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A resistance random access memory (ReRAM) is expected to be a next-generation nonvolatile memory because of its fast switching speed, and low power consumption. The device structure is composed of an oxide layer sandwiched with two metal electrodes. ReRAM exhibits repetitive transition between high resistance state (HRS) and low resistance state (LRS). It is considered that resistive switching phenomenon is caused by the formation and rupture of the conductive filament formed in the oxide layer. In this study, we investigated HfOx thickness dependence of current-voltage characteristics for Ni/HfOx/Pt ReRAM device. We fabricated Ni/HfOx/Pt devices with a simple crossbar structure. HfOx layer was deposited on the Pt bottom electrode by DC reactive sputtering. The I-Vcharacteristics were measured by two-probe method at room temperature. Red line in Figure 1 shows I-V characteristics of the Ni/ HfOx/Pt device with oxide thickness of 5.6 nm. Initial state of resistance is HRS. SET process (from HRS to LRS) took place at 2.4 V with a current compliance of 0.1 mA. RESET process (from LRS to HRS) occurred at -1.6 V. The resistance ratio of HRS to LRS is about 107 with 0.1 V readout bias. On the other hand, blue line shows the I-V characteristics of the Ni/ HfO2/Pt device with oxide thickness of 4.0 nm. Initial state of resistance is LRS. Forming process was not observed in this device. SET process took place at 1.0 V. RESET process was observed at -0.6 V. Switching voltage of this device is smaller than the former device with the thicker oxide layer. The resistance ratio of HRS/LRS is about 101 with 0.1 V readout bias. There was a significant difference in switching properties between these devices whose oxide thicknesses differ only 1.6 nm. It is suggested that leakage current increases when the oxide thickness decreases, thereby the on-state occurred initially for the device with 4.0 nm oxide. It should be noted that I-V characteristic is symmetrical for the device with 4.0 nm oxide, while it is asymmetrical for the device with 5.6 nm oxide. This suggests the mechanism of conductive filament formation and rupture is completely different between these devices. Further discussions and analysis will be presented in the symposium.

A resistive switching (RS) memory has been expected to be a next-generation memory owing to its high density, low power consumption, and high switching speed. The mechanism of the reversible RS phenomenon has been considered in terms of the formation and rupture of conductive filaments (CF). Recently, it has been reported that a ferromagnetic CF was formed in NiO-based RS memory. This suggested the possibility of the new device that has both RS memory and spintronics device functions. The presence of a ferromagnetic CF in the NiO layer was confirmed by measuring the magnetization of the large number of CFs at room temperature using a SQUID magnetometer. Although, it is not yet confirmed that a single ferromagnetic CF is formed by the SET process. The purpose of this study is to understand the physical properties of a single ferromagnetic CF by focusing on magnetoresistance (MR). We prepared Ni/HfO2/Pt memory device, having ferromagnetic Ni electrode and the oxide of a non-magnetic metal. The fabricated RS memory devices have a cross-bar structure. The crossed electrode size is 1×1 mm2. RS characteristic was measured with the bipolar operation mode. When current limit is 5 mA, the anisotropic magnetoresistance (AMR) was observed in the low resistance state. The direction of the external magnetic field is in-plane to the substrate, and the current is perpendicular to the magnetic field. The AMR showed a convex behavior with magnetic field, and the MR ratio was approximately 0.15%. It is strongly suggested that AMR came from a ferromagnetic CF formed in the oxide layer. Further discussions will be shown in the workshop.

We investigated resistive switching (RS) characteristics and temperature dependences of a resistance of the Ni/TiO2/Pt resistive switching memory. The device was operated in a unipolar mode. From the temperature dependence of resistance, it is shown that the conductive filament in the low resistance state (LRS) is metallic. Fabricated Pt/NiO/Pt RS memory to compare the RS characteristics showed similar RS phenomenon. The conductive mechanism of the Pt/NiO/Pt RS memory is also metallic. Temperature coefficients of resistance of various RS memory were compared.

The morphological changes of etched Si surfaces on the concentration of AgNO3 using single-step metal assisted etching and their reflectance were investigated. We used aqueous mixture solution containing silver nitrate (AgNO3) and hydrofluoric acid (HF) as single-step metal assisted etching solution. With changing concentration of silver ions in the etching solution, three types of Si surface morphologies (“nanowire”, “wall”, “polished”) were prepared. Then, phase diagram is developed for these morphologies obtained as a function of AgNO3 concentration and resistivity of silicon substrates. The measurement of reflectance for etched surface indicates that the nanowire is the lowest reflectance of three types of morphologies.

The electric conduction mechanism of electrochemically treated anodic porous alumina (APA) resistance switching (RS) memory is investigated. The current-voltage characteristics of the APA were measured for both of high resistance state and low resistant state at room temperature. The conduction mechanism of a high resistance state can be explained by separating into three regions of Ohm's law, space charge limited current (SCLC), and Fowler-Nordheim (F-N) tunneling. It was suggested that the F-N tunneling decreases the reproducibility of switching voltages.

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