Hangbing Lv

Real‐Time Observation on Dynamic Growth/Dissolution of Conductive Filaments in Oxide‐Electrolyte‐Based ReRAM

Evolution of growth/dissolution conductive filaments (CFs) in oxide-electrolyte-based resistive switching memories are studied by in situ transmission electron microscopy. Contrary to what is commonly believed, CFs are found to start growing from the anode (Ag or Cu) rather than having to reach the cathode (Pt) and grow backwards. A new mechanism based on local redox reactions inside the oxide-electrolyte is proposed.

Controllable Growth of Nanoscale Conductive Filaments in Solid-Electrolyte-Based ReRAM by Using a Metal Nanocrystal Covered Bottom Electrode

Resistive memory (ReRAM) based on a solid-electrolyte insulator is a promising nanoscale device and has great potentials in nonvolatile memory, analog circuits, and neuromorphic applications. The underlying resistive switching (RS) mechanism of ReRAM is suggested to be the formation and rupture of nanoscale conductive filament (CF) inside the solid-electrolyte layer. However, the random nature of the nucleation and growth of the CF makes their formation difficult to control, which is a major obstacle for ReRAM performance improvement. Here, we report a novel approach to resolve this challenge by adopting a metal nanocrystal (NC) covered bottom electrode (BE) to replace the conventional ReRAM BE. As a demonstration vehicle, a Ag/ZrO(2)/Cu NC/Pt structure is prepared and the Cu NC covered Pt BE can control CF nucleation and growth to provide superior uniformity of RS properties. The controllable growth of nanoscale CF bridges between Cu NC and Ag top electrode has been vividly observed by transmission electron microscopy (TEM). On the basis of energy-dispersive X-ray spectroscopy (EDS) and elemental mapping analyses, we further confirm that the chemical contents of the CF are mainly Ag atoms. These testing/metrology results are consistent with the simulation results of electric-field distribution, showing that the electric field will enhance and concentrate on the NC sites and control location and orientation of Ag CFs.

Reproducible unipolar resistance switching in stoichiometric ZrO2 films

The resistance switching characteristics of stoichiometric ZrO2 film were investigated for nonvolatile memory. The Al∕ZrO2∕Al device presents reliable and reproducible switching behaviors. The on/off ratio of two stable states is larger than 2×103. It is suggested that the current-voltage characteristics are governed by the Schottky conduction mechanism in high voltage region, while the filament conduction is suggested in low voltage region. The switching process is explained in terms of the spontaneous reversible reaction between electrode and ZrO2 films with the contribution of Joule heating effect by the external current. It provides a possible solution for low device yield of nonstoichiometric oxides.

Endurance enhancement of Cu-oxide based resistive switching memory with Al top electrode

We investigated the switching performance of Cu-oxide films with Al, Pt, and Ti electrodes. Compared with Pt electrode, the Al electrode shows better stability, preferable endurance, and larger resistance ratio. An interface AlOx layer is detected by transmission electron microscopy and Auger electron spectroscopy. This layer can strongly affect the movement of oxygen vacancies. However, the sample with pure Ti electrode almost has no switching characteristics. Ti/TiN electrode with thin Ti exhibits good switching behavior. The thickness control of Ti layer is quite critical. So we suggest that the oxygen diffusion in electrode is another important factor for switching performance.We investigated the switching performance of Cu-oxide films with Al, Pt, and Ti electrodes. Compared with Pt electrode, the Al electrode shows better stability, preferable endurance, and larger resistance ratio. An interface AlOx layer is detected by transmission electron microscopy and Auger electron spectroscopy. This layer can strongly affect the movement of oxygen vacancies. However, the sample with pure Ti electrode almost has no switching characteristics. Ti/TiN electrode with thin Ti exhibits good switching behavior. The thickness control of Ti layer is quite critical. So we suggest that the oxygen diffusion in electrode is another important factor for switching performance.

Resistive Memory Switching of

Poly crystalline CuxO films produced by plasma oxidation are investigated for nonvolatile memory applications. Reversible bistable resistive switching from a high-resistance state to a low-resistance state, and vice versa, is observed in an integrated Al/CuxO/Cu structure under voltage sweeping. More than 3000 repetitive cycles are observed in 180-mum memory devices with an on/off ratio of ten times. Data testing shows that the devices meet the ten-year retention requirement for the storage of programmed logic signals.

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For the first time, we report that the copper oxide (CuxO) based resistive random access memory (RRAM) cell can achieve 104 cycles (i.e. ~10x better) as a new record as well as elimination of the initial ¿forming¿ than reported in literature. The copper oxide is integrated in MIM (metal-insulator-metal) structure and is grown by plasma oxidation of Cu substrate, with CuO near upper surface and graded CuxO (i.e. increasingly Cu rich or O-vacancies rich) toward the Cu substrate. A thinner CuO upper layer can eliminate ¿forming¿ process and, for in turn, greatly enhance the endurance of resistive switching by eliminating the damage during the ¿forming¿ process.

Films for a Nonvolatile Memory Application

The electrochemical metallization cell, also referred to as conductive bridge random access memory, is considered to be a promising candidate or complementary component to the traditional charge based memory. As such, it is receiving additional focus to accelerate the commercialization process. To create a successful mass product, reliability issues must first be rigorously solved. In-depth understanding of the failure behavior of the ECM is essential for performance optimization. Here, we reveal the degradation of high resistance state behaves as the majority cases of the endurance failure of the HfO2 electrolyte based ECM cell. High resolution transmission electron microscopy was used to characterize the change in filament nature after repetitive switching cycles. The result showed that Cu accumulation inside the filament played a dominant role in switching failure, which was further supported by measuring the retention of cycle dependent high resistance state and low resistance state. The clarified physical picture of filament evolution provides a basic understanding of the mechanisms of endurance and retention failure, and the relationship between them. Based on these results, applicable approaches for performance optimization can be implicatively developed, ranging from material tailoring to structure engineering and algorithm design.

Enhancement of endurance for Cu x O based RRAM cell

In this letter, we dynamically investigate the resistive switching characteristics and physical mechanism of the Ni/ZrO2/Pt device. The device shows stable bipolar resistive switching behaviors after forming process, which is similar to the Ag/ZrO2/Pt and Cu/ZrO2/Pt devices. Using in situ transmission electron microscopy, we observe in real time that several conductive filaments are formed across the ZrO2 layer between Ni and Pt electrodes after forming. Energy-dispersive X-ray spectroscopy results confirm that Ni is the main composition of the conductive filaments. The ON-state resistance increases with increasing temperature, exhibiting the feature of metallic conduction. In addition, the calculated resistance temperature coefficient is equal to that of the 10–30 nm diameter Ni nanowire, further indicating that the nanoscale Ni conductive bridge is the physical origin of the observed conductive filaments. The resistive switching characteristics and the conductive filaments component of Ni/ZrO2/Pt device are consistent with the characteristics of the typical solid-electrolyte-based resistive random access memory. Therefore, aside from Cu and Ag, Ni can also be used as an oxidizable electrode material for resistive random access memory applications.

Evolution of conductive filament and its impact on reliability issues in oxide-electrolyte based resistive random access memory

In this letter, the resistive random access memory (RRAM) with metal-insulator-metal structure is investigated for the first time under radiation conditions. The fabricated Cu-doped HfO₂-based RRAM devices are found to have immunity from ⁶⁰Co γ ray of various dose ranges. The basic RRAM parameters such as high-resistance state, low-resistance state, SET/RESET voltages, operation speed, and endurance have nearly no degradation after ⁶⁰Co γ ray treatment with a total dose as high as 3.6 × 10⁵ rad (Si). Furthermore, a retention characteristic of 10⁵ s is also achieved during radiation. The highly stable characteristics of Cu-doped HfO₂ -based RRAM devices under radiation provide RRAM a great potential for aerospace and nuclear applications.

In situ observation of nickel as an oxidizable electrode material for the solid-electrolyte-based resistive random access memory

Negative-SET behavior is observed in various cation-based memories, which degrades the device reliability. Transmission electron microscopy results demonstrate the behavior is caused by the overgrowth of the conductive filament (CF) into the Pt electrode. The CF overgrowth phenomenon is suppressed and the negative-SET behavior is eliminated by inserting an impermeable graphene layer. The graphene-based devices show high reliability and satisfying performance.