Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices

Hafnium oxide (HfO_x)‐based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two‐terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfO_x/TiN‐based metal–insulator–metal structures as model systems, it is shown that a well‐controlled oxygen stoichiometry governs the filament formation and the occurrence of multiple switching modes. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. In addition, the engineering of oxygen vacancies stabilizes atomic size filament constrictions exhibiting integer and half‐integer conductance quantization at room temperature during set and reset. Identifying the materials conditions of different switching modes and conductance quantization contributes to a unified switching model correlating structural and functional properties of RRAM materials. The possibility to engineer the oxygen stoichiometry in HfO_x will allow creating quantum point contacts with multiple conductance quanta as a first step toward multilevel memristive quantum devices.     

32 × 32 Crossbar Array Resistive Memory Composed of a Stacked Schottky Diode and Unipolar Resistive Memory

Various array types of 1‐diode and 1‐resistor stacked crossbar array (1D1R CA) devices composed of a Schottky diode (SD) (Pt/TiO₂/Ti/Pt) and a resistive switching (RS) memory cell (Pt/TiO₂/Pt) are fabricated and their performances are investigated. The unit cell of the 1D1R CA device shows high RS resistance ratio (≈10³ at 1.5 V) between low and high resistance state (LRS and HRS), and high rectification ratio (≈10⁵) between LRS and reverse‐state SD. It also shows a short RS time of <50 ns for SET (resistance transition from HRS to LRS), and ≈600 ns for RESET (resistance transition from LRS to HRS), as well as stable RS endurance and data retention characteristics. It is experimentally confirmed that the selected unit cell in HRS (logically the “off” state) is stably readable when it is surrounded by unselected LRS (logically the “on” state) cells, in an array of up to 32 × 32 cells. The SD, as a highly non‐linear resistor, appropriately controls the conducting path formation during the switching and protects the memory element from the noise during retention.     

Impact of Bi Deficiencies on Ferroelectric Resistive Switching Characteristics Observed at p‐Type Schottky‐Like Pt/Bi1–δFeO3 Interfaces

This work reports a resistive switching effect observed at rectifying Pt/Bi_1–δFeO₃ interfaces and the impact of Bi deficiencies on its characteristics. Since Bi deficiencies provide hole carriers in BiFeO₃, Bi‐deficient Bi_1–δFeO₃ films act as a p‐type semiconductor. As the Bi deficiency increased, a leakage current at Pt/Bi_1–δFeO₃ interfaces tended to increase, and finally, rectifying and hysteretic current–voltage (I–V) characteristics were observed. In I–V characteristics measured at a voltage‐sweep frequency of 1 kHz, positive and negative current peaks originating from ferroelectric displacement current were observed under forward and reverse bias prior to set and reset switching processes, respectively, suggesting that polarization reversal is involved in the resistive switching effect. The resistive switching measurements in a pulse‐voltage mode revealed that the switching speed and switching ratio can be improved by controlling the Bi deficiency. The resistive switching devices showed endurance of >10⁵ cycles and data retention of >10⁵ s at room temperature. Moreover, unlike conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at Pt/Bi_1–δFeO₃ interfaces to nonvolatile memory.     

Resistive switching characteristics of solution-processed TiOx for next-generation non-volatile memory application; transparency, flexibility, and nano-scale memory feasibility

Solution-processed TiO_x layer was investigated as a candidate for next-generation resistive random access memory (ReRAM) application. TiO_x active layer was prepared by simple spin coating process of a titanium(IV) isopropoxide precursor using sol-gel chemistry. Through the introduction of indium-tin-oxide (ITO) coated glass and polyethersulfone (PES) substrates, tranparent and flexible ReRAM devices were demonstrated, respectively. In addition, using scalable via-hole structure with nano-scale active area, the feasibility for high-density memory application was investigated. All ReRAM devices formed using various substrates exhibited good memory performance, such as stable dc I-V, ac endurance, and retention characteristics during maintaining their own unique functions accomplished by substrate properties.     

Effects of Piezoelectric Potential of ZnO on Resistive Switching Characteristics of Flexible ZnO/TiO<Subscript>2</Subscript> Heterojunction Cells

Flexible resistance random access memory (ReRAM) devices with a heterojunction structure of PET/ITO/ZnO/TiO₂/Au were fabricated on polyethylene terephthalate/indium tin oxide (PET/ITO) substrates by different physical and chemical preparation methods. X-ray diffraction, scanning electron microscopy and atomic force microscopy were carried out to investigate the crystal structure, surface topography and cross-sectional structure of the prepared films. X-ray photoelectron spectroscopy was also used to identify the chemical state of Ti, O and Zn elements. Theoretical and experimental analyses were conducted to identify the effect of piezoelectric potential of ZnO on resistive switching characteristics of flexible ZnO/TiO₂ heterojunction cells. The results showed a pathway to enhance the performance of ReRAM devices by engineering the interface barrier, which is also feasible for other electronics, optoelectronics and photovoltaic devices.     

Interface‐Engineered Amorphous TiO2‐Based Resistive Memory Devices

Crossbar‐type bipolar resistive memory devices based on low‐temperature amorphous TiO₂ (a‐TiO₂) thin films are very promising devices for flexible nonvolatile memory applications. However, stable bipolar resistive switching from amorphous TiO₂ thin films has only been achieved for Al metal electrodes that can have severe problems like electromigration and breakdown in real applications and can be a limiting factor for novel applications like transparent electronics. Here, amorphous TiO₂‐based resistive random access memory devices are presented that universally work for any configuration of metal electrodes via engineering the top and bottom interface domains. Both by inserting an ultrathin metal layer in the top interface region and by incorporating a thin blocking layer in the bottom interface, more enhanced resistance switching and superior endurance performance can be realized. Using high‐resolution transmission electron microscopy, point energy dispersive spectroscopy, and energy‐filtering transmission electron microscopy, it is demonstrated that the stable bipolar resistive switching in metal/a‐TiO₂/metal RRAM devices is attributed to both interface domains: the top interface domain with mobile oxygen ions and the bottom interface domain for its protection against an electrical breakdown.     

Evidence for Chemicals Intermingling at Silicon/Titanium Oxide (TiOx) Interface and Existence of Multiple Bonding States in Monolithic TiOx

Metal oxides (MOs) are used in photovoltaics and microelectronics as surface passivating layers and gate dielectrics, respectively. The effectiveness of MOs predominantly depends on their structure and the nature of the semiconductor/MO (S/MO) interface. While some efforts are made to analyze interface behavior of a few MOs, greater fundamental understanding on the interface and structural behaviors of emerging MOs is yet to be established for enhanced scientific and technological developments. Here, the structure of atomic layer deposited titanium oxide (TiO_x) and the nature of the c‐Si/TiO_x interface on the atomic‐ to nanoscale are probed. A new breed of mixed oxide (SiO_x+TiO_x) interfacial layer with a thickness of ≈1.3 nm at the c‐Si/TiO_x interface is discovered, and its thickness further increases to ≈1.5 nm after postdeposition annealing. It is observed that both as‐deposited and annealed monolithic TiO_x films comprise multiple bonding states at varying film thickness, with an oxygen‐deficient TiO_x layer located close to the mixed oxide/TiO_x interface. The stoichiometry of this layer improves when reaching the middle and near surface regions of the TiO_x layer, respectively. This work uncovers several critical structural and interface aspects of TiO_x, and thus creates opportunities to control and design improved photovoltaic and electronic devices for future development.     

Self-Rectifying Effect in Resistive Switching Memory Using Amorphous InGaZnO

Resistance random access memory (ReRAM) has received attention as next-generation memory because of its excellent operating properties and high density integration capability as a crossbar array. However, the application of the existing ReRAM as a crossbar array may lead to crosstalk between adjacent cells due to its symmetric I–V characteristics. In this study, the self-rectifying effect of contact between amorphous In-Ga-Zn-O (a-IGZO) and TaO _x was examined in a Pt/a-IGZO/TaO _x /Al₂O₃/W structure. The experimental results show not only self-rectifying behavior but also forming-free characteristics. During the deposition of a-IGZO on the TaO _x , an oxygen-rich TaO _x interfacial layer was formed. The rectifying effect was observed regardless of the interface formation and is believed to be associated with Schottky contact formation between a-IGZO and TaO _x . The current level remained unchanged despite repeated DC sweep cycles. The low resistance state/high resistance state ratio was about 10¹ at a read voltage of ?0.5 V, and the rectifying ratio was about 10³ at ±2 V.     

Performance‐Enhancing Selector via Symmetrical Multilayer Design

Two‐terminal selectors with high nonlinearity, based on bidirectional threshold switching (TS) behaviors, are considered as a crucial element of crossbar integration for emerging nonvolatile memory and neuromorphic network. Although great efforts have been made to obtain various selectors, existing selectors cannot fully satisfy the rigorous standard of assorted memristive elements and it is in great demand to enhance the performance. Here, a new type of Ag/TaO_x/TaO_y/TaO_x/Ag (x < y) selector based on homogeneous trilayered oxides is developed to attain the required parameters including bidirectional TS operation, a large selectivity of ≈10¹⁰, a high compliance current up to 1 mA, and ultralow switching voltages under 0.2 V. Tunable operation voltages can be realized by modulating the thickness of inserted TaO_y. All‐TaO_x‐based integrated 1S1R (one selector and one memristor) cells, prepared completely by magnetron sputtering and no need of a middle electrode, exhibit a nonlinear feature, which is quite characteristic for the crossbar devices, avoiding undesired crosstalk current issues. The tantalum‐oxide‐based homojunctions offer high insulation, low ion mobility, and rich interfaces, which is responsible for the modulation of Ag conductive filaments and corresponding high‐performance cation‐based selector. These findings might advance practical implementation of two‐terminal selectors in emerging memories, especially resistive random access memories.     

Reliability/Uniformity improvement induced by an ultrathin TiO2 insertion in Ti/HfO2/Pt resistive switching memories

Reliability/Uniformity of resistance switching in Ti/HfO₂/Pt memory structure was improved by inserting an interfacial layer of 5 nm-thick TiO₂ between Ti and 45 nm-thick HfO₂. As a native oxide of Ti, TiO₂, effectively limits the disorder migration of oxygen from HfO₂ to Ti layer, and provides the more chemically-stable and morphologically-uniform interfaces with both the Ti electrode and the HfO₂ layer. Meanwhile, more stable resistive switching was observed in Ti/TiO₂/HfO₂/Pt than that in Ti/HfO₂/Pt memory, and the random variation during endurance test observed in Ti/HfO₂/Pt was also greatly limited in Ti/TiO₂/HfO₂/Pt memory. From these results, a thin TiO₂ insertion between the Ti electrode with the HfO₂ active layer, could greatly improve the reliability/uniformity of the Ti/HfO₂/Pt memory devices.     

Thermally Stable Transparent Resistive Random Access Memory based on All‐Oxide Heterostructures

An all‐oxide transparent resistive random access memory (T‐RRAM) device based on hafnium oxide (HfO_x) storage layer and indium‐tin oxide (ITO) electrodes is fabricated in this work. The memory device demonstrates not only good optical transmittance but also a forming‐free bipolar resistive switching behavior with room‐temperature R_OFF/R_ON ratio of 45, excellent endurance of ≈5 × 10⁷ cycles and long retention time over 10⁶ s. More importantly, the HfO_x based RRAM carries great ability of anti‐thermal shock over a wide temperature range of 10 K to 490 K, and the high R_OFF/R_ON ratio of ≈40 can be well maintained under extreme working conditions. The field‐induced electrochemical formation and rupture of the robust metal‐rich conductive filaments in the mixed‐structure hafnium oxide film are found to be responsible for the excellent resistance switching of the T‐RRAM devices. The present all‐oxide devices are of great potential for future thermally stable transparent electronic applications.     

Ferroelectricity-modulated resistive switching in Pt/Si:HfO2/HfO2-x/Pt memory

It is investigated for the effect of a ferroelectric Si:HfO₂ thin film on the resistive switching in a stacked Pt/Si:HfO₂/highly-oxygen-deficient HfO_2-x/Pt structure. Improved resistance performance was observed. It was concluded that the observed resistive switching behavior was related to the modulation of the width and height of a depletion barrier in the HfO_2-x layer, which was caused by the Si:HfO₂ ferroelectric polarization field effect. Reliable switching reproducibility and long data retention were observed in these memory cells, suggesting their great potential in non-volatile memories applications with full compatibility and simplicity.     

Resistive Random Access Memory Cells with a Bilayer TiO2/SiOX Insulating Stack for Simultaneous Filamentary and Distributed Resistive Switching

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO₂, SiO_X, and n⁺⁺Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle‐to‐cycle endurance (>1000 cycles), and low device‐to‐device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.     

The influence of crystallinity on the resistive switching behavior of TiO2

The resistive switching characteristics of Pt/TiO₂/Al devices were investigated. Amorphous and rutile TiO₂ samples were prepared and electrically characterized. The amorphous sample was sputtered at room temperature. The rutile phase of the TiO₂ was prepared by sputtering at elevated temperatures of 500 °C. Without an electroforming process, the amorphous sample showed bipolar resistive switching in low current mode. Switchable rectifying effects have been demonstrated in this mode. After applying the electroforming process on the rutile or amorphous device both samples exhibited a bipolar switching in high current mode. Amorphous titanium oxide showed low and high current mode bipolar switching in one device.     

Resistive Switching Memory Integrated with Nanogenerator for Self‐Powered Bioimplantable Devices

Resistive random access memory (ReRAM) devices powered by piezoelectric nanogenerators (NGs) have been investigated for their application to future implantable biomedical devices. Biocompatible (Na_0.5K_0.5)NbO₃ (NKN) films that are grown at 300 °C on TiN/SiO₂/Si and flexible TiN/Polyimide (TiN‐PI) substrates are used for ReRAM and NGs, respectively. These NKN films have an amorphous phase containing NKN nanocrystals with a size of 5.0 nm. NKN ReRAM devices exhibit typical bipolar switching behavior that can be explained by the formation and rupture of oxygen‐vacancy filaments. They have good ReRAM properties such as a large ratio of R_HRS to R_LRS as well as high reliability. The NKN film grown on flexible TiN‐PI substrate exhibits a high piezoelectric strain constant of 50 pm V^?1. The NKN NG has a large open‐circuit output voltage of 2.0 V and a short‐circuit output current of 40 nA, which are sufficient to drive NKN ReRAM devices. Stable switching properties with a large ON/OFF ratio of 10² are obtained from NKN ReRAM driven by NKN NG.     

A Polymer‐Electrolyte‐Based Atomic Switch

Studies on a resistive switching memory based on a silver‐ion‐conductive solid polymer electrolyte (SPE) are reported. Simple Ag/SPE/Pt structures containing polyethylene oxide–silver perchlorate complexes exhibit bipolar resistive switching under bias voltage sweeping. The switching behavior depends strongly on the silver perchlorate concentration. From the results of thermal, transport, and electrochemical measurements, it is concluded that the observed switching originates from formation and dissolution of a silver metal filament inside the SPE film caused by electrochemical reactions. This is the first report of an electrochemical “atomic switch” realized using an organic material. The devices also show ON/OFF resistance ratios greater than 10⁵, programming speeds higher than 1 μs, and retention times longer than 1 week. These results suggest that SPE‐based electrochemical devices might be suitable for flexible switch and memory applications.     

Materials and process aspect of cross-point RRAM (invited)

A survey of non-volatile and highly scalable cross-point memory in nanoscale resistive switching device is introduced. We present the basic operation of bipolar switching memory using combination between switching layer (HfO_x/PCMO) and oxygen reservoir layer (ZrO_x/AlO_x) and discuss the crucial issue for cross-point ReRAM. Based on the results, the applications of cross-point structure without any selection device were introduced. To evaluate the feasibility of cross-point ReRAM, read-out margin was calculated using PSPICE simulation. In addition, by the device scaling, three phenomena can be confirmed: (1) reset current reduction, (2) local heating effect and (3) significant improvement of uniformity.     

Low‐Dimensional Lead‐Free Inorganic Perovskites for Resistive Switching with Ultralow Bias

3D organic–inorganic and all‐inorganic lead halide perovskites have been intensively pursued for resistive switching memories in recent years. Unfortunately, instability and lead toxicity are two foremost challenges for their large‐scale commercial applications. Dimensional reduction and composition engineering are effective means to overcome these challenges. Herein, low‐dimensional inorganic lead‐free Cs₃Bi₂I₉ and CsBi₃I₁₀ perovskite‐like films are exploited for resistive switching memory applications. Both devices demonstrate stable switching with ultrahigh on/off ratios (≈10⁶), ultralow operation voltages (as low as 0.12 V), and self‐compliance characteristics. 0D Cs₃Bi₂I₉‐based device shows better retention time and larger reset voltage than the 2D CsBi₃I₁₀‐based device. Multilevel resistive switching behavior is also observed by modulating the current compliance, contributing to the device tunability. The resistive switching mechanism is hinged on the formation and rupture of conductive filaments of halide vacancies in the perovskite films, which is correlated with the formation of AgI_x layers at the electrode/perovskite interface. This study enriches the library of switching materials with all‐inorganic lead‐free halide perovskites and offers new insights on tuning the operation of solution‐processed memory devices.     

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.     

Effect of the top electrode material on the resistive switching of TiO2 thin film

Effect of the top electrode (TE) metal on the resistive switching of (TE)/TiO₂/Pt structure was investigated. It was confirmed that the potential barrier height between the metal and TiO₂ is an important factor on the resistive switching characteristics. When high Schottky barrier was formed with the TiO₂ film, using Pt or Au as a top electrode, both stable URS (unipolar) and BRS (bipolar resistive switching) characteristics were observed depending on the current compliance level. In the case of Ag, which forms a relatively low Schottky barrier, only BRS characteristics were observed, regardless of the current compliance level. In the case of Ni and Al, which have similar work function as Ag, unstable URS and BRS at very low current compliance levels were observed due to a chemical reaction at the interface. For the Ti electrode, resistive switching was not observed, because the work function of Ti is lower than that of TiO₂ and TiO phase was formed at the interface (Ti/TiO_x contact is ohmic).