Invited Speakers 

Gallium nitride (GaN) is central to modern optoelectronics and high-power electronics, but further progress depends on advances in crystal growth. This lecture reviews recent developments in bulk GaN crystallization, with emphasis on the basic ammonothermal method and its integration with halide vapor phase epitaxy (HVPE). Key challenges—including scalability, reactor design, and seed preparation—will be examined alongside issues in wafer processing and surface treatment. Particular focus will be given to hybrid growth strategies that combine the strengths of different techniques. Finally, future milestones toward large-diameter GaN substrates will be outlined, enabling next-generation lasers, transistors, and high-performance power 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). These templates are hereafter referred to as FFA Sp-AlN. 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. Various devices have been proposed using polarity inversion structures, such as second harmonic generation (SHG) devices based on quasi-phase matching, which exploits the nonlinear optical properties of nitride semiconductors. Leveraging the structural characteristics of the bilayer polarity-inverted FFA Sp-AlN, an SHG device with a transverse quasi-phase matched waveguide was fabricated. This device achieves a high nonlinear coupling coefficient by placing IDBs at the nodes of the second harmonic (SH) wave electric field distribution. The fabrication of a multi-layer polarity-inverted AlN structure with more than two layers is highly desirable. 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.

InGaN grown on (0001) vicinal surface shows In-content variations depending on the inclination angle. This property leads to polychromatic emission from InGaN quantum well emitters grown on 3D GaN templates. Controlling the 3D shape enables us to tune the emission spectra, which is beneficial for the fabrication of polychromatic, full color light emitters via single-step crystal growth.

Hexagonal boron nitride (h-BN), an insulating two-dimensional layered material, has attracted great attention due to its fascinating properties and promising applications across the fields of photonics, quantum optics, and electronics. Given the layered structure of h-BN, various polytypes exist with high-symmetry stacking sequences where successive layers are rotated or translated, leading to a variety of physical properties. Similarly to silicon carbide, polytypism in sp2-bonded boron nitride is to lead to a wealth of variations in the opto-electronic properties. I will report on the optical properties of a variety BN polytypes including bulk crystals grown by precipitation out of molten metallic solutions and epilayers by produced by metal-organic chemical vapor deposition on sapphire substrates [1,2]. The sp2-bonded layered compound BN exists in more than a handful of different polytypes (i.e. different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype single crystals extremely tricky. Co-existence of polytypes in a similar crystal leads to formation of many interfaces and structural defects having deleterious influence on the internal quantum efficiency of the light- emission and on the mobility of carriers. Light emission, when recorded in standard experimental illumination conditions, is in general inhomogeneous and its yield is dominated to radiative recombination to localized stacking faults [3], even for crystals with state-of-the- art, high structural quality [4]. However, despite of this, lasing operation was reported at 215 nm [5], which has shifted the interest of sp2-bonded BN from the laboratories of basic sciences to the fields of optoelectronic and electrical device applications. Here I present X-ray diffraction, electron microscopy and Raman light scattering experiments that were performed on a large variety of samples grown using the complementary techniques alluded to above and that we interpret with the help of group theory arguments and calculations of the phonon dispersion relations (in the context of quantum espresso). They revealed the three coherently ordered AA’ (normal), AB (Bernal), and ABC or rBN (Rhombohedral) and AA stackings, and the disordered turbostratic (tBN) one. Characterization of the different structural polytypes are completed by micro- photoluminescence experiment measurements that furnish a one-by-one, unambiguous relationship between the sp2-stacking and its photoluminescence spectrum [6,7,8,9]. These, together with reflectance experiment disentangle the nature of the light-matter interaction processes in sp2-bonded BN. All polytypes studied here display an indirect configuration of their fundamental bandgap, some of them (the AA and AB two-layer stackings and the rBN three-layer one) offer efficient Second Harmonic Generation of light performances.

References:

[1] Jiahan Li et al. ACS Nano 15, 7032 (2021).

[2] M. Chubarov et al., Crystal Growth Design 12, 3215 (2012)

[3] R. Bourrellier et al. ACS Photonics 1, 857 (2014).

[4] G. Cassabois et al. Nature Photonics 10, 262, (2016).

[5] K. Watanabe et al. Nature Materials 3, 404 (2004).

[6] M. Rousseau et al. Physical Review Materials 5, 064602 (2021).

[7] M. Moret et al. Applied Physics Letters 119, 262102 (2021).

[8] S.Moon et al. Nature Materials (2025) DOI10.1038/s41563-025-02173-2

[9] W.Desrat et al submitted to Physical Review Materials (under review)

We will present nano-scale correlation of structural, electronic and optical properties of nitride-based heterostructures using cathodoluminescence and electron-beam induced current directly performed in a scanning transmission electron microscope. Typical results exhibiting the complex nano-transport of excess carriers, in particular charge carriers and/or excitons, due to a specific heterostructure design of a device will be presented.

Polarization fields due to spontaneous and piezoelectric polarization are a distinguishing feature of nitride semiconductors. They are exploited in transistors to create high-density carrier gases, while in light emitters they need to be controlled to avoid efficiency loss. The impact of polarization on growth of nitrides has received less attention, however. We have recently developed a first-principles methodology that enables us to rigorously address polarization fields in surface calculations. The results show that spontaneous polarization affects the stability and nature of surface reconstructions. Compensation of the polarization fields requires the presence of significant amounts of charge, which manifests in the form of fixed charge associated with surface reconstructions or the presence of heterovalent impurities. Consequences for defect incorporation will be discussed.

Work performed in collaboration with C. E. Dreyer, J. Neugebauer, M. Todorova, S. H. Yoo, and D. Vanderbilt.

[1] S. H. Yoo, M. Todorova, J. Neugebauer, and C. G. Van de Walle, Phys. Rev. Appl. 19, 064037 (2023).

We developed III-nitride quantum dot (QD) uniform arrays using MOCVD, forming single InGaN QDs at the apex of six-fold and three-fold symmetric GaN pyramids via a selflimited growth mechanism. This method enables precise control of QD position, size, and symmetry, crucial for high-purity single-photon and polarization-entangled photon generation.

We also fabricated GaN hexagonal microrods and triangular prisms supporting whispering gallery and superscar modes, respectively, achieving room-temperature exciton-polariton condensation. These advances in QD symmetry control and microcavity engineering pave the way for integrated quantum photonic and polaritonic devices.

Gallium nitride (GaN) has become central to wide band gap power electronics, with decades of progress driving its adoption in sub-1kV applications and establishing it as a preferred choice for low-voltage systems. Silicon carbide (SiC) currently dominates the 1-3kV range, while gallium oxide (Ga2O3), with its ultra-wide band gap and higher breakdown strength, promises operation beyond both GaN and SiC. This raises a key question: which material will shape the future of high-voltage power electronics?

This talk will compare GaN, SiC and Ga2O3 across different voltage regimes, highlighting their performance advantages as well as key challenges in reliability, scalability and thermal management. Emphasis will be placed on high-voltage device architectures, defect-driven degradation, and emerging strategies such as heterogeneous integration, providing insights into pathways for next-generation, energy-efficient power conversion systems.

Deep ultraviolet LDs and LEDs around 260 nm, which are absorbed by the RNA and DNA of viruses and bacteria, are expected to be useful. To enhance the emission efficiency of these optical devices, controlling the flatness of the heterointerface at the atomic level is essential. In this study, we investigated the relationship between the device fabrication process, specifically the growth temperature of thin films, and the flatness of the heterointerface using a computational approach.

Far-ultraviolet C (Far-UVC) light-emitting diodes (LEDs) operating near 230 nm are emerging as promising sources for safe and effective disinfection, including inactivation of airborne pathogens and viruses. However, their external quantum efficiency (EQE) remains below 1% @ 230nm emission wavelength, primarily due to poor light extraction efficiency (LEE) and point defects. In this talk, we present recent theoretical and experimental progress at RIKEN in collaboration with National Yang Ming Chiao Tung University (Taiwan) toward overcoming this limitation through nano-patterned sapphire substrates (nanoPSS) and photonic crystal (PhC) structures. Finite-difference time-domain (FDTD) simulations reveal that optimized nanoPSS and reflective PhCs can synergistically enhance LEE by up to 6.7× compared to flat substrates. Experimentally, nanoPSS-based devices demonstrated a two-fold increase in EQE—from 0.15% to 0.32%—and doubled emission power. The experimental results of both the nanoPSS and PhC based LED will be revealed later. Furthermore, our 230 nm far-UVC LED modules successfully inactivated Dengue Virus (DENV), HIV-1, Influenza-A Virus (IAV), and SarsCoV-2 confirming their potential for real-world sterilization applications. Ongoing work aims to integrate optimized PhC designs with nanoPSS to achieve EQE >2% @ 230nm emission wavelength, paving the way for high-performance far-UVC LEDs for healthcare and environmental safety.

Point defects in crystalline materials can behave as artificial atoms, with optical and spin properties that enable their exploitation in quantum sensing. Compared to bulk materials, where point defects are generally located far from the surface (which improves their stability and minimizes surface-induced charge and spin noises), in 2D materials point defects lie necessarily close to the surface. This allows to place the sensing probes as close as possible to the test object, without necessarily degrading neither their stability nor their sensing performance. Among the plethora of 2D materials, hBN occupies an outstanding place, given its ubiquitous presence in almost every 2D-based device and heterostructure. Interestingly, hBN hosts several types of spin-defects; among them, the negatively-charged boron vacancy (VB-) is the most studied one because, even though it shows a relatively poor luminescence intensity, it works at 300K and its spin can be initialized and read-out. In this work we will introduce the use of VB- in hBN for quantum strain sensing [1]. The measurement of optically-detected magnetic resonance (ODMR) on VB- ensembles enables to achieve sub-micrometre spatial resolution that, combined with micro-Raman spectroscopy, can provide a spatially-resolved map of in-plane and out-of-plane strain distribution [1]. Subsequently, we will address the important issue of enhancing photon emission from VB- defects. To achieve such an enhancement, we will implement metallic (gold-based) nanotrenches in which the coupled effect of gap-plasmons and enhanced extraction efficiency results in a ~ 40-times emission enhancement with respect to a bare gold surface, while being compatible with ODMR measurements [2]. Finally, we will discuss the possibility of substituting the conventional optical detection by an electrical one, promoting thereby hBN as a practical real-field quantum sensor [3].

[1] X. Lyu et al., Nano Lett. 22 (2022) 6553

[2] H. Cai et al., Nano Lett. 23 (2023) 4991

[3] S. Ru et al., to be published

The developments towards higher wall-plug efficiency GaN-based VCSELs at Meijo University are described. By using high-quality AlInN/GaN DBR, lateral optical/current confinement, and in situ cavity length control, our GaN-based VCSELs show a WPE of 27% at 413 nm under RT CW operation.

Nitrogen (N) atoms doped in gallium oxide (Ga2O3) act as deep acceptors, and an energy barrier of about 3 eV can be formed at the N-doped p-Ga2O3/n-Ga2O3 junction. In this talk, we will present the effect of N radical irradiation on electrical properties of Ga2O3 Schottky barrier diodes and electrical properties of N-doped Ga2O3 thin films grown by molecular beam epitaxy.

In our study, the activities for the growth of large size single crystal diamond substrate have been conducting using the heteroepitaxy on the foreign substrate. In case of (100) plane, we achieved the heteroepitaxy of free-standing single crystal diamond wafer (size: 20x20 mm2) on the A-plane sapphire substrate in the last year [1-2], and are working on the heteroepitaxy of larger-size diamond wafer (>1inch).[3] Especially, the twin-free (111) single crystal diamond heteroepitaxy has been successfully demonstrated on the r-plane sapphire substrate, for the first time.[3]

In addition, the heteroepitaxial single crystal diamond-based device study has been successfully conducted in the area of doping and devices of p-SBDs and p-MESFETs [4-5].

Recently, the H-terminated diamond E-/D-mode MOSFETs have been simultaneously fabricated on the same heteroepitaxial diamond substrate [6]. The detailed advancements in the heteroepitaxial diamond wafer and power devices will be discussed in the conference.