Alloy perovskite oxide thin film as resistance switching non-volatile memory

Nonvolatile memory that permanently stores data is indispensable for computers and hand-held devices. In the last few years, resistance memory (RRAM) has emerged as an intriguing possibility that might replace flash memory one day, which is widely used in hand-held and portable-storage devices. The newest, rapidly growing interest in resistance switching is focused on semiconducting oxides and other related materials. In this dissertation, a novel material system for oxide RRAM that offers unique advantages over all the other existing oxide RRAM materials was designed and systematically investigated. The primary aim of these studies is to obtain a material system with the intrinsic property that allows electrically-induced metal-insulator transition, which is regulated by electron trapping and release at some interval sites. A series of alloy perovskite oxides thin film systems were designed by combining a wide band gap insulator (CaZrO3 or LaAlO3) and a conductor with a narrow bandwidth (SrRuO3 or LaNiO3 ), with the conductor concentration near the percolation threshold. These alloy perovskite oxides thin films are almost atomically flat without any defects, such as cracks or crosshatches, which is achieved using well controlled deposition conditions that favor domain-boundary relaxation of the large misfit strain. The bottom electrode is a single crystalline SrRuO 3 thin film, deposited on a single crystal substrate of SrTiO3 which exhibits high conductivity and ferromagnetic transition at ∼150K. The alloy thin films manifest an anisotropic percolation phenomenon: below a critical thickness a metallic conducting path always exists across the film thickness direction but not along the in-plane direction, which ensures electrical isolation between neighboring memory cells. These initially conducting films present excellent resistance switching properties: low switching voltages (1-3 V), high switching ratio (∼100), fast switching speed (50 ns), good switching reliability and long retention time. A variety of experiments have been conducted to explore the resistance switching mechanism. These include variation of film orientation, film strain and top electrode material, as well as UV irradiation and temperature dependent transport measurements. All the experimental results consistently suggest that the resistance switching is of an electronic nature. During the low resistance (LR) to high resistance (HR) switching, electrons are injected into the alloy film through the electrode with a lower work function, and the electrons are released through the same electrode during the HR to LR switching. Accompanying the electron trapping/release, disorder is introduced to or removed from the initially conducting paths due to electrostatic interaction. As the disorder raises or lowers the mobility edge above or below the Fermi level, the insulator-metal transition is electrically triggered enabling a robust non-volatile memory