arXiv subjects

Dennis Braun

Publications and source records attributed to Dennis Braun.

8 recordsLinked to original sources

arXiv2025

Reduced Variability in Threshold Switches Using Heterostructures of SiO${_x}$ and Vertically Aligned MoS${_2}$

Layered two-dimensional (2D) materials provide unique structural features, such as physical gaps between their layers that are only connected through van der Waals (vdW) forces. These vdW gaps can guide the migration of intercalated ions and thus regulate filament growth in resistive switching (RS) devices. Vertically aligned 2D materials and their heterostructures provide vdW gap-mediated ion transport in memristor crossbars, providing great potential for high-density integration and reliable RS performance. Nevertheless, the fundamental switching mechanisms and their contributions to the RS remain inadequately understood. In this work, we investigate silver (Ag) filament-based threshold switching (TS) in heterostructures comprising vertically aligned 2D molybdenum disulfide (VAMoS${_2}$) grown via sulfurization and silicon oxide (SiO${_x}$). Compared to SiO${_x}$-only devices, the SiO${_x}$/VAMoS${_2}$ devices exhibit TS with higher on-threshold and hold voltages, each approximately 0.4 V, faster switching times down to 356 ns under a 4 V pulse, and a lower cycle-to-cycle on-current variability of 3.0%. A physics-based, variability-aware model reveals that confined Ag ion migration within the vdW gaps in VAMoS${_2}$ forms ultrathin seed filaments, which guide filament growth in the SiO${_x}$ layer. These findings establish SiO${_x}$/VAMoS${_2}$ heterostructures as a promising concept for reliable TS in vertical device architectures for emerging memories and neuromorphic computing.

physics.app-ph↗

arXiv2025

Intermediate Resistive State in Wafer-Scale MoS${_2}$ Memristors through Lateral Silver Filament Growth for Artificial Synapse Applications

Memristors based on two-dimensional materials (2DMs) have garnered significant attention due to their fast resistive switching (RS) behavior and atomic-level thickness, which enables low power consumption, making them promising candidates for neuromorphic computing. Among these, memristors based on molybdenum disulfide (MoS${_2}$) have been extensively studied. Their RS has been attributed to the formation and rupture of conductive filaments (CFs). However, the underlying mechanism of filament formation remains underexplored, and the inherently stochastic nature of RS leads to high variability and limited reproducibility. Additionally, the lack of scalable fabrication techniques for 2DM-based memristors restricts their integration into standard semiconductor technology. Here, we demonstrate memristors based on metal-organic chemical vapor-deposited MoS${_2}$ on the wafer-scale. Our devices exhibit volatile and nonvolatile RS behavior, tunable by modulating the current compliance. Notably, we observe stable RS characteristics in an intermediate resistive state (IRS), featuring set and reset voltages within $\pm$1 V, an endurance exceeding 2500 cycles in direct current mode, and a state retention over 10${^6}$ s. The experimental data, complemented with simulations, suggest that the IRS originates from the lateral growth of the CF within the MoS${_2}$ layer. Furthermore, the devices successfully emulate synaptic plasticity with current responses on the microsecond timescale, highlighting their potential for large-scale integration in neuromorphic computing architectures.

physics.app-ph↗

arXiv2025

Threshold Switching in Vertically Aligned MoS${_2}$/SiO${_x}$ Heterostructures based on Silver Ion Migration

Threshold switching (TS) is a phenomenon where non-permanent changes in electrical resistance of a two-terminal device can be controlled by modulating the voltage bias. TS based on silver (Ag) conductive filaments has been observed in many materials, including layered two-dimensional (2D) transition metal dichalcogenides (TMDs). 2D TMDs are particularly promising for metal ion movement due to their van der Waals (vdW) gaps between their sheets, facilitating ion migration and filament formation without disturbing covalent chemical bonds. In this work, we demonstrate the heterostructure growth of vertically aligned molybdenum disulfide (VAMoS${_2}$) with an amorphous silicon oxide (SiO${_x}$) layer on top after sulfurization. We show that Ag ions migrate through this material stack, enabling TS. Our Ag/SiO${_x}$/VAMoS${_2}$/gold (Au) devices exhibit TS at low voltages of ~0.63 V, with high on-state currents over 200 $μ$A and stable switching exceeding 10${^4}$ cycles. Moreover, we identify two rate-limiting steps for filament formation through a physics-based dynamical model and simulate the switching kinetics. Our devices show a fast on-switching time of 311 ns and spontaneous relaxation in 233 ns. These findings deepen the understanding of SiOx/MoS${_2}$-based RS devices and demonstrate the promise for applications in emerging memories and neuromorphic computing systems.

cond-mat.mtrl-sci↗

arXiv2025

Volatile and Nonvolatile Resistive Switching in Lateral 2D Molybdenum Disulfide-Based Memristive Devices

Developing electronic devices capable of emulating biological functions is essential for advancing brain-inspired computation paradigms such as neuromorphic computing. In recent years, two-dimensional materials have emerged as promising candidates for neuromorphic electronic devices. This work addresses the coexistence of volatile and nonvolatile resistive switching in lateral memristors based on molybdenum disulfide with silver as the active electrode. The fabricated devices exhibited switching voltages of ~0.16 V and ~0.52 V for volatile and nonvolatile operation, respectively, under direct-current measurements. They also displayed the essential synaptic functions of paired-pulse facilitation and short- and long-term plasticity under pulse stimulation. The operation mechanism was investigated by in-situ transmission electron microscopy, which showed lateral migration of silver ions along the molybdenum disulfide between electrodes. Based on the experimental data, a macroscopic semi-classical electron transport model was used to reproduce the current-voltage characteristics and support the proposed underlying switching mechanisms.

cond-mat.mtrl-sci↗

arXiv2025

Influence of Humidity on the Resistive Switching of Hexagonal Boron Nitride-Based Memristors

Two-dimensional material-based memristors have recently gained attention as components of future neuromorphic computing concepts. However, their surrounding atmosphere can influence their behavior. In this work, we investigate the resistive switching behavior of hexagonal boron nitride-based memristors with active nickel electrodes under vacuum conditions. Our cells exhibit repeatable, bipolar, nonvolatile switching under voltage stress after initial forming, with a switching window > 10${^3}$ under ambient conditions. However, in a vacuum, the forming is suppressed, and hence, no switching is observed. Compact model simulations can reproduce the set kinetics of our cells under ambient conditions and predict highly suppressed resistive switching in a water-deficient environment, supporting the experimental results. Our findings have important implications for the application of h-BN-based memristors with electrochemically active electrodes since semiconductor chips are typically processed under high vacuum conditions and encapsulated to protect them from atmospheric influences.

cond-mat.mtrl-sci↗

arXiv2024

Volatile MoS${_2}$ Memristors with Lateral Silver Ion Migration for Artificial Neuron Applications

Layered two-dimensional (2D) semiconductors have shown enhanced ion migration capabilities along their van der Waals (vdW) gaps and on their surfaces. This effect can be employed for resistive switching (RS) in devices for emerging memories, selectors, and neuromorphic computing. To date, all lateral molybdenum disulfide (MoS${_2}$)-based volatile RS devices with silver (Ag) ion migration have been demonstrated using exfoliated, single-crystal MoS${_2}$ flakes requiring a forming step to enable RS. Here, we present volatile RS with multilayer MoS${_2}$ grown by metal-organic chemical vapor deposition (MOCVD) with repeatable forming-free operation. The devices show highly reproducible volatile RS with low operating voltages of approximately 2 V and fast switching times down to 130 ns considering their micrometer scale dimensions. We investigate the switching mechanism based on Ag ion surface migration through transmission electron microscopy, electronic transport modeling, and density functional theory. Finally, we develop a physics-based compact model and explore the implementation of our volatile memristors as artificial neurons in neuromorphic systems.

physics.app-ph↗

arXiv2023

Non-Volatile Resistive Switching of Polymer Residues in 2D Material Memristors

Two-dimensional (2D) materials are popular candidates for emerging nanoscale devices, including memristors. Resistive switching (RS) in such 2D material memristors has been attributed to the formation and dissolution of conductive filaments created by the diffusion of metal ions between the electrodes. However, the area-scalable fabrication of patterned devices involves polymers that are difficult to remove from the 2D material interfaces without damage. Remaining polymer residues are often overlooked when interpreting the RS characteristics of 2D material memristors. Here, we demonstrate that the parasitic residues themselves can be the origin of RS. We emphasize the necessity to fabricate appropriate reference structures and employ atomic-scale material characterization techniques to properly evaluate the potential of 2D materials as the switching layer in vertical memristors. Our polymer-residue-based memristors exhibit RS typical for a filamentary mechanism with metal ion migration, and their performance parameters are strikingly similar to commonly reported 2D material memristors. This reveals that the exclusive consideration of electrical data without a thorough verification of material interfaces can easily lead to misinterpretations about the potential of 2D materials for memristor applications.

cond-mat.mes-hall↗

arXiv2023

Resistive Switching and Current Conduction Mechanisms in Hexagonal Boron Nitride Threshold Memristors with Nickel Electrodes

The two-dimensional (2D) insulating material hexagonal boron nitride (h BN) has attracted much attention as the active medium in memristive devices due to its favorable physical properties, among others, a wide bandgap that enables a large switching window. Metal filament formation is frequently suggested for h-BN devices as the resistive switching (RS) mechanism, usually supported by highly specialized methods like conductive atomic force microscopy (C-AFM) or transmission electron microscopy (TEM). Here, we investigate the switching of multilayer hexagonal boron nitride (h-BN) threshold memristors with two nickel (Ni) electrodes through their current conduction mechanisms. Both the high and the low resistance states are analyzed through temperature-dependent current-voltage measurements. We propose the formation and retraction of nickel filaments along boron defects in the h-BN film as the resistive switching mechanism. We corroborate our electrical data with TEM analyses to establish temperature-dependent current-voltage measurements as a valuable tool for the analysis of resistive switching phenomena in memristors made of 2D materials. Our memristors exhibit a wide and tunable current operation range and low stand-by currents, in line with the state of the art in h-BN-based threshold switches, a low cycle-to-cycle variability of 5%, and a large On/Off ratio of 10${^7}$.

physics.app-ph↗