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We fabricate a Pt/Ag:SiO x N y /Ti programmable metallization cell exhibiting bidirectional threshold switching (TS) or nonvolatile resistive switching (RS) characteristics through a simple thermal annealing process. The cell composed of pristine Ag:SiO x N y layers showed self-limiting TS characteristics with high selectivity and extremely low OFF currents, whereas the same cell showed typical RS characteristics after thermal annealing at 250 °C. The operating mechanism was investigated using scanning transmission electron microscopy and X-ray photoelectron spectroscopy. Next, a Ag:SiO x N y -based one selector–one resistor device was fabricated by employing both TS and RS characteristics in a single cell, which exhibited excellent self-rectifying memory performance.

The recent progress of selector and self-rectifying devices for resistive randomaccess memory applications is reviewed. In particular, the performance of crossbar arrays based on resistive switching (RS) devices, the sneak-path current issue, and possible solutions is discussed. The parameters and requirements of selector devices are elucidated here, and several types of selector devices, such as a transistor-assisted transistor-one resistor, unipolar one diode-one resistor, bipolar one selector-one resistor, and threshold switching selectors, are comprehensively discussed. In the case of self-rectifying devices, the recent progress in complementary RS devices, vacancy-modulated conductive oxide-based devices, and tunneling barrier-based RS devices is reviewed. The switching mechanisms and the geometrical configuration of the selector and self-rectifying RS devices are emphasized. Furthermore, comparative assessments of the different devices are evaluated. Finally, an overview of the gaps in previously reported devices is presented and some key improvements for future research direction suggested.

We investigate the intrinsic thermoelectric (TE) properties of the metal-diffused aluminum oxide (AO) layer in metal/AO/metal structures, where the metallic conducting filaments (CFs) were locally formed in the structures via an electrical breakdown (EBD) process as shown by resistive switching memory devices, by directly measuring cross-plane Seebeck coefficients on the CF-containing insulating AO layers. The results showed that the Seebeck coefficients of the CF-containing AO layer in metal/AO/metal structures were influenced by the generation of the metallic CFs, which is due to the diffusion of the metal into the insulating AO layers when exposed to a temperature gradient in the direction of the cross plane of the sample. In addition, the increase in the Seebeck coefficients of the CF-containing AO layer when the number of EBD-processed patterns was increased is satisfactorily explained by the low-energy carrier (i.e., minority carriers) filtering through the metal-oxide interfacial barriers in the metal/AO/metal structures.

Electrochemical metallization-based threshold switching devices with active metal electrodes have been studied as a selector for high-density resistive random access memory (RRAM) technology in crossbar array architectures. However, these devices are not suitable for integration with three-dimensional (3D) crossbar RRAM arrays due to the difficulty in vertical stacking and/or scaling into the nanometer regime as well as the asymmetric threshold switching behavior and large variation in the operating voltage. Here, we demonstrate bidirectional symmetric threshold switching behaviors from a simple Pt/Ag-doped HfO x /Pt structure. While fabricating the Pt/Ag-doped HfO x /Pt film using a 250 nm hole structure, filaments composed of Ag nanoclusters were constructed through a low-temperature (∼200 °C) hydrogen annealing process where the shape of the film in a nanoscale via a hole structure was maintained for integration with 3D stackable crossbar RRAM arrays. Finite Ag filament paths in the HfO x layer led to uniform device-to-device performances. Moreover, we observed that the hydrogen annealing process reduced the delay time through the reduction of the oxygen vacancies in the HfO x layer. Consequently, the proposed Pt/Ag-doped HfO x /Pt-based nanoscale selector devices exhibited excellent performance of high selectivity (∼105), ultralow OFF current (∼10 pA), steep turn-on slope (∼2 mV/decade), and short delay time (3 μs).