This PDF file contains the front matter associated with SPIE Proceedings Volume 12421, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

A nitride semiconductor InGaN is a key material for visible optical devices such as light-emitting diodes (LEDs) and laser diodes. Recently, InGaN-based red LEDs attain increasing interest due to potential application in micro- LED displays. However, the emission efficiency is the highest in the blue spectral range and drastically decreases at longer emission wavelengths. One reason is the large lattice mismatch between the InGaN emitters and the host material of GaN. To circumvent lattice-mismatch-induced issues, ScAlMgO₄ substrates are attractive because the host material can be replaced from GaN to InGaN lattice matched to ScAlMgO₄, which can reduce strain in the InGaN emitters. Herein, we demonstrate far-red (∼700 nm wavelength) LEDs based on In_xGa_1−xN/InyGa_1−yN quantum wells (x < y) grown on lattice-matched In_yGa_1−yN/ScAlMgO₄ (0001) templates for the first time.

Cubic nitrides are candidate materials for next-generation optoelectronic applications as they lack internal fields and promise to cover large parts of the electromagnetic spectrum from the deep UV towards the mid infrared. This demands high-quality epitaxial growth of c-GaN as base material. We demonstrate the influence of pre-growth treatments and c- AlN buffer layers on the quality of c-GaN grown on 3C-SiC/Si substrates by molecular beam epitaxy (MBE). Optimized parameters yield extremely small surface roughness values below 1 nm of phase pure c-GaN layers with very limited stacking fault densities. Structural properties have been studied by X-ray diffraction and atomic force microscopy and surpasses the current standards, which allows for growth of more complex quantum structures for device application.

We present a new approach for detecting interlayers in gallium-nitride (GaN) semiconductor structures bonded on single or polycrystalline diamond. GaN-on-diamond is advantageous because it integrates and effective heat spreader into GaN devices, which is important for high power density operation. However, analyzing GaN/diamond interlayers can be difficult because of nonuniformity as well as atomic-scale roughness across the wafer. In this work, we studied GaN/diamond samples using Raman spectroscopy and employed adaptive principal component analysis (PCA) to explore silicon interlayer properties. For each GaN sample bonded to diamond, the presence of a silicon interlayer was confirmed using high resolution scanning transmission electron microscope (HR-STEM) and electron energy loss spectroscopy (EELS). For comparison, we collected Raman spectroscopic data at 25 °C (5 different locations across the wafer, 10 microns apart in each direction) using 10 Raman spectrum acquisitions at each location in the 300-800 /cm range. Raman spectra only show three distinct vibrational modes at ~482 /cm, ~568 /cm and ~735 /cm, associated with the external lamp, GaN E₂ (high) and A₁ (LO) phonon modes. When spectra were analyzed using adaptive PCA, results reveal buried information in the baseline pertinent to the interlayer, including the ~520 /cm (silicon) phonon mode as well as ~360 /cm, and ~760 /cm modes, owing to nanoscale nonuniformities. Improved analysis of ultra-thin silicon interlayers in GaN/diamond opens up new opportunities to examine the impact of growth/post-growth steps, carrier density, and structure for modeling thermal boundary conductance.

Gallium nitride (GaN) is a promising wide-bandgap material for high-power electronics, where GaN-on-GaN homoepitaxy is being developed for fabrication of compact high-voltage vertical devices. However, variation in GaN substrate quality strongly influences the properties of epitaxial layers grown on top, which in turn affects device performance and reliability. Hence, better knowledge of the surface electronic properties is needed, especially after wafer processing steps that can introduce surface contaminants and oxide layers. Photoemission-based techniques provide chemical and electronic information but are surface-sensitive; therefore, the formation of native oxides or contamination from ambient conditions can affect findings. Here, we present the initial results of various surface treatment methods on the electronic properties of p-type GaN epitaxial layers grown via metal-organic chemical vapor deposition (MOCVD) in preparation for photoemission electron spectroscopy and microscopy characterization. We use X-ray photoelectron spectroscopy (XPS) to evaluate changes in residual contamination after treatment. We find that piranha-based cleaning methods have large reductions in surface carbon contamination, while NH4OH and HCl-based treatments remove surface oxide. The elemental core levels and valence band correspondingly exhibit binding energy shifts with the different treatment methods, indicating reduced surface band-bending. Both XPS and initial photoemission electron microscopy results of the photoelectron yield suggest a deeper valence band edge location with respect to the Fermi energy measured for the forming gas plasma-cleaned sample. These results demonstrate that combined ex-situ treatments for carbon and oxygen removal are more effective, yet further in-situ cleaning is necessary for more complete contaminant removal.

We investigate the degradation of high-periodicity GaN-based InGaN-GaN multiple quantum wells (MQWs) solar cells submitted to stress under high excitation intensity and high temperature; stress conditions are chosen to investigate cell behavior in a harsh scenario, such as wireless power transfer systems, space applications and concentrator harvesting systems. By examining the decrease in the short-circuit current and electroluminescence of these devices and the increase in the forward current at low bias, we suggest the presence of a thermally-activated diffusion process of impurities from the p-side of the device toward the active region. This process favors the increase in the Shockley-Read- Hall (SRH) recombination rate. By employing the van Opdorp and t’Hooft model, we analyzed the time-variation of non-radiative Shockley-Read-Hall lifetime during aging, extracting the diffusion coefficient of the defect involved in the degradation; we also extracted the related activation energy by an appropriate fitting of the degradation kinetics according to Fick’s second law of diffusion. The obtained values suggest that degradation originates from the diffusion of hydrogen, whose severity depends also on the thickness of the p-GaN layer of these devices. The proposed analysis methods and the obtained results are useful for understanding the physics of multiple quantum wells (MQWs) solar cells during degradation. The results can be used to increase the performance and reliability in novel applications where these devices are proposed, such us additional layer in multi-junction (MJ) solar cells, and the application in harsh environments.

The market of Ultraviolet (UV) Light Emitting Diodes (LEDs) is expected to expand substantially in the coming years, thanks to the disinfection properties of the UV light; however, a detailed study on the reliability-limiting processes is a fundamental step, for an effective deployment of this technology. We investigated the degradation mechanisms of AlGaN-based UV Single Quantum Well (SQW) LEDs, with a nominal emission wavelength of 265 nm, an area of 0.1 mm² and a nominal current density of 100 A·cm^-2. By means of constant current stress test and Capacitance Deep Level Transient Spectroscopy (C-DLTS) we studied the main electrical, optical, spectral and capacitance characteristics of the devices, in order to understand the dominant causes of degradation. For aged devices the electrical characterization shows increased subthreshold leakage currents, due to the increase in Trap Assisted Tunneling (TAT) components, as well as an increase in drive voltage, which is ascribed to contact degradation or a decrease in injection efficiency. The optical output power showed a decrease especially at low current levels, which has been ascribed to an increase in non-radiative recombination and suggests the generation of defects in the LED active region. C-DLTS measurements showed in unaged devices the presence of two defects in the structure, both ascribed to magnesium (Mg), located at 475 meV and 150 meV from the respective band. Moreover, we detected the increase in concentration of a third defect during the stress test with an activation energy of 700 meV, that acts as a point defect, and could be ascribed to gallium vacancies or nitrogen antisites.

A sustainable and efficient approach to gallium nitride (GaN) through physical nitriding reactions instead of traditional chemical solvent methods. Solvent-less, non-toxic processes effectively reduce the impurities and parasitic reaction substances in the final product, with no harmful emission or environmental pollution.

We present the most recent results of photoluminescence (PL) studies, classification of defects in GaN and their properties. In particular, the yellow luminescence band (labeled YL1) with a maximum at 2.17 eV in undoped GaN grown by most common techniques is unambiguously attributed to the isolated C_N acceptor. From the Zero-Phonon Line (ZPL) at 2.59 eV, the −/0 level of this acceptor is found at 0.916 eV above the valence band. The PL also reveals the 0/+ level of the CN at 0.33 eV above the valence band, which is responsible for the blue band (BLC), with the ZPL at 3.17 eV. Another yellow band (YL2) with a maximum at 2.3 eV, observed only in GaN grown by the ammonothermal method, is attributed to the V_Ga3H complex. The nitrogen vacancy (V_N) causes the green luminescence (GL2) band. The VN also forms complexes with acceptors such as Mg, Be, and Ca. These complexes are responsible for the red luminescence bands (the RL2 family) in high-resistivity GaN. The results from PL studies are compared with theoretical predictions. Uncertainties in the parameters of defects are discussed.

We report the gate leakage current and threshold voltage characteristics of Al_0.3Ga_0.7N/GaN heterojunction field effect transistor (HFET) with metal-organic chemical vapor deposition (MOCVD) grown β-Ga₂O₃ as a gate dielectric for the first time. In this study, GaN channel HFET and β-Ga₂O₃ passivated metal-oxide-semiconductor-HFET (MOS-HFET) structures were grown in MOCVD using N₂ as carrier gas on a sapphire substrate. X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to characterize the structural properties and surface morphology of the heterostructure. The electrical properties were analyzed using van der Pauw, Hall, and the mercury probe capacitance-voltage (C-V) measurement systems. The 2-dimensional electron gas (2DEG) carrier density for the heterostructure was found to be in the order of ~10¹³ cm^-2. The threshold voltage shifted more towards the negative side for the MOSHFET. The high-low (Hi-Lo) frequency-based C-V method was used to calculate the interface charge density for the oxide-AlGaN interface and was found to be in the order of ~10¹² cm²eV^-1. A remarkable reduction in leakage current from 2.33×10^-2 A/cm² for HFET to 1.03×10^-8 A/cm² for MOSHFET was observed demonstrating the viability of MOCVD-grown Ga₂O₃ as a gate dielectric.

Cubic In_xGa_1-xN alloys are a candidate material for optoelectronic applications because they lack internal polarization fields and promise to cover a vast range of emission wavelengths. However, the large discrepancy in interatomic spacing and growth temperatures of c-GaN and c-InN hinder In_xGa_1-xN-growth. We report cubic In_xGa_1-xN layers grown by plasmaassisted MBE and achieve continuous miscibility of the indium content x(In) over the whole composition range. X-ray diffraction precisely monitors the composition, phase purity and miscibility of the thin films. Furthermore, we discuss the impact of the indium content on the crystallinity. Complementary, low-temperature photoluminescence studies elucidate the optical response of cubic In_xGa_1-xN layers.

Behaviors of vacancy-type defects in ion-implanted GaN were studied by means of positron annihilation. Si or Mg ions were implanted into GaN to obtain 300-nm-deep box profiles of the impurities. The ion-implanted samples were annealed up to 1480°C under a N₂ pressure of 1 GPa (ultra-high-pressure annealing: UHPA). For as-implanted GaN, the major defect species was identified as Ga-vacancy-type defects such as a divacancy (V_GaV_N). After annealing above 1000°C, vacancy clusters, such as (V_GaV_N)₃, were introduced, and they were found to be remained even after 1480°C annealing. For Mg-implanted GaN with [Mg]=10¹⁸ cm^-3, no large change in the depth distribution of Mg was observed before and after annealing at 1400°C. For the sample with [Mg]=10¹⁹ cm^-3, however, Mg diffused into the bulk, which was attributed to the over-doping of Mg and their vacancy-assisted diffusion. The Mg diffusion was suppressed by sequential N-implantation, which was attributed to the reaction between Mg and vacancies under a N-rich condition. Interactions between vacancies, Mg, and H during UHPA were also discussed.

GaN-based multi-quantum wells solar cells could be breakthrough devices for extreme applications like space environment and harsh photovoltaics. We performed a forward-bias stress on samples with 30 quantum wells with 15% indium content to better understand degradation mechanisms. DUTs were characterized by means of dark and illuminated IV, CV and Steady-State Photocapacitance measurements. First, we performed a step-stress experiment, by increasing current in 30 minutes steps up to 500 mA, when the device failed. We observed a variation in series resistance, a decrease in shunt resistance and a strong decrease in EQE, conversion efficiency and open-circuit voltage, especially at low excitation intensities. CV measurements showed an increase and then a decrease in free charge density in the device, that was correlated to the variation in trap-states density evaluated by SSPC. Based on the results of this step-stress experiment, we carried out a 100 hours constant-current stress at 40 mA. This stress showed a moderate decrease in series resistance and a decrease in EQE and conversion efficiency. An increase in charge density was observed and correlated with the decrease in conversion efficiency. The degradation of the device was related to the generation of defects, that may create a nearmidgap states, detected by SSPC measurement, and a shallow donor state that generates a change in free carrier density. These defects possibly migrate through the devices, as they are detected at different times by SSPC and CV measurements.

Semiconductor device developments are typically guided by simulations – what is the best epitaxial design, or how should field plates or edge terminations be designed. To assess how a particular device reflects what has been designed, comparing IV curves measured and simulated is typical, but for example different implantation or doping levels can lead to the same too low breakdown voltage, and there is then little guidance for example how to mitigate such a problem to optimize/enhance breakdown voltage. We illustrate the latest capability of a technique we developed, based on electricfield- induced optical second-harmonic generation (EFISHG), on vertical GaN-on-GaN pn junction examples, to directly access with submicron spatial resolution electric strength in a device, to aid improving device design and their implementation, and reducing product development cycles.

This paper reports the latest device performance of high-power blue and green edge-emitting Laser Diodes (LDs). The epitaxial layers 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. Fabricated every LD chip was mounted on a heat sink using a junction down method in a TO-Φ9 mm package. 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 5.90 W and 3.81 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 51.6 % at 3 A. This is the highest WPE reported so far. The peak WPE of the 455 nm LD was 52.4 % at the forward current of 2.2 A. And a new developed 525 nm green LD showed the optical output power and the voltage of 1.86 W and 4.12 V at the forward current of 1.9 A under CW operation. The wall-plug efficiency (WPE) of the 525 nm green LD was 23.8 % at 1.9 A. This is the highest WPE reported so far. The peak WPE of the 525 nm LD was 25.9 % at the forward current of 1.1 A.

InGaN lasers in the blue and green wavelength range have opened a wide variety of applications in the past years, which all require unique properties of the employed laser chips. In this paper we will show design and process developments for various InGaN laser designs, each optimized for its specific application. For applications which are very sensitive to energy consumption, like mobile AR/VR devices, we investigated InGaN laser chips with resonator lengths as short as 50 μm. To achieve this, we developed an etched facets technology to overcome the challenges of scribing and breaking for facet generation for such short resonator lengths. The etched facets of these devices are coated on-wafer with a dielectric mirror to achieve the desired reflectivity. Depending on the reflectivity chosen, these devices show ultra-low threshold currents below 3mA and output powers above 50 mW. Combined with a flip-chip design with both contacts on one side, such chips can be integrated into silicon wafer-based beam combiners to generate RBG PIC chips for VR/AR laser projection. For high power applications, we will present data of laser bars. Bars emitting at 430 nm achieved 100 W of continuouswave output power per bar and conversion efficiencies of 50%. Together with bars emitting at 450 nm, that were shown in previous publications, wavelength-multiplexing for materials-processing systems can be realized yielding blue laser light sources with multiple kilowatts of output powers.

We report the latest progress of gallium nitride-based Vertical-Cavity Surface-Emitting Lasers (VCSELs) containing a curved mirror. Highly uniform and efficient devices were developed. The average threshold current was 0.64 mA with a standard deviation of 0.043 mA. The peak wall-plug efficiency and output power were 13.4 % and 7.6 mW at operating currents of 5.2 mA and 12.8 mA, respectively. We obtained green VCSELs with milliwatt-class outputs and a wall-plug efficiency of 3.7%. We also report the progress of VCSELs with a single-cavity filtering mirror and a cavity length of approximately 25 μm showing highly varying reflectivity spectra, to demonstrate their single-longitudinal mode operation.

We demonstrate long cavity (60.5 ) GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a topside curved monolithic GaN lens, a buried tunnel junction (BTJ) current aperture, and a bottomside nanoporous GaN distributed Bragg reflector (DBR). Under pulsed operation (1% duty cycle, 1μs pulse), a VCSEL with a 9μm diameter aperture had a threshold current density of 6.6kA/cm², a maximum output power of 3mW, and a differential efficiency of 5.6% for a lasing mode at 411nm. Under CW operation, the threshold current density was 7.3kA/ cm2, the differential efficiency was 2.8%, and a peak output power of 1.1mW at rollover was reached. Preliminary farfield patterns are presented and discussed.

High-efficiency green laser diodes remain a challenge for researchers and manufacturers due to several factors intrinsic to the InAlGaN material system. Notably, the high indium content quantum wells (QW) are sensitive to thermal damage in the form of indium segregation, particularly when the QWs are exposed to the high-temperature growth steps of the ptype AlGaN cladding layers in MOCVD. This paper presents device results from blue distributed feedback (DFB) laser diodes grown entirely by MOCVD compared to material grown by MOCVD through the active region and overgrown with low-temperature p-AlGaN using remote plasma chemical vapor deposition (RPCVD). Preliminary data demonstrate comparable performance from MOCVD-only and MOCVD with RPCVD p-cladding, showing a full-width at half maximum emission of 3 pm and side mode suppression ratio of 19 dB. RPCVD enables the growth of high-quality ptype AlGaN layers at temperatures below the thermal damage threshold of In-rich QWs, leading to longer wavelength devices. The degree of thermal damage to the In-rich green QWs is assessed using fluorescence microscopy to directly compare the RPCVD-enhanced p-AlGaN growths at various growth temperatures over all-MOCVD n-side and green quantum wells.

Although AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been studied extensively, their quantum efficiency and optical output power still remain extremely low compared to the InGaN-based visible color LEDs. Electron leakage has been identified as one of the most possible reasons for the low internal quantum efficiency (IQE) in AlGaN based UV LEDs. The integration of a p-doped AlGaN electron blocking layer (EBL) or/and increasing the conduction band barrier heights with prompt utilization of higher Al composition quantum barriers (QBs) in the LED could mitigate the electron leakage problem to an extent, but not completely. In this context, we introduce a promising approach to alleviate the electron overflow without using EBL by utilizing graded concave QBs instead of conventional QBs in AlGaN UV LEDs. Overall, the carrier transportation, confinement capability and radiative recombination are significantly improved. As a result, the IQE, and output power of the proposed concave QB LED were enhanced by ~25.4% and ~25.6% compared to the conventional LED for emission at ~254 nm, under 60 mA injection current.

Micro-Light-Emitting Diode (μLED) displays have seen increasing interest over the past decade due to their promising advantages over other display technologies, especially in applications requiring extremely high resolutions such as virtual and alternate reality headsets. Most modern full-color μLED displays rely on red, green, and blue (RGB) pixels based on different material systems combined together on a thin-film transistor back panel, which is often costly and has poor yield. An alternative approach is to create a monolithic display in the GaN/InGaN material system, capable of covering the entire visible spectrum through tuning of quantum well (QW) Indium content or phosphor down conversion. However, monolithic GaN displays present the issue of pixel isolation, as the lack of truly insulating undoped GaN (u-GaN) makes it difficult to electrically isolate rows or columns of μLEDs from one another. In this work, we demonstrate a novel solution to this issue which utilizes photoresist to fill deep trench isolation features, enabling interconnection of μLED p-contacts in a single-color passive matrix display. A photoresist layer is used to fill deep trenches isolating columns of μLEDs from one another. This photoresist layer is patterned and baked at 250 °C for 30 minutes, crosslinking it and making it extremely durable. This process allows for formation of p-interconnects using liftoff, and avoids the issues involved in bridging high aspect ratio trenches. This photoresist planarized trench isolation process could contribute to creation of improved monolithic full color μLED displays which require multiple deep isolation features to be bridged by conductive interconnects.

The use of nanowires has recently emerged to go further in the development of nitride UV LEDs, especially to improve the efficiency or to make micro-sized UV-sources and flexible LEDs/photodetectors. Our current research interest is focused on the core-shell UV quantum wells grown by industrial MOVPE epitaxy tool. Several types of core-shell quantum wells have been successfully achieved: GaN/InAlN for UV-A, GaN/AlGaN or AlGaN/AlGaN for UV-A and UV-B. When the UV GaN/AlGaN QWs are embedded in a core-shell AlGaN p-n junction, the electroluminescence is demonstrated on single-wire μLEDs in the UV-A and UV-B. The further development of core-shell GaN/AlGaN monolayered system is also shown with demonstration of UV-B emission for single-wire μLEDs.

Suppressing the emission from (0001) plane region with low luminescence efficiency that exists on the top of nanowires (NWs) is necessary to realize highly efficient GaInN/GaN multi quantum shell (MQS) light-emitting diodes (LEDs). In this study, we attempted to improve the crystal growth by introducing electron blocking layers (EBLs) and to suppress the current leak in NW LEDs by using SiO₂ insulating film on top of NWs. The EBL structure features different thicknesses at each crystalline plane to reduce the current injection into (0001) plane, suppress red emission with low luminescence efficiency, and improve the light output by 2.4 times. Given that the luminescence from (0001) plane region remains, further optimization of EBL growth conditions, such as V/III ratio and Al composition, is essential. In addition, the luminescence from (0001) plane region and the current leakage can be reduced by forming SiO₂ insulating film on top of the NWs. Although even if an insulating film was formed on the top of NWs on which EBLs were grown, the SiO₂ adhering to (1-101) plane resulted in a decrease in light output and destruction due to high resistance. The results indicate the possibility of realizing highly efficient GaInN/GaN MQS-NW LEDs by inserting EBL structures between MQS and p- GaN shell.

We report the productive outcome of the design, fabrication, and testing of a new class of GaN/AlGaN-based ultrafast, sensitive photodiodes for deep-UV solar blind photodetection. Pt was employed as the interdigitated metal electrode to establish that while a Schottky contact was formed at the metal/semiconductor heterojunction, it was the quantum wells in the p-i-n vertical structure that produced the diode characteristics of the detectors. The metal‒semiconductor‒metal design retained its ultrafast property as expected in the p-i-n structure because the spacing between the interdigitated fingers was asymmetrically implemented to optimize the photogenerated carriers’ transit time from the quantum well to the external circuitry. Also, the vertical p-i-n structure provided its renowned efficient photocarrier generation, which was significantly enhanced by the delta quantum well architecture of the vertical epitaxial structure. The active area of the device was 300 μm × 300 μm with 5-μm finger width, and asymmetric electrode spacing of 2 μm, 3 μm, and 5 μm. The best device had peak responsivity of 3.5 A/W under 262-nm illumination with full width half maximum of 337.2-ps and 37.5-ps rise times when biased with 20 V.

Self-assembled strain-free growth of In droplets on GaN via droplet epitaxy (DE) technique was investigated. Controlling In droplet size and density as a function of substrate temperature is described. The highest density of 1.36 × 108 cm^-2 was observed at a very low substrate temperature of 30 °C. The resulting droplets are crystalline at room temperature and are characterized ex-situ by atomic force microscopy (AFM) and x-ray diffraction (XRD). The formation of quantum dots (QDs) through crystallization of In droplets grown using the DE method has many advantages over the strain driven Stranski-Krastinow technique, including the ability to control a wide range of QD shapes, sizes, and densities, as well as overcoming the limitations of lattice mismatched with substrates. This study explains the first stage of forming a controlled InN QD on GaN.