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Filamentary‐type resistive switching devices based on the valence change mechanism (VCM) have been studied extensively during the past decades, due to their great performance as nonvolatile memories and their promising nature for novel computing architectures. For many materials exhibiting the VCM, two different polarities can be observed, namely, counter‐eightwise and eightwise switching. The more technologically relevant materials, such as TaOx and HfOx, usually exhibit the counter‐eightwise resistive switching mode, however, the nature of its abrupt switching limits the potential for their implementation in more analog systems. On the contrary, the eightwise resistive switching mode exhibits a more gradual switching profile, which makes it a more suitable candidate for neuromorphic and in‐memory computing applications. Contrary to its counterpart, the eightwise switching mode has not been fully understood for several years, hampering the improvement of memristive devices based on this mode. This review presents the major advancements that have been made to understand the mechanism behind the eightwise switching mode and its implications on the properties of the memristive devices. The works presented here are based on the model system of SrTiO3, however, the obtained knowledge and the strategies to tailor their properties can be transferred to other technologically relevant systems. [ABSTRACT FROM AUTHOR]

For any application of redox-based resistive switching devices understanding of variability is of major interest. In this work, we analyze the dependence of the SET switching time in pulse measurements of TiN/HfO x/Pt devices on the initial state before switching. The distribution of the measured SET times covers up to seven decades in one cell. A stochastic as well as a deterministic variability component are identified by relating the initial HRS resistance - obtained from a READ pulse and from the SET pulse for comparison - to the SET time. The different resulting correlations to the SET time for the two descriptions of the initial resistance are analyzed and linked to the polarity and voltage dependence of the HRS. In addition, the origin of the resistance variation is discussed. [ABSTRACT FROM AUTHOR]

The polarity of the resistive switching (RS) characteristic of metal-oxide-metal devices from atomic layer deposited polycrystalline ZrO2 films was studied by means of impedance spectroscopy. Pt/ZrO2/Ti/Pt cells made with 10 nm Ti and 30 nm Pt capping top electrodes, served as unipolar switching (US) devices. Bipolar switching (BS) devices were represented by Pt/ZrO2/30 nm TiN cells. Temperature measurements of the ON-state resistances clearly show metallic and semiconducting behavior for the US and BS cells, respectively. The pristine and the ON and OFF states of the devices were analyzed by means of impedance spectroscopy. All ZrO2 based RS devices exhibited similar impedance characteristics in the pristine states. In contrast, after electroforming clear differences in the Nyquist-plots of the US and BS devices were observed. The effect of the device structure on the RS polarity is discussed under consideration of the pillar-shaped grainy microstructure of the ZrO2 thin films. An empirical model based on redox reactions between ZrO2 and the non-noble metal electrode is proposed emphasizing defect formation prior at the ZrO2 grain boundaries. [ABSTRACT FROM AUTHOR]

Thin ZrO2 films and ZrO2/TiO2 bilayers grown by atomic layer deposition (ALD) are integrated into metal-oxide-metal (MOM) structures for investigation of resistive switching (RS) properties. The films have different microstructure depending on the used ALD oxygen source and stacking sequence for the bilayers. Pt/ZrO2/Ti/Pt devices show unipolar RS for oxide thicknesses of 11-18 nm. The devices with O3 grown ZrO2 show higher yield in comparison to the ones with H2O processed oxide. The switching polarity of the Pt/ZrO2/Ti/Pt cells depends on the thickness of the Ti electrode layer. The increase of the Ti layer thickness leads to a change in switching polarity from unipolar to bipolar. The formation of ZrO2/TiO2 bilayers results in changes in the RS behavior of the MOM cells depending on the stacking sequence. [ABSTRACT FROM AUTHOR]

Epitaxial Sr1 − xCa xRuO3 thin films were grown on SrTiO3 and LaAlO3 substrates with use of high pressure dc-sputtering. A number of complementary methods: X-ray diffraction (XRD), Rutherford backscattering (RBS), high resolution transmission electron microscopy (HRTEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), and X-ray photoemission spectroscopy (XPS) was used to characterize both in-depth and lateral structure features. Fully strained thin films were obtained with a tetragonal lattice cell pronouncedly elongated in an out-of-plane direction, independent on details of deposition conditions. Growth types varied from a single layer-by-layer two-dimensional (2D) mode to three-dimensional (3D) spiral precipitations of different size. Interface layer between the substrate and the thin film as well as its surface suffer from considerable deviations from nominal stoichiometry, basically resulting from Ru deficiency. Samples stored under normal conditions undergo further compositional change of the surface region. The process of aging, extended over several days, involves incorporation of water and carbon dioxide present in an ambient atmosphere and a subsequent surface reconstruction. For samples deposited on SrTiO3 the valence band is indicative of a metallic character of the sample interior regardless of Ca content. The heterogeneous in-depth profile of the thin films is extremely important in fabrication of nanodevices, especially those based on ultrathin layers. [ABSTRACT FROM AUTHOR]

Devices made of amorphous thin films of the prototypical Mott‐insulator chromium‐doped V2O3 show a threshold and negative differential resistance effect after an electroforming step. Here, it is demonstrated that this effect is caused by the formation of a crystalline filament, in which a thermal runaway effect can occur. A compact model is developed that can describe the switching behavior as well as its inherent variability. The influence of the doping concentration and oxygen stoichiometry on the switching behavior is characterized and by fitting the model to the experimental data, the underlying physical changes are extracted. [ABSTRACT FROM AUTHOR]