MSE PhD Defense, Seong-Geon Park (Feb 7th, Monday, 10:30 am, CISX Auditorium)

University PhD Dissertation Defense

 

“The study of Resistive Switching Mechanism in TiO₂ using First Principles Calculation.”

 

Seong-Geon Park

 

Research Advisor: Prof. Yoshio Nishi

 

Monday, Feb 7^th, 2011,  10:30 am (Refreshments served at 10:15 am)

 

Location: Paul G. Allen Auditorium (CISX 101)

http://cis.stanford.edu/misc/directions.html

 

 

Abstract

Recently the interest in Resistive Random Access Memory (ReRAM) has been significantly increased, as it is now considered as the promising candidate for the next generation of non-volatile memory devices, due to its high density, low operating power, fast switching speed, and compatibility conventional CMOS process. Among many resistance switching materials, TiO₂ has been widely studied. However, the most challenging issue is that the underlying switching mechanism is lacking an in-debt understanding. It has been proposed that the resistance switching is strongly coupled to the presence and a preferential distribution of oxygen vacancies involving the formation of a conductive filament. Although many experiments have been done to address the switching mechanism during the last decade, it is hard to figure out what happens in microscopic level. Therefore systematic interpretation about the microscopic details of the role of oxygen vacancies in the formation of a conductive filament is essential.  To address the conduction and resistance switching mechanism, the effect of oxygen vacancies on the electronic structures in TiO₂ has been investigated using first principle study based on density functional theory.

 

In this talk, I will first discuss “ON”-state (Low Resistance State) conduction mechanism of rutile TiO₂ in terms of oxygen vacancies, and then the transition from “ON” to “OFF”-state (High Resistance State) will be demonstrated. Although it is known that TiO₂ exhibits n-type semiconducting conductivity with extra electron carriers generated by the formation of oxygen vacancies, “ON” and “OFF”-state conductivity during resistance switching cannot be explained by isolated single oxygen vacancy. I will demonstrate electronic characteristics such as density of states, electron localization function, band decomposed charge density distribution, and energy band structure, and show how they changes by oxygen vacancies. The influence of the number of oxygen vacancies and different configurations of multi vacancies on the resistance change will be discussed. Oxygen vacancy ordering and the diffusion of either vacancy or hydrogen has a significant impact on both the formation of a conductive filament and the transition from “ON” to “OFF”-state. Results from this work suggest “ON”-state conduction and resistance switching modeling that could be described by the formation and rupture of a conductive filament incorporating oxygen vacancy ordered structure.