The UV community for the fourth time took part in the International Workshop on Ultraviolet Materials and Devices (IWUMD4), which was organized in September 2019 by the Ioffe Institute, St. Petersburg, Russia. This event, following the successful meetings in Beijing, China (2016), Fukuoka, Japan (2017), Kunming, China (2018), was intended to show new horizons beyond the UV-blue LEDs based on wide-bandgap III-Nitrides. Material science, modeling and technology for the “UV gap” (200-300 nm) are at the frontier of applied physics. The studies are aimed at developing devices for such areas as medicine – for human-safely disinfection and rapid detection of viral threats, communication – for noise-protected and solar- blind UV-optical communication, nanotechnology – for high- resolution photolithography and for many other applications in the field of biology, pharmaceuticals and forensics.The Workshop was attended by 125 scientists from 15 coun- tries, including half from Asia and half from Europe and theDOI: 10.1002/pssa.202000378USA. The Program was very eventful and included 2 plenary talks by Prof. Chris G. Van de Walle (UCSB) and Prof. Michael Kneissl (TU Berlin&FBH), 4 keynotes, and 25 invited papers, as well as 35 oral presentations and 36 poster contributions displayed during two poster sessions. Prior to Workshop, the Tutorials on the basic properties of wide-gap semiconductor materials and nanostructures with lectures of outstanding experts in this field were held for one and a half days. The scope of the presentations at the Workshop was quite wide. Besides the fundamental properties of wide-gap semiconductor materials and nanostructures, their optical and structural characterization and applications, it highlighted new trends, such as the growth, basic properties and applications of Ga2O3 and physics of BN from bulk to 2D materials.This issue features selected reports from the Workshop participants. Recent results on the manufacture of high-quality templates of the nitrides and oxides by using differenttechnological approaches are presented. The fundamental problems of low-dimensional heterostructures and the techno- logical challenges of various epitaxial growth methods, such as molecular beam epitaxy and metalorganic chemical vapor deposition, are examined in detail to further improve the quan- tum efficiency and output power of UV light-emitting devices. A study of the optical properties of AlGaN layers and laser diode heterostructures are presented, which elucidate the nature and characteristics of spontaneous and stimulated emis- sions in AlGaN laser structures. In addition, the fabrication and radiation of GaN hexagonal microcavities are discussed. A variety of topics reflects the wide range of interests of the Workshop.The Editors would like to thank all the members of the International Advisory and Program Committees for their encouragement and great help during the Workshoporganization. We are grateful to the referees for their efforts and time devoted to the paper review process. We owe our best thanks to the official partners and sponsors of the workshop, as well as all participants at the symposium exhibition. Finally, we would like to express our sincere gratitude to Dr. Stefan Hildebrandt and the production team of physica status solidi for their friendly assistance and collaboration in the editorial process.The next 5th International Workshop on Ultraviolet Materials and Devices (IWUMD5) is planned to be held in November 2020 in Jeju, Korea. We wish the organizers a gorgeous and successful meeting.

we report the development of a scanning confocal microscope dedicated to photoluminescence in the 200 nm-wavelength range for samples at cryogenic temperatures (5-300K). We demonstrate the performances of our deep ultraviolet cryomicroscope in high- quality hexagonal boron nitride (hBN) crystals, although it can be utilized for biological studies in its range of operating wavelengths. From the mapping of the photoluminescence, we bring evidence for the suppression of extrinsic recombination channels in regions free from defects. The observation of emission spectra dominated by intrinsic recombination processes was never reported before in hBN, by means of photoluminescence spectroscopy. We show that photoluminescence tomography now competes with cathodoluminescence, and that deep ultraviolet cryomicroscopy by photoluminescence is a novel powerful tool in materials science applications, with the great advantage of an efficient non-invasive photo- excitation of carriers.

We review the recent progress regarding the physics and applications of boron nitride bulk crystals and its epitaxial layers in various fields. First, we high- light its importance from optoelectronics side, for simple devices operating in the deep ultraviolet, in view of sanitary applications. Emphasis will be directed towards the unusually strong efficiency of the exciton–phonon coupling in this indirect band gap semiconductor. Sec- ond, we shift towards nanophotonics, for the manage- ment of hyper-magnification and of medical imaging. Here, advantage is taken of the efficient coupling of the electromagnetic field with some of its phonons, those interacting with light at 12 and 6 μm in vacuum. Third, we present the different defects that are currently studied for their propensity to behave as single photon emitters, in the perspective to help them becoming challengers of the NV centres in diamond or of the double vacancy in silicon carbide in the field of modern and developing quantum technologies.

Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.

Nanotubes of transition metal dichalcogenides (TMDs), were firstsynthesized more than a quarter of a century ago; nevertheless, many of theiroptical properties have so far remained basically unknown. Herein, the stateof the art in the knowledge of the optical properties of TMD NTs is presented.First, general properties of multilayered crystals are evaluated, and availabledata on related NTs are analyzed. Then, the technology for the formation andthe structural characteristics of NTs are represented, focusing on thestructures synthesized by chemical transport reaction. The core of this work isthe presentation of the ability of synthesized TMD NTs to emit brightphotoluminescence (PL), which has been discovered recently. By means ofmicro-PL spectroscopy of individual tubes, we show that excitonic transitionsrelevant to both direct and indirect band gaps contribute to the emissionspectra of the NTs despite having the dozens of monolayers in their walls. Theperformance of the tubes as efficient optical resonators is highlighted, whereconfined optical modes strongly affect the emission. Finally, a brief conclusionis presented, along with an outlook of the future studies of this novel radiativemember of the NTs family, which have unique potential for differentnanophotonics applications

Hexagonal boron nitride (hBN) is attracting much attention due to its tremendous applications including nanophotonic and electronic devices, substrates for two-dimensional (2D) materials, heat management materials, etc. To achieve the best device performance, large area hBN single crystals are required. Herein, large area (>500 microns each) high-quality (defect density < 0.52/μm2) bulk hBN single crystals are grown from molten metal solutions with a temperature gradient. The narrow Raman linewidths of the intralayer E2g mode peak and the interlayer shear mode, the strong and sharp phonon-assisted transition photoluminescence peaks, and the high thermal conductivity demonstrate that the hBN produced by this method are high crystal quality with a low density of defects. Atomic force microscope images show that atomically flat layers of hBN can be produced by exfoliation. This study not only demonstrates a new strategy for growing large hBN single crystals, but also provides high quality thick and thin hBN layers for nano device applications

Fascinating optical properties governed by extremely confined excitons have been so far observed in 2D crystals like monolayers of transition metal dichalcogenides. These materials, however, are limited for production by epitaxial methods. Besides, they are not suitable for the development of optoelectronics for the challenging deep-ultraviolet spectral range. Here, we present a single monolayer of GaN in AlN as a heterostructure fabricated by molecular beam epitaxy, which provides extreme 2D confinement of excitons, being ideally suited for light generation in the deep-ultraviolet. Optical studies in the samples, supplemented by a group-theory analysis and first-principle calculations, make evident a giant enhancement of the splitting between the dark and bright excitons due to short-range electron–hole exchange interaction that is a fingerprint of the strongly confined excitons. The practical significance of our results is in the observation of the internal quantum yield of the room-temperature excitonic emission as high as ∼75% at 235 nm