Kristy A. Campbell

Silver chalcogenide based memristor devices

We have fabricated two-terminal chalcogenide-based devices containing Ge2Se3 and Ag that function as memristors. These devices have been electrically characterized at room temperature using quasi-static DC methods, AC sinusoidal methods, and AC pulse testing methods. In all cases, the devices exhibit memristive behavior.

Defending false data injection attack on smart grid network using adaptive CUSUM test

In modern smart grid networks, the traditional power grid is enabled by the technological advances in sensing, measurement, and control devices with two-way communications between the suppliers and customers. The smart grid integration helps the power grid networks to be smarter, but it also increases the risk of adversaries because of the currently obsoleted cyber-infrastructure. Adversaries can easily paralyzes the power facility by misleading the energy management system with injecting false data. In this paper, we proposes a defense strategy to the malicious data injection attack for smart grid state estimation at the control center. The proposed “adaptive CUSUM algorithm”, is recursive in nature, and each recursion comprises two inter-leaved stages: Stage 1 introduces the linear unknown parameter solver technique, and Stage 2 applies the multi-thread CUSUM algorithm for quickest change detection. The proposed scheme is able to determine the possible existence of adversary at the control center as quickly as possible without violating the given constraints such as a certain level of detection accuracy and false alarm. The performance of the proposed algorithm is evaluated by both mathematic analysis and numerical simulation.

Compact method for modeling and simulation of memristor devices: ion conductor chalcogenide-based memristor devices

A compact model and simulation methodology for chalcogenide based memristor devices is proposed. From a microprocessor design view point, it is important to be able to simulate large numbers of devices within the integrated circuit architecture in order to speed up reliably the development process. Ideally, device models would accurately describe the characteristic device behavior and would be represented by single-valued equations without requiring the need for recursive or numerically intensive solutions. With this in mind, we have developed an empirical chalcogenide compact memristor model that accurately describes all regions of operations of memristor devices employing single-valued equations.

Phase-change memory devices with stacked Ge-chalcogenide/Sn-chalcogenide layers

Non-volatile memory devices with two stacked layers of chalcogenide materials comprising the active memory device have been investigated for their potential as phase-change memories. The devices tested consisted of GeTe/SnTe, Ge2Se3/SnTe, and Ge2Se3/SnSe stacks. All devices exhibited resistance switching behavior. The polarity of the applied voltage with respect to the SnTe or SnSe layer was critical to the memory switching properties, most likely due to the voltage induced movement of either Sn or Te into the Ge-chalcogenide layer.

Reconfigurable Memristive Device Technologies

In this paper, we present a review of the state of the art in memristor technologies. Along with ionic conducting devices [i.e., conductive bridging random access memory (CBRAM)], we include phase change, and organic/organo-metallic technologies, and we review the most recent advances in oxide-based memristor technologies. We present progress on 3-D integration techniques, and we discuss the behavior of more mature memristive technologies in extreme environments.

Real-Time Detection of False Data Injection in Smart Grid Networks: An Adaptive CUSUM Method and Analysis

A smart grid is delay sensitive and requires the techniques that can identify and react on the abnormal changes (i.e., system fault, attacker, shortcut, etc.) in a timely manner. In this paper, we propose a real-time detection scheme against false data injection attack in smart grid networks. Unlike the classical detection test, the proposed algorithm is able to tackle the unknown parameters with low complexity and process multiple measurements at once, leading to a shorter decision time and a better detection accuracy. The objective is to detect the adversary as quickly as possible while satisfying certain detection error constraints. A Markov-chain-based analytical model is constructed to systematically analyze the proposed scheme. With the analytical model, we are able to configure the system parameters for guaranteed performance in terms of false alarm rate, average detection delay, and missed detection ratio under a detection delay constraint. The simulations are conducted with MATPOWER 4.0 package for different IEEE test systems.

Influence of Sn Migration on phase transition in GeTe and Ge2Se3 thin films

Phase transitions in GeTe/SnSe and Ge2Se3/SnTe are investigated using time resolved x-ray diffraction. GeTe exhibits a structural transition from rhombohedral to the cubic phase at 300 °C, which is ∼100 °C lower than that of pure GeTe. This is facilitated by incorporation of Sn from SnSe. Sn migration is observed explicitly in Ge2Se3/SnTe by separation of SnSe phase. Amorphous Ge2Se3 is also found to crystallize at a lower temperature of 300 °C resulting in orthorhombic GeSe and monoclinic GeSe2. Thus, inclusion of a Sn containing layer may offer a means to tailor phase transition in Ge-chalcogenide thin films for phase change memory applications.

W-2W Current Steering DAC for Programming Phase Change Memory

This paper presents the design and experimental results of W-2W current mirror binary-weighted current steering digital-to-analog converter (DAC) and its application for programming phase change memory (PCM). This new approach significantly reduces the layout area of current-mode DACs by the virtue of its compact size. The proposed DAC can replace the write driver circuits in phase change memories. Both 6-bit and 12-bit DACs have been fabricated in 0.5 mum CMOS technology. The layout size of the 12-bit and the 6-bit DACs is 0.09 mm 2 and 0.04 mm 2 respectively. Experimental results are presented and the limitations are discussed.

Density functional study of Ag in Ge 2 Se 3

We present density functional calculations of isolated Ag atoms in a crystalline model for Ge2Se3. We present defect levels for interstitial and substitutional sites. Absent other defects, we show that interstitial Ag donates an electron to the conduction band of Ge2Se3 because the highest occupied atomic level is well above the Kohn-Sham conduction band edge. Thus, we predict that Ag is always positively charged, although its charge state is fractional and significantly below +1. This results accounts for both the conductive properties of Ge2Se3:Ag and the persistence of field-driven Ag motion. The presence of three fold-coordinated Ge alters the physics considerably. The midgap Ge states are electron sinks for the conduction electrons donated by Ag. Finally, we present evidence that Ag will displace Ge into interstitial sites, creating both dangling Ge orbitals and substitutional Ag with a strong Ag-Ge bond.

Self-trapping of single and paired electrons in Ge2Se3

We report the theoretical prediction of single and paired electron self-trapping in Ge(2)Se(3). In finite atomic cluster, density functional calculations, we show that excess single electrons in Ge(2)Se(3) are strongly localized around single germanium dimers. We also find that two electrons prefer to trap around the same germanium dimer, rupturing a neighboring Ge-Se bond. Localization is less robust in periodic, density functional calculations. While paired electron self-trapping is present, as shown by wavefunction localization around a distorted Ge-Ge dimer, single-electron trapping is not. This discrepancy appears to depend only on the boundary conditions and not on the exchange-correlation potential or basis set. For single- and paired-electron trapping, we report the adiabatic barriers to motion and we estimate hopping rates and freeze-in temperatures. For the single trapped electron, we also predict the (73)Ge and (77)Se hyperfine coupling constants.