Vol 61, No 12 (2019)

This issue of Physics of the Solid State publishes the proceedings of the International Conference “Mechanisms and Nonlinear Problems of Nucleation and Growth of Crystals and Thin Films” (MGCTF-2019) dedicated to the memory of Vitalii Valentinovich Slezov, an outstanding theoretical ph-ysicist.

In the dynamic theory of martensitic transformations, the wave mechanism of controlling martensite crystal growth is determined by the superposition of wave beams of quasi-longitudinal (or longitudinal) waves carrying the “tensile–compression” deformation in the orthogonal directions. The wave beam formation is considered to be a result of the formation of excited (vibrational) states. The existence of transformation twins is interpreted as a result of a matched propagation with respect to long-wave (l waves) and short-wave (s waves) shifts. The matching condition is analyzed for the γ–α martensitic transformation in iron-base alloys. It is shown for the first time that the transition to a degenerate twin structure with the allowance for the medium discreteness enables one to estimate the dislocation density in crystals with habit {557}, which agrees with that observed experimentally.

In the model of radiation-induced segregation based on the first and second Fick laws and taking into account the inverse Kirkendall effect, concentration profiles of the components of the concentrated Fe‒Cr–Ni alloy and radiation point defects at various temperatures, dislocation densities, and dose gain rates were obtained. Calculation of concentration profiles was carried out from the moment of the beginning of irradiation to reaching the stationary mode. The sensitivity of the concentrations of the components of the alloy near the surface from the input parameters (migration energies of vacancies Cr, Ni, Fe, migration energy of interstitial atoms, etc.) is analyzed.

A complex study of the structure, phase composition, features of the surface topology, magnetostriction and thermal properties of the compound Tb_0.8Sm_0.2Fe₂ was carried out. Peculiarities of the structure at the micro- and nanolevel were found; information on the magnetic domain structure at room temperature was obtained. X-ray structural studies in a wide temperature range from 90 to 760 K, including the Curie point, are presented. Experimental data on the thermal expansion and magnetostriction in magnetic fields up to 12 kOe were obtained and analyzed. Anomalies on the thermal expansion curve Δl/l(T) and on the magnetostriction dependence λ(T) in the low-temperature region were revealed. The magnetostriction effect in the studied compound was found to be almost constant in a wide temperature range of 100–300 K and in fields up to 3.5 kOe.

The effect of the inversion of polarity in gallium nitride layers from the N-polar GaN layer to the Ga-polar GaN layer was discovered during the sequential growth of GaN films on hybrid SiC/Si(111) substrates by nitrogen plasma-assisted molecular beam epitaxy and chloride–hydride vapor phase epitaxy. A new method has been developed for the formation of crack-free Ga-polar GaN/AlN heterostructures on hybrid SiC/Si(111) substrates. The method includes two stages of growing gallium nitride layers. At the first stage, the transient N-polar GaN layer is grown on the SiC/Si(111) surface by nitrogen plasma-assisted molecular beam epitaxy. At the second stage, two layers are grown on the obtained N-polar GaN layer by chloride–hydride vapor phase epitaxy, namely, the AlN layer and then the GaN layer, which at this stage grows in the Ga-polar orientation. Etching in a KOH solution affects only the N-polar GaN transition layer and leads to its complete removal. This procedure separates the main Ga-polar GaN layer from the SiC/Si(111) substrate completely. The method enables one to grow crack-free and elastically unstressed thick GaN layers and transfer them to substrates of other materials.

The results of the growth of bulk crystals of aluminum and gallium nitrides on foreign seeds by the sublimation sandwich method are reviewed. The kinetics and mechanism of sublimation and condensation are analyzed depending on the growth conditions, vapor phase composition, crystal orientation, and the distance between the source and the seed. It is experimentally found that during joint annealing of aluminum nitride and silicon carbide, the sublimation rate of aluminum nitride substantially increases due to the formation of a liquid phase on the crystal surface. The inhomogeneous distribution of the liquid phase, localized mainly near structural and morphological defects, leads to the selective nature of surface etching and causes a deterioration in the quality of the growing crystal. A process of growth of bulk AlN crystals with simultaneous seed evaporation was implemented, which yields crystals without cracks and with improved parameters. Bulk crystals of aluminum nitride and gallium nitride up to 2 inches in diameter were grown on SiC seeds.

The low-resistance ohmic contact NiAu|p-type AlGaN|GaN was carefully investigated by high resolution electron microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS) after two-step annealing at 550 and 750°C. It is shown that complicate double-direction diffusion and reaction occur in the metal layer and underlying GaN layer. The stacks of Ni|Au|Ni|Au turn into one alloyed layer and an intimate relationship establishes at the NiAu|GaN boundary which should play a primary role in ohmic contact to lower the contact barrier. A great part of Ni is oxidized as dispersive NiO nanoclusters in the metal layer, which might have an effect to hinder Ga atoms migrating upward. So at the intimate interface, the metal layer close to the contact enriched with Ga and Au, and the GaN upper layer metallized by Au and Ni should reduce the lattice mismatch and the contact barrier. Dense vacancies in the upper GaN layer and dislocations connected with the contact boundary also have the effects to improve the current carrier transportation. So the low ohmic contact to p-GaN should be obtained by the combination of these microstructural characteristics.

The effect of γ-irradiation at the dose D_γ = 605.6 kGy on the temperature dependences of conductivity and dielectric permittivity of Cd_1 – xFe_xTe semimagnetic semiconductors were investigated. The character of the ε(T) dependences of the irradiated Cd_1 – xFe_xTe changes: there is a drop in the curves in the temperature range of 300–400 K at measurement frequencies 10 kHz–1 MHz, and ε increases by 20 times. In the σ(T) dependence, at all measurement frequencies a maximum appears at a temperature of 400 K and conductivity increases by 40 times. We assume that the character of the temperature dependences of dielectric permittivity and conductivity corresponds to the ionic conductivity.

In most cases, III–V compounds form a crystal structure, which is stable under certain experimental conditions. Meantime crystal phase of III–V nanowires may differ from the stable phase of bulk structures. In this work, we show that the elastic stress could be the sole factor responsible for nanowire growth in the metastable phase. Depending on the experimental conditions of GaP nanowire growth, the elastic stress contribution to nucleation barrier can be greater than the difference in the energy of the formation of the cubic and hexagonal phase, and thus, it causes the growth in metastable wurtzite crystal phase.

The results of a study of the influence of the crystal structure features of organic linear conjugated oligomers on the trends of their crystallization from solutions are given.

The formation of GaN nanocrystals on the graphene-like AlN (g-AlN) modification and graphene-like (g-Si₃N₃) silicon nitride by ammonia molecular beam epitaxy has been investigated. It has been found that the GaN growth on the g-Si₃N₃ surface leads to the formation of misoriented nanocrystals. During the GaN growth on the g-AlN surface, the epitaxial growth of similarly oriented GaN quantum dots of the graphite-like modification has been observed. The lattice parameters and energy structure of two graphite-like GaN modifications with the alternating AB (graphite structure) and AA⁺ (hexagonal boron nitride structure) layers have been calculated.

The main processes that occur during diffusion of gaseous carbon CO and silicon SiO monoxides through a layer of a single-crystal silicon carbide SiC of the cubic polytype are described. This problem arises as a single-crystal SiC layer is grown by the method of matched atomic substitution due to the chemical reaction of a crystalline silicon substrate with CO gas. The reaction products are an epitaxial SiC layer and SiO gas. CO and SiO molecules are shown to decompose in the SiC crystal into individual atoms. The oxygen atoms diffuse over interstitial sites only in the [110] direction with the activation energy 2.6 eV. The Si and C atoms displace by the vacancy mechanism in the corresponding SiC sublattices with activation energies 3.6  and 3.9 eV, respectively, and only in the [110] direction.

In this paper, we present the results of the study of thermoelectric materials formed by pulsed laser deposition on sapphire substrates and representing thin MnSi_1.74 films with intermediate germanium layers. A sharp decrease in the thermal conductivity coefficient of superlattices based on manganese silicides and germanium in comparison with single layers of manganese silicide with an equivalent thickness is shown. This allows significantly increasing the thermoelectric figure of merit. The obtained values of the coefficient of thermoelectric figure of merit are comparable with the known literature values that are typical for similar structures.

The electronic structure and the total energy of Ti₂AlC, Ti₂AlN, Ti₂SiC, and Ti₂SiN compounds have been explored by the methods of the density functional theory and pseudopotentials. Density curves of electronic states for the crystal systems and for the systems differing in the degree of order have been plotted. It is shown that the qualitative similarity of electronic structure with the electronic structure of corresponding crystalline compounds is observed even in completely disordered systems. This similarity increases with the ordering. The total energy of the studied systems grows with the increase in disorder in approximately the same way for all the systems investigated, except Ti₂SiC. In the latter case, the system is much more sensitive to the degree of disorder, most likely due to the greater role of the covalent component of the interatomic bonds.

Quartz sand produced by the Ramenskii mining and processing plant has been used as a raw material for the melting of quartz glass at OJSC DINUR. The performance of reactors (plasma torches) and the quality of the ingots obtained depends significantly on the quality of sand (content of impurity components, such as aluminum and iron oxides). It is established that the product of melting quartz sand from feedstock with a high content of impurities is characterized by reduced resistance to crystallization.

Amorphous Li–Nb–O films were deposited onto Si substrates by radio-frequency magnetron sputtering method. The fabricated heterostructures demonstrated the presence of effective oxide charge Q_eff having both negative and positive components. Thermal annealing (TA) of as-grown heterostructures leads to the crystallization of a Li–Nb–O system with the formation of LiNbO₃ films. The Q_eff changes with TA, reaching a minimum at the annealing temperature of about 470°C, corresponding to the entire film’s crystallization.

The influence of composition of glycine–nitrate systems on the completeness of the formation of solid solutions of barium–strontium metatitanate was studied. It was shown that the decomposition of all studied glycine–nitrate compositions starts at 180°C; however, its character depends on the glycine content and composition’s pH value. The volume combustion mode obtained by exceeding the content of the reducing agent over the oxidant in composition provides the optimum conditions for the formation of single-phase products. Such compositions allow obtaining nanoscale powders of Ba_1 – xSr_xTiO₃ solid solutions (0 ≤ x ≤ 0.4) by calcination at 550°C and introducing them into the composites based on porous magnetic glass, avoiding the interaction between the forming ferroelectric phase and glass matrix.

The nanocrystalline magnetically ordered materials based on bismuth orthoferrite doped with calcium are synthesized via glycine-nitrate combustion. X-ray diffractometry, helium pycnometry, BET, scanning electron microscopy, and elemental energy dispersive microanalysis data showed that they are isostructural to bismuth orthoferrite and have an average crystallite size of about 40 nm to form particles with an average size of 0.3 μm. Mössbauer spectroscopy and magnetometry data showed that their magnetic properties and those of pure bismuth orthoferrite are significantly differed. We found that the samples contain nanocrystals with other magnetic parameters and a low-dispersed phase fractions with a magnetic order.

Features of the microstructure and piezoelectric response of perovskite islands in a pyrochlore phase matrix in lead zirconate titanate thin films prepeared by the radio-frequency magnetron sputtering method are investigated. The radial inhomogeneity of the surface morphology and piezoresponse of the islands are revealed. The role of two-dimensional mechanical stresses acting on the thin film from the silicon substrate side is discussed.

Dielectric and polar properties of aluminum nitride (AlN) thin films epitaxially grown on differently oriented silicon substrates with the p-type conduction and a buffer silicon carbide (SiC) layer and on vicinal planes are investigated. The results of studies of the polar properties by two independent methods—the dynamic pyroelectric effect and the piezoresponse force microscopy—show that the SiC buffer layer application considerably improves polar properties of the aluminum nitride thin layer.

The growth of a faceted pore in a crystal by the Burton–Cabrera–Frank mechanism under action of a mechanical tensile load is considered. The growth is determined by the diffusion of vacancies existing in a crystal to the terraces and steps on the pore surface. The expressions for the speed of movement of a single step and a group of parallel steps, and also for the dependence of the normal growth rate of a pore by the spiral mechanism under action of applied load have been found. It is shown that, in a certain range of low tensile loads, the rate has a quadratic dependence on the load, while the dependence is linear at high loads. The influence of impurities on the crystal fracture by the pore growth mechanism under consideration is discussed.

The thermal conductivity of single-crystal solid solutions (ZrO₂)_{1 – x – y – z}(Sc₂O₃)_x(CeO₂)_y(Y₂O₃)_z (x = 0.08–0.10; y = 0.005–0.01; z = 0–0.005) was measured in the temperature range 50–300 K. Phase composition and defective structure of crystals were studied.

Three NaGd(WO₄)₂ single crystals were grown by the Czochralski method using charges of different chemical purity. The impurity compositions of the grown crystals were analyzed by inductively coupled plasma mass spectrometry. The optical absorption spectra of the crystals were measured, and the effect of uncontrollable impurities on their optical properties and color was analyzed. The green color encountered in NaGd(WO₄)₂ crystals in some cases was shown to be due to the uncontrollable impurities of d elements, among which the chromium impurity had the most significant effect.

Phase transitions in crystals of (Cs,NH₄)₄(HSO₄)₃(H₂PO₄) solid solutions have been investigated by synchronous thermal analysis and polarization microscopy at different temperatures. The temperatures of structural modifications are established; it is shown that the replacement of Cs cations with NH₄ in Cs₄(HSO₄)₃(H₂PO₄) crystals reduces the temperatures of phase transformations.

The computation models were created with the solid-liquid configuration method. The molecular dynamics simulations were performed to exhibit the crystallization process of liquid pure Ti to hexagonal close packed (HCP) and face-centered cubic (FCC) structure crystals. The results showed that both HCP and FCC crystallizations start from the solid–liquid interfaces and develop toward the middle. The system energy sharply falls at the beginning of crystallization, then is reduced slowly, and finally has a sudden down jump at the end of crystallization. The first peak of RDF is increased significantly with time and some new secondary peaks occur, which is consistent with configuration evolution. The HCP crystal has a longer crystallization process but lower stable energy than the FCC crystal.

Expressions for the nucleation and growth rates for condensed particles in a heterogeneous reaction of a definite kind have been derived, which allows the kinetics of the formation of the condensed phase in some complex systems to be described. Methods of the classical nucleation theory and an assumption about the reaction development mechanism were used.

Four modes of metal growth on a semiconductor substrate have been detected and distinguished on the basis of experimental data obtained under similar conditions using hot wall epitaxy. These modes are achieved at certain ratios between translational kinetic energy of vapor deposited onto substrate and its temperature. The mechanism of adaptation to the substrate of nanophase wetting coating of metal is proposed when the mode of pure metal growth is implemented, as is the structural model of this coating.

For the first time, the results on the solubility and growth of new oligomers with a single conjugated core from solutions of organic crystals are reported. The new oligomers consist of benzothiadiazole, phenylene, and oxazole units and have various terminal substituents. Using DSC and TGA methods, melting parameters are found and thermal stability upon heating is studied.

Ge-doped LiTi₂(PO₄)₃ has been synthesized by a conventional solid-state reaction. Compounds LiM\(_{2}^{{{\text{IV}}}}\)(PO₄)₃ with LTP-type structure present a different behaviour depending on nature of M^(IV). For M^(IV) = Ti and Ge, the structure shows the space group R3c, whereas for M^(IV) = Ge the space group is R3. Differences in behaviour of LiTi₂(PO₄)₃–LiGe₂(PO₄)₃ solid solutions are discussed in relation to the composition. Their structures LiTi_2 – xGe_x(PO₄)₃ (0 ≤ x < 2) were determined from X-ray powder diffraction method (XRD) using Rietveld analysis. A sharp change in the lattice parameter a is observed between the compositions with x = 1. The lattice parameter c increases as the Ge content increases in the whole range of composition. The space group R3c becomes R3 for the composition with x > 1. The SEM micrographs of the samples show relative porous microstructures due to the effect of the substitution.

Quasicrystalline coatings of the Al–Cu–Fe system were obtained by sputtering the cathode using a pulsed high-current low-pressure arc discharge followed by deposition on a hot (600°C) and cooled substrate (25°C). The surface morphology, chemical and phase composition of evaporation products in the powder form and the resulting coatings were studied. Powders and coatings were also annealed and the phase composition change was studied. It was found that during the evaporation process a significant change in chemical composition occurs in the form of loss of Al from the surface of particles and coating, that leads to a decrease in the content of the ψ-phase. However, subsequent annealing and spraying onto a hot substrate leads to an increase in the ψ-phase, as well as the density and hardness of the coatings.

Electronic structure of the ultrathin Cs, Ba/SiC(111)-4°, 8° interfaces have been studied in situ in an ultrahigh vacuum using synchrotron-based photoelectron spectroscopy. Change in the C 1s and Si 2p core level spectra was studied upon the adsorption of Cs or Ba within the submonolayer coverage range. The formation of a new, previously unknown carbon nanostructure was revealed under adsorption of Cs or Ba atoms. Data show that the nanostructure is formed exclusively on SiC vicinal surfaces in the presence of stabilizing adsorbed metal atoms, such as Cs or Ba atoms. It is established that the carbon nanostructure consists of carbon rings, in which chemical bonds are similar in nature to the bonds inherent in aromatic compounds.

The results of computer simulation of a new one-dimensional carbon structure representing chains composed of C₂₀ carbinofullerene fragments linked with carbon atoms are given. The bond energy values are determined. Their thermal stability is studied by the method of molecular dynamics. The tensile strength of these chains is also studied and compared to the tensile strength of C₂₀ carbinofullerene chains without intermediate carbon atoms. It is shown that the fusion of neighboring carbinofullerene moieties is a preferred channel for thermal decomposition and the separation of terminal carbinofullerene moieties from the chain is a preferable channel for deformational decay. The ultimate strains of the chains and the temperature dependences of their lifetime until the time of decay are determined. The values of activation energies and frequency factors of the Arrhenius law are determined for the decay process.

A Pb(Zr_0.52Ti_0.48)O₃–LaNiO₃–Si composition and LaNiO₃ thin films are synthesized using chemical solution deposition and studied by transmission electron microscopy. The polycrystalline, porous structure of LaNiO₃ is found to misalign the columnar structure of lead zirconate titanate. The effect that thermal treatment has on the structure and phase composition of lanthanum nickelate is addressed. The morphological features of LaNiO₃ film structure such as its layered character, porosity, and misalignment are observed in samples subjected to annealing at a temperature of 550°C and are more pronounced upon raising the temperature to 800°C.

Thick SiC_x films have been deposited on a c-Si surface by radiofrequency (rf) magnetron sputtering (150 W, 13.56 MHz, Ar flow 2.4 L/h, 0.4 Pa) of graphite and silicon targets. The X-ray diffraction study shows that fast annealing of the SiC_x film deposited on the c-Si surface for 3 h leads to the low-temperature (970°C) formation of hexagonal structural phases α-SiC (6H-SiC and other) along with the cubic modification of silicon carbide β-SiC. The IR spectroscopy has shown the formation of SiC nanocrystal nuclei due to the energy action of the rf plasma ions on the upper layer of the SiC film during its growth. The data of X-ray reflectometry demonstrate a high density of the films up to 3.59 g/cm² as a result of formation of dense C and SiC clusters in the layers under action of the rf plasma.

This work was devoted to studying the atomic structure and electron spectrum of a-SiO_x : H films created on silicon and glass substrates by means of plasma-enhanced chemical vapor deposition (PECVD). Depending on the conditions of oxygen supply into the reactor, the stoichiometric parameter x of the films was varied from 0.57 to 2. The structure of the films and the specific features of their electron structure were characterized depending on the parameter x with a complex of structural and optical methods and ab initio quantum-chemical simulation for the model SiO_x structure. The studied SiO_x : H films were established to consist predominantly of silicon suboxides SiO_y, SiO₂ clusters, and amorphous silicon. Based on the spatial fluctuations of their chemical composition, the model of bandgap width and potential fluctuations was proposed for SiO_x electrons and holes. The obtained data would provide the charge transport in a-SiO_x : H films with more precise modeling important for the creation of nonvolatile random-access memory (RAM) elements and memristors on their basis.

The thermal expansion properties of Mg–xSi (x = 1, 1.38, 2, 3, and 4 wt %) binary alloys and Mg–4Si–yCa (y = 0.2, 0.4, 0.6, 0.8, and 1.0 wt %) alloys over a temperature range of 25–300°C were systematically studied. The results show that the Mg–Si binary alloys consist of α-Mg and Mg₂Si phases. The volume fraction of Mg₂Si phase increases with the increase of Si content in Mg–Si alloys, from 30.72 to 46.50% when Si content increases from 1 to 4 wt % Si. The coefficient of thermal expansion (CTE) of Mg–xSi binary alloys decreases with the increase of Mg₂Si volume fraction. The addition of Ca element to Mg–4Si alloys has an obvious modification effect on the Mg₂Si phase. When the Ca content increases constantly, the CTE of the Mg–4Si–Ca alloys increases at first, then it continues to decline. The mechanism is mainly related to the formation of the CaMgSi phase.