Model of the pair phosphorus atom-interstitial silicon atom

Interstitial defects in silicon implanted with P and Si ions are investigated by x-ray diffraction. It is established that the interstitial complexes formed by implantation and in subsequent heat treatment do not contain a P atom. A model is proposed for the pair PI: P atom-institial Si atom. The pair PI consists of P and Si atoms at the same interstice which are not bound to one another by a covalent bond. The pair model accounts for the characteristic features of the diffusion of implanted phosphorus in silicon. Fiz. Tverd. Tela (St. Petersburg) 40, 1995–1998 (November 1998)     

Negatron effects in CdSe1 − x Te x and ZnS1−x Se x films

Various negatron effects in films of alloys of II–VI compounds deposited from solutions as a function of the deposition mode and heat treatment are studied. It is found that the negative photocapacitance effect, which was first discovered in ZnS_1?x Se _x films, and the slowly relaxing negative photoelectric effects, which are caused by the transition of electrons located in a nanoscale surface layer from the shallow energy levels of trapping centers to deeper levels with a lower polarizability and by the presence of nanoscale clusters in these materials, which play the role of a “reservoir” for minority charge carriers, occur according to a single mechanism. A model to explain the basic laws of negative photoconductivity in CdSe_{1 ? x} Te _x films deposited from a solution is proposed. Negative residual conductivity is explained in terms of double-barrier relief model, while negative differential photoconductivity is attributed to the presence of nanoscale electric domains.     

Ni-P alloy-carbon black composite films fabricated by electrodeposition

Ni-P alloy-carbon black (CB) composite films were fabricated by electroplating and their microstructures and properties were examined. The CB and phosphorus contents of the composite films were also investigated. The CB particles were found to be embedded in the Ni-P alloy matrix. The CB content in the deposits increased, reached a maximum value of 0.77 mass% with increasing CB concentration in the bath up to 10 g dm⁻³, and then decreased with a further increase in the CB concentration in the bath. Both before and after heat treatment, the composite films had higher hardnesses and lower friction coefficients than the Ni-P alloy films. Both before and after heat treatment, the friction coefficient of 0.77 mass% CB composite films was about half that of Ni-P alloy films without CB.     

Effect of the molecular structure of films of tetrapyrrole compounds on their photoelectrochemical characteristics for different types of sensitization

It was examined how the photoelectrochemical properties of thin films (50 nm) prepared from metal derivatives of tetrapyrrole compounds with various central atoms depend on the character of sensitization. It was demonstrated that the treatment of pigment films with oxygen and iodine vapor increases their photoactivity two- to threefold. A more pronounced effect is produced by deposition onto the films of layers of electron acceptors (o-chloranil, tetracyanoquinodimethane, etc.) or organic semiconductors (phenylselenodiimide) with a thickness of one to three monolayers. For tetrapyrrole compounds with the same structure of the macrocycle but with different central atoms, the sensitization effect may differ severalfold. For example, treatment of zinc tetraphenylporphyrin with iodine vapor increases the photocurrent by a factor of 14, with only a 1.5-fold increase being observed for cobalt tetraphenylporphyrin. The parameters of phthalocyanine metallocomplexes are less sensitive to sensitization compared to those of tetraphenylporphyrin metallocomplexes. The sequence of variation of the photoactivity of tetrapyrrole compounds according to the type of the central atom is not affected by the nature of the sensitizer, but the extent of change in the photoactivity is dependent on its properties, in particular, electron affinity.     

Effect of annealing on the self-poled state in thin ferroelectric films

It is shown that high-temperature treatment of self-poled lead zirconate-titanate films containing an excess of lead oxide, followed by prolonged storage at room temperature, results in a charge redistribution in the near-electrode regions of ferroelectric films. Such heat treatment destroys, as a rule, the self-poled state and removes the dielectric nonuniformity. A model of a thin-film ferroelectric capacitor is proposed which makes it possible to reproduce variations in the P-V hysteresis loop shape and capacity-voltage (C-V) characteristics, as well as in the frequency-dependent pyroelectric response (LIMM). The effect of the interface and grain boundaries on the onset of the self-poled state, its variation, and destruction is discussed in terms of the proposed model.     

Microhardness anisotropy and the density of atoms in the unit identity volume of crystals

A new criterion of the reticular and polar anisotropy of microhardness is proposed: the density of atoms in the unit identity volume, i.e., the ratio of the number of atoms of one species in the atomic planes (plane) contained within the limits of the unit identity volume to the sum of the areas of these planes (plane) containing atoms of that species. It is shown that these densities of lanthanide, actinide, and boron atoms correlate satisfactorily with the reticular and polar microhardness anisotropy of their tetraborides and hexaborides. This criterion can be used to predict the nature of the variation of the reticular and polar microhardness anisotropy for other classes of compounds as well. Fiz. Tverd. Tela (St. Petersburg) 40, 481–486 (March 1998)     

Preparation and characterizations of SiO2/TiO2/γ-glycidoxypropyltrimethoxysilane composite materials for optical waveguides

SiO₂/TiO₂/γ-glycidoxypropyltrimethoxysilane composite materials processed by the sol-gel technique were studied for optical waveguide applications. Waveguide films with thickness more than 1.7 μm were prepared on a silicon substrate by a single-coating process and low-temperature heat treatment from these high-titanium-content composite materials. Scanning electron microscopy (SEM), atomic force microscopy (AFM), thermal gravimetric analysis (TGA), UV-visible spectroscopy (UV-VIS), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the waveguide films. TGA curves showed that organic compounds in the composite materials would decompose in the temperature range from 200 °C to 480 °C. SEM, AFM and UV-VIS results showed that a dense, porous-free, and high transparency in the visible range waveguide film could be obtained at a low heat-treatment temperature. It was also noted that the carbon content in the film with higher titanium content heated at high temperature was evidenced by XPS. The waveguide propagation loss properties of the composite material films were also investigated and showed a dependence on the titanium molar fraction. Received: 13 June 2000 / Accepted: 21 June 2000 / Published online: 20 September 2000     

Ion-induced sputtering of a metal in the form of large clusters

A model is proposed for the ion-induced sputtering of a metal in the form of large clusters with a number of atoms N⩾5. The model is based on simple physical assumptions and is consistent with experiment. As an example, calculations are made of the relative cluster yield as a function of the number of atoms in the cluster as a result of the bombardment of various metals by singly charged 5 keV argon ions. A comparison is made with experimental data. Zh. Tekh. Fiz. 69, 64–68 (March 1999)     

Synthesis and microstructure of metal oxide thin films containing metal nanoparticles by liquid phase deposition (LPD) method

Au nanoparticles dispersed SiO₂-TiO₂ composite films have been prepared by a novel wet process, Liquid Phase Deposition (LPD) method. The composite films were characterized by XRD, XPS, TEM, ICP, SEM and UV-VIS absorption spectroscopy. The results showed that the SiO₂-TiO₂ composite films containing Au^III and Au^I ionic species were co-deposited from a mixed solution of ammonium silicofluoride, ammonium hexafluorotitanate, boric acid and tetrachloroauric acid. The heat treatment induced the reduction of Au ions and formation of Au nanoparticles in the film. TEM observation revealed that the Au nanoparticles with 5-10 nm in diameter were found to be dispersed uniformly in the SiO₂-TiO₂ matrix. The optical absorption band due to the surface plasmon resonance of dispersed Au particles were observed at the wavelength of 550 nm and shifted toward longer wavelength with increasing heat treatment temperature. Received 28 November 2000     

On the occupation of interstitial sites by hydrogen atoms in intermetallic hydrides: A quantitative model

A simple model to predict the relative occupation numbers of various interstitial sites in intermetallic hydrides is proposed. The model assumes that the occupation numbers are determined by Boltzmann distribution function as well as by the heat of formation of the “imaginary binary hydrides” formed between the hydrogen atoms and the host metallic atoms forming the interstitial sites. Comparison of our calculations with recent neutron scattering data on various hydrides indicate good agreement at T=300 K.     

Multiferroic and magnetoelectric properties of MnFe2O4/(Pb0.8Sr0.2)TiO3 composite films

Abstract

MnFe₂O₄/(Pb_0.8Sr_0.2)TiO₃ (MFO/PST20) heterostructured composite films with three different structures have been grown on Pt/TiO₂/SiO₂/Si substrates by metal–organic decomposition processing via spin coating technique. The structural analysis revealed that the crystal axes of the MnFe₂O₄ are aligned with those of the PST20 ferroelectric matrix with obvious interfaces and no diffusions exist in all the three composite films. These composite films exhibit simultaneously multiferroic and magnetoelectric responses at room temperature. The growth structure of MFO and PST20 layers has an effect on multiferroic and magnetoelectric coupling behaviours of the composite films. The bi- and four-layered MFO/PST20 composite films exhibit superior ferroelectric properties compared to the tri-layered film. The increasing MFO and PST20 layers in the composite films enhance ferromagnetic properties and are closely related to the strain release in MnFe₂O₄ phase. The MFO/PST20 bi-layered composite film shows a high magnetoelectric voltage co-efficient α_E ~ 194 mVcm^?1Oe^?1 at a dc magnetic field H_dc ~ 2.5 kOe. A significant decrease in α_E value has been observed for tri- and four- layered composite films. A close correlation between phase selective residual stress and magnetoelectric properties has been emerged. The results are reasonably encouraging for employing MnFe₂O₄ for growing multiferroic–magnetoelectric composite films.     

Significant positive magnetoresistance of graphene/carbon composite films prepared by electrospraying and subsequent heat treatment

Graphene/carbon composite films were prepared by electrospraying a graphene/polyacrylonitrile composite solution on SiO₂-coated silicon substrates and subsequent heat treatment. The as-produced graphene/carbon composite films had a porous structure comprising graphene layers. With a magnetic field applied perpendicularly to the sample, an unexpectedly significant positive magnetoresistance attributed to e–e interaction and weak localization has been observed, which constantly increases with the magnetic field in the temperature range of 300–50 K from 0 to 80 kOe.     

Finite size effect on critical transition temperature of superconductive nanosolids

A simple and unified model is developed for finite size effect on the critical transition temperature of superconductive nanosolids, which is based on the size-dependent Debye temperature of crystals within the McMillan expression. In the model, two material and structure dependent parameters of D₀ and α are used, which, respectively, are the critical size at which all atoms of a low-dimensional material are located on its surface, and the ratio of the mean square vibrational amplitude between surface atoms and interior atoms, In light of this model, the critical transition temperatures of superconductive nanosolids can decrease or increase with the dropping size of nanosolids depending on the bond strength changes of interfacial atoms. The predicated results are consistent with the available experimental results for superconductors MgB₂ and Nb thin films, Bi and Pb granular thin films and nanoparticles, Al thin films and nanoparticles.     

The spin-wave contribution to the heat capacity of ferromagnetic thin films with variable interlayer couplings

A Heisenberg model is solved within the spin-wave theory for thin films in which ferromagnetic monolayers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have included also a magnetic anisotropy in each manolayer. The temperature dependence of the spin-wave contribution to the heat capacity C _m in such composite systems is derived for different interlayer couplings and for different film thickness.     

Vacancy mechanism of the anomalous behavior of surface atoms at elevated temperatures

A model that takes into account the significant contribution of vacancies at moderate and high temperatures is proposed to explain the anomalous increase in the mean-square displacements of atoms in transition-metal surface monolayers. The dependence of the effects on the orientation of the surface is explained. The vacancy formation energies and the values of the Debye temperature are obtained within the proposed model for differently oriented surfaces. Zh. éksp. Teor. Fiz. 116, 2038–2044 (December 1999)     

New allotropic form of silicon

A new allotropic form of amorphous silicon with sp hybridization of the valence electrons is discovered. The new material consists of linear chains of atoms. A small fraction of the atoms are in the sp ² state. Acting as bridges, these atoms couple the linear chains into a single random network. This conclusion is based on an analysis of experimental data on the effect of annealing and ion implantation on the structure of the short-range order and the properties of amorphous-silicon films obtained by different methods. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 646–649 (10 May 1998)     

Misfit disclination dipoles in nanocrystalline films and coatings

A theoretical model is proposed for describing the special physical micromechanism of misfit stress relaxation in nanocrystalline (NC) films and coatings. According to this model, under certain conditions, grain boundary sliding occurs in NC films and coatings, which is accompanied by the formation of an ensemble of disclination dipoles (rotational defects). These dipoles produce elastic stress fields, which partially compensate misfit stresses in NC films and coatings. Using the proposed model, it is shown that the nucleation of disclination dipoles in a film (coating) can significantly decrease the total energy of the film/substrate composite for the AlN/6H-SiC and GaN/6H-SiC systems over a wide range of structural parameter values.     

A study of gas-sensitive properties of cobalt-modified polyacrylonitrile films by the methods of molecular modeling and quantum chemistry

The results of a theoretical study of the gas-sensitive properties of polyacrylonitrile (PAN) modified with cobalt compounds by using the methods of molecular modeling and quantum chemistry are presented. A surface cluster of cobalt-containing PAN formed by infrared heat treatment is simulated. The most stable configuration of the cluster has been determined, in which molecules of cobalt oxide and cobalt metahydroxide are located between two layers of PAN macromolecules without interacting with each other and with PAN molecules. This explains a composite structure of cobalt-containing PAN films. Molecular modeling demonstrated that the cobalt-modified PAN cluster is selectively sensitive to the molecules of chlorine, nitrogen dioxide, and carbon monoxide.     

Electronic structure of deep centers in LiNbO3

A model is proposed for photo-and thermally induced processes in lithium niobate crystals. The model is based on the presence of structural defects formed as a result of departure of the crystal composition from stoichiometry. Defects of the type Nb_Li-Nb_Li are introduced and the influence of oxidation-reduction heat treatment on the optical characteristics of the crystals is analyzed. Fiz. Tverd. Tela (St. Petersburg) 40, 1109–1116 (June 1998)     

The entropy of a single large finite system undergoing both heat and work transfer

Computing the entropy of a system from a single trajectory is discussed when the energy exchange with the environment includes both mechanical and thermal terms. The physical example chosen as an illustration is a cluster of atoms impacting a hard surface. Each atom of the cluster interacts with the smooth surface by a momentum transfer using the hard cube model [E. K. Grimmelmann, J. C. Tully and M. J. Cardillo, J. Chem. Phys. 72, 1039 (1980)]. Because of the thermal motion of the surface atoms the atoms of the cluster rebound from the surface with a (random) thermal component to their momentum. The change in the internal energy of the cluster has therefore both a mechanical, work, term and a heat transfer and the heat term contributes to the change in entropy of the cluster but the major contribution is the loss of potentially available work.