M. Sinder

RF heating of the conductor film on silicon substrate for thin film formation

Abstract A new method of rapid thermal process (RTP) is proposed which is based on an induction heating (IH) of a thin conducting film placed on a silicon substrate in a radio-frequency magnetic field perpendicular to the film surface. The characteristic property of the method is that no additional susceptor is required in contrast to the traditional IH methods, despite the fact that thickness of the conductor film/Si substrate structure is less than a skin-layer depth. Silicides, nitrides, borides, or conventional metallic films, etc. possessing a low electrical resistivity can be treated by the novel method. A theoretical model based on a numerical solution of quasi-stationary Maxwells equations coupled with the non-stationary thermal equation taking into account the dependence of the resistivity and the thermal capacity on temperature was developed. Theoretical calculations have shown the feasibility of the fabrication of conductor film/silicon substrate structures. Preliminary experimental results confirming the theoretical calculations for TiSi 2 film production are presented.

Penetration of magnetic field into high-temperature superconductors

The penetration of a monotonically time-increasing magnetic field in high-temperature superconductors (HTSC) slabs with the power law dependencies of the critical current density and resistivity on the magnetic field is investigated theoretically. It is shown that the asymptotic solutions of the extended critical state model correspond to the cases of the classical critical state model and the model of viscous flux flow without pinning. The ranges of the applicability of these models and the conditions for the transition from one model to the other are determined. These theoretical conclusions explain experimental data presented in scientific literature.

Reaction rate in reversible A↔B reaction-diffusion processes

The reversible A↔B reaction-diffusion process, where species A and B are initially mixed and diffuse with different diffusion coefficients, is usually simulated in the framework of the quasiequilibrium approximation. In this paper the reaction rate of such process is investigated. It is shown that despite the fact that a reaction-diffusion process is considered as the quasiequilibrium process the contribution of the reaction in changes of the species concentration is comparable with the diffusion contribution. Moreover, the ratio of the reaction to diffusion contributions is independent of time and coordinate. Using published experimental data the reaction rate and its profile are calculated.

Reversible and irreversible reaction fronts in two competing reactions system

Abstract A modeling of the non-equilibrium diffusion phenomena of the impurities in the semiconductors is based on the reaction–diffusion equations for local concentrations of the components. Through this approach a new feature, a reaction front, may be caused by reaction in diffusion profiles of the components. The asymptotic long-time properties of the reaction fronts in the system with initially separated components and two competing reactions: reversible A1+B↔C1 and irreversible A2+B→C2 are studied in this work. It is assumed that the backward constant of the reaction A1+B↔C1 is small. The dynamics of the system is described as a cross-over between the “irreversible” regime for small times and the “reversible” regime for large times. It is shown that the “irreversible” regime is characterized by single reaction zone, in which both reactions occur. The two reaction fronts, reversible A1+B↔C1 and irreversible A2+C1→A1+C2 appear in the “reversible” regime. Numerical computing of the mean-field kinetics equations confirms these asymptotic results. The experimental tests of the theoretical predictions relating to the diffusion phenomena in semiconductors are discussed.

Response of high-Tc superconductor totime-increasing magnetic field

Abstract Using the extended critical state model, we investigate theoretically the magnetic field penetration into a semi-infinite slab of a high-Tc superconductor in the flux flow region when an external magnetic field increases monotonically with time according to power law: H=At α . The dependencies of the flux flow resistivity and critical current density are taken to depend on magnetic field according to the generalized power law. The behavior of the solutions vs the exponent α and “amplitude” A is analyzed. It is shown that there is the critical value of the exponent α c separating the regions with different characters of the magnetic field penetration. The value of α c is determined by the exponents in the dependencies of the superconductor characteristics on magnetic field. For α α c , the electric field in a superconductor decreases with time and the magnetic field distribution tends to the one given by Bean’s critical state model. For α > α c the electric field increases with time and the pattern of magnetic field differs increasingly from Bean’s model; the behavior of a superconductor at large times is described by the model of a normal metal with the resistivity dependent on magnetic field. Thus, the response of a superconductor is described by different approximations on different stages of the process. This result is distinct from that presented in the literature. In the boundary case of α=α c , the character of the field penetration is time-independent.

An analytical solution for the extended critical state model

The equations of the extended critical state model are solved analytically for the case when an external magnetic field is increased according to the law . Distributions of magnetic and electric fields in a superconductor are found to depend on the relation between characteristic times of two processes: the increase of applied external magnetic field and the diffusion of magnetic field into a superconductor. If the diffusion time is much smaller than the characteristic time of the magnetic field increase, the solution reduces to that of the classical critical state model. In the opposite case, the solution tends to the solution for normal metals. In this case, the discrepancy between the results of the expended and classical critical state models increases with time.

Properties of Species Profiles during Oxygen Chemical Diffusion in Oxides

This is a theoretical study of species profiles during the oxygen chemical diffusion in an acceptor doped oxide crystal driven by large changes in the ambient oxygen partial pressure. The oxide crystal containing three species: mobile oxygen vacancy, mobile electron, immobile dopant ion, is considered. Our analysis is based on the expression of the chemical diffusion coefficient obtained in the framework of the concept of conservative ensembles (Maier J., 1993). It is shown that the dependence of chemical diffusion coefficient on ambient oxygen partial pressure in double-logarithmic coordinates is divided into distinct intervals. For each pressure interval the chemical diffusion equation is reduced to the diffusion equation with a diffusion coefficient which exhibits a power dependence on concentration. First, we analyzed the chemical diffusion under pressure inside each interval. As a result two singularities on the species diffusion profiles can be found: an internal reaction diffusion front, and an ambipolar diffusion front. This ambipolar diffusion front is characterized by a step of the electron concentration, moving inside a specimen. Afterwards, we consider a crystal in which the range of partial pressure spans all considered pressure intervals.

Oxidation Kinetics of Nitrogen Doped TiO2-δ Thin Films: Analysis on the Basis of Oxygen Activity Dependence of the Chemical Diffusion Coefficient

The model explaining the occurrence of the electron concentration step front during oxidation of nitrogen-doped TiO2-δ thin films is presented. This model is based on ambipolar chemical diffusion coefficient analysis, for which immobile and uniformly distributed nitrogen component is assumed. The diffusion species and oxygen activity (pressure) profiles are obtained by numerical and approximate analytical simulation of the chemical diffusion. The profiles indicate the presence of two separate singularities: the electron concentration step front, and the electron-hole recombination reaction front. The electron concentration step front relates to the singularity of the ambipolar diffusion of three types of charged species with essentially different diffusion coefficients.

Ag–B Thin Films Prepared by Magnetron Sputtering

The discovery of the new superconductor MgB 2 with critical temperature of T c = 39 K stimulated great activity in search for materials with higher T c . Theoretical works predicted the possibility of obtaining such superconducting materials in other borides also, in particular in AgB 2 . It was predicted in these works that diborides of metals having the AlB 2 type structure, among them AgB 2 may have superconductivity with a T c higher than 39 K. In an early very short report the lattice parameters of AgB 2 were evaluated in a synthesized specimen. However, no other indication exists that AgB 2 was ever produced. We report the preliminary results of our attempts to prepare AgB 2 films. Thin films of Ag-B were produced by magnetron cosputtering in Ar ambient from two separate targets that of B and Ag on Si (100) substrates. The specimens were subjected to X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photo spectroscopy (XPS), Auger analysis and optical microscope (OM) investigations. Resistivity measurements in a wide temperature range were measured. No superconductivity was yet observed in the range ∼10 – 200. K on the basis of resistivity measurements. Preliminary results indicate that AgB 2 might be an unstable phase.