Selector devices/architectures for ReRAM crossbar arrays

Abstract

Resistive Random Access Memories (ReRAMs) are amongst the most promising next generation memory technologies, due to their small feature size, low power consumption and capacity to be integrated in 3D when arranged in crossbar array. ReRAMs harvest the potential of the resistive switching phenomenon that allows to reversibly change their internal resistive state, from High to Low. This aspect, however, is also a problem when ReRAM cells are tesserated in crossbar array configuration: elements that are programmed in their low resistive state can create low resistance sneak paths in the array fabric making the readout operations strenuous. This challenge is usually mitigated by the use of selector devices. A selector must be able to suppress the currents owing in the sneak paths and, at the same time, allow for ReRAM reliable operations.

In this work, the sneak current problem is introduced and defined. Several solutions that have been studied in literature are presented and discussed, creating a solid background for the development of this work. Upon this strong basis, different selector devices technologies were developed. Ni/TiO₂/Ni selector devices were first designed, studied and benchmarked against other technologies showing improved performances in term of Voltage Margin without any adverse effect on the maximum current supplied. As this device stack has been designed to have a similar metal-insulator-metal architecture to ReRAM devices, the two were monolithically integrated and characterized.

The possibility to exploit volatile resistive switching was also introduced and discussed thoroughly, starting from a TiO₂/NiO stack exhibiting a promising threshold behaviour for selectors operations, fostering interests of the community on this approach. The limitations of this approach were studied via SPICE simulations, setting the framework and requirements for their implementation in crossbars: through this ongoing work, it was realised that the selector technologies should not be evaluated in isolation from ReRAM, which has led to contributions towards forming-free ReRAM devices. This research stream employed a TiO₂/AlO_y bi-layer stack devices able to perform resisting switching with a 2-orders of magnitude ON/OFF ratio and low cycle-to-cycle variability without requiring electroforming: this behaviour is presented and discussed.

Herein, overall this work has offered contributions towards reliable ReRAM/selector technologies operations for crossbar arrays in terms of both selector technology, improving existing approach and exploring new ones, and ReRAMs, providing the community with a promising path towards forming free operations.

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Identifiers

ORCID for Themistoklis Prodromakis: orcid.org/0000-0002-6267-6909

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