Gallium Nitride Materials and Devices XX

Attoseconds: When intense light interacts with a gas of atoms (or a transparent solid), electron wave packets are released. Attosecond pulse formation exploits the correlated electrons and holes, forcing the electron to return. Without the plasma connection, two of the most important strong-field process that accompany attosecond pulse formation—hot electron formation (inverse Bremsstrahlung) and non-sequential double ionization (collisional ionization)—seemed mysterious. These plasma-like processes lead to laser induced electron diffraction and orbital tomography. THz generation: Terahertz pulse formation by ionization has a similar linage. Using PIC codes to describe azimuthally polarized l=4 mm and 2 mm light interacting with a 150 µm thick jet of helium, we calculate THz pulses reaching 8.5 Tesla. But 10 Tesla is not a limit. 30 THz azimuthally polarized beams can be amplified in high-pressure CO2 reaching isolated magnetic fields of 1-gigagauss.

In recent years, topological phenomena in photonic systems have attracted much attention, with their striking features arising from robust states in the energy gaps of spatially periodic media. However, light waves are entities that extend in space as well as time, such that one may ask whether topological effects can also occur in the temporal domain, or even space-time. Intuitively, systems that are periodic in time may be gapped in momentum, leading to topological states localized at time interfaces. However, time - in contrast to space - exhibits a unique unidirectionality often referred to as the “arrow of time”. Inspired by these features, I will present our most recent experiments on topological states residing at temporal interfaces. Moreover, I will discuss the formation of spacetime-topological events and demonstrate unique features such as their limited collapse under disorder and causality-suppressed coupling.

Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in high-performance computing and simulation of quantum systems, as well as in sensor technology. However, the experimental realization of suitable system still poses considerable challenges. Current efforts in photonic quantum science target the implementation of practical devices and scalable systems, where the realization of quantum devices and controlled quantum network structures is key for envisioned future technologies. Here we present our progress on the engineering of integrated photonic systems, which can overcome current limitations for the realization of scalable photonic systems. Specifically, our research currently focuses on three different but complementary topics: integrated devices based on lithium niobate circuits, engineering and harnessing the temporal-spectral structure of quantum states of light, and photonic quantum computation.

Using our void-assisted separation (VAS) method based on hydride vapor phase epitaxy (HVPE), a 6-inch free-standing GaN wafer has been successfully realized. Extremely wide range of n-type doping from 1e15 to 1e20 /cm3 is available. High enough thermal conductivity around 150 W/mK, suitable for high-power device operation, was maintained even when the doping concentration was 1e20/cm3. In addition, we will also talk about newly developed method suitable for fabrication of large-sized GaN substrates, named pore-assisted separation (PAS) method, where GaN substrate is separated at a porous layer formed electrochemically inside a seed GaN-template.

ScAlN is a wide bandgap semiconductor with large piezoelectric and spontaneous polarization coefficients ensuring a very high charge density at the interface with GaN, which makes it a promising barrier layer for HEMTs in view of power switching and RF/mm-wave power amplifier applications. In this work, ammonia source molecular beam epitaxy was used to grow the heterostructures under nitrogen-rich regime. An optimum temperature was identified for the growth of ScxAl1-xN barriers quasi-lattice matched on GaN (x~14%). Functional transistors with 9 µm source-drain spacing have been fabricated on barriers as thin as 5 nm. Drain current density exceeds 700 mA/mm on 10 nm barrier and 1 A/mm on 25 nm barrier while a limited gate leakage current can be observed up to drain voltage of 100 V close to the buffer breakdown. This work is partly supported by Labex GaNeX (ANR-11-LABX-0014) and ECSEL JU project GaN4AP under Grant Agreement No. 101007310.

This presentation explores the epitaxial growth and application of AlScN and AlYN in electronic devices. High-quality AlScN and AlYN layers were grown by MOCVD, revealing benefits such as higher sheet carrier density and lattice-matched growth to GaN for improved transistor reliability. The structural and electrical properties of these layers were analyzed, and progress in HEMT performance, achieving output power beyond 8 W/mm at 30 GHz, will be discussed. Additionally, the ferroelectric properties of AlScN and AlYN layers will be compared to sputtered layers, highlighting the measured coercive field of 5.5 MV cm-1 for AlScN and challenges with AlYN.

The true band gap of InN was found to be around 0.7 eV in 2002, instead of the long-believed value of 1.9 eV, and the applicable areas covered by nitride semiconductors have expanded significantly. One of the promising devices in recent years is the red LED. The fabrication of high-quality GaInN-based structure, including a thick GaInN film and GaInN/GaInN quantum wells, on GaN substrate or template has still been a challenging topic, because of the serious problems on the phase separation and large lattice mismatch system of GaInN. RF-MBE is good at low-temperature growth, and in-situ monitoring is the powerful tool to understand the growth kinetics of GaInN. In this talk, we report the results and findings from in-situ monitoring using RHEED and XRD during the growth of GaInN by RF-MBE.

High hydrostatic pressure generated in diamond anvil cell is an efficient experimental tool that allows obtaining a lot of data which are difficult or not possible to get by other methods. In particular, it can be used to verify the mechanisms of light emission in semiconductor structures. An overview will be given on the high-pressure studies of phenomena influencing the radiative transition energy and emission efficiency in nitride (In,Al,Ga)N alloys and heterostructures. They include: ­- carrier localization processes and the presence of localized donor states, - conduction band filling and non-parabolicity effects, ­- effects related to built-in electric fields present in the wurtzite structures grown along the polar crystallographic direction, a consequence of a high piezo- and spontaneous polarization in these materials, The spectroscopic properties and the pressure dependence of light emission of different series of III-nitride-based samples will be discussed.

Scandium aluminum nitride has emerged as an alternative to conventional III-nitrides for designing optoelectronic devices. Wurtzite ScAlN is lattice-matched to GaN, but the exact lattice-matching composition remains a subject of debate. In this work, we demonstrate that ScₓAl₁₋ₓN is lattice-matched to GaN at x = 14.0 ± 1.0%. Using lattice constants derived from reciprocal space maps and alloy compositions estimated by Rutherford backscattering, we establish an inexpensive and fast method to extract alloy compositions of single-layer ScAlN coherently strained to GaN. Furthermore, the X-ray rocking curves of the ScAlN films discussed in this study exhibit the smallest linewidths reported in the literature, with a 300 nm Sc₀.₁₄Al₀.₈₆N film demonstrating an exceptionally low linewidth of 106 arcseconds..

InGaN-based devices are pivotal in the lighting market due to their high efficiency and long lifespan. However, emerging fields such as micro- and nano-displays require further improvement of their electronic and optical properties. The key factor in understanding the limits of InGaN is the influence of strain on its basic physical properties. Unfortunately, many studies focus on investigating plastically relaxed InGaN layers, which are not used in devices. In this study, we investigate elastic strain in InGaN layers on nanoporous GaN. The InGaN layers are grown epitaxially on GaN substrates and formed into pillars from 8 μm to 100 nm in diameter using electron beam lithography and reactive ion etching. After porosifying the GaN layer underneath using electrochemical etching, InGaN layers relax elastically. Using scanning X-ray diffraction microscopy (SXDM), we mapped the strain distribution within the InGaN nanostructures and correlated it with the luminescence spectrum, the built-in electric field and the emission energy shift. The work presented in this paper benefits from the support received from the Polish Ministry of Science and Higher Education, dec. no. 2021/WK/11

GaN has recently been shown to host bright, photostable, defect single-photon emitters in the 600–700 nm wavelength range that are promising for quantum applications. Our studies have revealed the optical dipole structure, mechanisms associated with optical dipole dephasing, and the spin structure of these emitters. We have also discovered optically detected magnetic resonance (ODMR) in two distinct species of defects. In one group, we found negative optically detected magnetic resonance of a few percent associated with a metastable electronic state, whereas in the other, we found positive optically detected magnetic resonance of up to 30% associated with the ground and optically excited electronic states. We also established coherent control over a single defect’s ground-state spin. In this talk, we will present our results on the basic physics of these defects and also discuss the spin physics associated with the observed ODMR.

Low energy photoemission (PE) and electro-emission (EE) experiments on nitride materials and devices provide a rich crop of results on band parameters, electron processes and carrier dynamics. We single out Auger-Meitner generation of hot carriers in the InxGa1-xN quantum wells as the main source of LED energy efficiency droop and the energy position of the second conduction band at ~1 eV above the main conduction band in GaN. Through a number of different experiments, the original assessments of Iveland et al, [Phys. Rev. Lett. 110, 177406, 2013] have proven correct. The hot electron peaks observed in EE from LEDs are shown to be generated within the active layers, changing with In composition and disappearing in GaN p-n junctions. They are due to Auger-Meitner processes, of intrinsic or extrinsic nature. Higher energy conduction bands are identified either through the accumulation of hot electrons, before full energy relaxation, in EE and PE electron distribution curves (EDCs), or as onsets of hot carrier photo-generation in PE measurements. The first excited conduction band is also directly observed through intervalley absorption.

InGaN/GaN quantum wells grown in the zincblende cubic phase are promising alternatives to conventional quantum wells grown in the wurtzite phase, benefiting from a ~100-fold faster rate of recombination. Here, temperature- and excitation-dependent photoluminscence measurements are used to study the emission properties of zincblende quantum wells grown by metal-organic chemical vapour deposition. Emission that can be tuned across the visible spectrum by varying the indium content is demonstrated. Quenching of the emission as the temperature is increased is significantly reduced as the emission wavelength is increased, consistent with thermal escape from the quantum wells being an important factor.

The development of UV lamps is primarily driven by their application in disinfection within the spectral ranges of 260-270 nm and 220-230 nm, the latter being safer for humans. The pursuit of higher efficiency at shorter wavelengths has motivated research on cathodoluminescent lamps, where light emission is achieved by exciting a semiconductor with an electron beam from a cold cathode. Key semiconductor requirements include high radiative efficiency at room temperature, and stability under electron beam pumping. In this context, we have analized 3 types of AlGaN/AlN nanostructures as potential anodes: Stransky-Krastanov quantum dots, nanowire-embedded dots and ultra-thin quantum wells. These nanostrutures, all of them synthesized by molecular beam epitaxy, were engineered within a 500 nm active region, with dimensions and composition tailored to cover the 280-230 nm spectral range. We present a comparative analysis of their spectral behavior, optical polarization of the emission, internal quantum efficiency and power efficiency.

The III-nitride semiconductors offer every building block needed for a universal photonic platform from the near-infrared to the visible and ultra-violet spectral range. Moreover, the photonic integration with III-nitrides offer many advantages in terms of monolithic integration as compared to other solutions de-pending on heterogeneous integration. We will show that photonic circuits with high quality factor resonators can be obtained either on silicon or sapphire substrates. One very peculiar property of the III-nitride semi-conductors is linked to the crystalline tetrahedral bonding in these wurtzite-type structures and their band structure leading to opposite signs for the second-order nonlinear susceptibility between GaN and AlN. We will show that this feature can provide efficient nonlinear interactions following a very simple monolithic approach. We will finally discuss the integration of other materials, like 2D materials, on this platform.

Modeling the forward bias I-V characteristics of multi-quantum well (MQW) GaN photodetectors provides crucial insights for optimizing device performance and reliability. This study presents an analytical model that incorporates radiative and nonradiative recombination mechanisms, thermionic current at the GaN/InGaN interface, and voltage drop across the quantum wells. The model, validated with experimental data from state-of-the-art MicroLEDs, reveals valuable information on carrier dynamics, defect states, and thermal behavior. These findings are essential for improving the design, fabrication, and operational reliability of GaN photodetectors in high-power and mixed-mode applications.

The monolithic integrated structure of LET-PD has been studied. The epitaxial wafer was designed and grown to enable a light emission, light detection and switching. The device was fabricated and characterized to verify all three operations within a single module. Furthermore, a simultaneous light emission and light detection operation was tested to check self-feedback capability. Thus, the monolithically integrated device has several advantages, including a precise self-examination of emitted light, delicate moderation of outputs through a voltage control, and downsized operation area by merging three devices into a single module, making the device a potential candidate for future display technology.

The emerging field of Quantum Information Sciences is enabled by high-performance laser sources. With the objective to replace large-sized laser systems with chip-based solutions, significant benefits in size, weight, power, and cost can be realized. We will report on chip-integrated GaN-based lasers with superior spectral performance, offering specifications conventionally only accomplished with large-sized external feedback lasers. The ultra-narrow emission linewidth with sub-MHz noise-levels makes them excellent candidates for laser Doppler cooling in atomic/ionic quantum applications, such as optical atomic clocks based on Ytterbium or Strontium. The low-noise, single-frequency emission is realized through laser self-injection locking of a single-mode ridge-waveguide AlGaInN Laser Diode coupled to a Photonic Integrated Circuit Chip with feedback from high-Q (0.2-0.5M) micro-resonators. This laser concept is demonstrated with both the CMOS-compatible SiN PIC and the crystalline AlN, AlGaN, and GaN PIC platforms. We will show chip-based laser operation in the blue, violet, and UV-A spectral range (383nm), with fast hysteresis-free frequency tuning via piezo-electric actuation.

AlGaN/GaN high electron mobility transistors (HEMTs) have gradually been used in RF/microwave and power electronics applications, mainly because of their high breakdown voltage, high power, and better thermal conductivity. Unlike InP or GaAs HEMTs, where the 2D electron gas (2DEG) results from modulation or delta doping, the 2DEG in GaN HEMTs is due to spontaneous polarization and piezoelectric polarization. Depending on the articles published so far, the polarization change due to thermally induced lattice mismatch has been reported to cause electron charge density changes, this is typically limited to the in-plane (x-y plane) direction. But the research on residual thermal strain in the out-of-plane (c/z direction) is very limited. Our study focuses on both in-plane and out-of-plane thermal strain in the epilayers of GaN and how electron charge density changes at different lattice temperatures. Empirical relationships among different physical parameters are given, and numerical simulations on a 0.75 μm gate device are studied.

In this work, leakage current reduction of GaN APDs is demonstrated utilizing shallow-bevel-mesa edge termination of ~1-2°. Metalorganic chemical vapor deposition was used to grow GaN APD device structures. Photoresist masks were formed by multi-step lithography followed by thermal reflow. The bevel-edged mesa was formed by inductively coupled plasma etching. A recessed window was formed by ICP etching in the center of the mesa to reduce optical losses in the p-GaN layer. The 300K J-V characteristics of GaN APDs were measured. The leakage current density was less than 10-9 A/cm2 up to a reverse bias of -70 V. Of 25 measured APD devices, the breakdown voltage (BV) is -104.05±1.25 V, and the leakage current density at the onset of reverse BV is 5.4×10-5±1.5×10-5 A/cm2 with the lowest value of 3.2×10-5 A/cm2. The temperature-dependent J-V analysis attests avalanche breakdown mechanism and reveals that trap-assisted tunneling current is the dominant leakage mechanism prior to avalanche breakdown.

We report the improvement in device performance of triboelectric touch sensors (TESs) fabricated with InN nanowires (NWs) additionally using a strain-nanoridge structure (S-NaRS). For the fabrication of the S-NaRS of the TESs, polydimethylsiloxane (PDMS) was first injected onto the InN NWs on Si(111), and the InN NWs/PDMS layer was mechanically peeled off from the Si substrate. And then, InN NW/PDMS was transferred onto a flexible indium-tin-oxide/polyethylene terephthalate substrate. When hammering the top surface of the TES with a human finger, the output voltage was measured to be 31.2 V, which is approximately three times higher than that (10.2 V) of the TES without InN NWs. The device performances of the InN-NW TES rarely changed under various measurement conditions such as operating moving frequency and operation time (up to 28 days).

We have grown high quality GaN and AlGaN by pulsed sputtering. We have also performed impurity doping of Si, Ge, Sn, and Mg and found that record low resistivity can be achieved for GaN and AlGaN. Heavily doped GaN samples also showed the record large optical bandgap of 3.84 eV due to the Moss-Berstein effect. We have tested these materials for the use of tunneling contacts for p-type layer of blue LEDs and obtained the same operational voltages as those for commercially available LEDs with TCO contacts. We have also test the sputtering materials as the source and drain epitaxial; regrowth materials for AlGaN/GN and AlN/AlGaN high electron mobility transistors and found that record low contact resistance can be achieved with this technique.

We have developed a method to precisely form a 4λ GaN cavity in a GaN-based VCSEL by using in situ reflectivity spectra measurement. By monitoring a clear reflectivity intensity oscillation at the center wavelength of a bottom DBR stopband during the epitaxial growth, we obtained a precise cavity thickness control within 0.5% error. In the cavity, we also inserted a GaInN underlayer which improved light output power from GaInN quantum wells even at high current density over 5 kA/cm2. We then obtained a maximum wall plug efficiency over 20% from the GaN-based VCSEL emitting an emission wavelength of 418 nm with a 5 µm aperture.

EXALOS recently demonstrated SLEDs exhibiting increased optical confinement factors (modal gains) at 512 nm by implementing an InAlN-based n-type cladding in the epitaxial structure. By leveraging the latter approach and by growth conditions optimization, here we demonstrate SLEDs devices at 525 nm and discuss their performance. A direct comparison with LDs realized from the same epitaxial wafer, exhibiting 10 nm longer emission wavelength, will be presented. This result, together with varied experimental data will allow us to elucidate the challenges faced when extending the SLED wavelength in the true green spectral range and beyond.

Surface-emitting superluminescent diodes (SLDs) with horizontal cavities in the visible spectrum offer a significant advancement in optoelectronics, combining the benefits of LEDs and LDs. Characterized by broad emission spectrum, directionality, and high output power, SLDs are ideal for applications requiring high-brightness, high spatial coherence, and low temporal coherence, such as imaging and projection systems due to their speckle-free emission. This study focuses on SLDs emitting in the blue spectral region using gallium nitride (GaN) materials with a novel epitaxial structure enhancing carrier injection and recombination efficiency. Advanced fabrication techniques yielded a surface-emitting architecture using ridge waveguiding with a 45° total internal reflector on the facets, achieving high power and broad spectral output. The SLDs demonstrated a peak emission wavelength around 4xx nm with FWHM of 2-8 nm, depending on cavity length, and peak optical output power up to 2W under pulse conditions. These features make SLDs cost-effective and suitable for next-gen optical systems, highlighting their potential for widespread adoption in high-performance applications.

Laser diodes in the deep-ultraviolet (DUV) wavelength range have rapidly developed in recent years and are expected to become a new, compact, and energy-efficient laser light source for wavelengths below 300 nm. Our group has successfully demonstrated continuous-wave lasing of an AlGaN-based DUV laser diode at 270 nm using a single-crystal AlN substrate. In this talk, we will review the critical technical challenges related to the material system and the breakthroughs in realizing these devices. Additionally, we will introduce recent progress of technology development and discuss the further issues of the device that need to be addressed for their practical application.

Recently, we have experimentally demonstrated far ultraviolet wavelength conversions in the wavelength range of 230 nm using transverse quasi-phase-matched AlN waveguide with a polarity-inverted bilayer epitaxially fabricated by sputtering with post-deposition face-to-face annealing. The device is operated under the modal-phase-matching condition between the lowest-order transverse magnetic mode for the fundamental waves and the higher-order mode for the harmonics. Also, phase matching along vertical direction can be simultaneously satisfied by maximizing an overlap between the nonlinear polarization and modal distribution of second harmonic wave. In this presentation, we will outline recent results from the introduction of two types of structural singularities to enhance FUV emission from AlGaN-based wavelength conversion devices: multiple polarity inversion at each node of transverse electromagnetic mode with second harmonic frequencies and strained layer superlattices to enhance the second-order nonlinearity via strong piezoelectric polarization.

Gain-guided lasers can be considered the simplest and most economical alternative to index-guided counterparts. However, in the case of AlInGaN semiconductors, gain-guided laser diode technology has not been extensively explored. In this work, we discuss the feasibility of using an extremely simplified processing technology for InGaN laser diodes. We fabricated conventional laser structures by MOVPE growth, with the active area consisting of two InGaN quantum wells, sandwiched between InGaN waveguides and AlGaN claddings. Current confinement was achieved by exposing the surface of the structure to oxygen plasma in a RIE reactor. No ridge structure was formed, and no isolation oxide was used, which favors single lateral mode operation. The lasers showed a lasing threshold of approximately 6.5 kA/cm² for a 5 µm-wide contact stripe, decreasing to 4 kA/cm² for a 20 µm stripe. The operating voltage was close to 4 V at 5 kA/cm², which is a satisfactory result. The threshold current of these devices is roughly twice as large as that of ridge waveguide devices made from the same wafer, which aligns well with numerical simulations of this type of waveguide. Although the processing scheme i

In this talk we will discuss the achievement of polariton lasing in a polariton GaN waveguide where vertical GaN/Air Bragg reflectors define horizontal cavities. After assessing the strong-coupling regime in these structures, we will demonstrate first quasi-CW polariton lasing operation at low temperature (70K). We will show that polariton lasing can be achieved by pumping the entire laser cavity, as in standard semiconductor edge-emitting lasers, but also when pumping a small fraction of the cavity, much shorter than 50% of the total length. Interestingly, when temperature is increased to 150K the behavior at threshold changes dramatically and the laser emission becomes inherently multimode, with about ten modes participating in the nonlinear emission. The onset of mode-locking can be explained by the specific self-focusing condition realized in a polariton waveguide, which combines a strong positive group velocity dispersion and a strong repulsive polariton-polariton interaction. Overall, these findings constitute a first demonstration of strong polariton nonlinearities in large bandgap semiconductors and open the way to exploit them at room-temperature.

We will present recent developments on gallium nitride-based edge-emitting lasers for high-power applications. We have been working on the development of GaN-based laser diodes (LDs) for projectors and copper processing. This time, we have improved the wall-plug efficiency of blue and green LDs for projectors to 53.5% and 25.0% respectively under continuous-wave (CW) operation. In addition, we achieved the highest output of over 25W per chip for processing under CW operation. We will report the details on the day of the presentation.

High-speed visible light communication (VLC) has been receiving increasing research attention recently owing to its unique advantages including unregulated channel, free of EMI. The modulation characteristics of GaN-based edge emitting laser diodes will be presented. The recent progress in design, fabrication and characterization of InGaN-based violet-blue laser diodes will be presented. The modulation bandwidth exceeding 7 GHz has been achieved.

In this work, we explore degradation mechanisms in nitride-based laser diodes. Despite the maturity of this technology, there is still no comprehensive model for the degradation processes. We previously proposed a mechanism for laser facet degradation due to oxidation in a water vapor-containing atmosphere, explaining surface degradation well. However, this is not the only important process for InGaN laser diode reliability. Point defects, likely cation and anion vacancies, are suspected of causing deterioration, but they are difficult to detect and identify. Epitaxial growth parameters define the type and concentration of these defects. We studied the relationship between p-type layer (GaN and AlGaN) growth parameters and laser diode reliability, finding that quantum well (QW) optical emission efficiency depends on p-type growth temperature, unrelated to the Mg doping level. This suggests native p-type point defects propagate towards the QWs.

In this work, we successfully demonstrated a strain relaxed template (SRT) on c-plane with reduced threading dislocation density (TDD) through a method called patterned SRT, which included patterning, etching and epitaxial lateral overgrowth (ELO) processes on SRT. InGaN blue edge emitting laser diodes (EELDs) on patterned SRT exhibited much improved laser characteristics compared to previous SRT. Moreover, an over 50% improvement in optical gain compared to conventional c-plane EELDs was realized. The enhanced optical gain was attributed to local anisotropic strain relaxation as suggested from the stress field simulation for inclined edge-type TDs found in SRT. In addition, optical polarization measurement utilizing micro-photoluminescence (µ-PL) with excitation spot less than 1 µm in diameter demonstrated linearly polarized emission for patterned SRT, verifying the local anisotropic strain relaxation. By achieving a 52.8% relaxed InGaN buffer on patterned SRT, we demonstrated a blue edge emitting laser diode (EELD) with a higher wall-plug efficiency than conventional c-plane EELDs.

In this work we discuss recent progress in nitride micro-light emitting diodes (µLEDs) grown by plasma assisted molecular beam epitaxy. The current path (and therefore dimensions) of these devices is defined by the size of ion implanted tunnel junctions (TJs). Such configuration of devices allows for study of carrier diffusion in quantum well (QW), since the walls of µLEDs are not etched. We demonstrate that, the electroluminescence area is larger than area of TJs, which is a proof of long-range diffusion of carriers in nitride InGaN QW. Lateral diffusion length was determined to be 11 µm at j=1 A/cm2 and decreased down to 2.4 µm for j=1000 A/cm2. We discuss the peculiarities of carrier diffusion in nitride µLEDs and its importance for size-dependent efficiency reduction observed in small µLEDs.

MicroLED is progressing towards mass production, emphasizing epitaxy as a crucial process for optimal display performance. Challenges and achievements are improving external quantum efficiency, ensuring wavelength uniformity, and enhancing yield through effective particle control. MicroLED TVs are now available in the market, with smartwatches soon to follow. Automotive manufacturers are also keen on MicroLED due to its high brightness and exceptional reliability, positioning it as a dominant solution for various display applications.

The development of next-generation communication networks seeks to offer faster, more reliable solutions in compact forms. Visible light communication (VLC) has emerged as a promising complement to traditional radio frequency (RF) communication due to its high efficiency, fast modulation speeds, and immunity to RF spectrum congestion. Recent advancements have focused on gallium nitride (GaN)-based micro-light-emitting diodes (μLEDs) for VLC, though these typically require high injection current densities and high-frequency operation, necessitating bulky independent drivers. This study introduces a novel solution by heterogeneously integrating 20x10 µm blue μLEDs using transfer printing onto GaN-based high electron mobility transistor (HEMT) targets grown on silicon carbide (SiC) substrates. This integration facilitates a compact, single-chip platform where a μLED's emission is modulated by adjusting the gate voltage of its GaN HEMT. The resulting on-chip system achieves a modulation bandwidth of 100 MHz, maintaining a small form factor. This approach shows significant potential for future large-scale VLC systems on a single chip.

Recent OLED advancements have enabled head-mounted displays like the ”Apple Vision Pro”, but its use of color filters highlights the need for better efficiency, resolution, color gamut, and brightness. GaInN-based μLEDs offer a promising solution, potentially achieving very small pixel sizes by stacking RGB LEDs on a single wafer through crystal growth and semiconductor processes. Challenges include implementation in driving circuits such as CMOS circuits, full colorization, and maintaining high resolution. This presentation details a method for forming stacked monolithic μLED arrays by realizing tunnel junctions between RGB LEDs during crystal growth and placing electrodes on etched n-type layers. By stacking GaInN-based blue, green, and red LEDs on one wafer, monolithic RGB μLED arrays are formed. While promising for head-mounted displays, blue and red LEDs have lower output than green LEDs and require precise wavelength control. The paper also will discuss guidelines for seamless device structures that support efficient mounting and operation in circuits.

High-quality and high-In-content InGaN is mandatory for efficient red LEDs and LDs. Our presentation reports some key technologies to grow such high-quality and high-In-content InGaN. The first is our novel MOVPE method to grow high-In-content InGaN with a Nitride MOVPE simulation technology. The second is strain-compensation by introducing AlN and AlGaN as barrier layers in the red QW region. The third is the usage of novel ScAlMgO4 substrates which have a-plane lattice constant lattice-matched with that of In0.17Ga0.83N. Such In0.17Ga0.83N is suitable for cladding layers for red LDs. Also, red LEDs on novel substrates will be reported.

A periodic arrangement of GaInN nanopyramids could be selectively grown by metal-organic vapor phase epitaxy (MOVPE) on the n-GaN template, which is followed by growths of red-light emitting GaInN/GaN multi-quantum shell (MQS) active region and the p-GaN shell. Currently, the InN molar fractions in the GaInN nanopyramid and the MQS are 21 and 38 %, respectively. After the crystal growth , the GaInN-nanopyramid-based red μ-LED, whose size was 100 X100 μm2, was fabricated, and highly bright red emission was observed. Detailed LED characteristics will be given at the presentation.

The efficiency of long wavelength (green to red) GaN light emitting diodes (LEDs) depends on the realisation of a uniform hole distribution between multiple quantum wells (QWs) of the active region. However, InGaN/GaN QWs are deep, and the interwell hole transport by thermionic emission is inefficient. To overcome this drawback, hole injection through V-defects has been proposed. In this work we have tested the viability of the hole injection through the V-defects and explored properties of this injection mechanism on multiple QW LEDs with a marker well. Multisided optical and structural studies showed that the hole injection through the V-defects does take place. Properties of the hole transfer from the semipolar to polar QWs, hole diffusion in the polar QWs, and nonradiative recombination at the V-defects and threading dislocations show that properly designed V-defect injectors can, indeed, increase the efficiency of long wavelength GaN LEDs.

Switching control transistors and their integration are key factors in achieving high-performance, efficient displays and communication systems. Integration of microLEDs with GaN switching devices provides an opportunity to control microLED output power with capacitive (voltage) control rather than current controlled schemes. This approach can greatly reduce system complexity for the driver circuit arrays while maintaining device opto-electronic performance. In this work, we discuss our recent work on integration of III-Nitride electronic and optical devices.

Phosphor-coated III-nitride light-emitting diodes (III-N LEDs) are currently the most efficient sources of white light. However, standard LEDs require a direct current (DC) supply, while the power in the electrical grid is distributed as alternating current (AC). Considerable efforts are therefore devoted to the development of directly AC-powered semiconductor light sources. In this paper, we demonstrate a III-nitride bidirectional light emitting diode (BD LED) that emits light under positive and negative bias from the same active region [1]. The operation of the BD LED is based solely on the epitaxial structure, symmetrical with respect to the active region, which is surrounded by two TJs on each side. Depending on the positive or negative bias, carriers are injected into the QW from different sides of the active region - electrons when a particular TJ is forward biased and holes when it is reverse biased. We also show that BD LEDs can be vertically stacked in one epitaxial process to multiply the optical power from a single device. References: 1. M. Żak et al., Nat. Commun., 14, 7562 (2023)

The effects of sidewall defects and the lateral carrier diffusion length on external quantum efficiency (EQE) are consistently investigated in InGaN/GaN-based blue and green micro-LEDs. Both blue and green micro-LEDs have the same mesa sizes: 20×20 and 60×60 μm2. The blue micro-LEDs have higher crystal quality and EQE than those green micro-LEDs. The EQE of green micro-LEDs is less size-dependent due to increase in carrier localization inside active region caused by higher In-composition. However, in blue micro-LEDs, the small-sized micro-LED has lower EQE than the bigger size due to nonradiative recombination via defects caused by the higher lateral carrier diffusion length.

InGaN/GaN µLEDs are being studied for visible light communication applications due to their ability to be fabricated at micron scales, their high brightness and their GHz-level bandwidth. To improve system performance and reduce power consumption, it is essential to achieve high bandwidth at low current densities. We optimized epitaxial layers using an optical approach that directly assesses the recombination lifetime in the quantum wells, providing quick feedback on the optimization process.

GaN-based micro-LEDs excel in display applications and visible light communication (VLC) due to their high resolution, low power consumption, high refresh rate, and broad bandwidth. Their efficiency, determined by internal quantum efficiency (IQE) and light extraction efficiency (LEE), is influenced by the quantum confined Stark effect (QCSE) in InGaN/GaN quantum wells. This study examined QCSE and LEE changes in GaN-based nano-LEDs to optimize structures. Using nanoimprint lithography, nanorods of 200nm and 500nm were patterned, and Inductively Coupled Plasma (ICP) etching was used to analyze damage in quantum wells and sidewalls. Chemical etching with KOH reduced defects and enhanced photonic extraction efficiency. Photoluminescence (PL), Raman spectroscopy, and X-ray Photoelectron Spectroscopy (XPS) were utilized for analysis. The effect of increasing Al2O3 thickness using Atomic Layer Deposition (ALD) on efficiency was also studied, with simulations analyzing light propagation based on size.

We discuss the device performance of triboelectric nanogenerators (TENGs) fabricated with GaN nanowires (NWs) as a response medium. The TENGs were fabricated by forming polydimethylsiloxane (PDMS) and a Pt/Cu electrode on the top and bottom of the GaN NW/Si(111) structure, respectively. To evaluate the effect of the PDMS layer on the performances of the TENG, the curing agent ratio in PDMS was varied to 0.05% (TENG1), 0.09% (TENG2) and 0.13% (TENG3). When touching the top surface of the TENG2 with a human finger, the maximum output voltage and power density were measured to be 6 V and 23.83 mW/m2, respectively, which are much higher than those of previous works. These results will be discussed using a theoretical model with vertical and spatial distribution of strain at the interface between the PDMS and GaN NWs.

We discuss the performances of flexible photosensors fabricated with GaN nanowires (NWs) and graphene used as a light-absorbing layer and a carrier channel, respectively. For the fabrication of photosensors, GaN NWs were horizontally and randomly dispersed on graphene with different number of stacks transferred onto a polyimide substrate. And then, single-layer graphene was transferred on the GaN NWs/graphene structure, forming so-called graphene-NW-graphene sandwich structure. The number of bottom graphene layers is varied from zero to three to investigate the effect of channel volume on device performances. The photocurrent and photoresponsivity of the photosensor with GaN NWs and double-layer bottom graphene were measured to be 1.1 mA and 224.5 mA/W at the light intensity of 100 mW/cm2, respectively. The device performance of the flexible photosensors will be discussed using conformal contact area, effective volume of a carrier channel, and the alignment of Fermi level between GaN NWs and graphene.

This study proposes etching the material in the channel region while retaining a certain width for implantation, known as the sidewall ion implantation process, aiming to achieve better insulation characteristics by using ion implantation technology to insulate the sidewall regions. It involves ion bombardment of the defect areas generated after plasma etching and the use of a passivation layer for protection. The isolation characteristics of μLED arrays processed by sidewall implantation exhibited better electrical isolation than those of μLED arrays processed only by plasma. The light output power, external quantum efficiency, and wall-plug efficiency were all superior for the sidewall implantation process when the device was miniaturized to 5 μm. Overall, the sidewall implantation process combined with plasma dry etching effectively improved the light output characteristics, with the enhancement ratio increasing as the device was miniaturized.

We report the improvement of photoelectrochemical water splitting (PEC-WS) using InGaN/GaN heterostructure NWs (HSNWs) coated with Ti3C2 (MXene) nanosheet. High-crystalline InGaN/GaN HSNWs were grown on a Si(111) substrate using a molecular-beam epitaxy. The weight ratio of MXene powder to isopropyl alcohol for coating InGaN/GaN HSNWs were varied to 0.06 (MX-HSNW1), 0.13 (MX-HSNW2), and 0.26 (MX-HSNW3). The maximum current density and applied-bias photon-to-current efficiency for the MX-HSNW2 photoelectrode were measured to be 7.6 mA/cm2 and 3.4%, respectively, under the light intensity of 100 mA/cm2. These results are approximately 4.5 times higher than that of the photoelectrode without the MXene layer. Also, the results are much higher than those of the previous results. The improvement in the PEC-WS of the HSNW-MXene photoelectrode will be discussed using an analytical model with the directional migration of carriers and the increase in carrier extraction rate at the interface between NW and electrolyte.

Gallium nitride (GaN) is well-known as stable and bandgap tunable materials for optoelectronics . Recently, micro-LED displays are constantly being researched as the increased demand for high-performance displays such as contrast ratio, resolution, response speed, and so on. In addition, nano-LED displays are also receiving a lot of attention in order to implement a resolution of thousands of PPI for application to augmented reality (AR), virtual reality (VR), and mixed reality (MR) displays. In this study, vertically aligned GaN nano-/micro-rod array are successfully fabricated on nanoimprinted substrate by two different strategies. First, MOCVD grown GaN film is patterned by nanoimprint lithography and post-dry/wet etching process. second, selective area growth of GaN nano-/micro-rod array is conducted by HVPE on sapphire substrate. Then, two different types of nano-/micro-rod is compared the quality of crystal structure, optical and electrical properties to apply as template for core-shell type nano-/micro-LED devices.

We demonstrated an efficiency increase of 220-230 nm AlGaN far-UVC LEDs fabricated on c-sapphire substrate. We have demonstrated 0.8-1.5% EQE for 232-236 nm LEDs by introducing polarization doping (PD) transparent p-contact layer. We also demonstrated record-high EQE of 0.32% for 228 nm LED by optimizing emitting and p-side layer structures. We also demonstrated efficiency increase by 4 times in 230 nm LED by introducing photonic crystal (PhC)-reflector on p-AlGaN/p-GaN contact layer. 80 chips of 230 nm LED were integrated to cupper heat sink and we demonstrated 220 mW power far-UVC light module.

Far-UVC light emitting diodes (LEDs) are of particular interest for disinfection and sensing applications, but the high-Al content AlGaN used to fabricate them introduces a number of performance-limiting technical challenges. We have overcome many of these issues by using short-period superlattices (SPSLs) consisting of monolayer to nanometre scale repeating AlGaN structures. This work focuses on further advancements in far-UVC SPSL LED technology by employing AlN templates, which lower the defect density in the devices by over an order of magnitude.

Aluminum nitride (AlN) is widely utilized in optoelectronic devices. We have successfully fabricated high-quality AlN templates by combining sputtering deposition of AlN (Sp-AlN) with high-temperature face-to-face annealing (FFA). Additionally, we achieved Al-polar and N-polar FFA Sp-AlN by annealing Sp-AlN films sputtered using a sintered AlN target and an Al metal target, respectively. By controlling the polarity, we fabricated a bilayer polarity-inverted AlN structure, where the Al-polar FFA Sp-AlN serves as the lower layer and the N-polar FFA Sp-AlN as the upper layer. In this bilayer polarity-inverted FFA Sp-AlN, an inversion domain boundary (IDB) with an abrupt interface, consisting of several monolayers (ML), and an atomically smooth surface were observed. In this study, we demonstrate the fabrication of a four-layer polarity-inverted AlN structure using multiple sputtering of AlN and FFA with the polarity control method of FFA Sp-AlN. It was found that the IDB structures vary depending on the sequence of polarity inversion.

We propose an overview of the degradation processes that affect state-of-the-art UV-C LEDs. In particular, we investigate the role of hydrogen in the electrical and optical degradation of 265 nm SQW devices, validating the results also by means of numerical simulations. The analysis is extended to far UV-C LEDs, whose degradation is analyzed in detail by means of defect spectroscopy analyses.

UV-C LEDs are widely used for sterilization and are expected to replace mercury lamps as a solid-state light source. However, their luminous efficiency remains inadequate, and improvement is required. One of the causes of low efficiency is low light extraction efficiency, and the other is a decrease in luminous efficiency during operation. In this study, we focused on the latter and elucidated the mechanism. UV-C LEDs were fabricated on a sapphire substrate. Two degradation modes were observed under current stress: a 30% power drop within 100 hours and a slower output power drop over a longer period. The effect of current stress on PL intensity and TRPL was measured. The results showed no change in the luminescence intensity and decay time of TRPL from the quantum wells under long-term current stress. However, SIMS revealed that the H drifted from the p-type layer to the n-type layer below QWs, resulting in a decrease in optical output. This suggests that point defects in the p-type layer, originally passivated by H, are activated by current stress. Developing a crystal growth technique that suppresses point defects is crucial for improving the degradation of UV-C LEDs.

Emission wavelength of light-emitting diodes (LEDs) have been shortened　by developing materials from red (800 nm-band, AlInGaP) to blue (450 nm-band, InGaN) UV-A (360 nm-band, AlGaN, with low Al compositions), UV-C (265 nm-band, AlGaN, with middle Al compositions) and far UV-C (230 nm band, AlGaN with high Al compositions) in these 50 years. Current frontier is far UV-C LED and its wall-plug efficiency (WPE) reached over 1% in 2024 by developing conductive and light-emissive AlGaN film layers. The pseudomorphic lattice-strained AlGaN films were grown by metal organic chemical vapor deposition method on 2-inch-high-quality single crystal semi-transparent AlN substrate to achieve high internal quantum efficiency at the emission layer, which are designed especially to increase carrier injection efficiency in QWs and reduce resistivity of n-AlGaN film. WPEs of the LEDs are 2.4% at 235 nm, 2.1% at 233 nm, 1% at 230 nm and 0.5% at 227 nm, which are very sensitive with emission wavelengths. From several theoretical models and analyses, it is estimated that the difference depends strongly on carrier injection efficiency rather than other factors.

AlGaN-based ultraviolet-C (UV-C) heterostructures are poised to revolutionize the UV lamp market by replacing bulky and toxic low-pressure mercury lamps. This is due to their compact size, long lifespan, lower direct current (DC) operating voltages, and the absence of toxic materials. However, the external quantum efficiency (EQE) of UV-C LEDs remains significantly lower than that of visible InGaN emitters. In this study, we have demonstrated that using nanopatterned sapphire substrates (NPSS) for UV-C epitaxial heterostructure growth can address these challenges. This approach reduces threading dislocations and improves TM-polarized light extraction through effective scattering, enhancing EQE and overall performance of UV-C LEDs. We also study the AlGaN heterostructure-based device performance under UV light irradiation for photodetection at reverse bias, highlighting the advantage of NPSS, and nanostructure fabrication for enhanced light absorption in the active layer and charge collection, thus presenting effective strategies for multifunctional integrated photonic devices.

Heterointegration of h-BN, a 2D wide bandgap nitride semiconductor, is an interesting solution for deep UV LEDs. It can be used as active materials as well as template substrate as described below. First, achieving transparency in the UV-C range for both n and p regions of the LED structure is crucial to EQE. Higher Al incorporation in AlGaN significantly diminishes device performance because of the high activation energy for Mg dopants in AlGaN. Here, we study AlGaN-based UV LEDs by replacing the conventional Mg-doped AlGaN p-layer with Mg-doped h-BN. UV emission around 290 nm is obtianed by electrical injection for the first time in a UV LEDs. Primarily, our study provides proof-of-concept that p-AlGaN can be replaced by p-h-BN. The obtained results will be presented. Secondly, with mainstream approaches, threading dislocations originating from the AlN/sapphire interface reduce the internal quantum efficiency. This issue could be mitigated by using lattice mismatch independent van der Waals epitaxial growth on 2-D h-BN. In this regard, we have reported the growth of AlN layers on h-BN on c-sapphire substrates and AlGaN quantum dots (QDs) on h-BN templates.

Nanowires have emerged as an alternative approach to develop novel UV LEDs. We have explored two kind of core-shell UV quantum well (QWs) systems grown by MOVPE: GaN/AlGaN and GaN/AlN. The UV wavelength is controlled by tuning the thickness of GaN QWs as well as by varying the Al-content of the AlGaN barriers. Single-wire UV µLEDs is fabricated and electroluminescence is tuned down to 310 nm using thinner QWs and Al-rich barriers. We have also developed the growth of core-shell GaN/AlGaN digital alloys with a demonstration of an UV LED at 310 nm. Finally, assemblies of nanowires embedded in PDMS are peeled off and transferred to fabricate flexible large UV LEDs. This work paves the way for the development of novel UV-LED devices, from µLEDs to flexible LEDs.

Research is being actively conducted to improve the efficiency and extend the lifetime of DUV LEDs in order to commercialize products for inactivating bacteria and viruses. As an approach to improve the operating voltage of DUV LEDs, lowering the resistance of n-type AlGaN:Si contact layer with 62% AlN content was implemented by cathodoluminescence analysis for point defects such as VIII-(SiIII)n. By using this low resistance technology, operating voltage of less than 7 V could be achieved at 63 A/cm2. The degradation mechanism of DUV LEDs lifetime has been recently reported by some research group. Based on the mechanism, by improving the crystal quality of the p-type AlGaN:Mg layers, L70 of the DUV LEDs lifetime are expected to more than 20,000 hours in the current stress test. The optical output power has been demonstrated to be over 150 mW class at 350 mA.

In this work, theoretical and experimental data is presented to advance UVC LEDs towards higher EQE and WPE. First, a ray tracing simulation of ultraviolet-c light-emitting diodes (UVC LEDs) is discussed. This simulation reveals that the primary limitation in light extraction efficiency (LEE) is not the refractive index contrast or total internal reflection (TIR), but rather absorption in the LED layers and substrate. Using this simulation work as a basis, actual UVC LEDs with Al based contacts were grown on AlN substrates, fabricated, and tested. Based on the experimental and simulation results, a clear pathway to UVC LEDs with EQE > 40% is presented.