The exciton states in wurtzite CdSe nanocrystals (NCs) embedded in a glass matrix are examined by spin-flip Raman scattering using resonant excitation at magnetic fields up to 8 T. The Land & eacute; g factors of the bright excitons of about 2.2 are found to be weakly dependent on the NC diameter in the range of 4.1-6.1 nm. For an explanation of the experimental results, a theory of acoustic phonon-assisted exciton spin flips is developed, and the exciton spin relaxation rates are calculated. The theory is used to model the spin-flip Raman spectra of spherical CdSe NCs, taking into account their random orientation and size dispersion in the ensemble. Our calculations show that, despite the strong anisotropy of the exciton g factors and the random NC orientation, the spin-flip Raman shifts are determined by the NCs whose wurtzite c axis is oriented along the magnetic field. The model results are in good agreement with our experimental data. From fitting the experimental data, the hole g factor gh = -1.17 is extracted.

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.

Migration of plasma erosion products in plasma facilities is studied experimentally and numerically within the framework of modeling transport of plasma-facing materials in the diagnostic ducts of fusion devices. Material transport simulation is discussed for two cases of low and high background neutral gas pressures. Monte Carlo software KITe was used to simulate transport at a neutral gas background pressure 0.1–0.5 Pa—typical during steady-state tokamak operation and during pressure pulses caused by edge localized modes (ELMs). The simulation approach was implemented to describe experiments at the MAGNUM-PSI facility. Fluid dynamic code FLUENT is used to simulate transport during pressure surges as high as 1000 Pa, which can occur in the case of severe disruptions in tokamak plasma discharges, such as vertical displacement events (VDE) or accidental events. The hydrodynamic approach was verified in simulation of target sputtering in the QSPA plasma gun facility.

Layers of ZnSe and ZnCdxSe (x ~ 0.32-0.35) grown on GaAs (001) substrates by molecular beam epitaxy method were investigated. The electron beam impact on changes in crystal structure of specimens under examination and on their luminescent properties was studied. Methods of cathode luminescence, transmission electron microscopy, and electron microprobe analysis were applied. It is found that irradiation of specimens in the transmission electron microscope results in stacking faults annealing accompanied by formation of ZnO precipitates with hexagonal crystal structure. Irradiation of specimens in the cathode luminescence plant results in decreased intensity of cathode luminescence layers of ZnSe and ZnCdxSe in question due to radiation-stimulated degradation processes. Key words: point defects, irradiation by electron beam, cathode luminescence, structural changes.

Исследовались слои ZnSe и ZnCdxSe (x ~ 0.32-0.35), выращенные на подложках GaAs (001) методом молекулярно-пучковой эпитаксии. Изучалось влияние электронного пучка на изменения кристаллической структуры изучаемых образцов и люминесцентные свойства. Исследования проводились методами катодолюминесценции, просвечивающей электронной микроскопии и методом рентгеноспектрального микроанализа. Установлено, что в результате облучения образцов в просвечивающем электронном микроскопе происходит отжиг дефектов упаковки, сопровождаемый образованием преципитатов ZnO c гексагональной кристаллической структурой. Облучение образцов в катодолюминесцентной установке приводит к уменьшению интенсивности катодолюминесценции исследуемых слоев ZnSe и ZnCdxSe из-за радиационно-стимулированных процессов деградации. Ключевые слова: точечные дефекты, облучение электронным пучком, катодолюминесценция, структурные изменения.

In the present paper we discuss correlations between crystal structure and magnetic properties of epitaxial ε-Fe2O3 films grown on GaN. The large magnetocrystalline anisotropy and room temperature multiferroic properties of this exotic iron oxide polymorph, make it a perspective material for the development of low power consumption magnetic media storage devices. Extending our recent progress in PLD growth of ε-Fe2O3 on the surface of technologically important nitride semiconductors, we apply reciprocal space tomography by electron and x-ray diffraction to investigate the break of crystallographic symmetry occurring at the oxide-nitride interface resulting in the appearance of anisotropic crystallographic disorder in the sub-100 nm ε-Fe2O3 films. The orthorhombic-on-hexagonal nucleation scenario is shown responsible for the development of a peculiar columnar structure observed in ε-Fe2O3 by means of HRTEM and AFM. The complementary information on the direct and reciprocal space structure of the columnar ε-Fe2O3 films is obtained by various techniques and correlated to their magnetic properties. The peculiar temperature dependence of magnetization studied by the small-field magnetization derivative method and by neutron diffraction reveals the existence of a magnetic softening below 150 K, similar to the one observed earlier solely in nanoparticles. The magnetization reversal in ε-Fe2O3 films probed by X-ray magnetic circular dichroism is found different from the behavior of the bulk averaged magnetization measured by conventional magnetometry. The presented results fill the gap between the numerous studies performed on randomly oriented ε-Fe2O3 nanoparticles and much less frequent investigations of epitaxial epsilon ferrite films with lattice orientation fixed by the substrate.

We report on structural and optical studies of metamorphic InAs(Sb)/In(Ga,Al)As quantum well (QW) heterostructures with different designs of the active region, grown by molecular beam epitaxy on GaAs substrates and emitting in the mid-IR spectral range (3.0–3.5 μm) at room temperature. The influence of the thickness of the InGaAs/InAlAs superlattice waveguide and design of the InSb/InAs/InGaAs QW on stress balance in such metamorphic structures, their luminescent properties, and density of extended defects in the active region is discussed. The peculiarities of electron and hole energy spectra of the active region vs stress and design are studied theoretically in the framework of the 8-band Kane model and verified experimentally by Fourier-transform infrared photoreflectance spectroscopy. Despite that optimized metamorphic heterostructures are characterized by the extended defect density in the active region of just about 107 cm−2, carrier confinement in the QW has a stronger impact on their mid-IR photoluminescence intensity at room temperature.

AlN/c-Al2O3 templates were grown by plasma-assisted molecular beam epitaxy using migration enhanced epitaxy (MEE) and metal modulated epitaxy (MME) employed for consequent growing the nucleation and buffer layers (NL and BL). Structural quality and stress evolving were compared using in situ stress measurements, x-ray diffraction, transmission and atomic force microscopies. Optimization of MEE mode of NL led to a high degree of initial nuclei coalescence and the weak tensile stress (<0.35 GPa) in the template. The optimal stoichiometric conditions were found for a double-stage MME of BL. During the first stage with the aluminum to active nitrogen flux ratio F-Al/F-N*=1.1 at 780 degrees C, the most effective bending of both screw and edge threading dislocations occurs, followed by their efficient filtration. This bending in the stretched BL is explained by the coarse grain AlN morphology, which can be smoothed dramatically during the top BL growth at higher temperature of 850 degrees C and F-Al/F-N*=2.1.

Self-organization mechanisms promoting elimination of cracks in thick GaN layers grown on sapphire substrates are considered on the basis of the experimental results on the fabrication of the layers by Hydride Vapor-Phase Epitaxy on MOCVD-grown GaN/Al2O3 templates. The obtained data support the supposition on the closure of tensile stress-related cracks via diffusion processes and demonstrate the strong contribution of bulk diffusion in addition to surface diffusion discussed earlier.

Metamorphic InAs(Sb)/InGaAs/InAlAs quantum-confined heterostructures with thin (1–5 nm) strongly mismatched GaAs and InAs inserts in a gradient metamorphic InxAl1−xAs buffer layer have been grown on GaAs (001) substrates by molecular beam epitaxy. It has been shown that the use of a 5-nm GaAs insert in the region of a metamorphic buffer layer at x ~ 0.37 almost doubles the photoluminescence intensity at 300 K (λ ~ 3.5 μm) from an InAs/InGaAs quantum well with a monolayer InSb insert. This is explained by an increase in the hole localization energy in InSb measured by photomodulation reflection Fourier transform infrared spectroscopy. This increase is due to increased elastic stresses in the quantum well because of a decreased density of threading dislocations in this structure caused by the introduction of an additional inverse step into the metamorphic buffer layer in the form of a 5-nm GaAs layer. The introduction of a 5-nm InAs layer into the metamorphic buffer layer violates its functions as a dislocation filter, leading to a higher density of dislocations in the quantum well region and to an order of magnitude decrease in the luminescence intensity.

Metamorphic laser heterostructures In(Sb, As)/In 0.81 Ga 0.19 As/In 0.75 Al 0.25 As with InSb/InAs/InGaAs composite quantum wells based on submonolayer InSb insertions in a 10-nm InAs layer have been grown by molecular beam epitaxy on GaAs (001) substrates. Stimulated emission at a wavelength of λ ~ 2.86 μm at temperatures of 10–60 K at optical pumping has been demonstrated in such structures without an optical cavity. The threshold pump power density is about 5 kW/cm 2 at a temperature of 10 K.

The metastable ε-Fe2O3 is known to be the most intriguing ferrimagnetic and multiferroic iron oxide phase exhibiting a bunch of exciting physical properties both below and above room temperature. The present paper unveils the structural and magnetic peculiarities of a few nm thick interface layer discovered in these films by a number of techniques. The polarized neutron reflectometry data suggests that the interface layer resembles GaFeO3 in composition and density and is magnetically softer than the rest of the ε-Fe2O3 film. While the in-depth density variation is in agreement with the transmission electron microscopy measurements, the layer-resolved magnetization profiles are qualitatively consistent with the unusual wasp-waist magnetization curves observed by superconducting quantum interference device magnetometry. Interestingly a noticeable Ga diffusion into the ε-Fe2O3 films has been detected by secondary ion mass spectroscopy providing a clue to the mechanisms guiding the nucleation of exotic metastable epsilon ferrite phase on GaN at high growth temperature and influencing the interfacial properties of the studied films.

A method for the growth of nanocomposite layers in stoichiometric amorphous silicon dioxide is proposed. It is shown that, after annealing at a temperature of 1150°C in nitrogen atmosphere, a layer containing silicon nanoclusters is formed. Silicon nanoclusters have a crystal structure and a size of 3–6 nm. In a film grown on a n-type substrate, a layer of silicon nanoclusters with a thickness of about 10 nm is observed. In the case of a film grown on a p-type substrate, a nanocomposite layer with a thickness of about 100 nm is observed. The difference in the formation of a nanocomposite layer in films on various substrates is associated with the doping of silicon dioxide with impurities from the substrate during the growth of the film. The formation of the nanocomposite layer was confirmed by transmission electron microscopy, XPS and local cathodoluminescence studies.

The goal of this work is to study the morphology and crytal structure of novel GaN epitaxial nanostructures (Y-shape nanotripods) grown on silicon (111) by plasma assisted molecular beam epitaxy (PA-MBE). Prior to the nanowire synthesis epitaxial GaN nanoparticles were formed on the Si(111) via nitridation of Ga nanodroplets on Si substrate surface (droplet epitaxy technique). Effect of the seeding layer on futher nanostructure growth, GaN/Si heterointerface formation, crystal structure and morphology is studied by use of different microscopic techniques.

We have investigated the plasma-chemical anodic oxidation of single-crystal GaAs under the action of products of the non-self-sustained dc Townsend discharge in a 98%Ar + 2%O2 gas mixture. In the experiments, an original design of a plasma-chemical microreactor with two discharge gaps and a plane-parallel arrangement of electrodes is used. In one of the gaps of the device, a self-sustained Townsend discharge is excited. The stability of its spatially uniform state is ensured by the fact that one of the electrodes (cathode) has a high resistance. The non-self-sustained Townsend discharge in the second gap is controlled by the state of the first gap, which is provided by a proper design of the device. The plasma-chemical anodic oxidation of a semiconductor in the microreactor is studied at room temperature while changing the current density and processing time. The thickness of the oxide films was determined by spectral ellipsometry and transmission electron microscopy. In accordance with the obtained data, the microreactor ensures the formation of nanoscale oxide layers, whose thickness can be varied by changing the parameters of the process. The results are compared with some literature data on dc gas-discharge anodic oxidation of GaAs. We believe that the method can be used in other plasma-chemical processes where gas-discharge products interact with a solid target.

AbstractThe results of optimization of the design and growth conditions of an In_ x Al_1– x As metamorphic buffer layer with a high In content ( x = 0.05–0.83) grown via MBE on GaAs(001) substrates with the purpose of optimizing its surface morphological characteristics and structural properties and lowering the surface density of threading dislocations. The lowest surface-pattern roughness RMS = 2.3 nm (on an area of 10 × 10 μm) and density of threading dislocations of 5 × 10^7 cm^–2 are found in the samples with a convex-graded metamorphic buffer layer.

We demonstrate that the use of a GaN seeding layer prepared prior to the growth of epitaxial GaN on Si (111) can lead to the formation of oriented arrays of Y-shaped nanoislands and nanowires and affects the surface density of the nanostructures.

Hybridization of semiconducting and magnetic materials into a single heterostructure is believed to be potentially applicable to the design of functional spintronic devices. In the present work we report epitaxial stabilization of four magnetically ordered iron oxide phases (Fe3O4, gamma-Fe2O3, alpha-Fe2O3 and most exotic metastable epsilon-Fe2O3) in the form of nanometer-sized single crystalline films on GaN(0001) surface. The epitaxial growth of four distinctly different iron oxide phases is achieved by a single-target pulsed laser deposition technology on a GaN semiconductor substrate widely used for electronic device fabrication. The discussed iron oxides belong to a family of simple formula magnetic materials exhibiting a rich variety of outstanding magnetic properties including peculiar Verwey and Morin phase transitions in Fe3O4 and alpha-Fe2O3 and multiferroic behavior in metastable magnetically hard epsilon-Fe2O3 ferrite. The physical reasons standing behind the nucleation of a particular phase in an epitaxial growth process is discussed in the present paper deserving interest from the fundamental point of view. The practical side of the presented study is to exploit the tunable polymorphism of iron oxides in order to create the ferroic-on-semiconductor heterostructures usable in spintronic devices. By application of a wide range of experimental techniques the surface morphology, crystalline structure and electronic and magnetic properties of the single phase iron oxide epitaxial films on GaN have been studied. A comprehensive comparison has been made to the properties of the same ferrite materials in the bulk and nanostructured form reported by other research groups.