Nanoscale 3D Stackable Ag-Doped HfO _x-Based Selector Devices Fabricated through Low-Temperature Hydrogen Annealing

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).