Investigation On The Switching Characteristic Of Binary Oxide Films

The semiconductor industry has long sought a nonvolatile memory with high-density, high-speed, low cost, and low power consumption, which retains its data even when the power is shutoff. Today, silicon based Flash memory represents the most widely nonvolatile memory due to its high density and low fabrication costs. However, it suffers from low programming speed, bad endurance, and high voltages required for the write or erase operations. Meanwhile, with the further miniaturization of silicon based devices down to 22-nm lateral feature projected for 2016, traditional memory technologies based on charge storage are expected to come against technical and physical limits. Ferroelectric and magnetic random access memories are also sharing struggle with scaling. The most challenge is that it becomes increasingly difficult to reliably retain sufficient electrons in the shrinking cells. Recently, a new memory concept called resistance random access memory has attracted considerable attention, due to its merits such as simple structure, fast speed, low operating voltage, low power consumption, high density and compatibility with 3-dimensional design. It is defined as that a metal-insulator-metal memory cell can be electrically switched between a high resistance state and a low resistance state. Up to now, resistive switching behavior has been discovered in various kinds of materials such as metal oxides, solid electrolytes and organic compounds. Among these materials, the binary oxides have gained particular interest, not only due to their simple composition and compatibility to CMOS technology, but also due to their resistance to thermal or chemical damages. However, the switching mechanism remains a controversial issue. The difficulty in understanding the resistive switching mechanism is a major obstacle to its future applications.In this work, we will focus on the resistive switching characteristics of binary oxide films. Both the unipolar resistive switching behavior and bipolar resistive switching behavior are investigated. We have explored the possible potential of several binary oxide films for memory application, with an emphasis on analyzing the switching mechanism. The main research results are summarized as follows:1. For the first time, we reported the unipolar resistive switching behavior of Co3O4 films. The "ON/OFF" operation of the Pt/Co3O4/Pt memory cells can be repeated more than 200 times at room temperature. The resistance ratio of the high-and low-resistance state is over 5 x 103. The resistance of the two states can be kept for more than 16 h without showing degradation. The temperature dependence of the resistance shows a metallic behavior at the low-resistance state, but a semiconductor-behavior at the high-resistance state. The formation and rupture of oxygen-deficient filaments is proposed responsible for the observed unipolar resistive switching behavior.2. The possible potential of amorphous lutetium oxide films for memory application was studied. As the semiconductor industry moves toward memory cells with 22-nm features, the cell size will become comparable to the size of grains. Due to the diverse distribution of grain boundaries, device-to-device variation in switching characteristics will emerge inevitably. Adopting amorphous film can effectively avoid such issues. So we focus on the switching characteristic of amorphous lutetium oxide films, which has been investigated as high-k candidate to replace silicon oxide as the gate dielectric and already proved to be compatible with the CMOS technology. Well unipolar switching behaviors of Pt/Lu2O3/Pt stacks were obtained. The memory cells exhibited a high resistance ratio over 1 x 103, fast programming speed within 30 ns, and no obvious degradation after an endurance of 300 switching cycles and a duration of 3.2x106s.3. For the switching mechanism of lutetium oxide film, our experimental result confirms that the migration of oxygen ions in the lutetium oxide film plays an important role in the switching process. The absence of grain boundaries in amorphous lutetium films helps us attribute the switching mechanism of such stacks to the possible redistribution of defects related to oxygen vacancies along the filamentary paths during the resistive switching process.4. Well-developed bipolar resistive switching behaviors have been revealed in Pt/GaOx/ITO stacks without an electroforming process. By substituting platinum with titanium as the top electrode, switching polarity changed from "counter-Figure-8" to "Figure-8". The modulation of Schottky barrier at the Pt/GaOx interface induced by migration of oxygen vacancies was proposed to explain the switching in Pt/GaOx/ITO stacks, while the switching in Ti/GaOx/ITO stacks was ascribed to the redox reaction at the Ti/GaOx interface. Our experimental result further confirms the migration of oxygen vacancies in the vicinity of the electrode area plays an important role in the resistive switching process.