Akihito Sawa

Resistive switching in transition metal oxides

Rapid advances in information technology rely on high-speed and large-capacity nonvolatile memories. A number of alternatives to contemporary Flash memory have been extensively studied to obtain a more powerful and functional nonvolatile memory. We review the current status of one of the alternatives, resistance random access memory (ReRAM), which uses a resistive switching phenomenon found in transition metal oxides. A ReRAM memory cell is a capacitor-like structure composed of insulating or semiconducting transition metal oxides that exhibits reversible resistive switching on applying voltage pulses. Recent advances in the understanding of the driving mechanism are described in light of experimental results involving memory cells composed of perovskite manganites and titanates.

Hysteretic current–voltage characteristics and resistance switching at a rectifying Ti∕Pr0.7Ca0.3MnO3 interface

We have characterized the vertical transport properties of epitaxial layered structures composed of Pr0.7Ca0.3MnO3(PCMO) sandwiched between SrRuO3(SRO) bottom electrode and several kinds of top electrodes such as SRO, Pt, Au, Ag, and Ti. Among the layered structures, Ti∕PCMO∕SRO is distinct due to a rectifying I–V characteristic with a large hysteresis. Corresponding to the hysteresis of the I–V characteristics, the contact resistance of the Ti∕PCMO interface reversibly switches between two stable states by applying pulsed voltage stress. We propose a model for the resistance switching at the Ti∕PCMO interface, in which the width and/or height of a Schottky-like barrier are altered by trapped charge carriers in the interface states.

Hysteretic current–voltage characteristics and resistance switching at an epitaxial oxide Schottky junction SrRuO3∕SrTi0.99Nb0.01O3

Transport properties have been studied for a perovskite heterojunction consisting of SrRuO3 (SRO) film epitaxially grown on SrTi0.99Nb0.01O3 (Nb:STO) substrate. The SRO/Nb:STO interface exhibits rectifying current–voltage (I–V) characteristics agreeing with those of a Schottky junction composed of a deep work-function metal (SRO) and an n-type semiconductor (Nb:STO). A hysteresis appears in the I–V characteristics, where high resistance and low resistance states are induced by reverse and forward bias stresses, respectively. The resistance switching is also triggered by applying short voltage pulses of 1μs–10ms duration.

Interface resistance switching at a few nanometer thick perovskite manganite active layers

We have studied the transport and resistance switching properties of Ti∕Sm0.7Ca0.3MnO3 (n unit cells)/La0.7Sr0.3MnO3 [Ti∕SCMO(n)∕LSMO] layered structures. The metal-to-metal contact of the Ti/LSMO junction (n=0) does not exhibit resistance switching effect, while the insertion of a very thin insulating SCMO layer (n⩾1) induces resistance switching effect. As the SCMO layer thickness (n) increases, the resistance switching amplitude grows and the response gets faster. This indicates that the SCMO layer as thin as several u.c. adjacent to the interface works as an active source for the resistance switching effect.

Tuning of the metal-insulator transition in electrolyte-gated NdNiO3 thin films

We demonstrate electrostatic control of the metal-insulator transition in the typical correlated-electron material NdNiO3 through a large effective capacitance of the electric double layer at the electrolyte/NdNiO3 interface. The metal-insulator transition temperature (TMI) of NdNiO3 is shown to decrease drastically with increasing hole concentration through the application of a negative gate voltage (VG). The shift in TMI (|ΔTMI|) is larger for thinner NdNiO3; for VG of −2.5 V, |ΔTMI| of 5-nm-thick NdNiO3 is as large as 40 K, and the resistivity change near 95 K is one order of magnitude. This study may be potentially applicable to Mott transistor devices.

Highly rectifying Pr0.7Ca0.3MnO3∕SrTi0.9998Nb0.0002O3 p-n junction

We have fabricated epitaxial Pr0.7Ca0.3MnO3∕SrTi0.9998Nb0.0002O3 (PCMO∕Nb:STO) junctions and characterized the interface electronic properties. The PCMO∕Nb:STO junctions show highly rectifying current density–voltage (J–V) characteristics without an apparent breakdown in the reverse bias up to 100V at room temperature. The J–V characteristics of the diodes agree well with the conventional diffusion theory for a p-n diode. The forward bias J–V and reverse bias capacitance-voltage characteristics result in an identical built-in potential of ∼0.7eV. Based on the experimental results, a plausible band diagram of the PCMO∕Nb:STO p-n diode is proposed.

Antiferromagnetic long-range order caused by nonmagnetic impurities; magnetization of single-crystal Cu1−xZnxGeO3

Abstract The magnetization was obtained as a function of temperature and magnetic field of a Cu 1− x Zn x GeO 3 single-crystal with x ∼ 0.04. The temperature dependence of the magnetic susceptibility in 0.1 T parallel to the c axis shows a clear cusp around 4.5 K and decreases rapidly below 4.5 K. There is no hysteresis between the data measured in zero-field-cooling and field-cooling processes. The field dependence of the magnetization parallel to the c axis at 2.0 K exhibits an apparent increase around 1.0 T, which corresponds to a spin-flop transition. The temperature and field dependences, the anisotropy, and the absence of the hysteresis undoubtedly prove the occurrence of an antiferromagnetic long-range order below 4.5 K instead of the spin-glass-like transition reported previously. Spins below the transition temperature are ordered almost parallel to the c axis. This is the first report of magnetic long-range order caused by nonmagnetic impurities.

Strain‐Mediated Phase Control and Electrolyte‐Gating of Electron‐Doped Manganites

A prototype Mott transistor, the electric double layer transistor with a strained CaMnO(3) thin film, is fabricated. As predicted by the strain phase diagram of electron-doped manganite films, the device with the compressively strained CaMnO(3) exhibits an immense conductivity modulation upon applying a tiny gate voltage of 2 V.

Colossal electroresistance effect at metal electrode/La1−xSr1+xMnO4 interfaces

We have studied the current-voltage (I-V) characteristics and resistance switching at the interface between metal electrodes M (=Pt, Au, Ag, Al, Ti, and Mg) and atomically flat cleaved (001) surfaces of La1−xSr1+xMnO4 (x=0–1.0) single crystals by using a three-probe method. Hysteretic I-V characteristics, indicating the appearance of the resistance switching, were observed in the junctions for M=Mg, Al, and Ti, which have relatively shallow work functions. The resistance switching ratio depends on the hole doping x and the optimal doping level is around x=0.5, verifying that the resistance switching property can be controlled by the doping level.

Fermi level shift in La1−xSrxMO3 (M=Mn, Fe, Co, and Ni) probed by Schottky-like heteroepitaxial junctions with SrTi0.99Nb0.01O3

The authors have studied electrical properties of perovskite heteroepitaxial junctions consisting of transition metal oxides La1−xSrxMO3 (LSMO: M=Mn, Fe, Co, and Ni) and an n-type semiconductor SrTi0.99Nb0.01O3 (Nb:STO). The junctions showed rectifying current-voltage characteristics that could be analyzed by taking into account a Schottky-like barrier formed in the Nb:STO at the interfaces. As the doping level x is increased, the Schottky barrier height and built-in potential increase as ∼x (eV), indicating the downward shift of the Fermi level position in the LSMO. The Fermi level position in the LSMO with the same doping level x tends to be deepened with increasing the atomic number of M, in the order of Mn, Fe, Co, and Ni.