Tsung-Ling Tsai

Forming-free bipolar resistive switching in nonstoichiometric ceria films

The mechanism of forming-free bipolar resistive switching in a Zr/CeOx/Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrOy layer at the Zr/CeOx interface. X-ray diffraction studies of CeOx films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeOx film and in the nonstoichiometric ZrOy interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).

Switching mechanism of double forming process phenomenon in ZrOx/HfOy bilayer resistive switching memory structure with large endurance

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.

Impacts of Co doping on ZnO transparent switching memory device characteristics

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 550u2009nm have been fabricated.

Enhanced switching uniformity in AZO/ZnO1−x/ITO transparent resistive memory devices by bipolar double forming

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 104u2009s at room temperature confirmed the reliability of optimized devices. Furthermore, highly transparent devices (transparency of approximately 85% in visible range) have been fabricated.

Unipolar resistive switching behavior in Pt/HfO2/TiN device with inserting ZrO2 layer and its 1 diode-1 resistor characteristics

Transition of resistive switching (RS) behavior from bipolar to unipolar is observed in Pt/ZrO2/HfO2/TiN device. Due to the lower oxygen vacancy concentration of the HfO2 layer, formation/rupture of the conducting filament is confined in the HfO2 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/ZrO2/HfO2/TiN resistive random access memory shows robust retention and nondestructive readout property at 85u2009°C.

Enhancing the memory window of AZO/ZnO/ITO transparent resistive switching devices by modulating the oxygen vacancy concentration of the top electrode

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 12xa0mTorr. 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 104 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.

A high performance transparent resistive switching memory made from ZrO2/AlON bilayer structure

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.

Unipolar resistive switching behaviors and mechanisms in an annealed Ni/ZrO2/TaN memory device

The effects of Ni/ZrO2/TaN resistive switching memory devices without and with a 400 °C annealing process on switching properties are investigated. The devices exhibit unipolar resistive switching behaviors with low set and reset voltages because of a large amount of Ni diffusion with no reaction with ZrO2 after the annealing process, which is confirmed by ToF-SIMS and XPS analyses. A physical model based on a Ni filament is constructed to explain such phenomena. The device that undergoes the 400 °C annealing process exhibits an excellent endurance of more than 1.5 × 104 cycles. The improvement can be attributed to the enhancement of oxygen ion migration along grain boundaries, which result in less oxygen ion consumption during the reset process. The device also performs good retention up to 105 s at 150 °C. Therefore, it has great potential for high-density nonvolatile memory applications.

Resistive Switching Characteristics of WO 3 /ZrO 2 Structure With Forming-Free, Self-Compliance, and Submicroampere Current Operation

The homogeneous switching of Ti/WO3/ZrO2/W resistive switching memory devices with stable resistive switching, forming-free, self-compliance, and multilevel operation characteristics are demonstrated. The area dependence of current at the low resistance and high resistance states confirms that the switching mechanisms of the devices are homogeneous conduction. The devices exhibit the stable bipolar resistive switching behavior with a low operating current (<;10-6 A) by inserting the WO3 layer with high electron affinity between the Ti top electrode and the ZrO2 layer and modulating the potential profiles at the WO3/ZrO2 interface. In addition, multilevel operation can be achieved by adjusting the magnitudes of set/ reset voltages.

Resistive switching characteristics of Pt/CeOx/TiN memory device

The resistive switching characteristics of Pt/CeOx/TiN memory devices are investigated for potential applications in nonvolatile resistive random access memory (RRAM). The X-ray diffraction characteristics of the sputtered CeOx layer indicate the formation of nanocrystalline single-phase CeO2 with a cubic fluorite structure. The existence of oxygen vacancies in the Pt/CeOx/TiN memory device was determined by X-ray photoelectron spectroscopic studies, while the presence of an interfacial layer between CeOx and the TiN bottom electrode was investigated by X-ray diffraction and high resolution transmission electron microscopy. The TiON layer formed at the TiN/CeOx interface seems to play a key role in the resistive switching mechanism of the device. The present CeOx-based device shows excellent bipolar resistive switching characteristics, including a low operation current (100 ?A), high ON/OFF resistance ratio (?105), and good retention/stress characteristics at both room temperature and 85 ?C.