Kate J. Norris

Electrical Performance and Scalability of Pt Dispersed SiO2 Nanometallic Resistance Switch

Highly reproducible bipolar resistance switching was recently demonstrated in a composite material of Pt nanoparticles dispersed in silicon dioxide. Here, we examine the electrical performance and scalability of this system and demonstrate devices with ultrafast (<100 ps) switching, long state retention (no measurable relaxation after 6 months), and high endurance (>3 × 10(7) cycles). A possible switching mechanism based on ion motion in the film is discussed based on these observations.

Anatomy of Ag/Hafnia‐Based Selectors with 1010 Nonlinearity

A novel Ag/oxide-based threshold switching device with attractive features including ≈1010 nonlinearity is developed. High-resolution transmission electron microscopic analysis of the nanoscale crosspoint device suggests that elongation of an Ag nanoparticle under voltage bias followed by spontaneous reformation of a more spherical shape after power off is responsible for the observed threshold switching.

Trilayer Tunnel Selectors for Memristor Memory Cells

An integrated memory cell with a memristor and a trilayer crested barrier selector, showing repeatable nonlinear current–voltage switching loops is presented. The fully atomic‐layer‐deposited TaN1+x/Ta2O5/TaN1+x crested barrier selector yields a large nonlinearity (>104), high endurance (>108), low variability, and low temperature dependence.

Physical origins of current and temperature controlled negative differential resistances in NbO2

Negative differential resistance behavior in oxide memristors, especially those using NbO2, is gaining renewed interest because of its potential utility in neuromorphic computing. However, there has been a decade-long controversy over whether the negative differential resistance is caused by a relatively low-temperature non-linear transport mechanism or a high-temperature Mott transition. Resolving this issue will enable consistent and robust predictive modeling of this phenomenon for different applications. Here we examine NbO2 memristors that exhibit both a current-controlled and a temperature-controlled negative differential resistance. Through thermal and chemical spectromicroscopy and numerical simulations, we confirm that the former is caused by a ~400 K non-linear-transport-driven instability and the latter is caused by the ~1000 K Mott metal-insulator transition, for which the thermal conductance counter-intuitively decreases in the metallic state relative to the insulating state.The development of future computation devices will be aided by a better understanding of the physics underlying material behaviors. Using thermoreflectance and spatially resolved X-ray microscopy, Kumar et al. elucidate the origin of two types of negative differential resistance in NbO2 memristors.

Thermally induced crystallization in NbO2 thin films.

Niobium dioxide can exhibit negative differential resistance (NDR) in metal-insulator-metal (MIM) devices, which has recently attracted significant interest for its potential applications as a highly non-linear selector element in emerging nonvolatile memory (NVM) and as a locally-active element in neuromorphic circuits. In order to further understand the processing of this material system, we studied the effect of thermal annealing on a 15 nm thick NbO2 thin film sandwiched inside a nanoscale MIM device and compared it with 180 nm thick blanket NbOx (x = 2 and 2.5) films deposited on a silicon dioxide surface as references. A systematic transmission electron microscope (TEM) study revealed a similar structural transition from amorphous to a distorted rutile structure in both cases, with a transition temperature of 700 °C for the NbO2 inside the MIM device and a slightly higher transition temperature of 750 °C for the reference NbO2 film. Quantitative composition analysis from electron energy loss spectroscopy (EELS) showed the stoichiometry of the nominal 15 nm NbO2 layer in the as-fabricated MIM device deviated from the target 1:2 ratio because of an interaction with the electrode materials, which was more prominent at elevated annealing temperature.

A niobium oxide-tantalum oxide selector-memristor self-aligned nanostack

The integration of nonlinear current-voltage selectors and bi-stable memristors is a paramount step for reliable operation of crossbar arrays. In this paper, the self-aligned assembly of a single nanometer-scale device that contains both a selector and a memristor is presented. The two components (i.e., selector and memristor) are vertically assembled via a self-aligned fabrication process combined with electroforming. In designing the device, niobium oxide and tantalum oxide are chosen as materials for selector and memristor, respectively. The formation of niobium oxide is visualized by exploiting the self-limiting reaction between niobium and tantalum oxide; crystalline niobium (di)oxide forms at the interface between metallic niobium and tantalum oxide via electrothermal heating, resulting in a niobium oxide selector self-aligned to a tantalum oxide memristor. A steady-state finite element analysis is used to assess the electrothermal heating expected to occur in the device. Current-voltage measurement...

Deterministic nanoparticle assemblies: from substrate to solution

The deterministic assembly of metallic nanoparticles is an exciting field with many potential benefits. Many promising techniques have been developed, but challenges remain, particularly for the assembly of larger nanoparticles which often have more interesting plasmonic properties. Here we present a scalable process combining the strengths of top down and bottom up fabrication to generate deterministic 2D assemblies of metallic nanoparticles and demonstrate their stable transfer to solution. Scanning electron and high-resolution transmission electron microscopy studies of these assemblies suggested the formation of nanobridges between touching nanoparticles that hold them together so as to maintain the integrity of the assembly throughout the transfer process. The application of these nanoparticle assemblies as solution-based surface-enhanced Raman scattering (SERS) materials is demonstrated by trapping analyte molecules in the nanoparticle gaps during assembly, yielding uniformly high enhancement factors at all stages of the fabrication process.

Indium phosphide nanowire network: growth and characterization for thermoelectric conversion

Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (MOCVD). InP nanowires grew in the structure of three-dimensional networks in which electrical charges and heat can travel over distances much longer than the mean length of the constituent nanowires. We studied the dependence of thermoelectric properties on geometrical factors within the InP nanowire networks. The InP nanowire networks show Seebeck coefficients comparable with that of bulk InP. Rather than studying single nanowires, we chose networks of nanowires formed densely across large areas required for large scale production. We also studied the role played by intersections where multiple nanowires were fused to form the nanowire networks. Modeling based on finite-element analysis, structural analysis, and transport measurements were carried out to obtain insights of physical properties at the intersections. Understanding these physical properties of three-dimensional nanowire networks will advance the development of thermoelectric devices.

Electron Energy-Loss Spectroscopy (EELS) Study of NbOx Film for Resistive Memory Applications

Niobium dioxide (NbO2) is one of the Mott insulators that exhibit current-controlled negative differential resistance, also referred as threshold switching, when used in two-terminal devices. This phenomenon is caused by a reversible insulator-to-metal phase transition, which is proposed to induce a conductive channel in the device that bridges the two electrodes [1][2]. As the amount and local distribution of oxygen vacancy play important roles during the switching, characterizing the composition with high spatial resolution at the atomic level is required for understanding the working mechanism and potential failure. Previous EELS study has been conducted on metallic Nb and stable Nb oxides (NbO, NbO2, Nb2O5) in terms of energy-loss near-edge structures (ELNES) of all relevant Nb edges and O-K edge for fingerprints of Nb in different formal oxidation states [3][4]. In this study, we provide a quantitative study on the amorphous NbOx thin-films by ion beam sputtering for resistive switching applications. The EELS quantification in NbOx film can provide the variation of local composition and chemical states across the film, which helps to understand the device behavior.

Two-step growth and fabrication of thermoelectric devices employing indium phosphide nanowire networks

The ability to make a good electrical/thermal contact to a large area filled with semiconductor nanowires has been a major engineering challenge in developing this type of thermoelectric devices. A practical fabrication process of a top electrical/thermal contact onto a network of randomly oriented intersecting semiconductor nanowires was designed by implementing a sequence of two separated metal organic chemical vapor deposition processes for indium phosphide. In the first step, a nanowire network was grown on a substrate with indium phosphide nanowires grown axially. Subsequently, growth temperature and pressure were altered to change the axial growth to lateral growth that promoted the formation of indium phosphide extending over multiple nanowires. Possible growth mechanisms during the lateral growth and structural properties of the laterally grown segment will be discussed.