Valence Change Mechanism Research Articles

Resistive Switching Effect in Titanium Oxides

Resistive switching (RS) phenomena have been vigorously investigated in a large variety of materials and highlighted for its preeminent potential for the future nonvolatile semiconductor memory applications or reconfigurable logic circuits. Among the various resistive switching materials, the binary metal oxides demonstrate more advantageous for micro- or nano-electronics applications due to their simpler fabrication process and compatibility with conventional CMOS technology, though the mechanisms are controversial due to the diversity of RS effects. This review mainly focuses on the current understanding of the microscopic nature of RS in titanium oxides, in which the working mechanisms can be categorized into thermochemical metallization mechanism, valence change mechanism, and electrostatic/electronic mechanism. The approaches developed to investigate the RS and the specific switching processes related to different mechanisms are addressed. Since titanium oxides are oxygen-vacancy doped semiconductors, the role of defects is analyzed in detail and possible effective strategies to improve the performance of RS are addressed.

Mechanisms of current conduction in Pt/BaTiO3/Pt resistive switching cell

The 80-nm-thickness BaTiO3 (BT) thin film was prepared on the Pt/Ti/SiO2/Si substrate by the RF magnetron sputtering technique. The Pt/BT/Pt/Ti/SiO2/Si structure was investigated using X-ray diffraction and scanning electron microscopy. The current–voltage characteristic measurements were performed. The bipolar resistive switching behavior was found in the Pt/BT/Pt cell. The current–voltage curves were well fitted in different voltage regions at the high resistance state (HRS) and the low resistance state (LRS), respectively. The conduction mechanisms are concluded to be Ohmic conduction and Schottky emission at the LRS, while space-charge-limited conduction and Poole–Frenkel emission at the HRS. The electroforming and switching processes were explained in terms of the valence change mechanism, in which oxygen vacancies play a key role in forming conducting paths.

Function by defects at the atomic scale – New concepts for non-volatile memories

A survey of non-volatile, highly scalable memory devices which utilize dedicated resistive switching phenomena in nanoscale chalcogenide-based memory cells is presented. We introduce the basic operation principle of the phase change mechanism, the thermochemical mechanism, and the valence change mechanism and we discuss the crucial role of structural defects in the switching processes. We show how this role is determined by the atomic structure of the defects, the electronic defect states, and/or the ion transport properties of the defects. The electronic structure of the systems in different resistance states is described in the light of the chemical bonds involved. While for phase-change alloys the interplay of ionicity and hybridization in the crystalline and in the amorphous phase determine the resistances, the local redox reaction at the site of extended defects, the change in the oxygen stoichiometry, and the resulting change in the occupancy of relevant orbitals play the major role in the thermochemical and the valence change mechanism. Phase transformations are not only discussed for phase-change alloys but also for both other types of switching processes. The switching kinetics as well as the ultimate scalability of switching cells is related to structural defects in the materials.

The role of defects in resistively switching chalcogenides

Abstract This overview describes the present understanding of resistive switching phenomena encountered in chalcogenide-based cells which may be utilized in energy-efficient non-volatile memory devices and in array-based logic applications. We introduce the basic operation principle of the phase change mechanism, the thermochemical mechanism, and the valence change mechanism and we discuss the crucial role of structural defects in the switching processes. We show how this role is determined by the atomic structure of the defects, the electronic defect states, and/or the ion transport properties of the defects. The electronic structure of the systems in different resistance states is described in the light of the chemical bonds involved. While for phase change alloys the interplay of ionicity and hybridization in the crystalline and in the amorphous phase determine the resistances, the local redox reaction at the site of extended defects, the change in the oxygen stoichiometry, and the resulting change in the occupancy of relevant orbitals play the major role in transition metal oxides which switch by the thermochemical and the valence change mechanism. Phase transformations are not only discussed for phase change alloys but also for redox-related switching processes. The switching kinetics as well as the ultimate scalability of switching cells are related to structural defects in the materials.

Redox-Based Resistive Switching Memories - Nanoionic Mechanisms, Prospects, and Challenges.

This review article introduces resistive switching processes that are being considered for nanoelectronic nonvolatile memories. The three main classes are based on an electrochemical metallization mechanism, a valence change mechanism, and a thermochemical mechanism, respectively. The current understanding of the microscopic mechanisms is discussed and the scaling potential is outlined..

Structure and superconductivity of Y (Ba1−xRx)2Cu3O7−δ (R=La, Pr, and Nd)

Powder X-ray diffraction and electrical-resistivity measurement were used to study the structure and the Tc variation of Y(Ba1−xRx)2Cu3O7−δ(0≤x≤0.2; R=La, Pr, and Nd). The solubility limit was estimated as x=0.05, 0.075, and >0.2 with R=Nd, Pr, and La, respectively, which depends on the ionic radius of dopants. All three series undergo an orthorhombic-to-tetragonal transition at x≈0.15-0.2. Meanwhile, the Tc increases slightly through a maximum at x≈0.025; then decreases rapidly with x, indicating an overdoped system for YBa2Cu3O7−δ. Whereas, the normal-state resistivity increases with x for all temperature ranges. The similarities and differences with various dopants are discussed in conjunction with ionic size, valence change and hole-filling mechanism.