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Aim to study the proliferative effect of various concentrations of sulfated glycosaminoglycans (sGAG) on the culture of human corneal epithelial cells. Material and methods. The study focused on the properties of a mixture of sGAG consisting of chondroitin-4-sulfate, chondroitin-6-sulfates and keratan sulfates obtained by isolation from the transparent unchanged stroma of the cornea of farm animals. The material for the in vitro study was the cells of the anterior epithelium of the human cornea. In the experimental group No.1, a mixture of sGAG was added to the culture medium at a concentration of 0.1%, in the experimental group No.2 at a concentration of 0.5%, in the experimental group No.3 at a concentration of 1.0%. In the control group No.1, the medium was replaced without the addition of solutions, cell seeding was carried out similarly to seeding with experimental samples. The negative control group included the holes of the tablet without the addition of medium and solution. Results. The highest levels of the cellular index (CI) were obtained in the experimental group No.3 (1.00% of sGAG) and amounted to 2.86 0.11 (the total increase in indicators was 60.67%). In the experimental group No.2 (0.50% sGAG), the cellular index levels were 2.650.24 (the total increase in indicators was 52.29%), which was significantly different from the experimental group No.1 (0.10% sGAG) and the control group No.1, where these indicators were 2.54 0.21 (35.81%) and 2.370.02 (25.39%) respectively. Conclusion. The final CI indicators show that solutions of the sGAG mixture at concentrations of 0.5% and 1.0% have a more pronounced proliferative effect on human corneal epithelial cells in vitro from the range of concentrations under study. The results obtained open up the prospects for the use of sGAG in the development of new approaches to the treatment of patients undergoing a chronic drug therapy or patients with concomitant corneal diseases.

Purpose. To evaluate immunological status in patients with metastatic forms of solid tumors before and at different intervals after stereotactic body radiation therapy (SBRT) of metastatic lesions depending on the dose and the number of irradiated metastasis. Materials and methods. A quantitative assessment and analysis of blood immunological parameters was conducted before irradiation, via 3-4 weeks and via 6-8 weeks after SBRT in patients with malignant tumors with oligometastases in the liver and lungs, in groups with a total focal dose (TFD ≤ 45 Gy ) and TFD> 45 Gy, and also in groups with irradiation of one metastases and two or more. All peripheral blood samples were analyzed by flow cytometry. Statistical analysis was performed using Friedman and Nemenyi criteria. Results. 3-4 weeks after the end of SBRT, when using higher doses (TFD> 45 Gy), we observed statistically significant increase of T-lymphocytes (CD3+CD19-); T-helpers (CD3+CD4+); activated T-helpers (CD3+CD4+HLA-DR+); activated cytotoxic T-lymphocytes (CD3+СD8+HLA-DR+). Decreasing of B-lymphocytes (CD3+CD19-) was observed in both dose groups (TFD ≤ 45 Gy and TFD > 45 Gy). We also noted the activation of the T-cell link of immunity both in the group with irradiation of one metastatic lesion, and in the group where 2 or more metastases were exposed to irradiation. Conclusion. Using of higher doses of SBRT is associated with a more activated antitumor T-cell immune response, while the analysis of groups of patients with different ammount of irradiated metastasis currently requires further research.

In 5 year 180 patients (89 men and 91 women) underwent radiosurgery of brain metastases. Most patients had breast cancer and less than 5 lesions. Median overall survival (MOS) was 12,5 months, median progression free survival (MPFS) - 6,1 months. Both values were higher in patients with solitary lesions: 22.5 vs 10 months (р=0.00703) and 9 vs 6 months (р=0.02787), respectively.MOS was 18 in women with breast cancer, 12 months in patients with non small cell lung cancer. 8 - with melanoma and only 4 months - for small cell lung cancer. MPFS was significantly shorter for lesions volumes above 15 cm3. In field progression was detected in 37 (2-24 months after treatment) of 180 cases (20,5%). Second radiosurgery was performed in 24 cases.

Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is $(6.8\pm0.2)$ eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of $(0.7\pm0.2)$ eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level., 50 Pages, 8 Figures, 100+ references

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.

Integration of graphene and hexagonal boron nitride (hBN) into lateral heterostructures has drawn focus due to the ability to broadly engineer the material properties. Hybrid monolayers with tuneable bandgaps have been reported, while the interface itself possesses unique electronic and magnetic qualities. Herein, we demonstrate lateral heteroepitaxial growth of graphene and hBN by sequential growth using high-temperature molecular beam epitaxy (MBE) on highly oriented pyrolytic graphite (HOPG). We find, using scanning probe microscopy, that graphene growth nucleates at hBN step edges and grows across the surface to form nanoribbons with a controlled width that is highly uniform across the surface. The graphene nanoribbons grow conformally from the armchair edges of hexagonal hBN islands forming multiply connected regions with the growth front alternating from armchair to zigzag in regions nucleated close to the vertices of hexagonal hBN islands. Images with lattice resolution confirm a lateral epitaxial alignment between the hBN and graphene nanoribbons, while the presence of a moiré pattern within the ribbons indicates that some strain relief occurs at the lateral heterojunction. These results demonstrate that high temperature MBE is a viable route towards integrating graphene and hBN in lateral heterostructures.

Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered increasing attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN via various bottom-up synthesis methods and directly through ion implantation, we provide direct evidence that the visible SPEs are carbon related. Room-temperature optically detected magnetic resonance is demonstrated on ensembles of these defects. We perform ion-implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of the simplest 12 carbon-containing defect species suggest the negatively charged $${\rm{V}}_{\rm{B}}{\rm{C}}_{\rm{N}}^ -$$ defect as a viable candidate and predict that out-of-plane deformations make the defect environmentally sensitive. Our results resolve a long-standing debate about the origin of single emitters at the visible range in hBN and will be key to the deterministic engineering of these defects for quantum photonic devices. Comparison of hexagonal boron nitride samples grown with different techniques and with varying carbon-doping content provides evidence that the defects emitting single photons in the visible range are carbon related.

Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band-gap but requires the formation of highly strained material and has not hitherto been realised. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and co-exists with a topologically-modified moiré pattern, and with regions of strained graphene which have giant moiré periods up to ~80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls, and also the topological defects where they terminate. We relate these results to theoretical models of band-gap formation in graphene/boron nitride heterostructures.

A new technique for measuring the thermopower anisotropy using polycrystals on the basis of repeated measurements of an array of randomly oriented microcrystals and statistical analysis of the data obtained is proposed. The thermopower anisotropy of a series of higher silicides of transition metals (β-FeSi2, doped MnSi1.75, ReSi1.75, and CrSi2) is measured using polycrystals. It is found that, in β-FeSi2 crystals, a very high thermopower anisotropy can be observed; in ReSi1.75 crystals, the thermopower can have different signs; and, in CrSi2, the absolute value of the thermopower depends heavily on the heat-treatment mode. The temperature dependences of the electrical conductivity and thermopower of CrSi2 needles are measured. The thermopower anisotropy in CrSi2 needles, as in bulk single crystals, is retained in a wide temperature range.

The reason why manganese monosilicide inclusions are formed during the growth of higher manganese silicide (HMS) crystals has not been studied in detail so far. Changes in the amount and density of these inclusions are greatly influenced by dopants. In this study, the structure of HMS crystals with various contents of germanium as a doping element is examined. It is found that raising the germanium content to 1 at % results in the fragmentation of layered manganese monosilicide inclusions and, simultaneously, leads to significant changes in the thermoelectric properties of HMS crystals. The results of the microstructural analysis of HMS crystals in relation to the germanium concentration may be of use for understanding the mechanism by which the manganese monosilicide phase is formed during the growth of higher manganese silicide crystals.

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require AlxGa1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of AlxGa1-xN bulk crystals by any standard bulk growth techniques has not been developed so far. There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by anystandard bulk growth techniques were not successful. Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite AlxGa1-xN wafers. Zinc-blende and wurtzite AlxGa1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and AlxGa1-xN wafers. At a thickness of ∼30 μm, free-standing GaN and AlxGa1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and AlxGa1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and AlxGa1 xN-based structures and devices. We have compared different RF nitrogen plasma sources for the growth of thick nitride AlxGa1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 μm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day. Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.

The use of surface active liquids facilitates intense stratification of mechanically strained Bi0.5Sb1.5Te3 crystallites. A Bi0.5Sb1.5Te3 heat element with specified thickness and structure is formed by layer-by-layer deposition of “thermoelectric ink” on its free surface. A heat treatment of the formed thermoelectric element in argon at a temperature of 800 K makes it possible to minimize radically the resistance of the grain boundaries introduced into its bulk.

In this work we present our recent results on the high-temperature plasma-assisted molecular beam epitaxy (PA-MBE) of hBN monolayers with atomically controlled thicknesses for 2D applications and on the growth of significantly thicker hBN layers for potential DUV applications. We also report our recent results on the high-temperature PA-MBE growth of hBN layers using a high-efficiency RF plasma source with high active nitrogen fluxes. Our results demonstrate that PA-MBE growth on highly oriented pyrolytic graphite (HOPG) substrates at temperatures ~1390°C can achieve mono- and few-layer thick hBN with a control of the hBN coverage and atomically flat hBN surfaces which is essential for 2D applications of hBN layers. The hBN monolayer coverage can be reproducible controlled by the PA-MBE growth temperature, time and B:N flux ratios. Significantly thicker hBN layers have been achieved at higher B:N flux ratios. We observed a gradual increase of the hBN thickness by decreasing the growth temperature from 1390°C to 1080°C. However, by decreasing the MBE growth temperature below 1250°C, we observe a rapid degradation of the optical properties of hBN layers. Therefore, high-temperature PA-MBE, above 1250°C, is a viable approach for the growth of high-quality hBN layers for 2D and DUV applications.

Monolayers of hexagonal boron nitride (hBN) are grown on graphite substrates using high-temperature molecular beam epitaxy (HT-MBE). The hBN monolayers are observed to grow predominantly from step edges on the graphite surface and exhibit a strong dependence of the morphology, including the dominant crystallographic edge, of the hBN monolayers, on the growth temperature, as well as systematic variations in growth rate and coverage, and significant differences in the growth at monolayer and multilayer graphite steps. At graphite monolayer steps hBN grows laterally across the surface on the lower terrace, but hBN growth on the upper side of the graphite step is more limited and is nucleated by three-dimensional clusters. Multilayer graphite steps exhibit a much higher density of non-planar hBN aggregates and growth on both the upper and lower terraces occurs. The results show that the hBN monolayer growth edge type, hBN island shape and the presence of hBN aggregates can be controlled in HT-MBE, with the highest quality layers grown at a substrate temperature of about 1390 °C. Sequential HT-MBE growth of hBN, graphene (G) and a second cycle of hBN growth results in the formation of monolayer thick lateral hBN–G–hBN heterostructures, in which a strip of G is embedded between monolayers of hBN.

Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commer-cially viable deep ultra-violet light sources. Due to a sig-nificant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 μm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day’s growth, which makes our MBE bulk growth technique commercially vi-able.

The possibility of obtaining composite materials in which the metal matrix is filled with CrSi2 needle microcrystals is considered. It is shown that it is possible in principle to obtain a regular structure of CrSi2 microcrystals in metal matrices of tin, aluminum, and their alloys. During chemical etching, a part of the matrix volume is dissolved, releasing “rods” of the semiconducting material. The thermoelectric parameters of such a system are estimated on the basis of the measured physical properties.

The results of studying the thermoelectric properties of p-type Bi0.5Sb1.5Te3 alloy samples prepared by melt spinning quenching are presented. The material after melt spinning is shaped as thin ribbons and has a quasi-amorphous structure. The thermoelectric properties (thermoelectric power and electrical resistance) and crystallization processes of as-prepared melt-spun ribbons are studied at 300–800 K for the first time. The stability range of the initial state, the crystallization-onset temperature, and the effect of thermal annealing on the thermoelectric-power factor of the alloy are determined.

The electrical resistivity and thermopower of cobalt monosilicide (CoSi) and dilute alloys of CoSi with iron at temperatures from 2 to 370 K are experimentally investigated. CoSi is a semimetal and considered to be a promising thermoelectric material. It crystallizes into the cubic structure without an inversion center. This feature suggests the existence of topologically nontrivial electronic states and makes CoSi a candidate for the Weyl semimetal class. The main goal of the study is to find experimental confirmation of this assignment. It is shown that the experimental temperature dependences of the electrical resistivity and thermopower of CoSi and Co1–xFexSi (x = 0.04) at low temperatures cannot be interpreted within the standard theory of conductivity in metals and may be related to topological features of the electronic structure of this compound.

Two In x Ga1-xN nanorod samples with graded In compositions of x = 0.5-0 (Ga-rich) and x = 0.5-1 (In-rich) grown by molecular beam epitaxy were studied using transmission electron microscopy. The nanorods had a wurtzite crystal structure with growth along and core-shell structures with an In-rich core and Ga-rich shell. Energy-dispersive x-ray analysis confirmed grading over the entire compositional range and showed that the axial growth rate was primarily determined by the In flux, and the radial growth rate by the Ga flux. There was no evidence of misfit dislocations due to grading, but the strain due to the lattice mismatch between the In-rich core and Ga-rich shell was relaxed by edge dislocations at the core-shell interface with Burgers vectors and.

Hexagonal boron nitride (hBN) has attracted a great deal of attention as a key component in van der Waals (vdW) heterostructures, and as a wide band gap material for deep-ultraviolet devices. We have recently demonstrated plasma-assisted molecular beam epitaxy (PA-MBE) of hBN layers on substrates of highly oriented pyrolytic graphite at high substrate temperatures of ~1400 &deg, C. The current paper will present data on the high-temperature PA-MBE growth of hBN layers using a high-efficiency radio-frequency (RF) nitrogen plasma source. Despite more than a three-fold increase in nitrogen flux with this new source, we saw no significant increase in the growth rates of the hBN layers, indicating that the growth rate of hBN layers is controlled by the boron arrival rate. The hBN thickness increases to 90 nm with decrease in the growth temperature to 1080 &deg, C. However, the decrease in the MBE temperature led to a deterioration in the optical properties of the hBN. The optical absorption data indicates that an increase in the active nitrogen flux during the PA-MBE process improves the optical properties of hBN and suppresses defect related optical absorption in the energy range 5.0&ndash, 5.5 eV.

Monolayer hexagonal boron nitride (hBN) tunnel barriers investigated using conductive atomic force microscopy reveal moiré patterns in the spatial maps of their tunnel conductance consistent with the formation of a moiré superlattice between the hBN and an underlying highly ordered pyrolytic graphite (HOPG) substrate. This variation is attributed to a periodc modulation of the local density of states and occurs for both exfoliated hBN barriers and epitaxially grown layers. The epitaxial barriers also exhibit enhanced conductance at localized subnanometer regions which are attributed to exposure of the substrate to a nitrogen plasma source during the high temperature growth process. Our results show clearly a spatial periodicity of tunnel current due to the formation of a moiré superlattice and we argue that this can provide a mechanism for elastic scattering of charge carriers for similar interfaces embedded in graphene/hBN resonant tunnel diodes.

Picosecond ultrasonics was used to study the photoelastic properties of zinc-blende (cubic) $c{\text{-Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ with $x$ around 0.5. The velocities for longitudinal sound in the alloys were measured using ultrafast UV pump-probe experiments with (AlGa)N membranes. Strong Brillouin oscillations were observed in (AlGa)N films attached to GaAs substrates. These oscillations are due to the dynamical interference of the probe beams reflected from the sample surface and interfaces and a picosecond-duration strain pulse propagating in the alloy layer. Optical and elasto-optical parameters including the complex refractive index and the fundamental band gap of the cubic nitride alloys are determined and compared with the values obtained by ellipsometry.

We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

GaNsub1-x/subSbsubx/subwith x∼5%-7% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photoelectrochemical cell for solar water splitting. In this study, we grew GaNsub1-x/subSbsubx/subunder conditions intended to induce phase segregation. Prior experiments with the similar alloy GaNsub1-x/subAssubx/sub, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb all suggested that GaNsub1-x/subSbsubx/suballoys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

Plasma-assisted molecular beam epitaxy (PA-MBE) is now widely used for the growth of group III-nitrides. Many years ago it became clear that during PA-MBE there is unintentional doping of GaN with boron (B) due to decomposition of the pyrolytic boron nitride (PBN) cavity of the RF plasma source. In this paper we discuss the unintentional B incorporation for PA-MBE growth of GaN and AlxGa1−xN using a highly efficient RF plasma source. We have studied a wide range of MBE growth conditions for GaN and AlxGa1−xN with growth rates from 0.2 to 3 µm/h, RF powers from 200 to 500 W, different nitrogen flow rates from 1 to 25 sccm and growth times up to several days. The chemical concentrations of B and matrix elements of Al, Ga, N were studied as a functions of depth using secondary ion mass spectrometry (SIMS). We demonstrate that boron incorporation with this highly efficient RF plasma source is approximately 1×1018 to 3×1018 cm−3 for the AlxGa1−xN growth rates of 2 – 3 µm/h.

We report present a study of the formation of hexagonal inclusions in thick (∼5 μm) and bulk (∼50 μm) cubic GaN (c-GaN) layers grown on GaAs (0 0 1) substrates. Surface analysis of both samples revealed the evidence of hexagonal grains on the surface. However, larger grains were formed in the bulk sample. In the XRD measurement, the signature of the h-GaN in the bulk sample was more visible than the thick sample. This is consistent with the PL measurement, of which the h-GaN peak emission was found to be more intense in the bulk sample with respect to the thick sample. Such observations showed that the fraction of hexagonal inclusions had increased by increasing the thickness of the c-GaN layer. As presented in the Raman spectroscopy, the signals of the h-GaN were dominant, but the transverse optical (TO) and longitudinal optical (LO) modes of c-GaN were still observable at ∼559 cm−1 and ∼739 cm−1, respectively. Finally, a micro-PL measurement showed that the average content of the hexagonal inclusions in the c-GaN layer up to ∼50 μm was less than 22%, suggesting that our samples have a lower hexagonal GaN content than some reported thin c-GaN samples.

Currently there is high level of interest in developing of vertical device structures based on the group III nitrides. We have studied n- and p-doping of free-standing zinc-blende GaN grown by plasma-assisted molecular beam epitaxy (PA-MBE). Si was used as the n-dopant and Mg as the p-dopant for zinc-blende GaN. Controllable levels of doping with Si and Mg in free-standing zinc-blende GaN have been achieved by PA-MBE. The Si and Mg doping depth uniformity through the zinc-blende GaN layers have been confirmed by secondary ion mass spectrometry (SIMS). Controllable Si and Mg doping makes PA-MBE a promising method for the growth of conducting group III-nitrides bulk crystals.

Optical properties of free standing cubic gallium nitride grown by molecular beam epitaxy system are investigated by a polarized infrared (IR) reflectance technique. A strong reststrahlen band, which reveals the bulk-like optical phonon frequencies, is observed. Meanwhile, continuous oscillation fringes, which indicate the sample consists of two homogeneous layers with different dielectric constants, are observed in the non-reststrahlen region. By obtaining the first derivative of polarized IR reflectance spectra measured at higher angles of incidence, extra phonon resonances are identified at the edges of the reststrahlen band. The observations are verified with the theoretical results simulated based on a multi-oscillator model.

We studied the thermoelectric properties, crystallization, and stability of amorphous and nanocrystalline states in Cr-Si composite films. Amorphous films, prepared by magnetron sputtering, were transformed into the nanocrystalline state by annealing with in situ thermopower and electrical resistivity measurements. We have found that the amorphous state is stable in these film composites to about 550 K. Prior to crystallization, the amorphous films undergo a structural relaxation, detected by peculiarities in the temperature dependences of the transport properties, but not visible in x-ray or electron diffraction. The magnitude and temperature dependences of electrical conductivity and thermopower indicate that electron transport in the amorphous films is through extended states. The amorphous films are crystallized at annealing temperatures above 550 K into a nanocrystalline composite with an average grain size of 10–20 nm.

We have studied time-integrated and time-resolved photoluminescence (PL) spectra and the transport properties of cubic GaN layers doped with different amounts of either magnesium (p-type) or silicon (n-type). In Mg doped material a broad blue peak was observed whose origin in cubic material has not been well established. Our temperature dependent and time resolved PL results as well as the observation of strong self-compensation in Mg-doped cubic GaN have allowed us to present a model for the origin of this emission in cubic material. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper demonstrates the growth of InGaN nanorods and lateral growth over nanorod arrays using molecular beam epitaxy. It is shown that nitrogen rich growth conditions result in a nanorod array and that, by changing to metal rich conditions, lateral growth may be enhanced to coalesce the nanorods into a continuous overgrown film. Energy dispersive X-ray spectroscopy has been used to demonstrate that the nanorods display a core-shell structure with In-rich cores and In-poor edges. Transmission Electron Microscopy has shown that the nanorods are free of dislocations. However, when lateral growth occurs basal plane stacking faults are generated. It is shown that this stacking fault generation leads to a change in structure from hexagonal to cubic. When coalescence has occurred large angle grain boundaries are present. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy has been used to grow continuous overlayers of In 0.5 Ga 0.5 N and InN on (1 1 1)Si, via growth of an intermediate nanorod arrays. Transmission electron microscopy showed that few threading dislocations are generated during nanorod growth, and are subsequently confined to twist grain boundaries 1–2 μm apart in the overlayer. In the In 0.5 Ga 0.5 N growth, the nanorods formed core–shell hexagonal wurtzite structures with In-poor surfaces, with lateral growth leading to grains with cubic zinc blende structure. High spatial resolution cathodoluminescence imaging showed that the In 0.5 Ga 0.5 N overlayers luminesced strongly, with evidence of compositional variations on the 1–2 μm scale. The reasons for compositional variations and the growth of cubic material are discussed.

In-situ Auger electron spectroscopy was used to determine the 1:1 flux ratios of Ga and N during growth of GaN by molecular beam epitaxy at low substrate temperatures. By linearly ramping the Ga-flux, while keeping the N-flux constant, and simultaneously measuring the chemical composition by monitoring N and Ga Auger peaks, the time of deviation from stoichiometry could be determined. The method was applied at very low substrate temperatures where reflection high-energy electron diffraction does not reveal clear growth mode changes. The importance of the N- vs Ga-rich conditions were confirmed with transmission electron microscopy which showed a distinct change in crystallinity between material at the top and bottom of the film, which are in agreement with previous findings.

Highly mismatched GaN 1− x Sb x alloys were grown under N-rich conditions at low substrate temperatures (325–550 °C) at a growth rates of ~0.09 μm/hr on sapphire. The alloys ranged in Sb composition from 0% to 16%, with the bandgap shifting from 3.3 to 1.6 eV in accordance with the band anticrossing (BAC) model. We compare these results to growths from another chamber, having a different N source, and using a faster growth rate (~0.24 μm/hr), much lower substrate temperatures (as low as 80 °C), different III/V ratios and absolute fluxes. Despite the range of morphologies obtained, all alloys follow the predictions of the BAC model with the bandgap only depending on the Sb composition.

The electronic structure of the conduction and valence bands of highly mismatched alloys (HMAs) such as GaN 1− x Sb x are well described by the band anticrossing model. The properties of this alloy, which has a large band gap range and controllable valence band positions, make it a candidate material for efficient solar energy conversion devices. We have examined the growth and structural properties of amorphous and crystalline GaN 1− x Sb x . These HMAs were grown by low temperature molecular beam epitaxy (MBE) under Ga-rich conditions. While there is a monotonic linear increase of Sb incorporation with Sb overpressure, there was no obvious dependence of Sb incorporation with growth temperature for the range of 10–470 °C. At growth temperatures lower than 100 °C, GaN 1− x Sb x HMAs lose crystallinity and become amorphous for Sb compositions at or exceeding ~5%. Ga-rich growth resulted in strong absorption at energies as low as 1 eV for GaN 1− x Sb x alloys of all compositions. The strong low energy absorption may arise from a Ga-related defect band.

We have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). We have designed and built a new growth chamber which allowed us to combine a plasma-assisted molecular beam epitaxy process with a liquid phase electroepitaxy. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from a liquid Ga melt at growth temperatures as low as ∼650 °C, with low nitrogen overpressures of ∼3×10−5 Torr.

We study thermoelectric properties, crystallization kinetics and stability of amorphous and nanocrystalline state in Cr–Si and Mn–Si composite films. The as-deposited amorphous films are transformed by annealing with in situ thermopower and electrical resistivity measurements into nanocrystalline state with continuous monitoring their state. Depending on the initial film composition, the films are transformed during the annealing into single phase or multi-phase nanocrystalline composites. A clear correlation between structural state and thermoelectric properties of the composites is found. Particularly, the thermoelectric power factor of nanocrystalline films is enhanced in comparison to the power factor of polycrystalline films, obtained by high-temperature re-crystallization of the nanocrystalline films, due to strong energy dependent charge carrier scattering.

We have studied the growth of GaN layers by plasma-assisted electroepitaxy (PAEE). We have used an RF-plasma source to produce high concentrations of N species in the Ga melt and a high DC electric current to transfer the N species from the Ga surface to the growth interface. We have achieved continuous GaN layers by PAEE at growth temperatures as low as ∼650oC. We have also investigated PAEE growth of GaN layers on GaN nanocolumn templates. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission Electron Microscopy of Mg doped GaN1-xAsx samples, grown by MBE at low temperatures, show substantial structural changes for samples that are semi-insulating and those with high or low conductivity. The conductive samples show p-type conductivity as evidence from the positive thermopower values. All the Mg doped samples show phase segregation: cubic GaAs and GaN grains (a mixture of cubic and some hexagonal) phases within an amorphous matrix. The best conductive samples show cubic GaAs grains with high density of stacking faults embedded into an amorphous matrix. The samples that are less conductive have lower ratio of the amorphous to the crystalline phase of the samples and much lower density of stacking faults. Higher Mg concentration is expected in the amorphous parts of the samples The semi-insulating samples that have either low Mg concentration or low As show grains of GaAs and GaN attached to each other with no evidence of the amorphous phase between them. There are no SFs in these grains. It is possible that the presence of the GaN between the GaAs grains lead to semi-insulating material properties since p-type doping of GaN is more difficult. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

The structural properties and electrical conduction mechanisms of p-type amorphous GaN1−x As x /n-type crystalline GaN PN junction diodes are presented. A hole concentration of 8.5 × 1019 cm−3 is achieved which allows a specific contact resistance of 1.3 × 10−4 Ω cm2. An increased gallium beam equivalent pressure during growth produces reduced resistivity but can result in the formation of a polycrystalline structure. The conduction mechanism is found to be influenced by the crystallinity of the structure. Temperature dependent current voltage characteristics at low forward bias (

We present high-resolution surface Brillouin scattering measurements on cubic GaN layers grown on GaAs substrate. By using a suitable scattering geometry, scattering by surface acoustic waves is recorded for different azimuthal angles, and the surface acoustic wave velocities are determined. A comparison of experimental results with numerical simulations of the azimuthal dependence of the surface wave velocity shows good agreement and allows a consistent set of elastic constants for c-GaN to be determined. © 2016 The Japan Society of Applied Physics., This work was partially supported by the Spanish MINECO under projects MAT2012-37276-C03-01 and MAT2015-71035-R, and from Comunidad de Madrid project S2013=MIT-2740 (PHAMA_2.0-CM). One of the authors (C. Kopittke) acknowledges the DAAD and its RISE program for financial support.

In this paper we describe detailed transmission electron microscopy studies of GaN1−xBix with 0.05 < x < 0.18 grown by low-temperature molecular beam epitaxy under Ga-rich conditions. Microstructural transformation from columnar growth separated by thin amorphous areas in the films with lowest Bi content (5%) to pseudo-amorphous structure with crystalline grains embedded in the amorphous matrix in the samples with higher Bi content (13% to 18%) was observed. In addition, metallic Bi segregation occurred in the samples with the highest Bi concentration. An abrupt decrease in absorption edge energy is found in samples with higher Bi content.

In this current paper we have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allowed us to combine the plasma-assisted molecular beam epitaxy process with a liquid-phase electroepitaxy system. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from liquid Ga melt at growth temperatures as low as ∼650 °C, with low nitrogen overpressures of ∼3×10 −5 Torr.

There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved.

In the current study we have demonstrated the feasibility of a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allows us to combine the plasma-assisted molecular beam epitaxy process with a liquid phase electroepitaxy system. We have demonstrated that it is possible to grow GaN layers by PAEE at growth temperatures as low as ∼650 oC and with low nitrogen overpressures of ∼3×10-5 Torr. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than ∼100 °C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1−xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1−xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1−xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of ∼1 eV.