Comprehensive study on unipolar RRAM charge conduction and stochastic features, a simulation approach

An in-depth analysis of resistive switching (RS) in unipolar devices is performed by means of a new simulator based on resistive circuit breakers of different features. The forming, set and reset processes are described in terms of the stochastic formation and rupture of conductive filaments of several branches in the dielectric. Both, the electric field and temperature dependencies are incorporated in the simulation. The simulation tool was tuned with experimental data of devices fabricated making use of the Ti/HfO2/Si stack. The variability and the stochastic behavior are characterized and reproduced correctly by simulation to understand the physics behind RS. Reset curves with several current steps are explained considering the rupture of different branches of the conductive filament. The simulation approach allows to connect in a natural manner to compact modeling solutions for the devices under study.

Cerium oxide capping on Y-doped HfO2 films for ferroelectric phase stabilization with endurance improvement

The effects of 1-nm-thick CeOx capping on 7.5-nm-thick Y-doped HfO2 films on the ferroelectric characteristics are investigated. From the ferroelectric characteristics of the samples annealed at different temperatures from 450 to 600oC and annealing durations, the time (τ) required to stabilize the ferroelectric phase at each temperature was shortened by the capping. The identical activation energy (Ea) of 2.65 eV for ferroelectric stabilization without and with capping suggests the same kinetics for phase transformation. However, an increase in the remnant polarization (Pr) was obtained. Only a few Ce atoms diffused into the underlying HfO2 film even after 600oC annealing. Ferroelectric switching tests revealed an improvement in endurance from 107 to 1010 by the capping, presumably owing to the suppression of conductive filament formation. Therefore, CeOx capping is effective in promoting the ferroelectric phase in HfO2 with high switching endurance.

Possible Equivalent Circuit Model and Physical Structures of Sputter-Deposited Silicon Oxide Film Showing Resistive Switching

Based on the results of experiments on the resistive switching behaviors of sputter-deposited silicon oxide films, this paper proposes a possible equivalent circuit model to characterize the switching behavior. It is revealed that frequency dispersion of the conductance component and capacitance component in the equivalent circuit model dominate the physical interpretation of the frequency-dependence of the components. The validity of the model and its physical interpretation are examined based on a theoretical model of the dielectric function of the conductive filament region. The polarizability of the conductive filament region suggests that the capacitance component of the conductive filament is insensitive to frequency in the low frequency range, whereas the conductance component of the conductive filament is proportional to frequency in the low frequency range. These theoretical results match experimental findings, and it is revealed that the equivalent circuit models and the frequency dispersion models for the capacitance and conductance component of the silicon oxide film are acceptable. In addition, this paper reveals the importance of the sub-oxide region and the Si precipitate region in determining the resistive switching behaviors of sputter-deposited silicon oxide film.

Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging

The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO2 has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO2-based memristors. Here, the conductive filament system in the amorphous HfO2-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf6O and its crystalline surroundings (monoclinic or tetragonal HfOx). The phase of the HfOx shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO2 is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO2-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.

High stability resistive switching mechanism of a screen-printed electrode based on BOBZBT2 organic pentamer for creatinine detection

The resistive switching (RS) mechanism is resulted from the formation and dissolution of a conductive filament due to the electrochemical redox-reactions and can be identified with a pinched hysteresis loop on the I–V characteristic curve. In this work, the RS behaviour was demonstrated using a screen-printed electrode (SPE) and was utilized for creatinine sensing application. The working electrode (WE) of the SPE has been modified with a novel small organic molecule, 1,4-bis[2-(5-thiophene-2-yl)-1-benzothiopene]-2,5-dioctyloxybenzene (BOBzBT2). Its stability at room temperature and the presence of thiophene monomers were exploited to facilitate the cation transport and thus, affecting the high resistive state (HRS) and low resistive state (LRS) of the electrochemical cell. The sensor works based on the interference imposed by the interaction between the creatinine molecule and the radical cation of BOBzBT2 to the conductive filament during the Cyclic Voltammetry (CV) measurement. Different concentrations of BOBzBT2 dilution were evaluated using various concentrations of non-clinical creatinine samples to identify the optimised setup of the sensor. Enhanced sensitivity of the sensor was observed at a high concentration of BOBzBT2 over creatinine concentration between 0.4 and 1.6 mg dL−1—corresponding to the normal range of a healthy individual.

Chitosan-Based Flexible Memristors with Embedded Carbon Nanotubes for Neuromorphic Electronics

In this study, we propose high-performance chitosan-based flexible memristors with embedded single-walled carbon nanotubes (SWCNTs) for neuromorphic electronics. These flexible transparent memristors were applied to a polyethylene naphthalate (PEN) substrate using low-temperature solution processing. The chitosan-based flexible memristors have a bipolar resistive switching (BRS) behavior due to the cation-based electrochemical reaction between a polymeric chitosan electrolyte and mobile ions. The effect of SWCNT addition on the BRS characteristics was analyzed. It was observed that the embedded SWCNTs absorb more metal ions and trigger the conductive filament in the chitosan electrolyte, resulting in a more stable and wider BRS window compared to the device with no SWCNTs. The memory window of the chitosan nanocomposite memristors with SWCNTs was 14.98, which was approximately double that of devices without SWCNTs (6.39). Furthermore, the proposed SWCNT-embedded chitosan-based memristors had memristive properties, such as short-term and long-term plasticity via paired-pulse facilitation and spike-timing-dependent plasticity, respectively. In addition, the conductivity modulation was evaluated with 300 synaptic pulses. These findings suggest that memristors featuring SWCNT-embedded chitosan are a promising building block for future artificial synaptic electronics applications.

Conduction and Resistive Switching Memory in Al/MoS2+PVP/Ag Devices Fabricated using Drop cast Method

Resistive switching in MoS2 embedded PVP composite-based ReRAM with Al and Ag electrodes is reported. A cost-free drop cast method was used to deposit active layers consisting of 30 wt%, 40 wt%, and 70 wt% of MoS2 in PVP. Each system exhibited unique electroforming and switching mode. Asymmetrical bipolar resistive switching occurring only in the positive voltage bias, a typical bipolar resistive switching and a typical ‘O-type’ resistive switching were observed for the 30 wt%, 40 wt%, and 70 wt% systems, respectively. Furthermore, injection of charge carriers at the electrode/active layer interface and electrochemical metalization mechanisms drove the formation of a nanoscale conductive filament in the device A and B. On the other hand, we attributed the conduction mechanism of device C to hopping conduction. Our results demonstrate the behaviour of MoS2 embedded PVP composite-based ReRAM has a strong dependence on the amount of MoS2 and that both the switching and conduction mechanism can be exploited by controlling the amount of MoS2 in the composite.