Advanced oxide based resistive random access memories (RRAMs) : switching mechanisms and material solutions

Abstract

This thesis reports a study of the resistive switching phenomena in oxide based material systems, practically hafnium and graphene oxide based Resistive Random Access Memories (RRAMs), in aspects of device fabrication, electrical and physical characterization, and theoretical calculation. Both hafnium and graphene oxide based RRAMs have been fabricated. Through variation of the configuration of material systems and deposition techniques, boosts of performance in terms of uniformity, forming-free property and self-compliance are realized in the hafnium oxide based RRAM. On the other hand, impact of electrodes, critical role for graphene oxide RRAM, has been studied for its influence to the switching directions. Electrical and physical characterization are employed to unravel the underlying transport and switching mechanisms. Frequency and temperature dependent dielectric loss characterization, implementing on the hafnium oxide RRAM, provides the evidence of the existence of small polaron adiabatic hopping in the high resistance state, consistent with the analysis of activation energy and field dependence. On the other hand, a weak metallic nature in transport is identified in the low resistance state. In graphene oxide RRAM, same technique reveals the fact that the combination of both oxygen related functional groups absorption/release and electrode metal ions diffusion are responsible for the multidirectional switching. Theoretical calculation, using hybrid functional in first principle simulation, is performed to interpret the observed metal-insulator transitions in the hafnium oxide based RRAM. Computational evidence is found for oxygen vacancy-bound small polarons migration and Mott type localization-delocalization transition. Adiabatic polaron hopping between equivalent threefold oxygen vacancies are of anisotropic barriers, in both the bulk and (-111) surface slab models, consistent with the characterization findings. Phonon correlation to the hopping of the small polaron is explored where the important roles of the vacancy neighboring threefold oxygen atoms are unveiled.