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Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit., Insulating hexagonal boron nitride (hBN) is theoretically expected to undergo a crossover to a direct bandgap in the monolayer limit. Here, the authors perform optical spectroscopy measurements on atomically thin epitaxial hBN providing indications of the presence of a direct gap of energy 6.1 eV in the single atomic layer.

We demonstrate room-temperature ferromagnetism with in-plane magnetic anisotropy in thin flakes of the CrTe$_2$ van der Waals ferromagnet. Using quantitative magnetic imaging with a single spin magnetometer based on a nitrogen-vacancy defect in diamond, we infer a room-temperature in-plane magnetization in the range of $M\sim 27$ kA/m for flakes with thicknesses down to $20$ nm. In addition, our measurements indicate that the orientation of the magnetization is not determined solely by shape anisotropy in micron-sized CrTe$_2$ flakes, which suggest the existence of a non-negligible magnetocrystalline anisotropy. These results make CrTe$_2$ a unique system in the growing family of van der Waals ferromagnets, as it is the only material platform known to date which offers an intrinsic in-plane magnetization and a Curie temperature above $300$ K in thin flakes., Comment: 9 pages, 5 figures

International audience; Hexagonal boron nitride (hBN) is an important insulator that is incorporated into numerous 2D electronic, optoelectronic, and photonic devices, whereas natural hBN is a mixture of 20% 10B and 80% 11B isotopes, and monoisotopic hBN is a variant with just a single boron isotope, either 10B or 11B. Consequently, monoisotopic hBN has a higher thermal conductivity and a stronger neutron absorption (in the case of h10BN), making it superior for neutron detectors, heat management materials in nano flexible electronic devices, and phonon polariton-based nanophotonics. Here we synthesized approximately monoisotopic hBN using boron powder containing a single boron isotope and nitrogen, and grew single crystals from a Fe–Cr metal flux at atmospheric pressure. Narrow Raman peaks from the shear (r1.3 cm

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.

International audience; 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

International audience; 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

International audience; I review advanced applications of boron nitride in the fields of optoelectronic and Infrared photonics and correlate them to electronic and phonon properties

For more than seven decades, hexagonal boron nitride (hBN) has been employed as an inert, thermally stable engineering ceramic; since 2010, it has also been used as the optimal substrate for graphene in nanoelectronic and optoelectronic devices. Recent research has revealed that hBN exhibits a unique combination of optical properties that enable novel (nano)photonic functionalities. Specifically, hBN is a natural hyperbolic material in the mid-IR range, in which photonic material options are sparse. Furthermore, hBN hosts defects that can be engineered to obtain room-temperature, single-photon emission; exhibits strong second-order nonlinearities with broad implications for practical devices; and is a wide-bandgap semiconductor well suited for deep UV emitters and detectors. Inspired by these promising attributes, research on the properties of hBN and the development of large-area bulk and thin-film growth techniques has dramatically expanded. This Review offers a snapshot of current research exploring the properties underlying the use of hBN for future photonics functionalities and potential applications, and covers some of the remaining obstacles. Hexagonal boron nitride (hBN) is highly sought after for mid-IR nanophotonics, nonlinear and quantum optics, and as an efficient UV emitter. This Review surveys its fundamental physical properties, applications and synthesis.

We investigate the highly anisotropic behavior of the in-plane and out-of-plane infrared-active phonons of hexagonal boron nitride by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high-quality single crystals show strict fulfillment of selection rules and an unusually long E1u[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure ϵ0= 6.96, ϵ∞= 4.95, ϵ 0= 3.37, and ϵ∞ = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30° allow us to observe both the transverse- and longitudinal-optic A2u modes. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab initio calculations. While Grüneisen parameters are generally small in this layered crystal, the A2u(TO) displays an exceptionally large and negative Grüneisen parameter that results in widening of the type I hyperbolic region with increasing pressure. Softening of the A2u(TO) mode is induced by dynamical buckling of the flat honeycomb layers. © 2019 American Chemical Society., This work has been supported by the Spanish MINECO/FEDER under contracts nos. MAT2015-71035-R and MAT2016-75586-C4-1-P. T.T. acknowledges support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI grant number JP15K21722. B.G. acknowledges support from the Government of the Russian Federation, contract #14.W03.31.0011 at Ioffe Institute.

Nanotubes (NTs) of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, were first synthesized more than a quarter of a century ago; nevertheless, many of their properties have so far remained basically unknown. This review presents the state of the art in the knowledge of the optical properties of TMD NTs. We first evaluate general properties of multilayered TMD crystals, and analyze available data on electronic band structure and optical properties of related NTs. Then, the technology for the formation and the structural characteristics of TMD NTs are represented, focusing on the structures synthesized by chemical transport reaction. The core of this work is the presentation of the ability of TMD NTs to emit bright photoluminescence (PL), which has been discovered recently. By means of micro-PL spectroscopy of individual tubes we show that excitonic transitions relevant to both direct and indirect band gaps contribute to the emission spectra of the NTs despite the presence of dozens of monolayers in their walls. We highlight the performance of the tubes as efficient optical resonators, whose confined optical modes strongly affect the emission bands. Finally, a brief conclusion is presented, along with an outlook of the future studies of this novel member of the family of radiative NTs, which have unique potential for different nanophotonics applications., 38 pages, 11 fugures, 106 references

International audience; Zn1-xMgxO microcrystals are produced in the 0.15 & x & 1 composition range by calcination of ZnO + Mg(OH)2 after hydro micro mechanical activation according to the patent WO2018065735A1. The structural properties of the samples have revealed the cohabitation of wurtzite and rock-salt phases for x values ranging from 0.15 up to 0.6 with a clear increase of the proportion of cubic phase with x. A single cubic phase is observed in the range 0.66

AlyGa1−yN quantum dots (QDs) have been grown by molecular beam epitaxy on AlxGa1−xN (0001) using a 2-dimensional–3-dimensional growth mode transition that leads to the formation of QDs. QDs have been grown for Al compositions y varying between 10% and 40%. The influence of the active region design [composition y, QD height, and bandgap difference (ΔEg) between the AlxGa1−xN cladding layer and the AlyGa1−yN QDs] is discussed based on microscopy, continuous wave photoluminescence (PL), and time-resolved PL (TRPL) measurements. In particular, increasing y leads to a shift of the QD emission toward shorter wavelengths, allowing covering a spectral range in the UV from 332 nm (UVA) to 276 nm (UVC) at room temperature (RT). The low-temperature (LT) internal quantum efficiency of the QD ensembles was estimated from TRPL experiments at 8 K and values between 11% and 66% were deduced. The highest internal quantum efficiency (IQE)-LT is found for the QDs with higher Al content y. Then, the PL spectrally integrated intensity ratios between RT and LT were measured to estimate the IQE of the samples at RT. The PL ratio is higher for larger ΔEg, for QDs with y of 0.1 or 0.2, and high PL intensity ratios up to 30% were also measured for QDs with larger y of 0.3 and 0.4. RT IQE values between 5% and 20% are deduced for AlyGa1−yN QDs emitting in the 276–308 nm range.AlyGa1−yN quantum dots (QDs) have been grown by molecular beam epitaxy on AlxGa1−xN (0001) using a 2-dimensional–3-dimensional growth mode transition that leads to the formation of QDs. QDs have been grown for Al compositions y varying between 10% and 40%. The influence of the active region design [composition y, QD height, and bandgap difference (ΔEg) between the AlxGa1−xN cladding layer and the AlyGa1−yN QDs] is discussed based on microscopy, continuous wave photoluminescence (PL), and time-resolved PL (TRPL) measurements. In particular, increasing y leads to a shift of the QD emission toward shorter wavelengths, allowing covering a spectral range in the UV from 332 nm (UVA) to 276 nm (UVC) at room temperature (RT). The low-temperature (LT) internal quantum efficiency of the QD ensembles was estimated from TRPL experiments at 8 K and values between 11% and 66% were deduced. The highest internal quantum efficiency (IQE)-LT is found for the QDs with higher Al content y. Then, the PL spectrally integrated ...

International audience; AlN epilayer properties (120 nm thick) grown by ammonia assisted molecular beam epitaxy on c-sapphire substrates with different low temperature AlN buffer layers (LT-BL) have been studied. The role of the LT-BL on the AlN structural and optical properties was investigated as a function of the LT-BL thickness and growth temperature. Optimum growth conditions were identified with LT-BL thickness of 3 nm and growth temperature between 480 degrees C and 520 degrees C. It was shown that by optimizing these conditions, a reduction of both mixed and edge threading dislocation densities up to 75% is achieved. The impact of the growth temperature of the AlN epilayer was also studied showing an additional improvement of the AlN crystal and morphological properties while growing at higher temperature. A correlation between the epilayer strain and the PL emission was also investigated. Finally, an Al0.7Ga0.3N:Si doped layer was grown on the top of the optimized AlN template showing a smooth surface with monoatomic steps and a roughness similar to 0.2 nm, confirming the potential of such templates for the fabrication of AlGaN based heterostructures.

International audience; Emerging part of condensed matter science, which deals with the systems of extreme two-dimensionality, renews the interest in natural 2D objects such as planar stacking faults (SFs) in semiconductor crystals. We report on the observation of an excitonic state localized at the 1D intersection of the SF with a high quality ZnSe quantum well (QW). The micro-photoluminescence measurements are performed in a specimen used for preceding transmission electron microscopy studies. We demonstrate that the observed narrow lines are polarized along SFs and their linewidths depend on the SFs length. For short SFs, the linewidth can be as low as 0.15 meV. Using the combination of the effective mass approach and the density functional theory calculations we show that the exciton localization is due to the intrinsic electric field inside the SF, which also leads to a spatial separation of electron and hole in the exciton. The 1D intersection of perfect natural and artificial 2D objects can serve as a promising playground for the study of subtle excitonic effects in single defects.

International audience; In this work, we investigate the impact of the quantum confined Stark effect and of the carrier localization on the internal quantum efficiency of polarized single or multiple InxGa1-xN/GaN quantum well(s), and semi-polar (11e22) multiple InxGa1-xN/InyGa1-yN quantum well. We find that increasing the influence of the quantum confined Stark effect at constant indium content with increasing the well-width induces a reduction of the internal quantum efficiency onsets for a decreasing value of the photoexcitation density. Similar but no so dramatic trend is reported when increasing the indium content and thus when increasing the localization of carriers to localized fluctuations of the chemical composition of the alloy. In addition, a change of the electric field internal to active layers (quantified by using time-resolved photoluminescence spectroscopy) realized by growing samples along a semi-polar orientation leads to experimental observation of a substantial enhancement of the threshold of photoexcitation density at which onsets the reduction of the internal quantum efficiency. A correlation is found through several orders of magni- tude between the photoexcitation density PT for the onset of the collapse of IQE and the values of the photoluminescence radiative decay time trad. A scaling law is found in the investigated samples: PT ~ trad-n with n 1⁄4 3/2 ± 0.15 which evidences that quantum confined Stark effect is the main origin for the efficiency droop in nitride light-emitting diodes based on InxGa1-xN active layers.

The optical properties of homoepitaxial non-polar (Zn,Mg)O/ZnO single quantum wells (QWs) grown by molecular beam epitaxy on a-plane (11-20) ZnO substrates are investigated by using reflectance and continuous wave photoluminescence spectroscopies. The reflectivity and CW-PL spectra measured at low temperature reveal strong in-plane optical anisotropies and clear reflectance structures. The signatures of confined excitons analogous to C-exciton and (A, B)-excitons in bulk ZnO, are detected using light polarized respectively along the c-axis and perpendicular to this axis. Temperature dependence of CW-PL has been investigated in the two polarizations. For electric field of the light perpendicular to the c-axis the total intensity of the PL lines decreases by about an order of magnitude when the temperature increases from 8 K to room temperature. At low temperature a line associated to an excitonic complex dominates the PL spectrum. When the temperature increases the intensity of this line decreases as the intensity of the line associated to the free exciton increases. In the other polarization for temperature above 100 K a new line appears in the spectrum. Its intensity increases with temperature. This feature is associated to the c-like free exciton. The degree of polarization of these lines is independent of temperature. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)