Abstract
Fast-emerging next-generation nonvolatile memories include resistive random access memory (ReRAM), phase-change random access memory (PRAM), magnetoresistive random access memory (MRAM), ferroelectric random access memory (FeRAM). In this dissertation, our purpose were the development of new functional material and novel structure for ReRAM application, aiming to improve the memristive characteristics. The first topic of this dissertation focuses on the electric induced resistive switching phenomena based on Cu-doped amorphous SiOx functional films (SCuO). Electrical switching of resistive memory is highly interface-dependent. We studied such a switching of Cu-doped amorphous SiOx thin-films in sandwich stacks Cu/SCuO/Pt. The stacks were prepared using radio frequency sputtering except Cu co-doping which utilized direct current (DC) power from 2 W to 15 W. We characterized electrical switching behavior by a Keithley 4200 semiconductor analyzer. Cu/SCuO/Pt devices with Cu-doping at DC-sputter 2 W exhibit the best switching performance showing reproducible forming-free and non-polar switching. The endurance is more than 102 cycles, electrical resistance ratio more than 10, and operating voltages as low as: ±0.75 V for SET and ±0.45 V for RESET. The switching mechanism of Cu/SCuO/Pt stacks is explained based on both filamentary conduction and diffusion of Cu ions/atoms in SiOx. Both ‘temperature coefficient of electrical resistance’ and ‘bonding status’ at different depth-profiles as analyzed by using X-ray photoelectron spectroscopy provide robust evidences of the mechanisms. Cu-doped amorphous SiOx thin-films are thus potential for resistive memory. The second topic of this dissertation focuses on the electric induced resistive switching phenomena based on Zn-doped amorphous SiOx films, (SZO). We demonstrated dual resistive switching capability of SZO films. Both mono-stable selector-switching and bi-stable memristive switching are tuning via Zn-doping content and appropriate operation conditions. Voltages of selector-switching in Pt/SZO/ITO stacks can be noticeably modulated by varying Zn-doping. The selector-switching is stable for more than 100 cycles with a resistance ratio of 104 at voltages within +3 V. Stable memristive switching is obtainable by current-controlled RESET and voltage-controlled SET. We found that selector-switching arises from generalized trap-assisted tunneling of electrons provided by zinc addition. The dual-switching-mode of SZO is proposed to facilitate implementation of cross-bar RRAM. The third topic of this dissertation, aims at developing a cost-effective method for ReRAM application. Cyclic voltammetry deposition (CV-D) was applied to deposit ZnO films on indium-tin-oxide (ITO) glass. The result is much superior coverage of the CV-D thin films as compared to those obtained by conventional electrochemical deposition. The Pt/CV-D ZnO/ITO devices in which ZnO prepared by CV-sweeping within ± 0.9 V for 6 cycles then fix-potential-deposited at - 0.75 V for 300 s show reproducible forming-free bipolar switching operation at voltages ≤ ±1 V. The cycle-life is at least 200 cycles. The electrical conduction belongs to space-charge-limited-current conduction mechanism, which is fitted to extract carrier mobility 0.97 cm2/Vs and carrier concentration 8.6x1018 cm-3. Gradient oxygen bonding status in ZnO film, typical of cyclic voltammetry deposition, was found to facilitate the electrical switching at low voltages. In the fourth topic of this dissertation, is to work out a simplest ReRAM configuration based on single layer transparent conducting oxides (TCO). Bias-polarity-induced transformation of point contact resistive switching memory is demonstrated on three kinds of TCO layers, including tin-doped indium oxides (ITO), fluorine-doped tin oxides (FTO), and aluminum-doped zinc oxides (AZO) as conducting electrode as well as memristive material by the controllably electrical field simultaneously. Voltage-controlled SET and current-controlled RESET were utilized to obtain much more stable endurance results. The special RS behavior based on the TCO single layer provides a new material selection and the simplest geometry to realize the highest stacking density at development of three dimensional (3D) point-contact ReRAM application.