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A finite element model consisting of a conducting filament with or without a gap was used to reproduce behavior of TaO$_x$-based resistive switching devices. The specific goal was to explore the range of possible filament parameters such a filament diameter, composition, gap width, and composition to reproduce the conductance and shape of I-V while keeping the maximum temperature within acceptable range allowing for ion motion and preventing melting. The model solving heat and charge transport produced a good agreement with experimental data for the oxygen content in the filament below TaO$_{1.3}$, the filament diameter range between 6 and 22 nm, and the gap oxygen content between TaO$_{1.7}$ and TaO$_{1.85}$. Gap width was not limited on either low or high sides by the criteria considered in this report. The obtained filament composition corresponds to oxygen deficiency an order of magnitude higher than one estimated by other modelling efforts. This was in a large part due to use of recent experimental values of conductivity as a function of composition and temperature. Our modelling results imply that a large fraction of atoms leaves and/or accumulates within the filament to produce large relative concentration change. This, in turn, necessitates inclusion of strain energy in the filament formation modelling. In addition, the results reproduce non-linear I-V without the necessity of assuming the Poole-Frenkel type of electrical conduction or presence of a barrier at the oxide/metal interface., Comment: 31 pages, 7 figures

Non-volatile memory devices have received a lot of interest in both industry and academia in the last decade. Transition metal oxide-based memories offer potential applications as universal memory and artificial synapses. Here we focus on the one-time conditioning of metal / oxide / metal structures leading to the formation of a conducting filament in TiN/TaxO1-x/TiN structures and develop a finite element model of this process. Model solved coupled equations for charge, heat, and mass transport with the last one including concentration, temperature, and stress gradients as driving forces. The results closely replicated available structural data such as diameter and composition of Ta-enriched core of the filament, O-enriched ring around it, and the dynamics of filament formation. The range of critical material parameters, namely heats of transport for Ta and O, is discussed., Comment: 34 pages, 12 figures

Resistive switching devices based on transition metal oxides require formation of a conductive filament in order for the device to be able to switch. Such filaments have been proposed to form by the reduction of the oxide due to application of the electric field, but this report seeks to rebut that interpretation. Frequently reported physical changes during electro-formation include delamination of electrodes, crystallization of functional oxide, intermixing of electrode and oxide materials, and extensive loss of oxygen presumably to the ambient. Here we show that most of these effects are not inherent to the formation and switching processes and instead are due to an experimental artifact: the discharge of parasitic capacitances in the forming circuit. Discharge of typical BNC cables can raise the temperature of the filament to between 2,000 and 5,000 K resulting in extensive physical changes. Discharge and associated effects mentioned above can be eliminated using an on-chip load element without affecting the ability to switch., Comment: 20 pages, 5 figures

In this paper Hybrid Ant Colony Optimization (HAntCO) approach in solving Multi--Skill Resource Constrained Project Scheduling Problem (MS--RCPSP) has been presented. We have proposed hybrid approach that links classical heuristic priority rules for project scheduling with Ant Colony Optimization (ACO). Furthermore, a novel approach for updating pheromone value has been proposed, based on both the best and worst solutions stored by ants. The objective of this paper is to research the usability and robustness of ACO and its hybrids with priority rules in solving MS--RCPSP. Experiments have been performed using artificially created dataset instances, based on real--world ones. We published those instances that can be used as a benchmark. Presented results show that ACO--based hybrid method is an efficient approach. More directed search process by hybrids makes this approach more stable and provides mostly better results than classical ACO., Comment: The final publication is available at Springer via http://dx.doi.org/10.1007/s00500-014-1455-x

The topographic and electronic structure of cleaved SrTiO3(001) surfaces were studied, employing samples that either had or had not been coated with Ti on their outer surfaces prior to fracture. In both cases, SrO- and TiO2-terminated terraces were present on the cleavage surface, enabling in situ studies on either termination. However, the samples coated with Ti prior to fracture were found to yield a rougher morphology on TiO2-terminated terraces as well as a higher density of oxygen vacancies during an annealing (outgassing) step following the coating. The higher density of oxygen vacancies in the bulk of the Ti-coated samples also provides higher conductivity which, in turn, improves a sensitivity of the spectroscopy and reduces the effect of tip-induced band bending. Nonetheless, similar spectral features, unique to each termination, were observed for samples both with and without the Ti coating. Notably, with moderate-temperature annealing following fracture, a strong discrete peak in the conductance spectra, arising from oxygen vacancies, was observed on the SrO-terminated terraces. This peak appears at slightly different voltages for coated and uncoated samples, signifying a possible effect of tip-induced band bending., Comment: 10 pages, 5 figures

The electronic structure of SrTiO3(001) surfaces was studied using scanning tunneling spectroscopy and density-functional theory. With high dynamic range measurements, an in-gap transition level was observed on SrO-terminated surfaces, at 2.7 eV above the valence band maximum. The density of centers responsible for this level was found to increase with surface segregation of oxygen vacancies and decrease with exposure to molecular oxygen. Based on these finding, the level is attributed to surface O vacancies. A level at a similar energy is predicted theoretically on SrO-terminated surfaces. For TiO2-terminated surfaces, no discrete in-gap state was observed, although one is predicted theoretically. This lack of signal is believed to be due to the nature of defect wavefunction involved, as well as the possible influence of transport limitations in the tunneling spectroscopy measurements., Comment: 17 pages, 8 figures