In this Letter, the mechanism of double forming process phenomenon revealing in ZrO2/HfO2 bilayer resistive random access memory structure is investigated. This phenomenon caused by the formation of TiON interfacial layer can be well explained by using the energy band diagram. The TiON interfacial layer will be a tunneling barrier during the first forming process when a negative voltage applied on the device, while it will breakdown when applying a positive voltage. Besides, due to the double forming process, an asymmetric conductive filament with narrower size at ZrO2/HfO2 interface is formed in the device. The point for formation and rupture of the conductive filament can be confined at the ZrO2/HfO2 interface, and it will suppress the consumption of oxygen ions during endurance test. Therefore, high speed (40 ns) and large endurance (107 cycles) characteristics are achieved in this device structure.

The resistive switching characteristics of indium tin oxide (ITO)/Zn1−xCoxO/ITO transparent resistive memory devices were investigated. An appropriate amount of cobalt dopant in ZnO resistive layer demonstrated sufficient memory window and switching stability. In contrast, pure ZnO devices demonstrated a poor memory window, and using an excessive dopant concentration led to switching instability. To achieve suitable memory performance, relying only on controlling defect concentrations is insufficient; the grain growth orientation of the resistive layer must also be considered. Stable endurance with an ON/OFF ratio of more than one order of magnitude during 5000 cycles confirmed that the Co-doped ZnO device is a suitable candidate for resistive random access memory application. Additionally, fully transparent devices with a high transmittance of up to 90% at wavelength of 550 nm have been fabricated.

The effect of a defect concentration-modified top electrode on the bipolar resistance switching of transparent Al-doped ZnO/ZnO/ITO [AZO(TE)/ZnO/ITO(BE)] devices was investigated. Different oxygen vacancy concentrations in the top electrode, Al-doped ZnO, can be simply controlled by modulating the sputtering working pressure condition from 1.2 to 12 mTorr. The oxygen vacancy concentration between AZO and ZnO may trigger oxygen diffusion at the interface and affect the switching characteristic. High oxygen release from a ZnO resistive layer caused by excessive oxygen vacancy concentration at the top electrode is responsible for reducing the memory window as a result of reduced oxygen available to rupture the filament. Top electrode based on lower oxygen vacancy concentration has a higher memory window and an asymmetric resistive switching characteristic. However, all set of devices have excellent endurance of more than 10 4 cycles. This study showed that an Al-doped ZnO top electrode helps not only to achieve a transparent device but also to enhance memory properties by providing a suitable oxygen vacancy concentration.

In this study, the switching properties of an indium tin oxide (ITO)/zirconium oxide (ZrO2)/ITO single layer device and those of a device with an aluminum oxynitride (AlON) layer were investigated. The devices with highly transparent characteristics were fabricated. Compared with the ITO/ZrO2/ITO single layer device, the ITO/ZrO2/AlON/ITO bilayer device exhibited a larger ON/OFF ratio, higher endurance performance, and superior retention properties by using a simple two-step forming process. These substantial improvements in the resistive switching properties were attributed to the minimized influence of oxygen migration through the ITO top electrode (TE), which can be realized by forming an asymmetrical conductive filament with the weakest part at the ZrO2/AlON interface. Therefore, in the ITO/ZrO2/AlON/ITO bilayer device, the regions where conductive filament formation and rupture occur can be effectively moved from the TE interface to the interior of the device.

The influence of single and double forming on the switching stability of AZO/ZnO1−x/ITO transparent resistive memory devices was investigated. Devices that underwent single forming exhibited severe switching instability, where as those that underwent double forming exhibited excellent switching uniformity. The quantity of conducting filaments can be limited by applying the two-step forming process. Consequently, the set/reset process can be controlled, enhancing switching stability. Satisfactory endurance with an acceptable ON/OFF ratio of 102 and satisfactory retention behavior of 104 s at room temperature confirmed the reliability of optimized devices. Furthermore, highly transparent devices (transparency of approximately 85% in visible range) have been fabricated.

In this study, metal diffusion barrier-dependent switching polarity in ZrO2-based conducting-bridge random access memory was investigated. The device without the barrier layer (BL) exhibited nonpolar switching characteristics. However, inserting TiW BL resulted in positive reset failure. This phenomenon depends on the size and shape of the conducting bridge and also on the defects that contribute to the formation and rupture of the bridge. Consequently, the properties of the conducting bridge govern the device switching performance. Cu- and oxygen vacancy-based conducting bridge during N-Set for a device with and without the BL was proposed. The effect of the insertion of BL on the switching performance was also discussed. The absence of BL resulted in switching instability and poor nonvolatility. By contrast, a device with BL exhibited enhanced uniformity and nonvolatility, and the retention was more than 105 s at 200 °C.

Transition of resistive switching (RS) behavior from bipolar to unipolar is observed in Pt/ZrO 2 /HfO 2 /TiN device. Due to the lower oxygen vacancy concentration of the HfO 2 layer, formation/rupture of the conducting filament is confined in the HfO 2 layer. To fulfill one diode and one resistor (1D1R) structure, the electrical relation between the RS device and diode is investigated. A Pt/InZnO/CoO/Pt/TiN oxide diode is fabricated to provide enough forward current and large forward/reverse current ratio to achieve unipolar RS behavior. The 1D-1R structure with Pt/ZrO 2 /HfO 2 /TiN resistive random access memory shows robust retention and nondestructive readout property at 85 C. V

The resistive switching characteristics of Pt/CeO x /TiN memory devices are investigated for potential applications in nonvolatile resistive random access memory (RRAM). The X-ray diffraction characteristics of the sputtered CeO x layer indicate the formation of nanocrystalline single-phase CeO 2 with a cubic fluorite structure. The existence of oxygen vacancies in the Pt/CeO x /TiN memory device was determined by X-ray photoelectron spectroscopic studies, while the presence of an interfacial layer between CeO x and the TiN bottom electrode was investigated by Xray diffraction and high resolution transmission electron microscopy. The TiON layer formed at the TiN/CeO x interface seems to play a key role in the resistive switching mechanism of the device. The present CeO x -based device shows excellent bipolar resistive switching characteristics, including a low operation current (100 µA), high ON/OFF resistance ratio (>10 5 ), and good retention/stress characteristics at both room temperature and 85°C.