Xiaohan Wu

A study of the interfacial resistive switching mechanism by proton exchange reactions on the SiOx layer

In this work, we investigated SiO(x)-based interfacial resistive switching in planar metal-insulator-metal structures using physical/chemical/electrical analyses. This work helps clarify the interfacial reaction process and mechanism in SiO(x), and also shows the potential for high temperature operation in future nonvolatile memory applications.

Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides

Recently, two-dimensional (2D) atomic sheets have inspired new ideas in nanoscience including topologically protected charge transport,1,2 spatially separated excitons,3 and strongly anisotropic heat transport.4 Here, we report the intriguing observation of stable nonvolatile resistance switching (NVRS) in single-layer atomic sheets sandwiched between metal electrodes. NVRS is observed in the prototypical semiconducting (MX2, M = Mo, W; and X = S, Se) transitional metal dichalcogenides (TMDs),5 which alludes to the universality of this phenomenon in TMD monolayers and offers forming-free switching. This observation of NVRS phenomenon, widely attributed to ionic diffusion, filament, and interfacial redox in bulk oxides and electrolytes,6-9 inspires new studies on defects, ion transport, and energetics at the sharp interfaces between atomically thin sheets and conducting electrodes. Our findings overturn the contemporary thinking that nonvolatile switching is not scalable to subnanometre owing to leakage currents.10 Emerging device concepts in nonvolatile flexible memory fabrics, and brain-inspired (neuromorphic) computing could benefit substantially from the wide 2D materials design space. A new major application, zero-static power radio frequency (RF) switching, is demonstrated with a monolayer switch operating to 50 GHz.

Dynamic conductance characteristics in HfOx-based resistive random access memory

Characteristics of HfOx-based resistive switching memory (RRAM) in Al/HfOx/Al and Al/AlOx/HfOx/Al structures were studied using a dynamic conductance method. Step-like RESET behaviors as well as pre- and post-RESET regions of operation were characterized. The results indicated that defects at the oxide interface caused cycling issues in the Al/AlOx/HfOx/Al structure. No such RESET behavior was observed for the Al/HfOx/Al structure. Current induced over-heating, which caused an early RESET event, could be avoided using current-sweep technique that caused less electrical and thermal stress in localized regions. The experimental results not only provided insights into potential reliability issues and power management in HfOx-based RRAM, but also helped clarifying the resistive switching mechanisms.

A universal model for interface-type threshold switching phenomena by comprehensive study of Vanadium oxide-based selector

For the first time, a comprehensive study of Vanadium oxide-based selector characteristics with a universal model observed by thermal and electrical induced threshold switching (TS) phenomena at interface is presented in this work. The model can explain that the resistance evolution by thermal temperature in TS behaviors, as well as the resistance gradually increases with cycling (“seasoning effect”). Compatible current density (107∼109 A/cm2) and selectivity (∼100) with physical understanding of evolution in energy barrier and MIT metallic state modulation are studied. The results show a promising design guideline for future storage-class memory (SCM) applications.

Comprehensive study of intrinsic unipolar SiOx-based ReRAM characteristics in AC frequency response and low voltage (< 2V) operation

Intrinsic unipolar SiOx-based Resistive-RAM (ReRAM) characteristics have been investigated. The cross-bar MIM structures have been examined under AC frequency response, by varying device area, temperature and current states. The results provide additional insights into the hopping/switching mechanisms. For the first time, by using SiOx/HfOx stacking engineering, we have developed a low-voltage operation (<; 2V) for SiOx-based ReRAM. The SiOx/HfOx stacking optimization not only maintains the RS behaviors even in air environment without any programming window degradation, but also further reduces the switching voltage below 2V.

Characteristics of Nb-doped SrTiO 3 and HfO 2 -based selector devices

Recently, resistive random access memory (RRAM) using various metal oxides (i.e., SiO 2 [1], HfO 2 , NiO[2], Al 2 O 3 , NbO) have attracted a lot of attentions since the current nonvolatile memory (NVM) approaching the scaling limits. Meanwhile, the selector devices are essential to address the sneak path issue which causing the reading errors in high-packing-density cross-bar RRAM array. Several types of selector devices with threshold switching (TS) behavior has been investigated, such as rectifying diode[3], varistors[4], ovonic TS (OTS)[5], metal-insulator transition (MIT)[6] etc. In this work, the characteristics of electrically driven MIT (E-MIT) in both Nb-doped SrTiO 3 and HfO 2 -based selector devices has been investigated.

Zero-static power radio-frequency switches based on MoS 2 atomristors

Recently, non-volatile resistance switching or memristor (equivalently, atomristor in atomic layers) effect was discovered in transitional metal dichalcogenides (TMD) vertical devices. Owing to the monolayer-thin transport and high crystalline quality, ON-state resistances below 10 Ω are achievable, making MoS2 atomristors suitable as energy-efficient radio-frequency (RF) switches. MoS2 RF switches afford zero-hold voltage, hence, zero-static power dissipation, overcoming the limitation of transistor and mechanical switches. Furthermore, MoS2 switches are fully electronic and can be integrated on arbitrary substrates unlike phase-change RF switches. High-frequency results reveal that a key figure of merit, the cutoff frequency (fc), is about 10 THz for sub-μm2 switches with favorable scaling that can afford fc above 100 THz for nanoscale devices, exceeding the performance of contemporary switches that suffer from an area-invariant scaling. These results indicate a new electronic application of TMDs as non-volatile switches for communication platforms, including mobile systems, low-power internet-of-things, and THz beam steering.The wide application of wireless communications in various technologies calls for the development of robust yet compact radio-frequency switches. Here, Kim et al. utilize MoS2 based non-volatile memristors to switch up to THz frequencies in sub µm2 areas, whilst the switches consume zero-static energy.

Resistive switching characteristics and mechanisms in silicon oxide memory devices

Abstract Intrinsic unipolar SiOx-based resistance random access memories (ReRAM) characterization, switching mechanisms, and applications have been investigated. Device structures, material compositions, and electrical characteristics are identified that enable ReRAM cells with high ON/OFF ratio, low static power consumption, low switching power, and high readout-margin using complementary metal-oxide semiconductor transistor (CMOS)–compatible SiOx-based materials. These ideas are combined with the use of horizontal and vertical device structure designs, composition optimization, electrical control, and external factors to help understand resistive switching (RS) mechanisms. Measured temperature effects, pulse response, and carrier transport behaviors lead to compact models of RS mechanisms and energy band diagrams in order to aid the development of computer-aided design for ultralarge-v scale integration. This chapter presents a comprehensive investigation of SiOx-based RS characteristics and mechanisms for the post-CMOS device era.