V. A. Klimov

Effect of grain sizes on the metal-semiconductor phase transition in vanadium dioxide polycrystalline thin films

Experimental data on the effect of grain sizes on the shape and width of the hysteresis loop characterizing a metal-semiconductor phase transition in vanadium dioxide films are analyzed in terms of the classical theory of nucleation. It is shown that the factors responsible for the changes in the shape and width of the hysteresis loop with variations in the size of the grains making up a film are associated with the heterogeneous character of nucleation of a new phase, on the one hand, and with the elastic stresses arising in the phase transition, on the other.

Effect of synthesis conditions on the metal-semiconductor phase transition in vanadium dioxide thin films

The effect of the conditions of synthesis and of the substrate material on the metal-semiconductor phase transition in thin vanadium dioxide films prepared using laser ablation has been studied. The broadening of the hysteresis loop is shown to be due to a decrease in the size of the crystal grains making up the film. Conjectures are put forward to explain the formation of asymmetric hysteresis loops.

Crystallization of thin polycrystalline PZT films on Si/SiO2/Pt substrates

This paper reports on a study of crystallization of thin lead zirconate-titanate films deposited on Si/SiO2/Pt substrates by RF magnetron sputtering at a low temperature and annealed at 540–580°C. In this temperature interval, one observes successively two first-order phase transitions: the low-temperature pyrochlore phase—perovskite-I phase and perovskite-I phase-perovskite-II phase transitions, which are accompanied by film volume shrinkage. The phase transformations have been studied by atomic force microscopy, scanning electron microscopy, X-ray diffraction and visual (optical) observation of the growth of islands of a new phase. It has been found that the dielectric parameters undergo substantial changes upon the transition from phase I to phase II. The origin of the observed effects has been discussed.

Effect of deformation on the metal-semiconductor phase transition in vanadium dioxide thin films

It has been demonstrated that a small plastic deformation of aluminum substrates with vanadium dioxide thin films deposited on the substrate surfaces is accompanied by the appearance of elastic stresses in the films. Depending on the deformation technique, the elastic stresses can have different signs and the range of the metal-semiconductor phase transition in VO2 shifts toward higher or lower temperatures as compared to the equilibrium phase temperature.

Influence of hydrogenation on electrical conductivity of vanadium dioxide thin films

The influence of hydrogenation on electrical conductivity of vanadium dioxide thin films has been investigated. It has been shown using measurements of the electrical conductivity that the hydrogenation of vanadium dioxide thin films leads to a decrease in the temperature of the phase transition from the tetragonal phase (with “metallic” conductivity) to the semiconducting monoclinic phase. It has been found that, upon doping of vanadium dioxide with hydrogen, the electrical conductivity of the monoclinic phase can increase by several orders of magnitude. Nonetheless, the temperature dependence of the electrical conductivity of hydrogenated films exhibits a typical semiconducting behavior in the temperature range where the monoclinic phase is stable.

Electrical conductivity of the semiconducting phase in vanadium dioxide single crystals

The electrical conductivity of the semiconducting phase of vanadium dioxide single crystals is studied over a wide range of temperatures. It is shown that the electrical conductivity varies with temperature as log σ ∼ T in the range 340–170 K and as log σ ∼ 1/T at temperatures below 120 K. The experimental results are described in the framework of the model in which the temperature dependence of the hopping conductivity of small-radius polarons is determined by the dependence of the resonance integral on the amplitude of thermal lattice vibrations.

Specific features of electrical conductivity of V3O5 single crystals

The electrical conductivity of V3O5 single crystals has been investigated over a wide temperature range, including the region of existence of the metallic phase and the region of the transition from the metallic phase to the insulating phase. It has been shown that the low electrical conductivity of metallic V3O5 is caused, on the one hand, by a lower concentration of electrons and, on the other hand, by a strong electronelectron correlation whose role with decreasing temperature increases as the phase transition temperature is approached. The temperature dependence of the electrical conductivity of the insulating phase of V3O5 has been explained in the framework of the theory of hopping conduction, which takes into account the effect of thermal vibrations of atoms on the resonance integral.

Effect of vacuum heat treatment on the metal-semiconductor phase transition in thin vanadium dioxide films

The effect of vacuum heat treatment of thin vanadium dioxide films on the parameters of the metal-semiconductor phase transition is studied. The results of heat treatment are compared with those obtained upon irradiation of the synthesized films by medium-energy electrons. The elemental composition of the films that is found by the Rutherford backscattering (RBS) method suggests that an observed change in the hysteresis loop of the films is associated with the reduction of the vanadium dioxide upon heating in a vacuum.

Specific features of the electrical conductivity of V4O7 single crystals

The electrical conductivity of V4O7 single crystals has been measured over a wide temperature range, including both the region of existence of the metallic phase and the region of the metal-insulator transition. It has been shown that the low conductivity of metallic V4O7 is due to the strong electron-electron correlation, whose role increases with decreasing temperature as the phase-transition temperature is approached. The temperature dependence of the conductivity of the insulator phase of V4O7 is explained in terms of the theory of hopping conduction taking into account the influence of atomic thermal vibrations on the resonance integral.

Investigation of photochromic cluster systems based on molybdenum oxides by ESR spectroscopy

The electron-spin resonance (ESR) spectra of cluster polyoxometalate systems—a finely dispersed powder of the (NH4)6[Mo7O24]-citrie acid complex, molybdic acid, and molybdenum(VI) oxide—are investigated. The initial samples are colored under exposure to ultraviolet (UV) irradiation (photochromic effect) and thermal annealing. The ESR signal (g⊥ = 1.94, g‖ = 1.92) which is observed for the (NH4)6[Mo7O24]-citric acid photocolored samples corresponds to an electron of the molybdenum atom. This is in agreement with the data derived from the electronic spectrum. In addition, the (NH4)6[Mo7O24]-citric acid colored system exhibits an ESR signal (g=2.02) which corresponds to a hole at the organic ligand. This confirms the previously advanced model of intramolecular electron transfer under UV irradiation. The thermally colored molybdic acid has a similar ESR spectrum (g=1.88, 1.92, 1.93, and 1.98). For the other samples, the ESR signal is not observed. It is demonstrated that an unpaired electron of molybdenum atoms is substantially delocalized over all metal atoms in the cluster.