Astrid Marchewka

Low-index discontinuity terahertz waveguides

A new type of dielectric THz waveguide based on recent approaches in the field of integrated optics is presented with theoretical and experimental results. Although the guiding mechanism of the low-index discontinuity (LID) THz waveguide is total internal reflection, the THz wave is predominantly confined in the virtually lossless low-index air gap within a high-index dielectric waveguide due to the continuity of electric flux density at the dielectric interface. Attenuation, dispersion and single-mode confinement properties of two LID structures are discussed and compared with other THz waveguide solutions. The new approach provides an outstanding combination of high mode confinement and low transmission losses currently not realizable with any other metal-based or photonic crystal approach. These exceptional properties might enable the breakthrough of novel integrated THz systems or endoscopy applications with sub-wavelength resolution.

Quantifying redox-induced Schottky barrier variations in memristive devices via in operando spectromicroscopy with graphene electrodes

The continuing revolutionary success of mobile computing and smart devices calls for the development of novel, cost- and energy-efficient memories. Resistive switching is attractive because of, inter alia, increased switching speed and device density. On electrical stimulus, complex nanoscale redox processes are suspected to induce a resistance change in memristive devices. Quantitative information about these processes, which has been experimentally inaccessible so far, is essential for further advances. Here we use in operando spectromicroscopy to verify that redox reactions drive the resistance change. A remarkable agreement between experimental quantification of the redox state and device simulation reveals that changes in donor concentration by a factor of 2–3 at electrode-oxide interfaces cause a modulation of the effective Schottky barrier and lead to >2 orders of magnitude change in device resistance. These findings allow realistic device simulations, opening a route to less empirical and more predictive design of future memory cells.

Anomalous Resistance Hysteresis in Oxide ReRAM: Oxygen Evolution and Reincorporation Revealed by In Situ TEM

The control and rational design of redox-based memristive devices, which are highly attractive candidates for next-generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky-type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so-called deep Reset.

Simulation of TaO x -based complementary resistive switches by a physics-based memristive model

Highly predictive memristive models of resistive switches are required to simulate the behavior of anti-serially connected resistive switches, so called complementary resistive switches (CRSs). As an emerging non-volatile device suited for ultra-dense memory architectures, CRS cells offer great potential also as content addressable memories. Here, we introduce a circuit model for TaOx-based resistive switches which we implemented in VerilogA. This model is capable of predicting CRS behavior correctly.

Determination of the electrostatic potential distribution in Pt/Fe:SrTiO3/Nb:SrTiO3 thin-film structures by electron holography

We determined the electrostatic potential distribution in pristine Pt/Fe:SrTiO3/Nb:SrTiO3 structures by electron holography experiments, revealing the existence of a depletion layer extending into the Nb-doped bottom electrode. Simulations of potential profiles in metal-insulator-metal structures were conducted assuming different types and distributions of dopants. It is found that the presence of acceptor-type dopant concentrations at the Fe:SrTiO3/Nb:SrTiO3 interface with a donor-doped insulating layer provides a good match to the measured profile. Such acceptor-type interface concentrations may be associated with Sr vacancies on the Nb:SrTiO3 side of the bottom interface.

Molecular dynamics simulations of oxygen vacancy diffusion in SrTiO3

A classical force-field model with partial ionic charges was applied to study the behaviour of oxygen vacancies in the perovskite oxide strontium titanate (SrTiO(3)). The dynamical behaviour of these point defects was investigated as a function of temperature and defect concentration by means of molecular dynamics (MD) simulations. The interaction between oxygen vacancies and an extended defect, here a Σ3(111) grain boundary, was also examined by means of MD simulations. Analysis of the vacancy distribution revealed considerable accumulation of vacancies in the envelope of the grain boundary. The possible clustering of oxygen vacancies in bulk SrTiO(3) was studied by means of static lattice calculations within the Mott-Littleton approach. All binary vacancy-vacancy configurations were found to be energetically unfavourable.

Physical simulation of dynamic resistive switching in metal oxides using a Schottky contact barrier model

We present a numerical drift-diffusion model of electronic-ionic transport combined with a Schottky contact barrier model to study resistive switching phenomena in ReRAM devices. Capturing the transition between Schottky and ohmic contact resistances upon temperature-accelerated ion migration, our model correctly describes the quasi-static I-V switching characteristics as well as dynamic set and reset events. It is shown to account for a transition between bipolar resistive switching and complementary switching when reducing the asymmetry between the contact barriers. Further, it is used to characterize the abrupt and gradual behavior of the set and the reset process, respectively.

Critical ReRAM Stack Parameters Controlling Complimentary versus Bipolar Resistive Switching

The thickness of the oxygen scavenging metal layer, forming the Ohmic contact in HfOx and TaOx VCM-type Metal-Oxide ReRAM cells, was found to be the critical experimental parameter controlling stable bipolar resistive switching versus the occurrence of single-cell complimentary switching. It is argued that the physically controlling parameter is the effective work function (a)symmetry between top and bottom electrode contact of the ReRAM cell. For a thin metal cap layer, oxidation increases the effective work function changing from Ohmic to a more blocking contact behavior.

Resistive switching near electrode interfaces: Estimations by a current model

The growing resistive switching database is accompanied by many detailed mechanisms which often are pure hypotheses. Some of these suggested models can be verified by checking their predictions with the benchmarks of future memory cells. The valence change memory model assumes that the different resistances in ON and OFF states are made by changing the defect density profiles in a sheet near one working electrode during switching. The resulting different READ current densities in ON and OFF states were calculated by using an appropriate simulation model with variation of several important defect and material parameters of the metal/insulator (oxide)/metal thin film stack such as defect density and its profile change in density and thickness, height of the interface barrier, dielectric permittivity, applied voltage. The results were compared to the benchmarks and some memory windows of the varied parameters can be defined: The required ON state READ current density of 105 A/cm2 can only be achieved for bar...

Low-Index Discontinuity THz Waveguides

A new type of dielectric THz waveguide, which evolved from a recent approach from the held of integrated optics, is presented with theoretical and experimental results. Due to the continuity of electric flux density at a dielectric interface, the THz wave is predominantly confined in the virtually lossless low-index air core of a high-index dielectric waveguide. Attenuation, dispersion, field enhancement and field confinement properties of the waveguide are discussed and compared to those of other THz waveguide approaches.