[en] This study examined the properties of Schottky-type diodes composed of Pt/TiO₂/Ti, where the Pt/TiO₂ and TiO₂/Ti junctions correspond to the blocking and ohmic contacts, respectively, as the selection device for a resistive switching cross-bar array. An extremely high forward-to-reverse current ratio of ∼ 10⁹ was achieved at 1 V when the TiO₂ film thickness was 19 nm. TiO₂ film was grown by atomic layer deposition at a substrate temperature of 250 deg. C. Conductive atomic force microscopy revealed that the forward current flew locally, which limits the maximum forward current density to < 10 A cm^-2 for a large electrode (an area of ∼ 60 000 μm²). However, the local current measurement showed a local forward current density as high as ∼ 10⁵ A cm^-2. Therefore, it is expected that this type of Schottky diode effectively suppresses the sneak current without adverse interference effects in a nano-scale resistive switching cross-bar array with high block density.

[en] This study examined the effects of electrical forming methods on the bipolar resistance switching (BRS) behavior in Pt/TiO₂/Pt sandwich structures. The BRS is confined to a region near the ruptured end of conducting nanofilaments, which are composed of a Ti_nO_2n-1 Magneli phase formed by electroforming. The intermediate phase with an oxygen vacancy concentration between the insulating TiO₂ and the residual conducting filament that formed at the interface region was considered to be the switching layer (SL). The change in filament shape caused by a variation in the compliance current during filament formation resulted in a different filament rupture location and SL configuration. Precise control of the filament formation and rupture process resulted in SLs connected in an anti-parallel configuration. It was possible to reconfigure the SLs in the same fashion without any restraints, which allowed an unlimited memristive operation to be achieved. This paper presents a new technique in voltage sweep mode that applies a compliance current as a tool to achieve a memristor with unlimited operation.

[en] Various types of bipolar resistive switching (BRS) at the filament ruptured region by the unipolar resistive switching (URS) reset in the structure Pt/TiO₂/Pt were categorized in terms of operation polarity and switching parameters. The differences in BRS behavior, even under identical current–voltage switching, are closely related to the previously performed URS reset parameter, especially the power consumed during the reset process. Various modes of BRS from the URS reset status in the structure Pt/TiO₂/Pt are reported, and interpreted in terms of a distinct oxygen vacancy configuration in the ruptured region of a Magnéli filament. (paper)

[en] The detailed mechanism of electronic bipolar resistance switching (BRS) in the Pt/TiO₂/Pt structure was examined. The conduction mechanism analysis showed that the trap-free and trap-mediated space-charge-limited conduction (SCLC) governs the low and high resistance state of BRS, respectively. The SCLC was confirmed by fitting the current-voltage characteristics of low and high resistance states at various temperatures. The BRS behavior originated from the asymmetric potential barrier for electrons escaping from, and trapping into, the trap sites with respect to the bias polarity. This asymmetric potential barrier was formed at the interface between the trap layer and trap-free layer. The detailed parameters such as trap density, and trap layer and trap-free layer thicknesses in the electronic BRS were evaluated. This showed that the degradation in the switching performance could be understood from the decrease and modified distribution of the trap densities in the trap layer.

[en] A tri-stable memristive switching was demonstrated on a Pt/TiO₂/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO₂/TiO_2−x/Pt cell. (paper)

[en] Electroforming behaviours of Ta₂O₅ resistance switching memory cell with a diameter of 28 nm and different thickness (0.5-2.0 nm) of Ta₂O₅ layer have been examined. The devices showed a constant forming electric field of 0.54 V/nm regardless of Ta₂O₅ thickness. The electroforming with negative bias to top TiN electrode was ascribed to electric field- driven migration of oxygen vacancies, originally residing near the bottom interface, toward the top electrode interface and formation of conducting filaments. The estimated electroforming energy (0.094-0.14 eV) was favourably compared with the hopping energy of electrons from the V_O site to a nearby Ta site. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] We observed bipolar switching behavior from an epitaxial strontium cobaltite film grown on a SrTiO₃ (001) substrate. The crystal structure of strontium cobaltite has been known to undergo topotactic phase transformation between two distinct phases: insulating brownmillerite (SrCoO_2.5) and conducting perovskite (SrCoO_3−δ) depending on the oxygen content. The current–voltage characteristics of the strontium cobaltite film showed that it could have a reversible insulator-to-metal transition triggered by electrical bias voltage. We propose that the resistance switching in the SrCoO_x thin film could be related to the topotactic phase transformation and the peculiar structure of SrCoO_2.5