F. Zeng

Bipolar resistive switching with self-rectifying effects in Al/ZnO/Si structure

We report the electrical characteristics of room-temperature-fabricated Al/ZnO/Si memory devices. Stable and reproducible clockwise bipolar resistive switching phenomena with self-rectifying effects in the low resistance state were observed in this complementary metal oxide semiconductor compatible memory structure. The current-voltage curve in different temperatures and the corresponding Arrhenius plot confirm the semiconducting conduction behavior of both the high resistance state and the low resistance state. The conduction mechanisms are explained by the Poole-Frenkel emission and space-charge-limited conduction mechanisms for the high resistance state and the low resistance state, respectively. It is proposed that the resistive switching originates from the formation and dissolution of the AlOx barrier layer which are induced by the migration of the oxygen ions.

Cr-substitution-induced ferroelectric and improved piezoelectric properties of Zn1−xCrxO films

(0001) oriented polycrystalline Cr-doped ZnO films have been prepared on n-Si(111) single-crystal substrates by nonequilibrium reactive magnetron cosputtering. The c-axis texture of the films weakens and a transformation of doping mechanism from CrZn to CrZn+Cri is indicated as the doping concentration increases. The Cr dopants are demonstrated to exist as Cr3+ ions in the films. Ferroelectric measurements show that the Ag∕Zn0.94Cr0.06O∕n-Si heterostructure displays well-defined hysteresis loop with a remanent polarization ∼0.2μC∕cm2 and a coercive field ∼50kV∕cm at room temperature. The capacitance-voltage curves with clockwise traces show typical memory windows, which symmetrically widen as the sweep amplitude increases. Ferroelectricity in Cr-doped ZnO was also established by a displacement-voltage “butterfly” loop. The observed ferroelectric behavior is attributed to the partial replacement of host Zn2+ ions by smaller Cr3+ ions, which occupy off-center positions and thereby induce permanent electric ...

Room temperature multiferroic behavior of Cr-doped ZnO films

Single-phase 9u2002at.u2009% Cr-doped ZnO film has been prepared on Pt(111)/Ti/SiO2/Si(100) substrates by reactive sputtering method. The film is found to present ferroelectric and ferromagnetic behaviors simultaneously at room temperature, and it undergoes transitions to paraelectric and paramagnetic phases at ∼368–373 and ∼495u2002K, respectively. It is considered that the local electric dipoles induced by the distortions of CrO4 tetrahedra should be responsible for the ferroelectricity. On the other hand, the ferromagnetic ordering could be explained by the interaction of the localized spins with statically occupied polaron states. The multiferroic behavior adds a dimension to the multifunction of ZnO.

Formation process of conducting filament in planar organic resistive memory

The formation process of conducting filaments (CFs) has been experimentally demonstrated in inorganics based memory devices, whereas the cation mobility dependent growth modes of the CFs remain an open question. Here, we provide direct evidence on this process in Ag/poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)/Pt planar device. The CFs, composed of partially sulfurized Ag clusters, are unexpectedly verified to nucleate initially at the middle region of the planar device and locate on the surface of the organic layer. These phenomena can be attributed to the appropriate cation mobility and the relatively lower activation energy for diffusion on the surface of the organic layer.

Migration of interfacial oxygen ions modulated resistive switching in oxide-based memory devices

Oxides-based resistive switching memory induced by oxygen ions migration is attractive for future nonvolatile memories. Numerous works had focused their attentions on the sandwiched oxide materials for depressing the characteristic variations, but the comprehensive studies of the dependence of electrodes on the migration behavior of oxygen ions are overshadowed. Here, we investigated the interaction of various metals (Ni, Co, Al, Ti, Zr, and Hf) with oxygen atoms at the metal/Ta2O5 interface under electric stress and explored the effect of top electrode on the characteristic variations of Ta2O5-based memory device. It is demonstrated that chemically inert electrodes (Ni and Co) lead to the scattering switching characteristics and destructive gas bubbles, while the highly chemically active metals (Hf and Zr) formed a thick and dense interfacial intermediate oxide layer at the metal/Ta2O5 interface, which also degraded the resistive switching behavior. The relatively chemically active metals (Al and Ti) can...

Intrinsic and extrinsic origins of room temperature ferromagnetism in Ni-doped ZnO films

The structural, electrical and magnetic properties of Ni-doped ZnO films with different Ni concentrations (x = 0–0.11, x: Ni concentration) and thicknesses (d = 15–330u2009nm, d: film thickness) prepared by radio-frequency magnetron sputtering have been systematically investigated. The structural characterizations indicate that Ni ions in the 2+ valence state, uniformly distributed in the film, almost substitute for the Zn ions when x ≤ 0.07, whereas when x increases up to 0.11, a second phase Ni is formed. Room temperature (RT) ferromagnetism (FM) has been observed for all the Ni-doped ZnO wurtzite films. The saturated magnetization varies drastically with the Ni concentration and the film thickness. A large magnetic moment of 2.80 μB/Ni is obtained in the 15u2009nm thick Zn0.96Ni0.04O film at RT. The results show that the FM observed is intrinsic for Ni-doped ZnO films and can be explained in terms of the bound magnetic polaron mechanism based on the presence of defects. In addition, the Ni precipitates owing to the excessive doping of Ni in ZnO, as an extrinsic origin, also contribute to the ferromagnetic properties in highly doped samples.

Strain engineering induced interfacial self-assembly and intrinsic exchange bias in a manganite perovskite film

The control of complex oxide heterostructures at atomic level generates a rich spectrum of exotic properties and unexpected states at the interface between two separately prepared materials. The frustration of magnetization and conductivity of manganite perovskite at surface/interface which is inimical to their device applications, could also flourish in tailored functionalities in return. Here we prove that the exchange bias (EB) effect can unexpectedly emerge in a (La,Sr)MnO3 (LSMO) “single” film when large compressive stress imposed through a lattice mismatched substrate. The intrinsic EB behavior is directly demonstrated to be originating from the exchange coupling between ferromagnetic LSMO and an unprecedented LaSrMnO4-based spin glass, formed under a large interfacial strain and subsequent self-assembly. The present results not only provide a strategy for producing a new class of delicately functional interface by strain engineering, but also shed promising light on fabricating the EB part of spintronic devices in a single step.

Switching mechanism transition induced by annealing treatment in nonvolatile Cu/ZnO/Cu/ZnO/Pt resistive memory: From carrier trapping/detrapping to electrochemical metallization

ZnO/Cu/ZnO trilayer films sandwiched between Cu and Pt electrodes were prepared for nonvolatile resistive memory applications. These structures show resistance switching under electrical bias both before and after a rapid thermal annealing (RTA) treatment, while it is found that the resistive switching effects in the two cases exhibit distinct characteristics. Compared with the as-fabricated device, the memory cell after RTA demonstrates remarkable device parameter improvements including lower threshold voltages, lower write current, and higher Roff/Ron ratio. A high-voltage forming process is avoided in the annealed device as well. Furthermore, the RTA treatment has triggered a switching mechanism transition from a carrier trapping/detrapping type to an electrochemical-redox-reaction-controlled conductive filament formation/rupture process, as indicated by different features in current-voltage characteristics. Both scanning electron microscopy observations and Auger electron spectroscopy depth profiles r...

Strain-induced ferromagnetism enhancement in Co:ZnO films

The structural and ferromagnetic properties of Zn0.95Co0.05O films grown on Si and LiNbO3 (LNO) substrates have been studied as a function of thickness (15–900 nm). The structural characterizations indicate that the c-axis lattice constant and Co–O bond length slightly decrease with the increase in film thickness, implying the progressive relaxation of the tensile strain. The magnetic measurements show that a larger strain can result in an enhancement of room temperature ferromagnetism. The thinnest films (15 nm) with the largest lattice strains possess the highest saturated magnetic moments, i.e., 5.52 and 2.96μB/Co in Co:ZnO/LNO and Co:ZnO/Si films, respectively. As the film becomes thicker, the saturated ferromagnetism rapidly decreases, which is about two orders of magnitude smaller than that of the 15-nm-thick film when its thickness is 900 nm. The enhancement of ferromagnetism in Co:ZnO films originates from the combination of enlarged Co–O bond length and increased defect amount induced by strain.

Multilevel resistance switching in Cu/TaOx/Pt structures induced by a coupled mechanism

We report on multilevel bipolar resistance switching in Cu/amorphous-TaOx/Pt structures controlled by a coupled mechanism. The devices could be reproducibly programmed into three nonvolatile resistance states, and the on/off ratios between all neighboring states are >20. It is speculated that the switching between the high resistance state and the intermediate resistance state originates from a phase transformation between Ta2O5 and TaO2, while the low resistance state is induced by the formation of Cu filaments. This structure might be promising for developing multilevel logic and memory devices with high reliability. It may also serve as a model system for investigating the coupling effect between different switching mechanisms.