Iulia Salaoru

Memory Impedance in TiO2 based Metal-Insulator-Metal Devices

Large attention has recently been given to a novel technology named memristor, for having the potential of becoming the new electronic device standard. Yet, its manifestation as the fourth missing element is rather controversial among scientists. Here we demonstrate that TiO2-based metal-insulator-metal devices are more than just a memory-resistor. They possess resistive, capacitive and inductive components that can concurrently be programmed; essentially exhibiting a convolution of memristive, memcapacitive and meminductive effects. We show how non-zero crossing current-voltage hysteresis loops can appear and we experimentally demonstrate their frequency response as memcapacitive and meminductive effects become dominant.

Resistive switching of oxygen enhanced TiO2 thin-film devices

In this work, we investigate the effect of oxygen-enhanced TiO2 thin films on the switching dynamics of Pt/TiO2/Pt memristive nanodevices. We demonstrate that such devices can be used as resistive random access memory (RRAM) cells without required electroforming. We experimentally demonstrate that devices based on TiO2 films fabricated via sputtering with partial pressures of Ar/O2 6/6 sccm and 2/10 sccm show OFF/ON ratios of six and two orders of magnitude, respectively. Additionally, it was found that a lower O2 flow during sputtering of TiO2 allows for lower energy requirements for switching the devices from a high to low resistive state.

Memristive devices as parameter setting elements in programmable gain amplifiers

In this paper, we investigate the AC performance of a variable gain amplifier that utilizes an in-house manufactured memristor as a gain setting element. Analysis includes frequency and phase responses as the memristor is programmed at different resistive states. A TiO2-based solid-state memristor was employed in the feedback branch of an inverting voltage amplifier and was programmed externally. We have also observed indications of memcapacitive effects and a correlation with resistive states is presented. We demonstrate that our TiO2 memristive devices, although possessing relatively low ROFF/RON switching ratios (∼10), are versatile and can be used reliably in programmable gain amplifiers.

Pulse-induced resistive and capacitive switching in TiO2 thin film devices

In this study, we exploit the non-zero crossing current–voltage characteristics exhibited by nanoscale TiO2 based solid-state memristors. We demonstrate that the effective resistance and capacitance of such two terminal devices can be modulated simultaneously by appropriate voltage pulsing. Our results prove that both resistive and capacitive switching arise naturally in nanoscale Pt/TiO2/Pt devices under an external bias, this behaviour being governed by the formation/disruption of conductive filaments through the TiO2 thin film.

Origin of the OFF state variability in ReRAM cells

This work exploits the switching dynamics of nanoscale resistive random access memory (ReRAM) cells with particular emphasis on the origin of the observed variability when cells are consecutively cycled/programmed at distinct memory states. It is demonstrated that this variance is a common feature of all ReRAM elements and is ascribed to the formation and rupture of conductive filaments that expand across the active core, independently of the material employed as the active switching core, the causal physical switching mechanism, the switching mode (bipolar/unipolar) or even the unit cells’ dimensions. Our hypothesis is supported through both experimental and theoretical studies on TiO2 and In2O3 : SnO2 (ITO) based ReRAM cells programmed at three distinct resistive states. Our prototypes employed TiO2 or ITO active cores over 5 × 5 µm 2 and 100 × 100 µm 2 cell areas, with all tested devices demonstrating both unipolar and bipolar switching modalities. In the case of TiO2-based cells, the underlying switching mechanism is based on the non-uniform displacement of ionic species that foster the formation of conductive filaments. On the other hand, the resistive switching observed in the ITO-based devices is considered to be due to a phase change mechanism. The selected experimental parameters allowed us to demonstrate that the observed programming variance is a common feature of all ReRAM devices, proving that its origin is dependent upon randomly oriented local disorders within the active core that have a substantial impact on the overall state variance, particularly for high-resistive states.

Investigation of the Switching Mechanism in TiO2-Based RRAM: A Two-Dimensional EDX Approach

The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).

Stochastic switching of TiO2-based memristive devices with identical initial memory states

In this work, we show that identical TiO2-based memristive devices that possess the same initial resistive states are only phenomenologically similar as their internal structures may vary significantly, which could render quite dissimilar switching dynamics. We experimentally demonstrated that the resistive switching of practical devices with similar initial states could occur at different programming stimuli cycles. We argue that similar memory states can be transcribed via numerous distinct active core states through the dissimilar reduced TiO2-x filamentary distributions. Our hypothesis was finally verified via simulated results of the memory state evolution, by taking into account dissimilar initial filamentary distribution.

Electrical bistability in a composite of polymer and barium titanate nanoparticles

Growth in the use of organic materials in the fabrication of electronic devices is on the rise. Recently, some attempts have been undertaken to manufacture polymer memory devices. Such devices are fabricated by depositing a blend (an admixture of organic polymer, small organic molecules and nanoparticles) between two metal electrodes. These devices show two electrical conductivity states (‘high’ and ‘low’) when a voltage is applied, thus rendering the structures suitable for data retention. In this paper, we describe an attempt to fabricate memory devices using ferroelectric nanoparticles embedded in an organic polymer. This paper also discusses issues related to the observed memory effect.

Coexistence of memory resistance and memory capacitance in TiO2 solid state devices

This work exploits the coexistence of both resistance and capacitance memory effects in TiO2-based two-terminal cells. Our Pt/TiO2/TiOx/Pt devices exhibit an interesting combination of hysteresis and non-zero crossing in their current-voltage (I-V) characteristic that indicates the presence of capacitive states. Our experimental results demonstrate that both resistance and capacitance states can be simultaneously set via either voltage cycling and/or voltage pulses. We argue that these state modulations occur due to bias-induced reduction of the TiOx active layer via the displacement of ionic species.

Electrically re-writable non-volatile memory device - using a blend of sea salt and polymer.

Intensive research is currently underway to exploit the highly interesting properties of nano-sized particles and organic molecules for optical, electronic and other applications. Recently, it has been shown that nano-sized particles and small organic molecules embedded in polymer matrices can be used to realise memory devices. Such memory devices are simple to fabricate via the spin-on technique. This work presents an attempt to use sea salt, embedded in polyvinyl acetate, in the making of the memory devices. A polymer blend of polyvinyl acetate and sodium chloride (NaCl) was prepared in methanol and spin coated onto a glass substrate marked with thin Al tracks and a top contact was evaporated onto the blend after drying - this resulted in a metal-organic-metal (MOM) structure. The current-voltage (I-V) behaviour of MOM devices shows that the devices can be switched from a high conductivity state to a low conductivity state, by applying an external electric field - this property can be exploited to store data bits. The possible charging mechanism, based on the electric dipole formation, is presented in this work. Polymer blends of polyvinyl acetate with nano-particles of BaTiO3 are also investigated to further our understanding of charging mechanism(s).