Resistive Switching in Transition Metal Oxides for Integrated Non-volatile Memory

Abstract

Transition metal oxides (TMOs) exhibit characteristic resistance changes when subjected to high electric fields due to the creation, drift and diffusion of defects, and this resistive-switching response is of interest for future non-volatile memory applications. Indeed, resistive random access memories (ReRAM) are considered promising alternatives to conventional charge storage-based devices because of their low production cost, simple fabrication, and excellent scalability. However, the realization of reliable ReRAM devices and their integration in large-scale arrays requires further understanding of the switching mechanisms and the development of new strategies for improving integrated device functionality. The aim of this work is to understand the role of the material structure on device reliability and to investigate the integration of passive selector elements with memory devices for use in memory cross-bar arrays. The thesis begins by investigating the properties of relevant oxide films (ALD HfO2 and plasma deposited NbOx) and then addresses three technologically relevant problems. Specifically these include: 1) understanding how the roughness of metal/dielectric interfaces affects dielectric breakdown and switching behaviour; 2) exploring methods for reducing the operating current of selector and memory/selector devices and 3) investigating the effect of operating conditions on the switching response of devices. The first of these studies is based on Pt/Ti/HfO2/Pt devices and combines experimental methods and finite element modelling to understand the effect of the Pt/HfO2 interface roughness on the electroforming and switching response. Atomic force microscopy (AFM) showed that the roughness of Pt electrodes deposited by electron-beam evaporation increased with film thickness due to facetted grain growth. Results show that roughness leads to a reduction in the electroforming voltage of HfO2, an increase in the failure rate of devices, and a corresponding reduction in resistive switching reliability. Conventional wisdom suggests that these effects result from local electric field enhancement in the vicinity of electrode asperities. However, the effect on electroforming voltage is much less than estimated from simple geometric considerations. Comparison with finite-element modelled showed high-aspect-ratio asperities can produce field enhancements of more than an order of magnitude but that the generation and redistribution of defects moderates this effect prior to dielectric breakdown. As a consequence, the effect of field enhancement is less than anticipated from the initial electric-field distribution alone. It is argued that the increase in the device failure rate with increasing electrode roughness derives partly from an increase in the film defect density and effective device area and that these effects contribute to the reduction in breakdown voltage. The second study showed that the leakage current in NbO2-x selector (1S) elements is shown to be reduced by the properties of an adjacent memory (1M) element when integrated into a hybrid selector-memory device structure. This is shown to result from current confinement in conductive filaments formed in the memory layer. Finite element modelling of the selector-memory structures is used to confirm the observations and to explore material dependencies. The thermal and electrical conductivities of the memory layer are shown to influence the threshold current, but the dominant effect is due to current confinement. The final study explores the effect of device operating conditions on its operation and identifies an alternative approach for reducing the forming and RESET current in integrated memory/selector devices. This study is based on Pt/Nb/HfO2/Pt devices which require a very "soft" electroforming process. Such devices are shown to undergo configurable switching controlled by the SET compliance current. When operated at a low compliance-current (~100 µA), devices show uniform bipolar resistive switching behaviour. As the compliance current is increased (~500 µA), the switching mode changes to integrated threshold-resistive (1S1M) switching, and at still higher currents (~1 mA), it changes to symmetric threshold switching (1S) characteristic of threshold switching in NbO2-. These switching transitions are shown to be consistent with the development of an NbO2- interlayer at the Nb/HfO2 interface that is limited by the set compliance current due to its effect on oxygen transport and local Joule heating. The proposed mechanism is supported by finite element modelling of the 1S1M response assuming the presence of such an interlayer. These findings help to understand role of interface reactions in controlling device performance and provide a means for the self-assembly of integrated 1S1M resistive random access memory structures