Yusuke Nishi

Effects of 6H-SiC surface reconstruction on lattice relaxation of AlN buffer layers in molecular-beam epitaxial growth of GaN

Growth of GaN on on-axis 6H-SiC (0001)Si substrates with an AlN buffer layer was performed by molecular-beam epitaxy. The effects of SiC surface reconstruction on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers were studied. High-temperature HCl-gas etching followed by HF chemical treatment resulted in an atomically flat SiC surface with a 1×1 structure. The AlN layer grown on the surface showed slow lattice relaxation. GaN grown on the AlN buffer layer exhibited the narrowest (0002) x-ray rocking curve of 70 arcsec and 107 cm−2 screw-type dislocation density, which was two orders of magnitude smaller than that of GaN grown on as-received substrates.

Temperature dependence of conductance in NiO-based resistive switching memory showing two modes in the forming process

Two modes were observed in the forming process in a resistive switching cell based on NiO deposited by radio-frequency reactive sputtering. After semi-forming, the first step of the forming, the conductance of the cell exhibited several discrete values characterized by integer n multiples of the quantized conductance G0. n was 1 just after semi-forming and increased depending on the applied voltage. Nonpolar-type resistive switching behaviors were confirmed after both the first and the second step of the forming. We investigated temperature dependence of conductance in several states, the structural properties and oxygen composition in NiO, and a model for resistive switching phenomena was discussed.

Microscopic Investigation of the Electrical and Structural Properties of Conductive Filaments Formed in Pt/NiO/Pt Resistive Switching Cells

Conductive filaments formed in Pt/NiO/Pt resistive switching (RS) cells were characterized by conductive atomic force microscopy (C-AFM) and cross-sectional transmission electron microscopy (XTEM) after the removal of top electrodes. The area and current of the filaments were directly detected by C-AFM. By examining various RS cells with different cell resistances in the low-resistance state (RLRS), it has been revealed that the RLRS variation in the range of 10–220 Ω is mainly caused by the variations in the area and shape of the filaments. XTEM was carried out on a filament that dominantly determines RLRS. The area of the filament significantly varied along the thickness direction of a NiO film, which was consistent with the speculation from C-AFM analysis. Furthermore, the filaments observed in this study consist of reduced NiO with an inclusion of platinum, the electrode material, owing to Joule heating by the current during forming.

Correlation between Oxygen Composition and Electrical Properties in NiO Thin Films for Resistive Random Access Memory

Admittance spectroscopy measurement has been performed on NiOx thin films with various oxygen compositions (x = 1.0–1.2) in order to characterize localized defect levels. The activation energy and concentration of localized defect levels in NiOx films with low oxygen composition (x≤1.07) are 120–170 meV and lower than 2×1019 cm-3, respectively. The result of current–voltage measurement of Pt/NiOx/Pt structures indicated that samples with high oxygen composition (x≥1.10) did not show resistance switching operation, whereas samples with low oxygen composition (x≤1.07) did. The best oxygen composition of NiOx thin films for realizing repeatable and stable resistance switching operation turned out to be 1.07.

Dominant conduction mechanism in NiO-based resistive memories

The conduction characteristics of Pt/NiO/Pt resistive switching cells were investigated systematically through DC and AC conductance measurements in each of three resistance states: the initial state, the low resistance state (LRS), and the high resistance state (HRS). In the initial state, two different conduction processes are dominant, although this dominance is temperature-dependent. The activation energy (Ea) of the DC conductance was approximately 4.3 meV below 170 K. In addition to the low Ea, the frequency and temperature dependence of the AC conductance confirmed that the conduction originated from hopping between isolated sites. However, above 170 K, the Ea of the DC conductance was 330 meV, and this conductance was independent of frequency at low frequencies. The dominant conduction process above 170 K was found to be band conduction from analysis of the AC conductance. The resistance in the LRS varied linearly with temperature and was almost independent of frequency, thus representing the char...

Effect of NiO crystallinity on forming characteristics in Pt/NiO/Pt cells as resistive switching memories

Resistive switching (RS) in metal/oxide/metal stack structures plays a key role in resistive RAM. The formation and rupture of conductive filaments have been widely accepted as an origin of RS mechanism especially in binary transition metal oxides. Forming exhibits some analogies with a dielectric breakdown of SiO2 thin films. In this study, Time-Dependent Forming (TDF) characteristics of Pt/NiO/Pt stack structures have been investigated. The results revealed that the formation of conductive filaments at the forming process by applying constant voltage followed a weakest-link theory and that the weakest spots were almost randomly distributed in NiO thin films according to the Poisson statistics. Furthermore, the distribution of TDF characteristics depends on NiO crystallinity. A small variation of initial resistance tends to result in a large variation of time to forming and vice versa.

Effects of Heat Treatment on the Resistive Switching Characteristics of Pt/NiO/Pt Stack Structures

Effects of heat treatment on the resistive switching characteristics of Pt/NiO/Pt cells were investigated. Heat treatment was carried out under various conditions, and different resistive switching characteristics were obtained depending on the oxygen desorption from NiO films. After heat treatment by which the oxygen composition of the NiO films was not changed, the resistance ratio was markedly improved owing to the increase in the initial resistance (RIni). It was revealed that the increase in RIni is linked to the increase in its activation energy, which may be related to carrier excitation from defect states. On the other hand, many of the cells were initially in the low-resistance state after heat treatment by which the oxygen desorption from NiO films took place. This result indicates the formation of conductive filaments by oxygen desorption.

Conductance fluctuation in NiO-based resistive switching memory

In nonvolatile resistive memory and neuromorphic computing, the formation and rupture of a conductive filament after the forming process causes a reversible resistance transition between low- and high-resistance states. We confirm herein that conductance fluctuations by sweeping the applied voltage before and after “semi-forming” appear in Pt/NiO/Pt resistive switching cells through an investigation of nonpolar resistive transitions after semi-forming and “second forming.” The increase in conductance owing to conductance fluctuations originates from the modification of oxygen-vacancy densities at grain boundaries in the NiO layer. Moreover, this modification may disappear at 470 K. The influence of a significant Joule heating caused by current through a fat filament created by second forming determines whether the cell conductance after the reset takes on the value of the conductance just after second forming or that of the initial conductance. Thus, Joule heating seems to be the driving force behind the reset in Pt/NiO/Pt cells.In nonvolatile resistive memory and neuromorphic computing, the formation and rupture of a conductive filament after the forming process causes a reversible resistance transition between low- and high-resistance states. We confirm herein that conductance fluctuations by sweeping the applied voltage before and after “semi-forming” appear in Pt/NiO/Pt resistive switching cells through an investigation of nonpolar resistive transitions after semi-forming and “second forming.” The increase in conductance owing to conductance fluctuations originates from the modification of oxygen-vacancy densities at grain boundaries in the NiO layer. Moreover, this modification may disappear at 470 K. The influence of a significant Joule heating caused by current through a fat filament created by second forming determines whether the cell conductance after the reset takes on the value of the conductance just after second forming or that of the initial conductance. Thus, Joule heating seems to be the driving force behind the ...