Study On SrZrO₃and Ni/NiO/Ni/Co-Pt For Nonvolatile Memory Applications

In the last decade, new types of nonvolatile memories have attracted considerable attention. Non-volatile strategies recently proposed include Ferroelectric Random Access Memory (FeRAM), Magnetoresistive Random Access Memory (MRAM), Phase Change Memory (PCM) and Resistive Random Access Memory (ReRAM). Among these, ReRAMs have been regarded as promising candidates for the next generation of non-volatile memory due to their simple structure and compatibility with metal oxide semiconductor technologies. For MRAM, Current-Perpendicular-to-Plane Giant Magnetoresistance (CPP-GMR) is expected to replace the Tunneling Magnetoresistance (TMR) because of its small resistance-area product (RA). However, the change in resistance area product (ΔRA) is a major concern for using the CPP-GMR reading head. It is reported that the current-confined-path (CCP) effect could improve the MR ratio. The CCP-CPP-GMR structure includes a several nanometer thick layer of nano-oxide, where many metallic nano-contacts exist. Electrons can then slip through these nano-contacts and thus the GMR effect can be enhanced.In this paper, the main focus is on ReRAM and CCP-CPP-GMR. The first part is about the Pt/SrZrO3/Pt structure for ReRAM applications, which includes the development process, resistive switching behaviors and conduction mechanisms. The results are summarized as the following:1. Stoichiometric Pt/SZ-A/Pt and Zr-deficient Pt/SZ-B/Pt capacitors are successfully prepared by sol-gel process. Both capacitors have unipolar resistive switching characteristics. The average ratio between the high-resistance state (HRS) and the low-resistance state (LRS) of both capacitors is larger than10,000to1. Zr-deficient Pt/SZ-B/Pt capacitor can stay stable for up to600cycles,10times better than that of the stoichiometric Pt/SZ-A/Pt capacitor. The performance of SZ-B capacitors is among the best reported SrZrO3-based devices.2. Conduction mechanisms in LRS and HRS of Zr-deficient Pt/SZ-B/Pt capacitor are Ohmic and SCLC, respectively. This is different from the stoichiometric Pt/SZ-A/Pt capacitor, which shows the Ohmic behavior for both LRS and HRS. The unipolar resistive switching of SrZrO3thin films is attributed to the formation and annihilation of conducting filamentary paths composed of oxygen vacancies in the structure. The different conduction mechanisms of SZ-A and SZ-B might be because the different levels of oxygen vacancies in the films. Research shows that Zr-deficiency in SrZrO3improves its life-time by introducing more oxygen vacancies. Therefore, the control of the amount of oxygen vacancies may play a major role in ReRAM applications of SrZrO3thin films.The second part is about CCP-CPP-GMR. In this study, we proposed a structure of CCP-CPP-GMR with NiO layer. By using it, we can achieve a higher SNR ratio, a denser areal density and a faster operation speed. Fabrication process, magnetic properties and GMR effects on CPP-CPP-GMR devices consists of a Ni/NiO/Ni nanostructure are addressed. The main results are the following:1. We fabricated nanodots with Au(20nm)/Ni(5nm)/NiO(3nm)/Ni(1nm)/Co83Pt17(15nm) multilayer by ion beam sputtering, electron beam lithography and lift-off technique on Ru(10nm)/Cu(280nm)/Cr(10nm) electrode. Nanodots with a diameter of36.6nm can be fabricated successfully.2. Magnetic measurements of Au(20nm)/Ni(5nm)/NiO(3nm)/Ni(1nm)/Co83Pt17(15nm)multilayer were performed by vibrating sample magnetometer (VSM). The M-H loop shows a double step hysteresis loop, which indicates that the magnetization of the Ni(5nm) and Co83Pt17(15nm) layers is anti-parallel at250and-250Oe. From the M-H loop, we can expect the GMR effect in this structure.3. By using the cantilever of conductive-AFM as top, we successfully observed the MR effect in an isolated dot, with an MR ratio up to40-60%. This may provide an effective way to fabricate high-density CPP-GMR devices for industrial applications.