Xiang-Shu Li

Atomic structure of conducting nanofilaments in TiO2 resistive switching memory

Resistance switching in metal oxides could form the basis for next-generation non-volatile memory. It has been argued that the current in the high-conductivity state of several technologically relevant oxide materials flows through localized filaments, but these filaments have been characterized only indirectly, limiting our understanding of the switching mechanism. Here, we use high-resolution transmission electron microscopy to probe directly the nanofilaments in a Pt/TiO(2)/Pt system during resistive switching. In situ current-voltage and low-temperature (approximately 130 K) conductivity measurements confirm that switching occurs by the formation and disruption of Ti(n)O(2n-1) (or so-called Magnéli phase) filaments. Knowledge of the composition, structure and dimensions of these filaments will provide a foundation for unravelling the full mechanism of resistance switching in oxide thin films, and help guide research into the stability and scalability of such films for applications.

Electrical manipulation of nanofilaments in transition-metal oxides for resistance-based memory.

The fabrication of controlled nanostructures such as quantum dots, nanotubes, nanowires, and nanopillars has progressed rapidly over the past 10 years. However, both bottom-up and top-down methods to integrate the nanostructures are met with several challenges. For practical applications with the high level of the integration, an approach that can fabricate the required structures locally is desirable. In addition, the electrical signal to construct and control the nanostructures can provide significant advantages toward the stability and ordering. Through experiments on the negative resistance switching phenomenon in Pt-NiO-Pt structures, we have fabricated nanofilament channels that can be electrically connected or disconnected. Various analyses indicate that the nanofilaments are made of nickel and are formed at the grain boundaries. The scaling behaviors of the nickel nanofilaments were closely examined, with respect to the switching time, power, and resistance. In particular, the 100 nm x 100 nm cell was switchable on the nanosecond scale, making them ideal for the basis for high-speed, high-density, nonvolatile memory applications.

Observation of electric-field induced Ni filament channels in polycrystalline NiOx film

For high density of resistive random access memory applications using NiOx films, understanding of the filament formation mechanism that occurred during the application of electric fields is required. We show the structural changes of polycrystalline NiOx (x=1–1.5) film in the set (low resistance), reset (high resistance), and switching failed (irreversible low resistance) states investigated by simultaneous high-resolution transmission electron microscopy and electron energy-loss spectroscopy. We have found that the irreversible low resistance state facilitates further increases of Ni filament channels and Ni filament density that resulted from the grain structure changes in the NiOx film.

Resistive memory switching in epitaxially grown NiO

Epitaxial NiO films have been fabricated on SrRuO3 films prepared on SrTiO3 single-crystal substrates. The x-ray diffraction spectra and transmission electron microscopy confirm the epitaxial growth of NiO with atomically flat surfaces on the SRO electrode. The I-V measurements of epitaxial NiO show the resistive memory switching behavior with a change in the polarity of the voltage bias, in contrast with the switching behavior of polycrystalline NiO by a single polarity. The I-V characteristics of epitaxial NiO prepared under various synthesis conditions and electrodes are presented, which suggests an important role of interfaces between NiO and electrodes on the resistive switching behavior.

Fractal Dimension of Conducting Paths in Nickel Oxide (NiO) Thin Films During Resistance Switching

A resistance-switching model in nickel oxide thin film is proposed based on Poisson distribution of electrical switching power. Conductive percolating network in soft breakdown surface may be the source of resistance switching. The main body of network may remain unchanged, but a portion of network is broken and healed repeatedly during switching. Dependence of reset current on electrode area is explained by fractal dimension.

Investigation of Interface Formed between Top Electrodes and Epitaxial NiO Films for Bipolar Resistance Switching

We investigated the resistance switching (RS) phenomenon in epitaxial NiO (epi-NiO) films by employing different types of top electrodes (TEs). Epi-NiO showed successive bipolar RS when Pt and CaRuO3 (CRO) were used as the TEs, but not when Al and Ti were used. We studied the temperature dependence of the current–voltage (I–V) characteristics for various TEs and resistance states to understand the conduction properties of TE/epi-NiO. Pristine CRO/epi-NiO showed metallic behavior, while pristine Pt/epi-NiO and Al/epi-NiO showed insulating behavior. Pt/epi-NiO and Al/epi-NiO, however, switched to a metallic or non-insulating state after electroforming. Transmission electron microscopy (TEM) images revealed the presence of a distinct stable interfacial AlOx layer in pristine Al/epi-NiO. On the other hand, the interfacial metal oxide layer was indistinguishable in the case of pristine Pt/epi-NiO and CRO/epi-NiO. Our experimental results suggested that epi-NiO has an oxygen defect on its surface and therefore the various TE/epi-NiO interfaces characterized in this study adopt distinctive electrical states. Further, the bipolar RS phenomenon can be explained by the voltage-polarity-dependent movement of oxygen ions near the interface.

Structural Properties and Resistance-Switching Behavior of Thermally Grown NiO Thin Films

We investigated the structural and electrical properties of polycrystalline NiO thin films on Pt electrodes formed by thermal oxidation. A Ni–Pt alloy phase was found at the interface, which could be explained by the oxidation kinetics and reactions of Ni, NiO, and Pt. An increase in the oxidation temperature decreased the volume of the alloy layer and improved the crystalline quality of the NiO thin films. Pt/NiO/Pt structures were fabricated, and they showed reversible resistance switching from a high-resistance state (HRS) to a low-resistance state (LRS) and vice versa during unipolar current–voltage measurements. The oxidation temperature affected (did not affect) the HRS (LRS) resistance of the Pt/NiO/Pt structures. This indicated that the transport characteristics of HRS and LRS should be different.

Exciton Recombination, Energy-, and Charge Transfer in Single- and Multilayer Quantum-Dot Films on Silver Plasmonic Resonators

We examine exciton recombination, energy-, and charge transfer in multilayer CdS/ZnS quantum dots (QDs) on silver plasmonic resonators using photoluminescence (PL) and excitation spectroscopy along with kinetic modeling and simulations. The exciton dynamics including all the processes are strongly affected by the separation distance between QDs and silver resonators, excitation wavelength, and QD film thickness. For a direct contact or very small distance, interfacial charge transfer and tunneling dominate over intrinsic radiative recombination and exciton energy transfer to surface plasmons (SPs), resulting in PL suppression. With increasing distance, however, tunneling diminishes dramatically, while long-range exciton-SP coupling takes place much faster (>6.5 ns) than intrinsic recombination (~200 ns) causing considerable PL enhancement. The exciton-SP coupling strength shows a strong dependence on excitation wavelengths, suggesting the state-specific dynamics of excitons and the down-conversion of surface plasmons involved. The overlayers as well as the bottom monolayer of QD multilayers exhibit significant PL enhancement mainly through long-range exciton-SP coupling. The overall emission behaviors from single- and multilayer QD films on silver resonators are described quantitatively by a photophysical kinetic model and simulations. The present experimental and simulation results provide important and useful design rules for QD-based light harvesting applications using the exciton-surface plasmon coupling.