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There is a question whether the conductive filament (CF) formed in the oxide layer of a resistive-switching random access memory is made of oxygen vacancies or metallic atoms. We investigated the CF of Ni/TiO2/Pt device using temperature coefficient of resistance (TCR), anisotropic magnetoresistance (AMR), and cross-sectional transmission electron microscopy with energy dispersive X-ray analysis (TEM-EDX). The low resistance state (LRS) of the device showed metallic property by TCR measurement. Furthermore, the device in the LRS showed AMR, which was a direct evidence of the formation of ferromagnetic CF. The cross-sectional TEM-EDX observation revealed that a nano-sized Ni precipitation existed in the area nearby a conductive spot. It is intensively suggested that Ni atoms migrated from the adjacent Ni electrode to TiO2 layer to form the nano-sized ferromagnetic CF.

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.

We have studied variation of switching voltage of resistive random access memory (ReRAM). Basically, the switching voltage at every switching has a different value because there are many conduction paths, which are randomly chosen at every switching with different conductivity in an oxide insulator between top and bottom electrodes. Limitation of the number of conductive paths is expected to lead the suppression of the variation of switching voltage. In this study, Ni nanorods buried into the nanoholes of an anodic aluminum oxide (AAO) film have been fabricated to restrict the formation of many filaments and the switching behaviors of the In/NiO/Ni nanorods has been investigated.

This paper discusses the impact of the tunneling probability on the variability of tunnel field-effect transistors (TFETs). Isoelectronic trap (IET) technology, which enhances the tunneling current in TFETs, is used to suppress the variability of the ON current and threshold voltage. The simulation results show that suppressing the tunneling rate fluctuations results in suppression of the variability. In addition, a formula describing the relationship between the tunneling rate fluctuations and the electric field strength is derived based on Kane's band-to-band tunneling model. This formula indicates that the magnitude of the tunneling rate fluctuations is proportional to the magnitude of the fluctuations in the electric field strength and a higher tunneling probability results in a lower variability. The derived relationship is universally valid for any technologies that exploit enhancement of the tunneling probability, including IET technology, channel material engineering, heterojunctions, strain engineering, etc.

The switching process of the conductive filament formed in Ni/HfOx/Pt resistive random access memory (ReRAM) devices were studied. We evaluated the oxide thickness dependence and temperature dependence of voltage for the Forming, Set and Reset operations for HfOx layers whose thickness are between 3.3 and 6.5 nm. The resistance of conductive filaments showed typical metallic behavior, which suggests Ni filament formation in the HfOx layer. There is a clear dependence of switching voltages for the Set and Reset processes on oxide thickness, which implies that the formation and rupture of conductive filaments occur in the entire thickness range of the HfOx layer. This finding differs from that of a previous study by Yang, which suggests the existence of a constant-thickness switching region. It is suggested that the thickness of the switching region in HfOx may be larger than 6.5 nm.

Resistive change memory has been expected to be a next generation memory due to high-speed response. However, wide variations of switching voltages are often found in the current–voltage (I–V) characteristics and hinder the practical applications. We have tried suppressing the variation by three tactics based on the filament model; confinement of conductive filaments, concentration of electric field, and preparation of many routes for conductive filaments with the same electrical properties. In this study, we have investigated the combination of concentration of electric field method and preparation of many routes for conductive filaments in the insulator by electrochemical treatment (ET) using sulfuric acid. The I–V curves depend on the ET time. The best result is obtained in the long ET time. With regards to switching voltage distribution, the combination of two methods provide better switching characters than those of single method.

An attempt to reduce the SET voltage and RESET current of resistive switching (RS) memory was made using a geometric array of nanopeak (NP) structures. Bottoms of anodic porous alumina were used to form the NP structures that act as guides for the formation of conductive filaments that effectively concentrate the electric field. Samples were fabricated with flat surfaces (FS) and with two types of NP structure with different NP pitch. The NP samples provided SET voltages less than 2 V with narrow distributions and the RESET current was lower than that with the FS sample.

We have tried suppressing variation of switching voltages which hinder the practical application of resistive switching random access memory. On the basis of a filament model, fabrication of the top electrode with small enough size against the filament size and confining the filament are thought to be the solutions. In this study, we have investigated the confinement effects on the switching voltage variations using anodic porous alumina with ordered nano holes. Variation of the voltage has a trend to reduce with enlarging the diameters of nano holes by chemical etching. The confinement is very effective to improve the reproducibility of the switching voltages.

We investigated the resistive switching characteristics and temperature dependence of resistance of a Ni/NiOx/Pt resistive switching memory. The device was operated in a unipolar operation mode. From the temperature dependence of resistance, it is suggested that the conduction mechanism of the low-resistance state is metallic. Moreover, a deformed site that may correspond to the conductive filament appeared after the forming process. Cross-sectional transmission electron microscopy with energy dispersive X-ray analysis showed that the Ni atomic content at the NiOx of the deformed area was larger than those of other areas. It is strongly suggested that the conductive filament formed in the NiOx layer is composed of Ni atoms.

The control of the crystal orientation and diameter of vertically grown epitaxial Si nanowires was demonstrated using a combination of a vapor–liquid–solid (VLS) growth technique and the use of an anodic aluminum oxide (AAO) template on a single-crystal Si substrate. The [100], [110], and [111] nanowires were selectively obtained by choosing the Si substrate with appropriate crystal orientation. The diameter of a Si nanowire in the AAO template could be controlled by the modification of the pore size of the AAO template with anodic voltage during anodization.

An investigation of current–voltage (I–V) characteristics and the temperature dependence of resistance in a resistive switching (RS) memory with a Cu/SiO2/Au device was performed. Moreover, conductive spots were observed by scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM). Both unipolar and bipolar operation modes were obtained. From the temperature dependence of resistance, it is suggested that the conduction mechanism of the low resistance state (LRS) is metallic conduction and that of the high resistance state (HRS) is variable-range hopping (VRH) conduction. The results of observing the device support this suggestion. We propose the switching mechanism in the Cu/SiO2/Au device from these results.

Resistance change random access memory (ReRAM) has been expected to be a next generation non-volatile memory. However, poor reproducibility of threshold voltage at which the dramatic change of the resistivity occurs hinders the practical application. We have attempted to improve the reproducibility of switching voltages using anodic porous alumina whose nanoholes are quite useful to restrict the filament forming area on the basis of the filament model. In this study, we have reported the pore size and film properties dependences of the variation width of the switching voltages. Two kinds of oxide films prepared by oxalic and sulfuric acids with two different anodic times were used as the insulating layer. Contrary to our expectation, just the sulfuric samples indicate good improvement about the switching voltages. Considering that the size of the effective contact area is not enough small against the filament size, the changing the film properties seems to be important for the suppressing the variation of switching voltages.

We have investigated the current–voltage characteristics of a resistive switching memory (ReRAM), especially the reproducibility of the switching voltage between an insulating state and a metallic state. The poor reproducibility hinders the practical use of this memory. According to a filament model, the variation of the switching voltage may be understood in terms of the random choice of filaments with different conductivities and lengths at each switching. A limitation of the number of conductive paths is expected to lead to the suppression of the variation of switching voltage. In this study, two strategies for the limitation have been proposed using an anodic porous alumina (APA). The first is the reduction of the number of conductive paths by restriction of the contact area between the top electrodes and the insulator. The second is the lowering of the resistivity of the insulator, which makes it possible to grow filaments with the same characteristics by electrochemical treatments using a pulse-electroplating technique.