Nakaoka Toshihiro

Abstract The shape of conductive filaments in CBRAM is important for resistance switching and conductance modulation, especially in applications like neuromorphic and reservoir computing that use conductance as weight. We report on RF-induced modulation of CBRAM using Ge₂Sb_3.4Te_6.2 with sheet-like filaments and compared it to those with dendritic filaments. RF input below 100 MHz reduced SET and RESET voltages, similar to CBRAM with dendritic filaments, but showed significantly different resistance changes. Repeated RF on/off input gradually increased the resistance of low-resistance state, unlike the dendritic filament CBRAM, where the resistance decreased. The increased resistance suggests RF-induced denser sheet-like filaments. Furthermore, the resistance of the high-resistance state showed a peculiar RF-induced resistance change not observed in dendritic filaments. The resistance decreased during RF input and increased to nine times the initial value when RF was switched off. The results show that the conductance modulation by RF input strongly depends on the filament type.

The electronic spins of rare-earth materials are attractive candidates for spin qubits and quantum memories. To access individual spins, tuning of the g-factor is desirable. Here, we report on local strain-dependent g-factors of the ⁵D₀–⁷F₂ transitions of Eu³⁺ centers in GaN:Eu thin films. We have found a clear correlation between the effective g-factor and the emission energy shift induced by the local strain. The combination of micro-photoluminescence and scanning electron microscope/electron backscattering diffraction measurements has revealed that the compressive strain of 0.2%–0.4%, relative to a surrounding reference point, induces an energy shift of about 3 meV. The strain decreases the g-factor of the emission at 1.991 eV from 2.5 to 1.5, while the strain increases the g-factor of the emission at 1.994 eV from 1.1 to 1.7. The result suggests that the g-factor can be tuned by the local strain. On the basis of the strain-induced energy shift and the g-factor, we have identified the optical sites. The ⁵D₀–⁷F₂ transitions observed in this study consist of three optical sites with C_3v symmetry and one site with C_1h symmetry.

We report on the spectroscopy of single InGaN/GaN nanocolumns that are site-controlled nanostructures allowing for pixel-like large-scale integration. A single nanocolumn shows several narrow photoluminescence peaks at around 600 nm at 20–90 K, the linewidth of which ranges between 0.3 and 10 meV. We have observed an interesting temperature dependence of the integrated intensity, the maximum being around 40–70 K, which suggests the existence of efficient carrier injection channels from localized states. The results show that the optical properties of single nanocolumns are favorable for the future large-scale integration of single-photon emitters and qubits.