Excellent Switching Uniformity Research Articles

Carrier-doping-driven insulator-metal transition in disordered materials for memristive switching with high uniformity

Attaining highly uniform operations in a disordered system presents a persistent challenge. The utilization of ion migration in amorphous materials to trigger the resistive switching process of the material usually results in inferior uniformity of the memristive device. Here, we demonstrate that the resistive switching behavior can be activated through carrier doping in the disorder system, and highly ordered resistance modulation is achieved in Ag-doped albumen. By manipulating the doping level of the carrier, the localization of the free electron wavefunction can be tuned, leading to multi-level variations in resistance. This memristive switching behavior is in all electronic and displays excellent switching uniformity, holding great potential for applications in high-density memories and neuromorphic computing chips.

Improving the selector characteristics of ovonic threshold switch via UV treatment process

In this study, we investigated the influence of ultraviolet (UV) treatment on the ovonic threshold switch (OTS) to improve its selector properties. Our findings demonstrate that iteratively applying UV treatment during the film deposition phase considerably improves device characteristics compared to a single UV treatment. Consequently, this process provided a significant decrease in the forming voltage, maintaining outstanding switching features, with an off-state current of approximately 2 nA. Furthermore, the refined UV treatment process resulted in an impressive 45% improvement in threshold voltage drift characteristics and facilitated excellent switching uniformity. X-ray photoelectron spectroscopy analysis revealed alterations in the bonding structure of the Si–Te–As–Ge film after UV exposure. Specifically, a transition was observed from unstable homopolar bonds, such as As-As or Te–Te, to their more stable heteropolar equivalents, such as As–Te. These results highlight the potential of UV treatment as an effective method for enhancing the OTS performance.

Enhanced Switching Characteristics of an Ovonic Threshold Switching Device With an Ultra-Thin MgO Interfacial Layer

In this study, the effect of a MgO interfacial layer on the electrical characteristics of an ovonic threshold switching (OTS) device was investigated. Compared to the control OTS device, the device with the MgO interfacial layer showed excellent switching uniformity and ultralow off-state leakage current (<inline-formula> <tex-math notation="LaTeX">$\sim 440$ </tex-math></inline-formula>pA) without noticeable changes in the electrical forming process. By extracting the activation energy before and after the forming process, we confirmed that the enhanced switching characteristics were obtained by reducing the interaction between the OTS film and electrodes. Furthermore, the 1S-1R operation with TaO_x-based resistive random access memory (ReRAM) showed successful suppression of the leakage current, which is suitable for a cross-point memory array application.

Homo-layer hafnia-based memristor with large analog switching window

The fast development of high-accuracy neuromorphic computing requires stable analog memristors. While filamentary memory switching is very common in binary oxides, their resistive switching usually involves abrupt changes due to the rupture or reformation of metallic filaments. In this work, we designed a memristor consisting of dual-layer HfOy/HfOx, with different concentrations of oxygen vacancies (y &amp;gt; x). During the electroforming process, both the migration of existing oxygen vacancies in HfOx and the generation of new oxygen vacancies in HfOy occur simultaneously, leaving a semiconducting part close to the HfOy/HfOx interface. The resulting filament is not metallic as a whole, as revealed by first principles calculations. Such a device demonstrates excellent switching uniformity as well as highly gradual resistance change, ideal for neuromorphic computing. Through fine tuning of the filament structure, the device achieves low variation, high speed, gradual SET and RESET processes, and hundreds of stable multi-level state behaviors. The long-term synaptic plasticity was further achieved, showing good linearity and large analog switching window (ΔG as high as 487.5 μS). This works affords a route toward a gradual resistance change in oxide-based memristors.

Improved Turn-Off Speed and Uniformity of Atomic Threshold Switch Device by AgSe Electrode and Bipolar Pulse Forming

To improve the turn-off speed and uniformity of atomic threshold switching (TS) devices, we propose the use of the AgSe electrode and controlled bipolar pulse forming method. Compared with TS devices with Ag and AgTe electrodes, TS device with the AgSe electrode shows an extremely fast turn-off speed, which can be explained by the limited injection of Ag into the switching layer. By applying positive bias (Icc = 500 nA, 1 ms) followed by negative bias (−0.1 to −0.2 V, $10~\mu \text{s}$ ), the TS device exhibits excellent switching uniformity. It can be explained by the formation of atomic-scale filament under the positive bias at low Icc and drift-back of excessive Ag by the negative bias. By designing the shape of the filament and concentration of the residual Ag, the TS device with the AgSe electrode shows promise for selector applications.

Realization of resistive and magnetization switching in sol-gel derived yttrium iron garnet thin films

In this work, large-area ferrite thin films of ferromagnetic yttrium iron garnet (Y3Fe5O12, YIG) were synthesized on Pt/Ti/SiO2/Si (Pt) substrates by a sol-gel method to investigate the resistive and magnetization switching properties. The synthesized YIG thin films acquire a single garnet structure. The Pt/YIG/Pt stack illustrates unipolar resistive switching behavior with excellent switching uniformity, large memory window (102), stable cycle-to-cycle endurance, and good data storage retention (104 s). The ~46% saturation magnetization variation could be realized via the conversion between high and low resistance states by manipulating the electric field. Schottky emission is governed in the high-field region for the high resistance state. Temperature dependence of resistance and magnetization variation confirms that oxygen vacancies conductive filament model and valence state change (Fe2+ and Fe3+) are responsible for the resistive and magnetization switching mechanisms. These results indicate that YIG ferrite based stack is suitable to design the electro-magnetic coupling multifunctional nonvolatile memory devices.

AlxTe1−x selector with high ovonic threshold switching performance for memory crossbar arrays

We report the fabrication of a bidirectional selector based on threshold switching (TS) material AlxTe1−x. By modulating the composition and the thickness of the AlxTe1−x film layer, an optimized bidirectional selector with the advantages of being electroforming-free, with sufficient operating current (1 mA), satisfactory selectivity (ca. 5.9 × 103), appropriately small threshold voltage (ca. ±0.7 V), and excellent switching uniformity was fabricated. The trap-limited conduction model was employed to explain the TS characteristics of the W/AlxTe1−x/W device. The application of a high electric field to the devices is considered to induce the tunneling of the high-electric field-derived carriers from deep traps to shallow traps, switching the device to the on-state.

Influence of argon and oxygen pressure ratio on bipolar-resistive switching characteristics of CeO2−x thin films deposited at room temperature

Cerium oxide (CeO2−x) film was deposited on Pt/Ti/SiO2/Si substrate by rf magnetron sputtering at room temperature. Resistive switching characteristics of these ceria films have been improved by increasing oxygen content during deposition process. Endurance and statistical analyses indicate that the operating stability of CeO2−x-based memory is highly dependent on the oxygen content. Results indicate that CeO2−x film-based RRAM devices exhibit optimum performance when fabricated at an argon/oxygen ratio of 6:24. An increase in the oxygen content introduced during CeO2−x film deposition not only stabilizes the conventional bipolar RS but also improves excellent switching uniformity such as large ON/OFF ratio (102), excellent switching device-to-device uniformity and good sweep endurance over ~ 500 repeated RS cycles. Conduction in the low-resistance state (LRS) as well as in the low bias field region in the high-resistance state (HRS) is found to be Ohmic and thus supports the conductive filament (CF) theory. In the high voltage region of HRS, space charge limited conduction (SCLC) and Schottky emission are found to be the dominant conduction mechanisms. A feasible filamentary RS mechanism based on the movement of oxygen ions/vacancies under the bias voltage has been discussed.

Excellent selector performance in engineered Ag/ZrO2:Ag/Pt structure for high-density bipolar RRAM applications

A high-performance selector with bidirectional threshold switching (TS) characteristics of Ag/ZrO2/Pt structure was prepared by incorporating metallic Ag into the ZrO2 matrix. The bidirectional TS device exhibited excellent switching uniformity, forming-free behavior, ultra-low off current of &amp;lt;1 nA and adjustable selectivity (from 102 to 107). The experiment results confirmed that metallic Ag clusters were penetrated into the ZrO2 matrix during the annealing process, which would function as an effective active source responsible for the bidirectional TS. The volatile behavior could be explained by the self-dissolution of unstable filaments caused by minimization of the interfacial energy and thermal effect. Furthermore, a bipolar-type one selector-one resistor (1S-1R) memory device was successfully fabricated and exhibited significant suppression of the undesired sneak current, indicating the great potential as selector in a cross-point array.

Low-cost fabrication and polar-dependent switching uniformity of memory devices using alumina interfacial layer and Ag nanoparticle monolayer

A facile and low-cost process was developed for fabricating write-once-read-many-times (WORM) Cu/Ag NPs/Alumina/Al memory devices, where the alumina passivation layer formed naturally in air at room temperature, whereas the Ag nanoparticle monolayer was in situ prepared through thermal annealing of a 4.5 nm Ag film in air at 150°C. The devices exhibit irreversible transition from initial high resistance (OFF) state to low resistance (ON) state, with ON/OFF ratio of 107, indicating the introduction of Ag nanoparticle monolayer greatly improves ON/OFF ratio by four orders of magnitude. The uniformity of threshold voltages exhibits a polar-dependent behavior, and a narrow range of threshold voltages of 0.40 V among individual devices was achieved upon the forward voltage. The memory device can be regarded as two switching units connected in series. The uniform alumina interfacial layer and the non-uniform distribution of local electric fields originated from Ag nanoparticles might be responsible for excellent switching uniformity. Since silver ions in active layer can act as fast ion conductor, a plausible mechanism relating to the formation of filaments sequentially among the two switching units connected in series is suggested for the polar-dependent switching behavior. Furthermore, we demonstrate both alumina layer and Ag NPs monolayer play essential roles in improving switching parameters based on comparative experiments.

Enhanced switching uniformity in AZO/ZnO1−x/ITO transparent resistive memory devices by bipolar double forming

The influence of single and double forming on the switching stability of AZO/ZnO1−x/ITO transparent resistive memory devices was investigated. Devices that underwent single forming exhibited severe switching instability, where as those that underwent double forming exhibited excellent switching uniformity. The quantity of conducting filaments can be limited by applying the two-step forming process. Consequently, the set/reset process can be controlled, enhancing switching stability. Satisfactory endurance with an acceptable ON/OFF ratio of 102 and satisfactory retention behavior of 104 s at room temperature confirmed the reliability of optimized devices. Furthermore, highly transparent devices (transparency of approximately 85% in visible range) have been fabricated.

The band-gap energy dependence of metal oxides on non-linear characteristics in the HfO2-based resistive random access memory

In this paper, the influence of the band-gap energy of metal oxide layers on the non-linearity of the device had been investigated. The band-gap energy of the metal oxide layer determines barrier height of tunneling between metal oxides and electrodes for tunneling mechanisms. The optimum barrier height between the metal oxides and electrodes exhibits high non-linear characteristics of the device for low leakage current of the cross-point array applications with excellent switching uniformity.

3D integration of planar crossbar memristive devices with CMOS substrate

Planar memristive devices with bottom electrodes embedded into the substrates were integrated on top of CMOS substrates using nanoimprint lithography to implement hybrid circuits with a CMOL-like architecture. The planar geometry eliminated the mechanically and electrically weak parts, such as kinks in the top electrodes in a traditional crossbar structure, and allowed the use of thicker and thus less resistive metal wires as the bottom electrodes. Planar memristive devices integrated with CMOS have demonstrated much lower programing voltages and excellent switching uniformity. With the inclusion of the Moiré pattern, the integration process has sub-20 nm alignment accuracy, opening opportunities for 3D hybrid circuits in applications in the next generation of memory and unconventional computing.

Highly uniform and reliable resistance switching properties in bilayer WOx/NbOx RRAM devices

AbstractMemory performances, especially uniformity and reliability of resistance random access memory (RRAM) devices with W/NbOx/Pt structures were investigated. Scaling down the active device area (ø = 250 nm) can significantly minimize extrinsic defects related to nonuniform switching and also demonstrate low‐voltage SET/RESET operations due to increased Joule heating. Electromigration of oxygen ions under the bipolar electric field, bilayer formation, and lightning‐rod effect localized at WOx/NbOx interface can explain the improved switching behavior in this novel stack. Excellent device characteristics such as lower switching voltage, fast switching speed (100 ns), high‐temperature retention (&gt;104 s, 85 °C), stable cycling endurance (107 cycles), almost 100% device yield and excellent switching uniformity are obtained.

Excellent Selector Characteristics of Nanoscale $ \hbox{VO}_{2}$ for High-Density Bipolar ReRAM Applications

We herein present a nanoscale vanadium oxide (VO2) device with excellent selector characteristics such as a high on/off ratio (>; 50), fast switching speed ( ; 106 A/cm2). Owing to extrinsic defects, a large-area device with a 20-nm-thick VO2 layer underwent an electrical short. In contrast, after scaling the device active area (<; 5 × 104 nm2), excellent switching uniformity was obtained. This can be explained by the reduced defects and the metal-insulator transition of the whole nanoscale VO2. By integrating a bipolar resistive random access memory device with the VO2 selection device, a significantly improved readout margin was obtained. The VO2 selection device shows good potential for cross-point bipolar resistive memory applications.

Resistive-Switching Characteristics of $\hbox{Al}/ \hbox{Pr}_{0.7}\hbox{Ca}_{0.3}\hbox{MnO}_{3}$ for Nonvolatile Memory Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A systematic study on the switching mechanism of an <formula formulatype="inline"><tex Notation="TeX">$\hbox{Al}/ \hbox{Pr}_{0.7}\hbox{Ca}_{0.3}\hbox{MnO}_{3}$</tex></formula> (PCMO) device was performed. A polycrystalline PCMO film was deposited using a conventional sputtering method. A thin Al layer was introduced to induce a reaction with the PCMO, forming aluminum oxide <formula formulatype="inline"><tex Notation="TeX">$(\hbox{AlO}_{x})$</tex></formula>. Transmission electron microscopy analysis of the interface between Al and PCMO showed that resistive switching was governed by the formation and dissolution of <formula formulatype="inline"><tex Notation="TeX">$\hbox{AlO}_{x}$</tex></formula>. Some basic memory characteristics, such as good cycle endurance and data retention of up to <formula formulatype="inline"><tex Notation="TeX"> $\hbox{10}^{4}$</tex></formula> s at 125 <formula formulatype="inline"><tex Notation="TeX">$^{\circ}\hbox{C}$</tex></formula>, were also obtained. It also showed excellent switching uniformity and high device yield. </para>