Design of selector-based insulator-metal transition model for TiO2 bipolar resistive random access memory

Abstract

In this study, a NbO2-based selector was designed that can change high resistance states to low resistance states due to the insulator–metal transition (IMT). A one-selector-one-resistor cell for a 3D crossbar array composed of this selector and a TiN/TiO2/TiN bipolar resistive random access memory (RRAM) in series was modeled using the COMSOL finite element multiphysics software package. First, the temperature dependencies of the electrical conductivity (σ), thermal conductivity (kth), and mass specific heat (CP) were used to compare two IMT selectors, which showed that the NbO2-based selector better matched the RRAM due to its appropriate hysteresis width (hw), threshold voltage (Vth), and low off-state current (Ioff). Second, the effect of size variations for the NbO2 on the electrical performance of the IMT selector was investigated. Vth, hold voltage (Vhold), threshold current (Ith), Ioff, and Ith/Ioff ratio of the IMT selector were sensitive to thickness (L) and filament radius (r) of NbO2. L and r were designed by comparing the electrical characteristics of the NbO2 selectors of different sizes to make a connection with the TiO2-based bipolar RRAM. Finally, a TiN/NbO2/TiN/TiO2/TiN stack structure was simulated, and the results show a significantly reduced leakage current (1 μA), high read window (15.8), and improved readout margin (>27 word lines), which are suitable for next-generation high-density memory devices. This study provides guidance for the design of selectors connected to the RRAM, which is beneficial for circuit designs and process manufacturing.