Yutaka Abe

Crystallographic Features of ZnO Single Crystals

The atomic arrangements of ZnO single crystals have been investigated in detail using ion channeling methods. The ZnO crystals were grown by a flux method, and we focused on the channeling properties along the axis. In particular, to obtain information on O atoms in the crystal, a nuclear reaction induced by D+ ions was observed. As a result, it was revealed that the atomic arrangement of O atoms along the direction is better than that of Zn atoms in contrast with simple anticipation. This is quite remarkable but is regarded as one of the structural features of the crystal.

Bottom-gate amorphous InGaZnO4 thin-film transistor pH sensors utilizing top-gate effects

We discuss the sensitivity enhancement of bottom-gate type amorphous InGaZnO4 thin-film transistor (a-InGaZnO TFT) pH sensors from the viewpoint of top-gate effects. Comparing the top-gate effects in a-InGaZnO TFTs having TaOx and SiOx ion-sensitive insulators, we draw an analogy between the operations of dual-gate TFTs and TFT pH sensors. Our new concept for enhancing pH sensitivity is characterized by a high capacitance ratio of the ion-sensitive insulator to the bottom-gate insulator and pH sensing utilizing threshold-voltage shifts in bottom-gate transfer characteristics. The close similarity between top-gate effects and pH sensitivity strongly suggests that a common mechanism underlies the phenomena. We discuss the mechanism on the basis of the material properties of a-InGaZnO and the silicon-on-insulator (SOI) model that relates bottom- and top-gate electric fields in fully depleted operations. We believe that the pH-sensitivity enhancement utilizing top-gate effects is one of the potential applications that would make the most of the intrinsic features of a-InGaZnO TFTs.

Gate-to-source voltage response in high-sensitivity amorphous InGaZnO4 thin-film transistor pH sensors

In this paper, we discuss our top-gate-effect-based high-sensitivity amorphous InGaZnO4 thin-film transistor (a-InGaZnO TFT) pH sensor from the viewpoint of gate-to-source voltage (Vgs) response to small pH step variations. The a-InGaZnO TFT pH sensor, whose sensitivity is as high as 450 mV/pH, shows Vgs response to a pH step change of 0.1 with negligible hysteresis and good linearity. Because the high sensitivity is based on the enhancement of parallel shift in the transfer characteristics through the top-gate effect, the Vgs range for detecting is randomly selected.

Fabrication of suspended amorphous indium–gallium–zinc oxide thin-film transistors using bulk micromachining techniques

In this article, we propose a novel application of amorphous indium–gallium–zinc oxide (a-InGaZnO) thin films to micro electromechanical systems (MEMS). For this purpose, we investigated the residual stress in the a-InGaZnO thin films deposited by magnetron sputtering. The films with various residual stresses were characterized using atomic force microscopy, scanning electron microscopy, and X-ray diffraction. We found that the residual stress strongly depended on the sputtering gas pressure. In addition, we discovered a relationship between the residual stress and the microstructure in a-InGaZnO thin films. To gain more insight into the a-InGaZnO thin films with various residual stresses, we fabricated a-InGaZnO thin-film transistors (TFTs), and measured their transfer characteristics. We also fabricated a-InGaZnO TFTs on self-supported membranes, combining the typical TFT fabrication process and bulk micromachining techniques, for the application of a-InGaZnO to MEMS devices.

Demonstration of Detecting Small pH Changes Using High-Sensitivity Amorphous InGaZnO 4 Thin-Film Transistor pH Sensor System

In this brief, we discuss our top-gate-effect-based high-sensitivity amorphous InGaZnO4 thin-film transistor (a-InGaZnO TFT) pH sensor system. The pH sensor system consists of an a-InGaZnO TFT pH sensor and a driving circuit that was designed to bring out the high-sensitivity performance of the sensor. We present a primary demonstration for the a-InGaZnO TFT pH sensor system that has the potential for accurately detecting stepwise pH changes as small as 0.02.

Reduction of the Supply Current of Single-Flux-Quantum Time-to-Digital Converters by Current Recycling Techniques

We have been developing superconducting time-of-flight mass spectrometry systems that utilize superconducting strip particle detectors and single-flux-quantum (SFQ) time-to-digital converters (TDCs). We previously demonstrated a 24-bit SFQ TDC with a 3 × 24-bit first-in first-out (FIFO) buffer using the AIST Nb standard process (STP2) with a time resolution of 100 ps. In this study, we improved the SFQ TDC by using a current recycling technique. The technique enables us to increase the FIFO buffer capacity without increasing the total supply current, and the result is an improvement of ion count rates. We designed and demonstrated the SFQ TDC with a 12 × 24-bit FIFO buffer, whose total supply current was reduced by 71% with the current recycling technique.