Thermally robust HfN_x-based bidirectional diode and its integration with RRAM for crossbar array application

High-density nonvolatile memories can be realized by fabricating resistive random-access memories (RRAMs) as 4F² (F: minimum feature size) cells in a crossbar array (CBA) configuration. However, the CBA configuration induces leakage currents in the nearby cells, which significantly reduces the read-out margin and limits the CBA size extension. One solution to this problem is to integrate an RRAM with a bidirectional selector. Research on the various type of selector devices has been extensively explored so far, however, a device that satisfies all requirements has not been developed. In this paper, we propose a novel bidirectional diode selector with a Pt/HfN_x/Pt stack. We implement a one-selector–one-resistor configuration in a 10 × 10 CBA by integrating Pt/HfN_x/Pt with a Ti/HfO₂/Pt RRAM stack. We confirm the stable resistive switching operation and good thermal stability of the selector and its integrated device. Chemical analyses aided by Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal the gradient-oxidation of the HfN_x film, which explains the nonlinear and asymmetric current–voltage characteristics. This novel nonlinear selector with simple structure and high thermal stability can be applied to build CBAs for high-density nonvolatile memories or neuromorphic computing elements, in the future.