In order to fabricate gallium nitride (GaN) substrates, bulk crystals of the highest structural quality have to be grown. Today, three GaN crystallization methods are mainly applied for this purpose: Halide Vapor Phase Epitaxy (HVPE), sodium-flux, and ammonothermal. This work will cover basic ammonothermal and HVPE crystallization of bulk GaN. Properties of crystals and substrates will be shown. Limiting factors as well as recent progress in the two approaches will be presented.

It is demonstrated that Borrmann effect X-ray topography (XRT), based on the anomalous transmission of X-rays for high-perfection and high-absorption crystals, is a robust method to benchmark the structural perfection of GaN substrates. GaN substrates prepared by ammonothermal method or HVPE were investigated. Substrates for these growth methods were analyzed on the basis of native seed or alternatively on the basis of foreign seed. The study shows that only GaN crystals prepared with a native seed approach, both by ammonothermal method and HVPE, exhibit a Borrmann effect and proves the high perfection of the substrate produced in this way.

Niobium nitride (NbN) is widely used in single-photon detectors, hot electron bolometer mixers, and superconducting quantum bits. The structural similarity between NbN and III-nitrides offers the possibility that high-quality NbN superconductor thin films can be epitaxially grown on III-nitride devices. However, the characteristics of epitaxial NbN films are still unveiled. In this presentation, we will discuss the structural and electrical properties of NbN films epitaxially grown on AlN by sputtering. The formation mechanism of NbN twins on AlN will be shown, and techniques to reduce the NbN twin boundaries will be presented.

A nitride-based light-emitting structure composed of a GaN nanowire core and a GaInN/GaN multi-quantum shell (MQS) is promising for high performance optoelectronic devices. By growing high crystalline quality MQS on the nonpolar (m-plane) sidewall of the nanowires, an improvement of luminous efficiency is expected. In this work, we induced the sputtering growth of n++-GaN shell on the tunnel junction/p-GaN/MQS/nanowire structures. By performing sputtering by the optimized condition, we were able to demonstrate a device with an operating voltage of about 1.0 V lower than that of the sample without sputtering.

As we reduce the emission wavelength of III-Nitride materials, and delve deeper into the UV region, the struggle to keep the material at a high internal quantum efficiency escalates. A reduction in the quantum confined Stark effect and an improvement in strain engineering are just two of the challenges that wurtzite boron nitride (wz-BN) could play a key role in. In this presentation, we investigate the possibility of incorporating wz-BN into ternary and quaternary multiple quantum wells serving as the active region for UV emitters. This work was funded by Science Foundation Ireland (IPIC and PIADs.)

Epitaxial lateral overgrowth (ELO) of GaN is a well-established technology to reduce the dislocation density of III-Nitride films grown on sapphire. One of the longstanding problems with ELO has been the so-called wing tilt. This wing tilt is a result of a tilt of the GaN crystal in the regions not grown on the SiO2 opening which results in high dislocation densities in the coalescence region of the film. In this study, we demonstrate that wing tilt in ELO is result of the SiO2 used to pattern GaN templates. We show that wing tilt occurs immediately at the start of the ELO growth process. Furthermore, we show that wing tilt can be avoided completely if an etched template is used in combination with facet controlled epitaxial lateral overgrowth (FACELO). Results are based on homoepitaxial GaN growth in combination with TEM, XRD, and PL studies.

Realization of ohmic contacts to p-GaN-based optical devices is challenging because of the absence of metals with sufficiently large work function, good adhesion and thermal stability. In order to to extend the lifetime of optoelectronic devices is necessary to increase the perfection of metal/semiconductor system. The technological aspects of contact formation should be understand on the atomic scale. In present work we will discuss the details of Pd-based/p-GaN system characteristics in terms of atomic structure and correlations with electrical properties of these contact layers after different type of annealing parameters (temperature, atmosphere) in order to reduce the contact resistivity and improve metallization stability.

A lateral p+n GaN junction with a 10 µm drift layer has been characterized by combined Cathodoluminescence (CL) and Electron-Beam-Induced Current (EBIC) measurements performed at different temperatures. A vertical CL linescan across the pn-junction shows the evolution of luminescence in growth direction. The distinct changes of local emission in the space charge region are correlated with temperature dependent EBIC profiles. In particular in the drift zone, a mono-exponential behavior with a large characteristic length was observed. These profiles are correlated to the calculated band profiles for quantifying the evolution of electrical fields in the space charge region and drift zone.

The measurement of low-magnitude shear forces is essential for material characterization and in-hand object pose estimation for robotic applications. Previous work had demonstrated the ability to directly measure force using an optical GaN nanopillar sensor when combined with Multiphysics simulations. To investigate the accuracy of Multiphysics simulations, we use a commercially available six-axis force/torque sensor as our stress applicator with a polymer stamp mounted on the force sensing element. By investigating the accuracy of the Multiphysics simulations, we generated a mapping between applied shear force and our nanopillar tactile sensor response. We can use this mapping for robotic gripping applications.

The carrier dynamics in InGaN/GaN LEDs are directly tied to their efficiency and maximum modulation speed, which are important metrics for solid-state lighting, displays, and optical communication. We measure the carrier dynamics of nonpolar, semipolar, and commercial c-plane InGaN/GaN LEDs using small-signal electroluminescence methods. Rate equations and a small-signal circuit are used to model the carrier dynamics. The model is fit to the optical frequency response and input impedance of the LEDs to extract the carrier lifetimes and the recombination rates. The results offer insight into the underlying causes of efficiency droop and the green gap, and inform device design strategies.

III-nitride semiconductor system is widely used in many electronic and optoelectronic applications. The presence of extremely high piezoelectric field in quantum wells (QWs) is known to cause severe separation of electron and hole wavefunctions and limits the thickness of QWs used in devices. We have recently shown that wide QWs are also a viable solution in optoelectronic devices. In this paper we will discuss the physics of recombination in wide InGaN QWs. We will show that the piezoelectric field is fully screened in case of wide InGaN QW and light emission occurs through excited states with high wavefunction overlap.

Wide well blue InGaN LEDs and laser diodes show high internal quantum efficiency and high differential optical gain. We investigate the contribution of recombination from ground and excited confined states in MBE grown LED or laser structures with 2.6 nm, 7.8 nm and 15 nm wide InGaN wells by low and room temperature µEL and µPL. For the 7.8 nm well, the blue-shift caused by piezoelectric field screening (QCSE) and transition from ground to higher confined states are observed separably. The 25 nm emits at the band edge of the fully screened InGaN region and shows pronounced state filling.

Spontaneous and stimulated emission spectroscopies were performed on transparent aluminum nitride (AlN) substrates grown by hydride vapor-phase epitaxy. The stimulated emission was observed from cryogenic to room temperatures and the origin was assigned by the spontaneous emission spectra and existing theories. AlN exhibited a purely excitonic stimulated emission at cryogenic temperatures, whereas the stimulated emission mechanism at room temperature originated from an exciton-electron scattering process. The temperature-induced and excitation-power-induced stimulated emission crossovers were found and interpreted in terms of the peculiar excitonic structure of AlN.

Aluminium gallium nitride ((Al,Ga)N) quantum wells have attracted significant interest for optoelectronic device applications in the ultraviolet (UV) spectral range. Here, we present an atomistic theoretical analysis of the impact of random alloy fluctuations on the electronic and optical properties of c-plane (Al,Ga)N/AlN quantum wells. Special attention is paid to the impact of the Al content on the results. Our studies show that already random alloy fluctuations are sufficient to lead to strong carrier localization effects. Furthermore, our calculations reveal that the degree of optical polarization is strongly affected by alloy fluctuations.

The results of complimentary time-resolved photoluminescence and positron annihilation measurements on Mg-implanted GaN on GaN fabricated using various I/I sequences will be shown to identify the species and quantify the concentrations and minority carrier capture coefficients of major midgap recombination centers (MGRCs) created by the I/I processes. Because vacancy clusters comprised of Ga vacancies (VGa) and N vacancies (VN) such as (VGaVN)3 were assigned as major vacancy-type defects and the room-temperature photoluminescence lifetime for the NBE emission increased with decreasing their concentration, (VGaVN)3 are assigned as major nonradiative recombination centers with electron capture coefficient of 5×10-6 cm3s-1, which is an order of magnitude larger than the hole capture coefficient of VGaVN in n-GaN (6×10-7 cm3s-1). Financial supports: CSTI-SIP, MEXT (JPJ005357, JPJ009777, JP16H06427, JP21H01826), PNCRD TECHMATSTRATEG-III/0003/2019-00 and PNSC 2018/29/B/ST5/00338.

Three dopants (Be, Zn, Mg) were analysed in terms of diffusion through the crystal lattice of HVPE-GaN. Different crystallographic directions were investigated: [0001], [10-10] and [11-20]. Ion implantation was employed to create a thin layer of strongly doped GaN which acted as the diffusion source. Annealing in high nitrogen pressure was performed. Secondary ion mass spectrometry (SIMS) was used to measure the post-annealing depth profiles of implanted species. The measured profiles were used for calculation of diffusion coefficients and activation energies for all dopants and crystallographic directions. A strong dependence of diffusion on crystallographic orientation and impurities composition was observed.

In this work we presents effect of ultra high pressure annealing on Si-implanted GaN n-type and p-type epilayers on ammonothermally grown bulk GaN substrates. Samples were blanked implanted with different Si ion fluences from 3x1014 cm^-2 to 3x1015 cm^-2 and then annealed using UHPA at temperature of 1200, 1300 and 1400°C for 5 minutes at 1 GPa. Ion distribution before and after annealing where investigated using SIMS method showing no Si diffusion in p-type GaN along with Mg diffusion from epilayer and very low Si diffusion in n-type GaN epilayers. X-ray diffraction studies shows that not all defects were recovered after annealing, especially for high ion fluences. Annealing at 1400°C causes changes in implanted GaN morphology. The surface roughness where increased after annealing especially for samples implanted with 3x1015 cm^-2Si dose. Our results shows that more work is needed to optimize UHPA parameters for defect recovery in Si-implanted GaN especially for high ion fluences.

Selective doping technique by ion implantation was applied to power devices. A novel activation method for p-type doping named as UHPA (Ultra High-Pressure Annealing) was recently developed. Using this method, edge termination of FLR (Field Limiting Rings) structure by Mg ion implantation for a pn diode was formed, and the improvement of the breakdown voltage was confirmed. JBS (Junction Barrier Schottky) diodes were also fabricated, which showed high performance such as breakdown voltage of 675V, specific on-resistance of 0.67mΩ·cm2 and avalanche capability. The activation of Mg using UHPA has enabled p-type selective doping even in GaN power devices. Financial supports: MEXT (JPJ005357 and JPJ009777)

We will present nano-scale correlation of structural, electronic and optical properties of GaN-based power devices by cathodoluminescence directly performed in a scanning transmission electron microscope. The two-dimensional electron gas (2DEG) of a lateral AlGaN/GaN field-effect transistor has been investigated directly probing the local origin of the 2DEG by its spectral luminescence fingerprint. This characteristic 2DEG luminescence is locally observed nanometers inside the GaN - close to the GaN/AlN/AlGaN interface. Furthermore, a lateral p-n+ superjunction will be presented, where excitonic and donor-acceptor transitions directly visualize the space charge region evidencing the exciton dissociation in the built-in electric field.

Low angle bevelled-mesa structures are crucial for development of high quality GaN p-n high voltage diodes and photodetectors. However, there is lack of details of development of such a process in the literature. Here in this work, we present results of optimization of bevelled mesa fabrication process for vertical GaN p-n diodes using plasma etching through photoresist mask prepared using reflow process. Developed process of formation of low angle bevelled mesa structures was integrated in the vertical GaN p-n diodes on bulk GaN substrates fabrication process. Very low leakage current density below 10-9 A/cm² and very high Ion/Ioff current ratio over 1013 was obtained. Low values of ideality factor down to 1.5 were obtained as well. These prove applicability of developed process in technology of vertical GaN p-n diodes on bulk gallium nitride substrates.

Recently much attention gained development of “buffer free” AlGaN/GaN HEMT structures with thin high quality AlN nucleation layer for better carrier confinement in the transistor channel mitigating short channel effects and with reduced thermal resistance. In this work results of development of low resistivity Ti/Al/TiN/Au ohmic contacts to such a structures will be presented . The impact of annealing temperature and different metal layer thickness on the ohmic contact formation, morphology and structural and electrical properties was studied. Low contact resistance of 0.28 Ωmm was obtained for metal stack with Ti/Al 20nm/80nm thickness after annealing at 750°C. Developed ohmic contacts were integrated in the AlGaN/GaN HEMT fabrication process. Good electrical characteristics were obtained showing high on-state current up to 0.95 A/mm. These prove applicability of developed process in technology of buffer-free AlGaN/GaN high electron mobility transistors.

In this talk, we present watt-class high-power, high-beam-quality operation of GaN-based photonic crystal lasers (PCSELs) at blue wavelengths, having a low threshold current density. First, we design a device layer structure to enhance the 2D photonic-crystal resonant effect in GaN-based materials, considering their low refractive index contrast. Next, we develop a new nano-fabrication method for realizing GaN/air structures with sufficiently small disorders. Then, we introduce an asymmetric unit cell based on a double-lattice structure in order to enhance the extraction efficiency of light toward the optical facet for high-power operation. Based on these optimizations, we successfully develop GaN PCSELs with high, watt-class (>1 W) output power and circular, single-lobed beam with a very narrow (~0.2°) divergence angle. In addition, we demonstrate CW operation with a high output power (~320 mW) and a high beam quality (M2~1).

We present the realization of low-absorptive GaN waveguide for cost-effective, ultra-compact visible light Photonic Integrated Circuit (PICs) applications. Within this framework, in order to achieve on chip fabrication of both active and passive parts on GaN platform, we realized the well-defined sensitivity of indium incorporation to InGaN layers (Quantum wells) to substrate misorientation during epitaxial growth of device structure with MOCVD. Fabrication of GaN waveguides on locally high misoriented areas (1 degree- 4 degree) coupled with neighboring laser diodes on flat areas (0.2 degree) allowed us to locally shift the absorption edge of InGaN quantum wells enabling efficient light transmission through the fabricated waveguides.

Moiré photonic crystals can provide flat, dispersion-free bands that localize light with high Q-factors and low mode volumes, presenting opportunities to address nanoscale emitters with great specificity for applications in low-threshold nano-lasers. We report on the realization of moiré photonic crystal lasers in Gallium Nitride (GaN). We demonstrate fabrication of suspended moiré photonic crystals in GaN with embedded indium gallium nitride quantum wells and quantum dots, which emit in the violet-blue regime. We characterize moiré lattices of varying sizes, and photoluminescent spectra reveal evidence of moiré-induced flat-band modes, indicating a path toward a new kind of highly efficient, GaN-based nano-laser.

AlGaN-based UVC laser diodes operating at wavelengths are expected to be a low-cost, environmentally friendly, and highly efficient laser light source for a variety of applications. In this study, we further reduced the threshold gain by improving the optical confinement and improved the threshold current density while lowering drive voltage. A threshold current density of 2.8 kA/cm2 was achieved in pulsed current operation, and an operating voltage as low as 9.6 V was also achieved. Furthermore, the packaged device successfully emits CW light at a wavelength of 274.8 nm with a DC current operation of over 110 mA at 5°C.

We have demonstrated RT pulsed operation of AlGaN-based UV-B LDs. In this presentation, we would like to introduce our recent efforts to achieve higher output power. To increase the output power of LDs, it is essential to increase the external differential quantum efficiency ηd, increase the input current, and reduce the threshold current. The ηd is determined by the carrier injection efficiency ηi, internal losses and mirror losses in the LD. In LD with the polarization doped structure, the polarized positive fixed charge formed at the interface of the pn junction allows free electrons to easily overflow to the p-AlGaN cladding layer side. Therefore, improving the ηi is one of the most effective approaches to increase the ηd. In this study, we investigated the increase of ηi through device simulators and actual experiments. Through these investigations, we have succeeded in achieving a peak power of over 150 mW in RT pulsed driving of UV-B LDs.

Although the polarization doping is a break-through technology for deep UV emitters, it is also very useful for enhancing p-type conductivity and lower the resistance in the classical InGaN laser diodes operating in the visible part of the spectrum. We were able, additionally, to show that these devices can operate in broader temperature range, especially at low (cryogenic) temperatures. We also show the drastic reduction in hydrogen content in top layers of the laser diodes, which may influence their reliability.

Many efforts for GaN-based VCSEL developments have been pursued so far. One of the most promising reflectors for the VCSELs is a lattice-matched AlInN/GaN DBR. In principle, simple device process after the epitaxial growth and a vertical current injection through a conductive DBR are expected. We proposed two new approaches to obtain high quality conductive AlInN/GaN DBRs, one is an implementation of a high-temperature-grown AlGaN graded layer instead of a low-temperature-grown AlGaInN graded layer, and the other is an introduction of a hydrogen treatment instead of a thermal treatment. Both the approaches are useful to obtain high-quality Si-doped conductive AlInN/GaN DBRs.

Electrochemical etching of III-nitride-materials is a fast-developing research field. This method is used to selectively porosify or completely etch such materials and thereby opens up a new design space for both photonic and electronic devices. Here we will focus on complete lateral electrochemical etching for substrate removal to realise thin-film vertical-cavity surface-emitting lasers (VCSELs) and light emitting diodes (LEDs). Key challenges that will be addressed are how to achieve etched surfaces as smooth as the as-grown material and how to protect fully processed and highly doped device structures such as tunnel junctions, during substrate removal.

We investigate the role of dislocations in nitride light emitters by comparing a set of laser diodes grown on GaN substrates with their counterparts grown on sapphire. Studied structures emit light in the range 383-477nm. We observe decrease in intensity of electroluminescence for samples with low indium composition and high dislocation density. To understand this effect we measured cathodoluminescence and investigated thermal stability of chosen structures. Results show that significance of dislocation related nonradiative recombination increases for low indium content structures due to shorter diffusion path of carriers. Results of TEM and SEM indicate that edge dislocations are the main source of nonradiative recombination in our structures.

In this work, the device performances of three laser architectures are examined. All the laser epitaxial structures are grown on freestanding m-plane GaN substrates by Metalorganic Chemical Vapor Deposition (MOCVD), with a peak emission wavelength of about 405 nm. The three laser device architectures are shallow-etch ridge design, which the ridges are defined by etching into the p-GaN layer and not through the active region, and the deep-etch ridge structure that etches through the active region with or without Atomic Layer Deposition (ALD) sidewall passivation. By utilizing ALD sidewall passivation, the optical and electrical characteristics show significant improvements than the other two device designs.

Broad area InGaN laser diodes have gained importance as versatile high-power sources in the blue spectral range. Lateral multi-mode operation naturally occurs in a ridge waveguide that is more than 3-5 µm broad, while also the formation of longitudinal mode bunches is amplified by a large active region. We investigate a series of broad-ridge InGaN laser diodes with different cavity length and ridge width and characterize their spectral-lateral-temporal behavior. Lateral modes of different order show sequential onset dynamics and can be observed at different wavelengths. We characterize their interplay with longitudinal mode dynamics as well as the emerging lateral-longitudinal mode pattern depending on cavity length and ridge width.

GaN based edge-emitting lasers traditionally rely on index contrast from InGaN or AlGaN cladding for modal confinement. However, defects introduced by lattice mismatch limit the feasible composition and layer thickness of alloyed cladding layers. Alternatively, porous GaN offers high refractive index contrast while remaining lattice-matched, making it a suitable candidate for cladding in green lasers. Edge-emitting laser diodes with nano-porous cladding were fabricated and reached electrical injection at 524 nm. In this work, a deep ridge waveguide structure was used to improve electrochemical etch selectivity of the porous cladding, resulting in higher efficiency and lower loss compared to shallow ridge devices.

In this study laser diodes with different InGaN quantum well thickness up to 25 nm are analyzed. For those devices efficient screening of the piezoelectric field and operation on excited states was reported. We observe the time-dependent behavior of the intensity and emission spectrum below threshod operation, which can give insights about the QW tilt and the wave function overlap. In particular we observe a strong rise in intensity and a wavelength shift at the trailing edge of the pulse. Furthermore, we investigate the laser operation of the different quantum well width diodes showing unusual spectral-temporal behavior especially for the wide QW devices.

Recent advances in optimizing the efficiency and lifetime of far-UVC LEDs with emission wavelengths below 240 nm are presented. The design of the semiconductor heterostructure is considered as well as the chip layout. Cross-comparisons are used to draw general conclusions about degradation mechanisms in UV LEDs and to identify development strategies to minimize them. Furthermore, it is discussed which chip packaging is particularly suitable for a combination of far-UVC LEDs with spectral filters. Finally, far-UVC irradiation systems for skin-friendly irradiation of the human body are presented and their performance is illustrated with selected medical and biological data.

Since the intensity of deep ultraviolet (DUV) light with the wavelength below 300 nm is nearly zero in the sun spectrum, optical wireless communications (OWC) based on DUV light can have a unique “zero-background” characteristics. We recently demonstrated LED-based DUV OWC at 280-nm band over a 1.5-m direct line-of-site channel realizing the effective data rate of as fast as >2 Gbps under standard room lighting and >1 Gbps under direct sun with outdoor experiments. These experiments were performed off-the-shell DUV AlGaN light-emitting diodes, which was grown by metalorganic vapor-phase epitaxy using an AlN/sapphire template with dense macrosteps.

To fabricate deep-ultraviolet(DUV)-LEDs with high efficiency, the crystallinity of AlGaN must be improved, and is significantly influenced by that of the underlying AlN template. The face-to-face annealed sputter-deposited AlN templates (FFA Sp-AlN) have achieved one order of magnitude lower TDD than that of the typical MOVPE-grown AlN on sapphire substrates. The EQE of the UV-C LED fabricated on FFA Sp-AlN increased with the TDD reduction of the FFA Sp-AlN. Maximum EQE of 8.0% and output power of 6.6 mW at a 20-mA input were achieved with the peak emission wavelength of 263 nm.

Light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) are of great interest for monitoring gases, pollutants in water as well as the in-vivo inactivation of multi-drug-resistant bacteria. This paper reviews advances in development of AlGaN-based DUV-LEDs, including the realization of low defect density AlN on sapphire. DUV-LEDs near 230 nm with output powers of more than 3 mW will be demonstrated and the root causes for the efficiency drop at shorter UV wavelength will be explored, including changes in the polarization of light emission, the role of point defects as well as carrier injection in AlGaN MQWs.

We investigated electrical characteristics of the Mg-doped AlGaN contact layers for DUV LEDs. We found that the contact resistances were exponentially increased from 0.14 to 15.1 Ω∙cm2 with an increase of AlN mole fraction from 0.37 to 0.58. Also, the offset voltages were linearly increased from 0.22 to 3.62 V. While the LED with the AlGaN contact showed higher light output power, its operating voltage at 0.5 mA was 0.8 V higher than that with the GaN contact, showing a reasonable agreement with the abovementioned offset voltage. Further decreases of the offset voltage are important to obtain high-efficiency DUV LEDs.

We have developed highly efficient InGaN red LEDs via the strain-compensated InGaN SQW structure. The LED structure is an InGaN single-quantum-well with AlGaN barriers to compensate for the compressive strain in the InGaN well layer. The red LEDs exhibited an EQE of 4.3%, a light output of 1.7 mW, and a wavelength of 621 nm at 20 mA (10 A/cm²) under 2.96 V. We applied the hydrogen passivation method to pixelize the planar-type micro-LEDs and to fabricate the efficient mesa-type micro-LEDs by suppressing the carrier surface recombination.

High-density micro-light-emitting diode (μ-LED) arrays are the key to next-generation ultrahigh-resolution displays. We have demonstrated a full-color LED consisting of Eu-doped GaN and InGaN quantum wells (QWs). The full-color LED exhibited a remarkably wide color gamut and its maximum luminance reached over 3000 cd/m2. However, due to the quantum-confined Stark effects, the wavelength of a green LED shifted to a shorter wavelength as the injected current increased, which resulted in a reduced color gamut. There are two strategies in this regard; the use of a semi-polar substrate with reduced internal electric field and Tb-doped III-nitrides as a green emitter.

Achieving high quantum efficiency in long-wavelength nitride LEDs has proved challenging due to stress induced from high In%, piezoelectric polarization, and defect generation. A leading technique for increasing the efficiency in long-wavelength InGaN is V-defect engineering which uses naturally occurring pyramidal (10-11) defects to achieve deeper hole injection through lateral pathways on the semi-polar sidewalls of the V-defects. In this work we demonstrate V-defect engineered red LEDs achieving a peak EQE of 6.5% at 600 nm on patterned sapphire. We discuss formation and distribution of V-defects as well as luminescence properties and efficiency of InGaN red LEDs.

Hexagonal boron nitride (h-BN), a two-dimensional (2D) ultrawide-bandgap semiconductor, has seen fast progress in the last decade and attracted tremendous attention and intensive investigation for its association with III-Nitride heterostructures. More specifically, the weak van der Waals interactions between the III-Nitrides heterostructures and 2D h-BN layers is a major enabler because it allows a mechanical release and transfer of freestanding membranes of III-Nitrides heterostructures to foreign substrates which open new pathways for III-Nitride heterogeneous integration and flexible devices. In this talk we will present the results of our work on LEDs grown on h-BN and their transfer to foreign substrates.

Micro-light emitting diodes (micro-LEDs) have attracted lots of attention in recent years due to their compact size, high efficiency, and good reliability for display applications. More and more research communities and industries are putting efforts into exploring micro-LEDs to achieve high-performance micro-LED display which can be used in monitor, watch, AR/VR, etc. Here we present a novel application of nanoporous GaN with quantum dots (NPQD®)_in the field of micro-LEDs. We demonstrate fabricating full-color monolithic micro-LEDs with loading green and red QDs inside nanoporous GaN. Strong light scattering induced by nanoporous GaN structure greatly enhanced the absorption of the blue pumping light by QDs. Nearly 50% power conversion efficiency from blue to red has been achieved at 2.5µm sub-pixel size. A full-color micro-LED display driven by CMOS with a sub-pixel size of less than 5um has been fabricated using NPQD® technology.

As the new display technology, micrometer-sized light emitting diodes (micro-LEDs) have the advantages of high resolution, and high luminance. In this work we now present a new realization method of micro-LEDs based on this bottom-up process. A comprehensive analysis of the electro-optical properties of this new type of micro-LED has been performed. In addition, we will discuss one of the major challenges for active matrix micro-displays: the transfer of micro-LED platelets on a host substrate. Therefore, our approach will provide a promising route for the fabrication of bottom up, highly efficient micro-LED based displays.

The innovative method of μLEDs fabrication is presented. The light emission area was defined by a size of the tunnel junction (TJ) embedded inside diode. The epitaxial structures were grown entirely by plasma assisted molecular beam epitaxy (PAMBE) on (0001) bulk GaN crystals. The PAMBE grown LED structure emitting light at 450 nm was capped with TJ region and 100 nm n-type GaN. The emission size of μLEDs was defined by ion implantation of n-type GaN and TJ region. The entire surface of the wafer is atomically flat, ready for the next epitaxial process, which is important e.g. for TJ µLEDs red-green-blue displays with a stack of 3 µLEDs.

In order to drop the dislocation density and reduce the manufacturing costs, GaN can be deposited on substrates that possess a high density of growth sites. In our present strategy, we simultaneously focus on overcoming the misorientation and dislocation problems. To achieve this goal, nanopatterning of GaN/AlN layers deposited on Silicon-On-Insulator (SOI) substrates is used in order to fabricate nanopillar arrays, using nanoimprint lithography (NIL) and plasma etching. This will be reflected by the high-quality epitaxial layer of GaN. The latter will be used as an active layer to produce a LED array having a better emission quality.

By combining nanomasking with thermally resistant materials and sublimation in a molecular beam epitaxy reactor, porous (In)GaN layers can be obtained. The advantages and disadvantages of this technique compared to classical electrochemistry methods will be discussed. The porosity can be adjusted from 0 to 1 and the pore depth can be controlled by the sublimation temperature and time. Preferential sublimation occurs at the dislocation position which strongly enhance the photoluminescence properties. As the porosification process by sublimation does not depend on the doping, fully porous light emitting diodes can be demonstrated.

This conference poster presentation was prepared for the Photonics West OPTO 2023 Symposium.

This conference poster presentation was prepared for the Photonics West OPTO 2023 Symposium.

This conference poster presentation was prepared for the Photonics West OPTO 2023 Symposium.

Excitons in InGaN quantum wells are a complex phenomenon: due to the interplay between pronounced Coulomb interaction and strong alloy disorder, they possess a hybrid nature in-between the textbook cases of free and fully-localized excitons. Studying them also represent a significant computational challenge. We will present a thorough numerical investigation of these excitons, based on statistical exploration. We will show how the details of the quantum well structure can lead to a wide range of binding energies and wavefunction behaviors. We will discuss prospects for experimental observation.

Interest in Be as a shallow acceptor in III-Nitrides is growing. Recently, we reported the first systematic study of MOCVD growth of GaN:Be. It is well known that impurities tend to segregate at threading dislocations in GaN. Despite the relatively high quality of MOCVD-grown GaN, lattice mismatch with foreign substrates such as sapphire usually results in dislocation density on the order of 108 cm-2. In this study, we investigate the impact of substrate quality on the GaN:Be. The effects of the substrate lattice mismatch and dislocation density on total Be incorporation, optical characteristics, and Be activation efficiency are discussed.

This conference presentation was prepared for Photonics West, 2023.

This conference presentation was submitted for Photonics West, 2023.

III-nitride semiconductors (GaN and related alloys) triggered the 21st century lighting revolution thanks to the achievement of blue LEDs exhibiting remarkable performance, e.g., a wall-plug efficiency of 80%. Another striking feature of those compound semiconductors is their capability of covering the whole visible spectrum when alloying GaN with indium. De facto, a single material platform could be used to make RGB displays based on direct light emission. In addition, InGaN alloy - which is behind all blue and green LEDs- exhibits strong alloy disorder, which is highly beneficial for micron-size LEDs thanks to reduced carrier diffusion length.

In this talk, I will first present the current understanding of the physics of blue LEDs. This will bring me to the question of the “green-gap” and its relevance in light of recent results reported on InGaN red LEDs. Eventually, I will discuss challenges and opportunities offered by III-nitrides for RGB microLED displays.