Random Telegraph Noise in Metal-Oxide Memristors for True Random Number Generators: A Materials Study

Some memristors with metal/insulator/metal (MIM) structure have exhibited random telegraph noise (RTN) current signals, which makes them ideal to build true random number generators (TRNG) for advanced data encryption. However, there is still no clear guide on how essential manufacturing parameters like materials selection, thicknesses, deposition methods, and device lateral size can influence the quality of the RTN signal. In this paper, an exhaustive statistical analysis on the quality of the RTN signals produced by different MIM-like memristors is reported, and straightforward guidelines for the fabrication of memristors with enhanced RTN performance are presented, which are: i) Ni and Ti electrodes show better RTN than Au electrodes, ii) the 50 μm × 50 μm devices show better RTN than the 5 μm × 5 μm ones, iii) TiO₂ shows better RTN than HfO₂ and Al₂O₃, iv) sputtered-oxides show better RTN than ALD-oxides, and v) 10 nm thick oxides show better RTN than 5 nm thick oxides. The RTN signals recorded have been used as entropy sources in high-throughput TRNG circuits, which have passed the randomness tests of the National Institute of Standards and Technology. The work can serve as a useful guide for materials scientists and electronic engineers when fabricating MIM-like memristors for RTN applications.     

How Does Moisture Affect the Physical Property of Memristance for Anionic–Electronic Resistive Switching Memories?

Memristors based on anionic–electronic resistive switches represent a promising alternative to transistor‐based memories because of their scalability and low power consumption. To date, studies on resistive switching have focused on oxygen anionic or electronic defects leaving protonic charge‐carrier contributions out of the picture despite the fact that many resistive switching oxides are well‐established materials in resistive humidity sensors. Here, the way memristance is affected by moisture for the model material strontium titanate is studied. First, characterize own‐processed Pt|SrTiO_3‐δ|Pt bits via cyclic voltammetry under ambient conditions are thoroughly characterized. Based on the high stability of a non‐volatile device structures the impact of relative humidity to the current–voltage profiles is then investigated. It is found that Pt|SrTiO_3‐δ|Pt strongly modifies the resistance states by up to 4 orders of magnitude as well as the device's current–voltage profile shape, number of crossings, and switching capability with the level of moisture exposure. Furthermore, a reversible transition from classic memristive behavior at ambient humidity to a capacitively dominated one in dry atmosphere for which the resistive switching completely vanishes is demonstrated for the first time. The results are discussed in relation to the changed Schottky barrier by adsorbed surface water molecules and its interplay with the charge transfer in the oxide.     

Storage of Electrical Information in Metal–Organic‐Framework Memristors

Single crystals of a cyclodextrin‐based metal–organic framework (MOF) infused with an ionic electrolyte and flanked by silver electrodes act as memristors. They can be electrically switched between low and high conductivity states that persist even in the absence of an applied voltage. In this way, these small blocks of nanoporous sugar function as a non‐volatile RRAM memory elements that can be repeatedly read, erased, and re‐written. These properties derive from ionic current within the MOF and the deposition of nanometer‐thin passivating layers at the anode flanking the MOF crystal. The observed phenomena are crucially dependent on the sub‐nanometer widths of the channels in the MOF, allowing the passage of only smaller ions. Conversely, with the electrolyte present but no MOF, there are no memristance or memory effects.     

Nanoscale Resistive Switching in Amorphous Perovskite Oxide (a‐SrTiO3) Memristors

Memristive devices are the precursors to high density nanoscale memories and the building blocks for neuromorphic computing. In this work, a unique room temperature synthesized perovskite oxide (amorphous SrTiO₃: a‐STO) thin film platform with engineered oxygen deficiencies is shown to realize high performance and scalable metal‐oxide‐metal (MIM) memristive arrays demonstrating excellent uniformity of the key resistive switching parameters. a‐STO memristors exhibit nonvolatile bipolar resistive switching with significantly high (10³–10⁴) switching ratios, good endurance (>10⁶I–V sweep cycles), and retention with less than 1% change in resistance over repeated 10⁵ s long READ cycles. Nano‐contact studies utilizing in situ electrical nanoindentation technique reveal nanoionics driven switching processes that rely on isolatedly controllable nano‐switches uniformly distributed over the device area. Furthermore, in situ electrical nanoindentation studies on ultrathin a‐STO/metal stacks highlight the impact of mechanical stress on the modulation of non‐linear ionic transport mechanisms in perovskite oxides while confirming the ultimate scalability of these devices. These results highlight the promise of amorphous perovskite memristors for high performance CMOS/CMOL compatible memristive systems.     

A Memristive Element Based on an Electrically Controlled Single‐Molecule Reaction

The exponential proliferation of data during the information age has required the continuous exploration of novel storage paradigms, materials, and devices with increasing data density. As a step toward the ultimate limits in data density, the development of an electrically controllable single‐molecule memristive element is reported. In this device, digital information is encoded through switching between two isomer states by applying a voltage signal to the molecular junction, and the information is read out by monitoring the electrical conductance of each isomer. The two states are cycled using an electrically controllable local‐heating mechanism for the forward reaction and catalyzed by a single charge‐transfer process for the reverse switching. This single‐molecule device can be modulated in situ, is fully reversible, and does not display stochastic switching. The I–V curves of this single‐molecule system also exhibit memristive character. These features suggest a new approach for the development of molecular switching systems and storage‐class memories.     

Origin of the Ultra‐nonlinear Switching Kinetics in Oxide‐Based Resistive Switches

Experimental pulse length–pulse voltage studies of SrTiO₃ memristive cells are reported, which reveal nonlinearities in the switching kinetics of more than nine orders of magnitude. The results are interpreted using an electrothermal 2D finite element model. The nonlinearity arises from a temperature increase in a few‐nanometer‐thick disc‐shaped region at the Ti electrode and a corresponding exponential increase in oxygen‐vacancy mobility. The model fully reproduces the experimental data and it provides essential design rules for optimizing the cell concept of nanoionic resistive memories. The model is generic in nature: it is applicable to all those oxides which become n‐conducting upon chemical reduction and which show significant ion conductivity at elevated temperatures.     

Ternary Conductance Switching Realized by a Pillar[5]arene-Functionalized Two-Dimensional Imine Polymer Film

Nowadays, most manufacturing memory devices are based on materials with electrical bistability (i. e., “0” and “1”) in response to an applied electric field. Memory devices with multilevel states are highly desired so as to produce high-density and efficient memory devices. Herein, we report the first multichannel strategy to realize a ternary-state memristor. We make use of the intrinsic sub-nanometer channel of pillar[5]arene and nanometer channel of a two-dimensional imine polymer to construct an active layer with multilevel channels for ternary memory devices. Low threshold voltage, long retention time, clearly distinguishable resistance states, high ON/OFF ratio (OFF/ON1/ON2=1 : 10 : 10³), and high ternary yield (75 %) were obtained. In addition, the flexible memory device based on 2DP_TPAZ+TAPB can maintain its stable ternary memory performance after being bent 500 times. The device also exhibits excellent thermal stability and can tolerate a temperature as high as 300 °C. It is envisioned that the results of this work will open up possibilities for multistate, flexible resistive memories with good thermal stability and low energy consumption, and broaden the application of pillar[n]arene.     

Photoelectric Multilevel Memory Device based on Covalent Organic Polymer Film with Keto–Enol Tautomerism for Harsh Environments Applications

Covalent organic polymers (COPs) memristors with multilevel memory behavior in harsh environments and photoelectric regulation are crucial for high-density storage and high-efficiency neuromorphic computing. Here, a donor–acceptor (D–A)-type COP film (Py-COP-3), which is initiated by keto–enol tautomerism, is proposed for high-performance memristors. Satisfactorily, the indium tin oxide (ITO)/Py-COP-3/Ag device demonstrates multilevel memory performance, even in high temperatures, acid-base corrosion, and various organic solvents. Moreover, the performance can be modulated by the photoelectric effect to maintain a great switching behavior. By contrast, Py-COP-0, with similar structure and chemical composition to Py-COP-3 but without keto–enol tautomerism, exhibits binary storage performance. Further studies unravel that both the formation of conductive filaments and charge transfer within D-A Py-COP-3 film contribute to the resistive switching behavior of memory devices.