In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges.

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Status and Prospects of ZnO-Based Resistive Switching Memory Devices

Reversible chemical and structural changes induced by ionic motion and reaction in response to electrical stimuli leads to resistive switching effects in metal-insulator-metal structures. Filamentary switching based on the formation and rupture of nanoscale conductive filament has been applied in non-volatile memory and volatile selector devices with low power consumption and fast switching speeds. Before the mass production of resistive switching devices, great efforts are still required to enable stable and reliable switching performances. The conductive filament, a bridge of microscopic metal-insulator-metal structure and macroscopic resistance states, plays an irreplaceable part in resistive switching behavior, as unreliable performance often originates from unstable filament behavior. In this Review, departing from the filamentary switching mechanism and the existing issues, recent advances of the switching performance improvement through the conductive filament modulation are discussed, in the sequence of material modulation, device structure design and switching operation scheme optimization. In particular, two-dimensional (2D) nanomaterials with excellent properties including and beyond graphene, are discussed with emphasis on performance improvement by their active roles as the switching layer, insertion layer, thin electrode, patterned electrode, and edge electrode, etc.

Resistive Switching Performance Improvement via Modulating Nanoscale Conductive Filament, Involving the Application of Two-Dimensional Layered Materials.

The ability for artificially reproducing human brain type signals' processing is one of the main challenges in modern information technology, being one of the milestones for developing global communicating networks and artificial intelligence. Electronic devices termed memristors have been proposed as effective artificial synapses able to emulate the plasticity of biological counterparts. Here we report for the first time a single crystalline nanowire based model system capable of combining all memristive functions - non-volatile bipolar memory, multilevel switching, selector and synaptic operations imitating Ca2+ dynamics of biological synapses. Besides underlying common electrochemical fundamentals of biological and artificial redox-based synapses, a detailed analysis of the memristive mechanism revealed the importance of surfaces and interfaces in crystalline materials. Our work demonstrates the realization of self-assembled, self-limited devices feasible for implementation via bottom up approach, as an attractive solution for the ultimate system miniaturization needed for the hardware realization of brain-inspired systems.

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Self-limited single nanowire systems combining all-in-one memristive and neuromorphic functionalities

Recent Developments and Perspectives for Memristive Devices Based on Metal Oxide Nanowires

A Review of Resistive Switching Devices: Performance Improvement, Characterization, and Applications

Resistive random access memory (RRAM) is one of the most promising nonvolatile memory technologies because of its high potential to replace traditional charge-based memory, which is approaching its scaling limit. To fully realize the potential of the RRAM, it can be important to develop a unique device with current self-rectification, which provides a solution to suppress sneak current in crossbar arrays, and self-compliance, which eliminates the current limiter. In this paper, self-rectifying resistive switching is demonstrated in Ga-doped ZnO single nanowire devices; the current is not only self-rectifying but also self-compliance for Sb-doped single nanowire devices. Multilevel resistive switching has also been achieved for Sb-doped ZnO single nanowire devices by using different SET voltages. Furthermore, the doping of Ga and Sb narrows the switching voltage distribution greatly.

Resistive switching in Ga- and Sb-doped ZnO single nanowire devices

Write-once-read-many-times memory (WORM) devices were fabricated using ZnO and ZnO/MgO as active layers on Si. Devices fabricated with ZnO show a different memory effect at different current compliances such as WORM at 100 $\mu \text{A}$ , 500 $\mu \text{A}$ , and 1 mA, resistive switching (RS) instead of WORM at 5 and 10 mA, and WORM and RS coexisting at 20, 50, and 100 mA, while devices fabricated with ZnO/MgO show WORM only at all current compliances. A few nanometers of MgO layer play a major role in preventing devices from reset at all current compliances because the much lower drift velocity of oxygen vacancy in MgO and accumulation of negatively charged O2− ions at the interface between ZnO and MgO prevent the conducting filaments composed of oxygen vacancies from breaking.

Bo Wang

Papers

Resistive switching in Ga- and Sb-doped ZnO single nanowire devices

Metal/ZnO/MgO/Si/Metal Write-Once-Read-Many-Times Memory