Nanoscale characterization of resistive switching phenomena in HFO2-based stacks using transmission electron microscopy and atomistic simulation.

Abstract

This thesis presents a comprehensive study combining electrical characterization, physical analysis, and atomistic simulation on the mechanism of resistive switching. The metal-insulator-semiconductor (MIS) stack based on conventional transistor was used in this study as an effective test structure to understand the chemical origin of switching behavior in the metal-insulator-metal (MIM) stack. The objective of this thesis is to understand the fundamental mechanism governing the nucleation and rupture of the nanosized conductive filaments in resistive random-access memory (RRAM). To study the chemistry of the localized nanoscale conductive path, electrical characterization techniques were employed to pinpoint the location of the conductive path, then advanced nanoscale analysis tools such as transmission electron microscopy (TEM) along with electron energy loss spectroscopy (EELS) were used to study the elemental composition of the conductive path; finally first-principles calculations were used to further understand the band structure change of the switching material. It is found that the conductive filament consists of oxygen vacancies and the metal filament. There are two stages for the formation of the conductive filament: the initial stage, which is commonly referred to as the soft breakdown stage in MIS gate stack reliability study where the oxygen vacancies are the physical defects responsible for the formation of a percolation path; In the second stage, the metal atoms from the top gate electrode migrate along the oxygen deficient breakdown path driven by the high current density and temperature enhanced metal atom diffusion, forming a metal-rich filament at the central core of the breakdown spot. For the ruptured conductive filament, our physical analysis results show that metal fragments still remain in the dielectric even after the conductive filaments have been electrically switched-off. It is very likely that the residual metal in the dielectric bonds with the O2- ions forming an insulator.