Daisuke Matsumura

Retinal vein occlusion is a disease that causes vision disorder due to occlusion of the retinal vein which is one kind of micro-vascular spreading to the fundus. Most of the current treatments are aimed at suppressing symptoms caused by vascular occlusion. In this study, we focused on the impact pressure of micro-jet generated by the collapse of the electrical-induced bubbles. This pressure can be worked as local and minimally invasive physical stimuli from the outside of the blood vessel to resolve the vascular occlusion itself. Capacitance measurements were conducted to sense the approach of the device to the target. In addition, we conducted experiments to eliminate thrombus in vascular occlusion model in vitro and in vivo to establish a new treatment method for retinal vein occlusion targeting vascular occlusion.

Atomic-layer-deposition (ALD) Al₂O₃ films are promising as gate insulators of non-Si semiconductor devices. Although they allow relatively small leakage currents just after deposition, ALD Al₂O₃ films formed at low temperatures are subject to high temperature during fabrication or operation of devices. Therefore, the effect of post-deposition annealing (PDA) on the properties of Al₂O₃ films is investigated in this study. ALD Al₂O₃ films formed using H₂O oxidant at low temperatures are compacted by PDA, but their mass density and dielectric constant remain approximately unchanged or slightly decrease owing to the desorption of methyl groups contained in the films as impurities. In accordance with these results, the wet etching rate of Al₂O₃ films is not much reduced by PDA. The conduction current in ALD Al₂O₃ films formed on Si is reduced by PDA and becomes smaller than that in films formed at the same ALD temperatures as those of PDA. The conduction current for PDA temperatures above 250 °C, however, increases and, accordingly, spoils the merit of low-temperature ALD. Therefore, given that the dielectric constant of annealed films remains low, high-temperature ALD is practically more significant than applying PDA to low-temperature ALD Al₂O₃ films from the viewpoint of leakage current under the same thermal budget. Space-charge-controlled field emission analysis revealed that, at the aforementioned threshold temperature, PDA abruptly increases the Al₂O₃/SiO₂ interfacial dipoles and simultaneously reduces the amount of the positive charge near the interface. The so-called negative-charge buildup by PDA might be caused by this decrease in the positive charge.

We fabricated and characterized black polycrystalline diamond field effect transistors. By implementing a C-H bonded channel with a wide gate-drain length up to 20 μm, a breakdown voltage of 1.8 kV was achieved, which is the highest value reported for a diamond field effect transistor (FET) to date. Several of our devices achieved a breakdown voltage/wide gate-drain length ratio > 100 V/μm. This is comparable to the performance of lateral SiC and GaN FETs. We investigated the effects of voltage stress up to 2.0 kV, and showed that the maximum current density fell to 26% of its initial value of 2.42 mA/mm before the device eventually broke down at 1.1 kV.

To develop high-performance, high-reliability gate insulation and surface passivation technologies for wide-bandgap semiconductor devices, the effect of atomic layer deposition (ALD) temperature on current conduction in Al₂O₃ films is investigated based on the recently proposed space-charge-controlled field emission model. Leakage current measurement shows that Al₂O₃ metal-insulator-semiconductor capacitors formed on the Si substrates underperform thermally grown SiO₂ capacitors at the same average field. However, using equivalent oxide field as a more practical measure, the Al₂O₃ capacitors are found to outperform the SiO₂ capacitors in the cases where the capacitors are negatively biased and the gate material is adequately selected to reduce virtual dipoles at the gate/Al₂O₃ interface. The Al₂O₃ electron affinity increases with the increasing ALD temperature, but the gate-side virtual dipoles are not affected. Therefore, the leakage current of negatively biased Al₂O₃ capacitors is approximately independent of the ALD temperature because of the compensation of the opposite effects of increased electron affinity and permittivity in Al₂O₃. By contrast, the substrate-side sheet of charge increases with increasing ALD temperature above 210 °C and hence enhances the current of positively biased Al₂O₃ capacitors more significantly at high temperatures. Additionally, an anomalous oscillatory shift of the current-voltage characteristics with ALD temperature was observed in positively biased capacitors formed by low-temperature (≤210 °C) ALD. This shift is caused by dipoles at the Al₂O₃/underlying SiO₂ interface. Although they have a minimal positive-bias leakage current, the low-temperature-grown Al₂O₃ films cause the so-called blisters problem when heated above 400 °C. Therefore, because of the absence of blistering, a 450 °C ALD process is presently the most promising technology for growing high-reliability Al₂O₃ films.

The hydrogen-terminated diamond surface (C-H diamond) has a two-dimensional hole gas (2DHG) layer independent of the crystal orientation. A 2DHG layer is ubiquitously formed on the C-H diamond surface covered by atomic-layer-deposited-Al₂O₃. Using Al₂O₃ as a gate oxide, C-H diamond metal oxide semiconductor field-effect transistors (MOSFETs) operate in a trench gate structure where the diamond side-wall acts as a channel. MOSFETs with a side-wall channel exhibit equivalent performance to the lateral C-H diamond MOSFET without a side-wall channel. Here, a vertical-type MOSFET with a drain on the bottom is demonstrated in diamond with channel current modulation by the gate and pinch off.