Adaptable diode

Abstract

Resistive switching in oxide materials has been studied for decades[1]. The phenomenon occurs in different materials with different characteristics. The reason of reversible hysteric change in resistance was not clearly understood. With the characteristic pinched hysteresis loop, resistive switching was included as a special case of the fourth passive circuit element, memristor[2]. Memristor was a long missing element since its mathematical proof of existence. The scientific community has been trying to find detail mechanism and explore the applications. The main direction is nonvolatile memory for computation. Other potential applications include digital and analog system and neuromorphic circuits. There have been evidences that memristors can enable biological brain emulation[3-5]. The memristor is a two terminal passive device, besides the capacitor, resistor and inductor. The property of the device cannot be duplicated by the rest three components. For a flux controlled memristor, the conductance of the device is a integral of flux. Its resistance is thus a history of voltage through the device. The data stored is also non-volatile that retains after unplug from electricity. This property allows the device to store data especially to replace the silicon flash memory or for long term potentiation in biological synapse emulation. The phenomenon was believed to be related to movement of oxygen vacancy[6]. Whilst the detailed principle of electric field induced resistive switching is unknown, the memristors can be categorized into interfacial type and filamentary type. The filamentary type memristor involves the formation and disruption of conduction pathways formed in the thin film. Interfacial type memristor has the whole surface contributing to the switching event. In this dissertation, manganese doped zinc oxide thin film was used to make interfacial type memristors and film characterizations were performed. 2% Mn-doped ZnO target pellet was first made by standard solid state reaction. Thin film was deposited by pulsed laser deposition under different conditions. The grain size was calculated from the result of X-ray diffraction. Atomic force microscope showed the smoothness of the film. Resistivity and carrier density were measured by Hall system. Resistive switching was demonstrated. A vibrating sample magnetometer test was used to measure the ferromagnetic properties. Finally, electrical switching was investigated. The properties of Mn-doped ZnO, especially the room temperature ferromagnetism and resistive switching were shown suitable for neuromorphic applications.