Dashan Shang

Improvement of reproducible resistance switching in polycrystalline tungsten oxide films by in situ oxygen annealing

The electric-field-induced resistance switching in polycrystalline tungsten oxide films was investigated. Compared with the as-deposited film, the film annealed in an oxygen atmosphere shows a more stable switching behavior, a higher low-to-high resistance ratio, and a better endurance and retention. Based on the x-ray photoemission spectroscopy analysis, the resistance switching was attributed to the change in the interfacial barrier potential, due to the electron trapping/detrapping in the surface states, and the switching improvement was attributed to the decrease in the surface density of states. The present work suggests a possible approach controlling the resistance switching property by surface modification.

Resistive switching properties in oxygen-deficient Pr0.7Ca0.3MnO3 junctions with active Al top electrodes

Current-voltage characteristics, conduction mechanisms, and resistive switching properties are investigated in Al/Pr0.7Ca0.3MnO3 (PCMO)/Pt junctions. The junction resistance exhibits an irreversible increase from 2 to 90 MΩ in the forming process, the first several repeated bias sweeps. In contrast to the PCMO junctions involving inert top electrode (TE), the active Al-TE-based junctions show very large junction resistance and opposite cycling directions. It is found that the junction resistance sequence is qualitatively consistent with the standard Gibbs energies ΔG0 for the formation of corresponding TE oxides, rather than the Schottky barrier heights. Current-voltage fits indicate that the conduction processes in high and low resistance states are controlled by Poole–Frenkel emission and space-charge-limited conduction, respectively. The junctions show asymmetric switching thresholds with the minimal switching voltages are +1 V at the positive and −4 V at the negative side. Resistance retention tests i...

Retention behavior of the electric-pulse-induced reversible resistance change effect in Ag–La0.7Ca0.3MnO3–Pt sandwiches

Materials showing reversible resistance switching at room temperature are attractive for today’s semiconductor technology with its wide interest in nonvolatile random access memories. In this letter, the retention behavior of the electric-pulse-induced reversible (EPIR) resistance change effect in Ag–La0.7Ca0.3MnO3–Pt sandwiches was demonstrated. The results suggest that the retention property of the EPIR materials depends on both the resistance states and the resistance switching history, and it can be modified using the proper modes of applied electric pulse.

Bipolar Resistance Switching in Fully Transparent ZnO:Mg-Based Devices

Transparent indium–tin-oxide/ZnO:Mg/F-doped SnO2 devices that show bipolar resistance switching have been successfully fabricated. In addition to the transmittance above 80% for visible light, the devices show a high-to-low resistance ratio greater than 2.5, an endurance more than 105 cycles, and a resistance retention longer than 5000 s even at the temperature of 110 °C. The field-induced resistance change can be explained based on the formation/rupture of conduction filaments, due to the migration of structural defects in electric field. The present work shows the potential application of resistive random access memory to invisible electronics.

Photoresponse of the Schottky junction Au/SrTiO3:Nb in different resistive states

A systematic study on photovoltaic effects has been performed for the Schottky junction Au/SrTiO3:0.05 wt %Nb, the resistance of which can be tuned, by applied electric pulses, between ∼1 and ∼200 MΩ. It is found that, despite the great change in junction resistance, the photocurrent across the junction is constant when the power and wavelength of incident light are fixed. The corresponding Schottky barrier, deduced from the photoresponse data is ∼1.5 eV, independent of junction resistance. This result suggests the invariance of the interfacial barrier during resistance switching and the occurrence of filamentary conduction channels.

The polarity origin of the bipolar resistance switching behaviors in metal/La0.7Ca0.3MnO3/Pt junctions

Bipolar resistance switching behaviors in the M/La0.7Ca0.3MnO3/Pt (M=Pt, Ag, Cu, Al, Ti, and W) junctions were investigated. We found that the switching polarities of the junctions for M=Pt, Ag, and Cu were opposite to those for M=Al, Ti, and W. This phenomenon was attributed to the different Gibbs free energy of the metal oxide formation. Based on Auger electron spectroscopy measurement of the M/La0.7Ca0.3MnO3 interfaces, the switching mechanisms were further discussed in terms of metal electrode redox reaction for M=Al, Ti, and W and oxygen vacancy generation/annihilation in the La0.7Ca0.3MnO3 film for M=Pt, Ag, and Cu, respectively.

Local resistance switching at grain and grain boundary surfaces of polycrystalline tungsten oxide films

Resistance switching behavior has been investigated in as-prepared and oxygen-annealed polycrystalline tungsten oxide films using conductive atomic force microscopy. The oxygen-annealed film appeared more insulative than the as-prepared films. The local current distributions demonstrated the lower conductivity at the grain boundaries than at the grains in the oxygen-annealed films. Reversible resistance switching behavior only occurred at the grain surface region of the oxygen-annealed films and the resistance switching process was described by the local valence change of tungsten ions induced by electrochemical migration of protons or oxygen vacancies. This different resistance switching behavior between the grain and grain boundary surface was attributed to the different oxygen vacancy density caused by the post-annealing process. The present results would be especially meaningful for the fabrication of nanoscale resistive nonvolatile memory devices.

Electronic transport and colossal electroresistance in SrTiO3:Nb-based Schottky junctions

Current-voltage characteristics and colossal electroresistance (CER) have been experimentally investigated in the temperature range from 293 to 454 K for the Schottky junctions Au/SrTiO3:0.5 wt % Nb and Au/SrTiO3:0.05 wt % Nb. Both junctions show electron tunneling-dominated transport behavior. Postannealing of SrTiO3:0.05 wt % Nb in oxygen atmosphere causes a transition of the transport behavior from electron tunneling to thermionic emission. The CER effect appears in the junctions with the transport behavior dominated by electron tunneling and greatly weakens when thermionic emission prevails after postannealing. This result reveals the presence of a close relation between CER and electron tunneling.

Temperature dependence of current-voltage characteristics of Ag–La0.7Ca0.3MnO3–Pt heterostructures

Temperature dependence of the current-voltage characteristics of Ag–La0.7Ca0.3MnO3–Pt heterostructures was investigated and the related current conduction mechanism was discussed. Poole-Frenkel emission and trap-controlled space charge limited current mechanisms are employed to explain the carrier transport at low and high temperature regions, respectively. Obvious hysteresis is observed only in the region dominated by space charge limited current with traps exponentially distributed in energy. This can be explained by the retention behavior of trapped carriers at the Ag∕La0.7Ca0.3MnO3 interface layer. It is proposed that the resistance switching can be optimized for device application by the electrode/film interface defect engineering.

Roles of silver oxide in the bipolar resistance switching devices with silver electrode

Three devices, Ag/WO3−x/Pt, Ag/AgOx/Pt, and Ag/AgOx/WO3−x/Pt, were investigated to elucidate the influence of the silver oxide on the bipolar resistive switching behavior. The silver oxide films were obtained by depositing silver at oxygen atmosphere. We find that the resistive switching behavior was determined by the silver oxide layer. Bulk and interface resistive switching were observed in the Ag/AgOx/Pt and Ag/AgOx/WO3−x/Pt devices, respectively. By the micro-x-ray photoemission spectroscopy analysis, it was demonstrated that the electrochemical redox reaction occurred in the AgOx layer is responsible for the resistive switching behavior at silver/oxide interface.