This paper describes different growth modes of AlN layers on micro-cone patterned c-sapphire substrates (μ-PSSs) using plasma-assisted molecular beam epitaxy. Ordered arrays of AlN nanoprisms and microrods were selectively grown on the tips of μ-PSS's microcones according to a bottom-up formation mechanism using sequential migration enhanced and metal-modulated epitaxy (MME) under metal-rich growth conditions at 820 °C. Transmission electron microscopy revealed structurally perfect AlN regions above the tips of the μ-PSSs, which initiate as inverted nanopyramids with {1011¯} side faces, evolving into hexagonal nanoprisms with orientations of {11¯00} and (0001) for side and top surfaces, respectively. The diameter and height of these ordered hexagonal nanoprisms, which have a 60% probability of nucleating, were about 1 μm. Long-term MME growth of these nanoprisms in both vertical and lateral directions led to the formation of AlN(0001) microrods with a maximum possible diameter of two micrometers and a height of up to 6 μm. Atomic force microscopy revealed a mixed step-flow and 2D nucleation growth mechanism for the flat tops of these AlN nanoprisms and microrods with an average surface roughness of 1–2 monolayers. Micro-Raman spectroscopy demonstrated narrow E2 (high) linewidths of 3.8 and 4.2 cm−1 for essentially stress-free AlN nanoprisms and microrods, respectively.

Using electrons with energy about 15 keV, electron source based on a ferroelectric cathode and GaN/AIN MQW Structures the following values of power of UV-pulse radiation were obtained: $50 \mathrm{~W}(\lambda=240 \mathrm{~nm}, J=2240 \mathrm{~mA}), 69 \mathrm{~W}$ $(\lambda=248 \mathrm{~nm}, J=2500 \mathrm{~mA}), 71 W(\lambda=267 \mathrm{~nm}, J=2340 \mathrm{~mA})$

The paper reports on heterostructures for mid-ultraviolet (UVC) emitters with multiple (up to 400 periods) and single two-dimensional (2D)-GaN/AlN quantum disks/quantum wells with a nominal thickness below the critical thickness of ~2 monolayers (MLs) characterizing the transition of the 2D growth mode to 3D.The structures were grown by plasma-assisted molecular beam epitaxy (PA MBE) using low growth temperatures (~690C) in a wide range of gallium and activated nitrogen flux ratios Ga/N2* = 0.6  2.2 on various AlN/csapphire templates fabricated either by PA MBE or MOCVD.This made it possible to vary the surface topography from a 3D type under nitrogen-rich conditions to various types of 2D topographies in the structures grown under metal(Ga)-rich conditions.The absence of a Stranski-Krastanov transition in the latter structures was confirmed by a streaky RHEED pattern throughout the growth of QWs and barrier layers.The growth runs were monitored also by multi-beam optical stress sensor, which revealed an unusual stress relaxation in the ML-thick GaN/AlN heterostructures.Structural properties of GaN/AlN heterostructures were studied using X-ray diffraction analysis, including measurement of X-ray reflectance curves, atomic force microscopy, and high-resolution transmission microscopy.The results of these studies, together with the measurements of photoluminescence spectra, both cw and time-resolved, made it possible to suggest the formation of twodimensional GaN quantum disks with a thickness of either 1 or 2 ML and different lateral sizes on the stepped surface of the AlN barrier layers, which can lead to effective carrier localization.Moreover, we demonstrate a unique functional property of these atomically thin QW to maintain stable excitons, resulting in a particularly high radiation yield at room temperature.As a result, the emission energy (wavelength) from GaN/AlN 400QW structures could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) and was connected with a simultaneous increased of charge carrier localization.Using electron-beam pumping with a plasma cathode ferroelectric electron gun ensuring a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W, as shown in Fig. 1.In addition, we discussed the optical properties of cylindrical nanorods with diameters from 50 to 5000 nm, fabricated by the topdown technology from ML-thick GaN/AlN single QWs using a combination of wet and reactive ion etching.Photoluminescence measurements in an ultra-small QW region enclosed in a nanorod revealed narrow lines of individual excitons localized on potential fluctuations attributed to GaN quantum nano-disks 2-3 MLs high, which appear in a QW with an nominal thickness of 1.5 ML.A model that takes into account dark and bright exciton states was used to explain the features in the PL spectra, including their behavior with increasing temperature.This research was funded by

Powerful emitters of ultraviolet C (UVC) light in the wavelength range of 230–280 nm are necessary for the development of effective and safe optical disinfection technologies, highly sensitive optical spectroscopy and non-line-of-sight optical communication. This review considers UVC emitters with electron-beam pumping of heterostructures with quantum wells in an (Al,Ga)N material system. The important advantages of these emitters are the absence of the critical problem of p-type doping and the possibility of achieving record (up to several tens of watts for peak values) output optical power values in the UVC range. The review consistently considers about a decade of world experience in the implementation of various UV emitters with various types of thermionic, field-emission, and plasma-cathode electron guns (sources) used to excite various designs of active (light-emitting) regions in heterostructures with quantum wells of AlxGa1−xN/AlyGa1−yN (x = 0–0.5, y = 0.6–1), fabricated either by metal-organic chemical vapor deposition or by plasma-activated molecular beam epitaxy. Special attention is paid to the production of heterostructures with multiple quantum wells/two-dimensional (2D) quantum disks of GaN/AlN with a monolayer’s (1 ML~0.25 nm) thickness, which ensures a high internal quantum efficiency of radiative recombination in the UVC range, low elastic stresses in heterostructures, and high-output UVC-optical powers.

GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs' thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission.

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

We report non-contact local doping of a monolayer WSe2 transferred onto a piezoelectric substrate having surface potential wells (SPWs) induced by structural inhomogeneities. We used epitaxial GaN and InP/GaInP2 structures, in which there are SPWs ∼0.2 V deep and 0.1–2 μm in size. Using surface topography and potential scanning probe microscopy, as well as optical reflectance, photoluminescence, and Raman spectroscopy measurements, we observed strong enhancement of charged exciton emission and Raman intensity in the SPW regions of the monolayer WSe2, which indicate on piezoelectric doping at a level n ≥ 1012 cm−2 on a length scale ∼0.2–1 μm. Our results can be used to create electron/hole quantum puddles with anyon states in transition metal dichalcogenides, promising for the development of room temperature and magnetic-field-free fault-tolerant topological quantum computing.

Semiconductor heterostructures form the basis of modern electronics and optoelectronics, and the study of physical phenomena in them, along with the development of technological methods for their manufacture, is actively carried out all over the world to ensure progress in the output parameters of devices [...]

Al x Ga1−x N layers (x = 0.6−0.75) grown using plasma‐assisted molecular beam epitaxy with alternating metal‐enriched stoichiometric conditions using an off‐centered nitrogen flux demonstrate sharp compositional modulation with the formation of monolayer (ML)‐thick Ga‐enriched quantum disks embedded in a Ga‐depleted AlGaN matrix. These structures have a constant modulation period of ≈3 ML over the entire surface of a 2‐inch substrate, and the modulation amplitude increases from zero to maximum with distance from the center of the substrate. This is confirmed experimentally by conventional and scanning transmission electron microscopes, as well as studies of optical absorption and photoluminescence (PL) mappings. The PL measurements also show a high efficiency of ultraviolet‐C (UVC) radiative recombination at room temperature in these layers with an atomically smooth surface topology, emitting in the spectral range 250–290 nm with the maximum ratio of PL intensities measured at high (310 K) and low(10 K) temperatures up to 58%. Moreover, these layers demonstrate stimulated emission with the lowest threshold optical power density of 240 kW·cm−2 (at 287 nm) for the Al0.6Ga0.4N layer with the highest degree of compositional modulation. The results obtained can be used to develop technologies for growing ML‐scale heterostructures in (Al,Ga)N material system.

We demonstrate non-contact local doping of a monolayer WSe2 transferred onto a piezo-electric substrate having structural inhomogeneities using surface topography and potential scanning probe imaging along with photo-luminescence (PL) and Raman spectra mapping. We used a GaN epilayer grown by molecular beam epitaxy, in which surface potential wells having the depth of ~0.2 V and the size of ~1 μm, induced by inversion domains and Ga droplets are formed. In the monolayer WSe2 flake mechanically exfoliated on the epilayer, we observed a strong enhancement of the trion emission and the A1g phonon Raman intensity in the well region, which indicate a local piezo-electric doping of WSe2 at a level n>1012 cm-2. We show that our results can be used to create quantum Hall puddles having fractionally charged magneto-electron/anyon states at room temperature, which are promising for the development of fault-tolerant topological quantum computing.

The decay kinetics of low-temperature exciton photoluminescence in a heterostructure with multiple GaN/AlN monolayer quantum wells, which is prepared by molecular beam epitaxy, is studied. Measured radiation decay curves are theoretically simulated within a three-level model. The relaxation of dipole-allowed "bright" excitons spatially confined in GaN monolayers is determined as exciton relaxation with a characteristic time of ~3 ps, which is accompanied by spin flip and by transformation to dipole-forbidden "dark" excitons whose levels lie by ~60 meV below in energy. It has been shown that exciton states at temperatures above 50 K are two-dimensional.

Time-resolved pump-probe measurements characterizing the ultrafast carrier dynamics of intersubband transitions in GaN/AlN MQWs.

Stress evolution was studied in up to ~3 µm thick AlN templates, comprising ∼65 nm thick AlN nucleation layers (NLs) and thick buffer layers (BLs), grown using different growth modes and conditions by plasma-assisted molecular beam epitaxy (PA MBE) on c-Al2O3. Growth of both the NL and BL in a standard PA MBE mode at N-rich conditions (at flux ratio Al/N2* ∼ 0.5) led to instant generation of a relatively high tensile stress (∼1.5 GPa) which is maintained throughout the entire growth. On the contrary, NLs, grown using a migration-enhanced epitaxy (MEE), demonstrated a transition from the initial compressive stress to stress-free growth, which is usually observed in the Volmer–Weber films. Further growth of thick AlN BLs on the MEE–NLs at various Al/N2* ratios revealed a wide variety of stress evolution processes. The BL growth by using metal-modulated epitaxy (MME) at Me-rich conditions with Al/N2* ∼ 1.33 led to a gradual decrease in the initial compressive stress in the 2D AlN layers, whereas standard PA MBE growth of 3D BL at N-rich conditions (Al/N2* ∼ 0.92) exhibited a fast transition from the initial compressive stress to tensile stress. Moreover, we succeeded in achieving the quasi-stress-free growth of a 3.1 µm thick AlN BL using the MME growth mode at the optimum flux ratio Al/N2* = 1.05. These results were compared with the results of other authors and explained using a kinetic approach to description of stress evolution during PA MBE of AlN/c-Al2O3 templates, taking into account several simultaneously acting competitive mechanisms of continuous generation of tensile and compressive stresses.

Monolayer (ML)-scale GaN/AlN multiple quantum well (MQW) structures for electron-beam-pumped ultraviolet (UV) emitters are grown on c-sapphire substrates by using plasma-assisted molecular beam epitaxy under controllable metal-rich conditions, which provides the spiral growth of densely packed atomically smooth hillocks without metal droplets. These structures have ML-stepped terrace-like surface topology in the entire QW thickness range from 0.75–7 ML and absence of stress at the well thickness below 2 ML. Satisfactory quantum confinement and mitigating the quantum-confined Stark effect in the stress-free MQW structures enable one to achieve the relatively bright UV cathodoluminescence with a narrow-line (~15 nm) in the sub-250-nm spectral range. The structures with many QWs (up to 400) exhibit the output optical power of ~1 W at 240 nm, when pumped by a standard thermionic-cathode (LaB6) electron gun at an electron energy of 20 keV and a current of 65 mA. This power is increased up to 11.8 W at an average excitation energy of 5 µJ per pulse, generated by the electron gun with a ferroelectric plasma cathode at an electron-beam energy of 12.5 keV and a current of 450 mA.

We present an extensive theoretical and experimental study to identify the effect on the Raman spectrum due to interface interdiffusion between GaN and AlN layers in short-period GaN/AlN superlattices (SLs). The Raman spectra for SLs with sharp interfaces and with different degree of interface diffusion are simulated by ab initio calculations and within the framework of the random-element isodisplacement model. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, showed that the bands related to A1(LO) confined phonons are very sensitive to the degree of interface diffusion. As a result, a correlation between the Raman spectra in the range of A1(LO) confined phonons and the interface quality in SLs is obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy.

Recent progress in the development of monolayer (ML)‐thick GaN/AlN multilayer heterostructures for deep‐ultraviolet (UV) optoelectronics is reviewed. Analysis of both plasma‐assisted molecular beam epitaxy and metal–organic vapor phase epitaxy shows that extreme interface sharpness and sub‐ML accuracy in setting the layer thickness are attractive features of the former, whereas the lowest density of threading dislocations and wide possibilities for the implementation of various 2D growth mechanisms are the advantages of the latter. The structural properties of ML GaN/AlN heterostructures are evaluated not only by standard X‐ray diffraction and scanning transmission electron microscopy, but also by Raman spectroscopy. Theoretical and experimental studies of the optical properties of ML‐thick GaN/AlN quantum wells (QWs) reveal that quenching of the quantum‐confined Stark effect, suppression of transverse electric transverse magnetic polarization switching, as well as the excitonic nature of UV‐radiative recombination in ultrathin (1–2 ML) QWs ensure in such structures a high internal quantum yield of 75% for UV radiation at 235 nm at room temperature. The possibilities of using ML‐GaN/AlN heterostructures to fabricate UVC emitters of spontaneous and stimulated emissions in a wide range of output powers with various pumping techniques are considered, and the most important problems are formulated.

We report the results of experimental and theoretical studies of phonon modes in GaN/AlN superlattices (SLs) with a period of several atomic layers, grown by submonolayer digital plasma-assisted molecular-beam epitaxy, which have a great potential for use in quantum and stress engineering. Using detailed group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals is established. Ab initio calculations in the framework of the density functional theory, aimed at studying the phonon states, are performed for SLs with both equal and unequal layer thicknesses. The frequencies of the vibrational modes are calculated, and atomic displacement patterns are obtained. Raman spectra are calculated and compared with the experimental ones. The results of the ab initio calculations are in good agreement with the experimental Raman spectra and the results of the group-theoretical analysis. As a result of comprehensive studies, the correlations between the parameters of acoustic and optical phonons and the structure of SLs are obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy. The results obtained can be used to optimize the growth technologies aimed to form structurally perfect short-period GaN/AlN SLs.

We realize and investigate a nonlinear metasurface taking advantage of intersubband transitions in ultranarrow GaN/AlN multi-quantum well heterostructures. Owing to huge band offsets, the structures offer resonant transitions in the telecom window around 1.55 µm. These heterostructures are functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second-harmonic generation at normal incidence which is completely absent for an antenna array without the multi-quantum well structure underneath. While the second harmonic is originally radiated only into the plane of the quantum wells, a proper geometrical arrangement of the plasmonic elements permits the redirection of the second-harmonic light to free-space radiation, which is emitted perpendicular to the surface.

The results of joint theoretical and experimental studies aimed at revealing features in the Raman spectra, which can be used for evaluation of the interface quality between GaN and AlN layers in short-period GaN/AlN superlattices (SLs) are presented. The Raman spectra for SLs with sharp interface and with different degree of interface diffusion are simulated by ab initio calculations and within the framework of random-element isodisplacement model, respectively. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, leads to conclusion that the spectral region of the A1(LO) confined phonons is very sensitive to the degree of interface sharpness. As a result of comprehensive studies, the correlations between the parameters of the A1(LO) confined phonons and the structure of SLs are obtained. The results of the complex studies can be used to optimize the parameters of the growth process in order to form structurally perfect short-period GaN/AlN SLs.

Structural and dynamic properties of short-period GaN/AlN superlattices with the thicknesses of the constituent layers varying from two to several monolayers, grown using the method of submonolayer digital molecular beam epitaxy, are experimentally and theoretically studied. It is established that in the grown samples there are two types of periodicity. One type is formed by periodic sequence of GaN and AlN layers in the superlattice, and the second one is related with periodic interruptions in the growth of superlattice for the evaporation of excess Ga metal. The dependences of the positions and intensities of the lines in the Raman spectra on the period of the superlattice are determined, and microscopic nature of optical phonon modes is established. The doublet structure of the E(TO) lines localized in the GaN and AlN layers of superlattice, genetically related to the E 2(high) and E1 phonon branches of the bulk crystal, is first discovered and explained. A strong dependence of the polar modes localized in the AlN layer on the thickness of layer forming the superlattice is revealed. The results of complex studies will improve the accuracy of quantitative estimation of important parameters of superlattice structures and can be used to optimize growth parameters for the fabrication of structurally perfect short-period GaN/AlN superlattices.