arXiv subjects

Derek A. Stewart

Publications and source records attributed to Derek A. Stewart.

4 recordsLinked to original sources

arXiv2023

A Compact Model of Threshold Switching Devices for Efficient Circuit Simulations

In this paper, we present a new compact model of threshold switching devices which is suitable for efficient circuit-level simulations. First, a macro model, based on a compact transistor based circuit, was implemented in LTSPICE. Then, a descriptive model was extracted and implemented in MATLAB, which is based on the macro model. This macro model was extended to develop a physical model that describes the processes that occur during the threshold switching. The physical model derived comprises a delay structure with few electrical components adjacent to the second junction. The delay model incorporates an internal state variable, which is crucial to transform the descriptive model into a compact model and to parameterize it in terms of electrical parameters that represent the component's behavior. Finally, we applied our model by fitting measured i-v data of an OTS device manufactured by Western Digital Research.

cond-mat.mes-hall↗

arXiv2020

Interface controlled thermal properties of ultra-thin chalcogenide-based phase change memory devices

Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data storage and processing towards overcoming the von Neumann bottleneck. In PCMs, the primary mechanism for data storage is thermal excitation. However, there is a limited body of research regarding the thermal properties of PCMs at length scales close to the memory cell dimension and, thus, the impact of interfaces on PCM operation is unknown. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in thermal boundary resistance as GST transitions from one crystallographic structure (cubic) to another (hexagonal) and as the thickness of tungsten contacts is reduced from five to two nanometers. Simulations reveal that interfacial resistance between the phase change unit and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~40% and ~50%, respectively. The resultant phase-dependent and geometric effects on thermal boundary resistance dictate that the effective thermal conductivity of the phase change unit can be reduced by a factor of four, presenting a new opportunity to reduce operating currents in PCMs.

physics.app-ph↗

arXiv2009

Oxide layer boron leads to reduced symmetry spin filtering magnetic tunnel junctions

Experimental studies of FeCoB/MgO/FeCoB tunnel junctions indicate that boron diffuses into MgO during rf-sputtering and forms polycrystalline Mg-B-O regions. These tunnel junctions provide high tunneling magnetoresistance values and low RA products. However the crystal structure of the Mg-B-O region remains unknown. Using density functional techniques, I examine three potential Mg(B) oxides including Mg$_{2}$B$_{2}$O$_{5}$ (monoclinic and triclinic) and the orthorhombic mineral Kotoite (Mg$_3$B$_2$O$_6$). Kotoite is the best candidate for formation in magnetic tunnel junctions. The (100) surface of Kotoite has a good lattice match with (001) MgO and could template neighboring FeCo into bcc layers during annealing. Complex band structure analysis of Kotoite shows that the C$_{2v}$ $\tildeΔ_1$ band has a much smaller imaginary k component than the C$_{2v}$ $\tildeΔ_4$ band. Based on symmetry analysis, the majority spin $Δ_1$ band in FeCo should couple well with the Kotoite $\tildeΔ_1$ band, while the minority FeCo $Δ_5$ will couple partially with the $\tildeΔ_4$ band. Kotoite provides a new route to high tunneling magnetoresistance based on spin filtering by a lower symmetry oxide region.

cond-mat.mtrl-sci↗

arXiv2007

Ab-initio investigation of phonon dispersion and anomalies in palladium

In recent years, palladium has proven to be a crucial component for devices ranging from nanotube field effect transistors to advanced hydrogen storage devices. In this work, I examine the phonon dispersion of fcc Pd using first principle calculations based on density functional perturbation theory. While several groups in the past have studied the acoustic properties of palladium, this is the first study to reproduce the phonon dispersion and associated anomaly with high accuracy and no adjustable parameters. In particular, I focus on the Kohn anomaly in the [110] direction.

cond-mat.mtrl-sci↗