Abstract:Titanium dioxide thin films are extensively studied for applications in solid state gas sensor devices. Their gas sensing properties are strongly dependent on deposition technique, annealing temperature, film thickness and consequent properties like crystalline structure, grain size or amount of defects and impurities. In this work we report the gas sensing properties of TiO 2 thin films prepared by reactive magnetron sputtering technique and subsequently annealed at temperatures 600°C and 900°C. The films were exposed to different concentrations of H 2 gas up to 10 000 ppm. Their sensitivity to gas at various operating temperatures, ranging from 250°C to 450°C, was obtained by measuring their resistance. 07.07.Df, 68.47.Gh, 73.25.+i, 82.47 PACS (2008):

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa 2 Cu 3 O 6+c (YBCO) thin films with different crystallographic orientations have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.The resistive switching (RS) effect observed in capacitor-like metal/insulator/metal junctions belongs to the most promising candidates for the next generation of the memory cell technology based on a sudden change of the junction resistance caused by an electric field applied to the metal electrodes. Despite a burst of activities triggered by increasing technological interest for this phenomenon 1,2 , details of its physical mechanism are still debated. The most studied are highly insulating binary transition metal oxides, where a key ingredient of the RS effect is believed to be a redistribution of oxygen ion vacancies which results in the formation (low-resistive state -LRS) and rupture (high-resistive state -HRS) of conductive filaments within the insulating media. 1,2 For more complex oxides like perovskites, the process of the RS effect seems to be more complicated. In this case the switching was usually bipolar whereas in simple binary oxides bi-and unipolar behavior has been observed. [3][4][5] In the following we are studying the RS effect in contacts based on conducting YBa 2 Cu 3 O 6+c (YBCO) films, compounds with a crystal structure which promotes electronic transport primarily within a twodimensional layer of atoms. One of the ways to shed light on their basic properties is to study an electricfield effect in the normal state. Up to now, two mutually exclusive field-effect mechanisms have been proposed. The first one, fundamentally electronic, is based on a conventional approach which takes into account the Coulomb interaction of an applied electric field with mobile current carriers. 6 It is fast, symmetric with respect to the bias polarity and results in the enhancement or depletion of the number of charge carriers within a few near-surface atomic layers. 6 The second mechanism 7 is related to direct interaction of oxygen ions with an applied electric field which causes significant charge carrier rearrangement due to a comparatively small oxygen migration energy and high density of vacancies in the oxygen sub-lattice in an optimal doping state. Such a process is characterized by a slower time constant and can be unequal in magnitude at positive and negative voltage biases of identical absolute value. 7 To choose between the two mechanisms of field-induced changes in normal-state...

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Resistive switching (RS) effect observed in capacitor-like metal/insulator/metal junctions belongs to the most promising candidates for next generation of memory cell technology. It is based upon a sudden change of the junction resistance caused by an electric field applied to the metal electrodes. The aim of this work was to study this effect on the structure metal/YBCO 6 /YBCO 7 , where YBCO 7 is a metallic phase and YBCO 6 is an insulator phase which arises spontaneously by out-diffusion of oxygen from a few nanometers wide YBCO surface layer. Oriented YBa 2 Cu 3 O 7 thin films were prepared by the method of magnetron sputtering and consequently planar structures with metal-YBCO junction were made by the means of the optical lithography, ion etching and vacuum evaporation. On these junctions we have studied the temperature dependence of the RS effect with I-V and dI/dV-V transport measurements down to liquid He temperature. We have determined temperature dependence of the RS effect threshold voltage in the temperature range 100-300 K and showed that this dependency is compatible with common idea of oxygen ions migration under electric field within the YBCO surface layer.

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