrum at high currents for blue and green LEDs is also observed.

In this study, ArF laser activation annealing of an Mg-doped GaN four-point probe small mesa device is investigated. Fabricated mesa device has 2-μm-high small mesa structures with In/Au contacts formed using standard semiconductor device process. In our setup, the ArF excimer laser was directed onto the sample using a series of mirrors and was focused through an aspheric lens to the sample, which is mounted on to a movable stage. The mesa device and the laser were aligned while being viewed on the computer monitor screen using a UV CCD camera. The dependence of the resistivity to the laser fluence and irradiation time were investigated by multiple-irradiation measurement cycles to a single mesa device. Results reveal that laser annealing at 530 mJ/cm² for 25 min (at 150 Hz repetition rate) decreases the resistivity from 11.5 (before irradiation) to 5.6 Ω·cm (after irradiation). This value is comparable to resistivity achieved by RTA at 800 ◦C for 2 min, suggesting successful Mg-doped GaN activation.