A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

The local electronic properties of tantalum oxide (TaO x , 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO 3 ) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO x films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO 3 , we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO x and SrRuO 3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides. KEYWORDS: resistive switching, strontium ruthenate, tantalum oxide, scanning tunneling microscopy, electric field effect R edox-based resistance switching random access memories (ReRAMs) are considered as the next-generation memory devices to replace the present flash-based technology.1,2 ReRAMs have a simple metal−solid electrolyte− metal architecture, storing binary code information using the change in the resistance induced by filament formation and rupture, defining the low-resistive ON state (also denoted as LRS) and the high-resistive OFF state (or HRS), respectively. High scalability, CMOS compatibility, switching times in the subnanosecond range, excellent endurance and retention, and low power consumption are key but otherwise difficult-toduplicate features of ReRAM devices.

The oxygen incorporation/extraction kinetics of the potential solid oxide fuel cell (SOFC) cathode material Bi(1-x)Sr(x)FeO(3-δ) with x = 0.5 and 0.8 was studied by electrochemical impedance spectroscopy on geometrically well-defined pore-free thin film electrodes. The oxygen exchange rate was found to be higher than that of La(1-x)Sr(x)FeO(3-δ) and-among cobalt-free perovskites-only surpassed by Ba(1-x)Sr(x)FeO(3-δ) which is however known to be unstable in a SOFC environment.

The potential solid oxide fuel cell cathode material Bi1-xSrxFeO3-δ exhibits low electrical conductivities (1-12 S cm-1 at 750°C) and is therefore especially susceptible for significant sheet resistance effects when investigated in thin film form. Impedance spectra of circular thin film microelectrodes of Bi1-xSrxFeO3-δ (x = 0.2, 0.5 and 0.8) show remarkable features such as an increase in apparent ohmic resistance (high-frequency intercept) and the appearance of a semicircle at intermediate frequencies, due unlikely to an interfacial resistance contribution. To understand the origin of these features, a 2D empirical numerical model developed by Lynch et al. to account for sheet resistance in thin electrode films was applied to simulate the experimental spectra. All spectral features were reproduced qualitatively and, in most cases, nearly quantitatively indicating high sheet resistance to be the reason for the observed peculiarities. In the presence of a lateral electrical potential gradient, the apparent chemical capacitance strongly underestimates the real material properties, whereas the electrode surface resistance remains largely unaffected.

The four-component atomic intermediate-Hamiltonian Fock-space coupled cluster (IHFSCC) code of Landau et al. [J. Chem. Phys. 115, 6862 (2001)] has been adapted to two-component calculations with relativistic pseudopotentials of the energy-consistent variety. Recently adjusted energy-consistent pseudopotentials for group 11 and 12 transition elements as well as group 13 and 14 post-d main group elements, which were fitted to atomic valence spectra from four-component multiconfiguration Dirac-Hartree-Fock calculations, are tested in IHFSCC calculations for ionization potentials, electron affinities, and excitation energies of a variety of atoms and ions. Where comparison is possible, the deviations from experimental data are in good agreement with those found in previously published IHFSCC all-electron calculations: experimental data are usually reproduced within a few hundred wavenumbers.