Toshihiko Maemoto

Rectification Effects of ZnO-Based Transparent Nanodiodes on Glass and Flexible Plastic Substrates

Self-switching nanodiodes (SSDs) using zinc oxide (ZnO) were fabricated on glass substrates. The SSDs using ZnO have attracted significant attention as transparent devices because of their low cost, abundance in nature, and so on. Rectification characteristics in the SSDs were resemblance to the characteristics of conventional diodes with use of a doping junction or a barrier structure. The changes in characteristics depending on the shape of SSDs were investigated. Channel widths in the SSD of 230 and 190 nm and turn-on voltages of 5 and 8 V were obtained. On the other hand, it was found that the channel length influences the current strength. Moreover, after coating the devices with HfO2 to enhance the electric field coupling, the rectification behavior was maintained while the device current increased dramatically. The SSDs were fabricated using ZnO on flexible plastic substrates. For channel widths of 250 and 200 nm, turn-on voltages of 4 and 6 V were obtained, respectively. We also obtained clear rectification and observed the dependence of the turn-on voltage on the channel width.

Fabrication of Superconducting Transistors using InAs/(AlGa)Sb Quantum Wells

We report on a fabrication process of superconducting weak links and superconducting transistors, which contain two-dimensional electrons in InAs/(AlGa)Sb quantum well. The superconducting transistors with lead alloy superconducting contacts were fabricated by using electron beam lithography, lift-off, and wet chemical etching processes. We have succeeded in fabricating a superconducting transistor with 0.4 µ m channel length and 0.17 µ m gate length between superconducting electrodes. Although the ideal performance of superconducting transistors has not been achieved yet, we present the results of superconducting weak links. The process of reducing contact resistance between lead alloy and InAs is also discussed, which is crucial to demonstrate the performance of the superconducting transistor.

Quantum anti-dot arrays and quantum wire transistors fabricated on heterostructures

We report on a new fabrication process of anti-dot and multiple quantum wire transistors based on InAs/AlGaSb heterostructures with conventional photolithography and wet chemical etching. Starting from the square arrays of a mask, anti-dot arrays of circular shape were fabricated utilizing the unique etching properties of this material system, which are quite different from those of its counterpart GaAs/AlGaAs. Successful formation of the anti-dot structures was confirmed by magnetotransport measurements. For the fabrication of multiple quantum wire transistors, we introduced a GaAs regrowth as a hole barrier between the gate electrode and the InAs channel layer to eliminate the inherent gate leakage current for this material system and to obtain good device performance. By optimizing the fabrication process, we succeeded in forming quantum wires with a width as narrow as 100 nm. The device performance of the quantum wire transistor is compared with that of conventional two-dimensional electron gas transistors.

Magneto-transport properties of InAs/AlGaSb open quantum dot structures

We report on the magneto-transport properties of InAs/AlGaSb open quantum dot structures. The open dot structures have been fabricated by using electron beam lithography. The open dot structures are characterized by magnetoresistance measurements at 4.2 K. In addition to the Shubnikov-de Hass oscillations, aperiodic magnetoresistance fluctuations have been observed at low magnetic fields below 1 T. The amplitude of the fluctuations was quite large even at 4.2 K in comparison with the case of the GaAs/AlGaAs system. Electron-wave interference effect and phase-breaking times in InAs mesoscopic structures are discussed from the analyses using correlation functions of magneto-conductance fluctuations.

Structural and optical properties of ZnMgO thin films grown by pulsed laser deposition using ZnO-MgO multiple targets

We report on structural and optical properties for Zn1-xMgxO (ZMO) thin films produced by pulsed laser ablation. ZMO thin films were grown on a-plane Al2O3 substrates at 400°C. In order to efficiently incorporate Mg into ZnO thin films, we used multiple ZnO-MgO ablation targets. Pulses from a Nd:YAG laser (4th harmonic generation: 266 nm) were directed on the ZnO-MgO ablation targets, which consisted of MgO single crystals mounted on ZnO ceramic targets. The ZMO films were characterized by x-ray diffraction, optical transmittance and cathodeluminescence (CL) measurements. Highly c-axis oriented ZMO(0002) reflections corresponding to the wurtzite-phase were observed. The c-axis lattice constants of the films were determined from the ZnMgO(0002) peak. The c-axis length of the ZMO films decreased linearly with Mg content. From the optical transmittance spectra of ZMO films, we observed a blue shift in the absorption edge with increasing Mg content. Band gap energies of ZMO thin films were determined from the optical transmittance and CL spectra. We found that the band gap energy changed from 3.27 eV to 3.95 eV. The Mg content of ZMO films increased monotonically with the number of laser pulses which struck the MgO target. These results show that laser ablation using multiple targets of ZnO and MgO is effective for band engineering of ZMO.

A potentiometric immunosensor based on a ZnO field-effect transistor

The characteristics of an aptamer-modified Ta2O5/ZnO field-effect transistor as a prototype label-free immunosensor for the potentiometric electrical detection of immunoglobulin G (IgG) were studied. The Ta2O5/ZnO film was grown on a glass substrate by a facing-target sputtering method. Stable operation in electrolyte solution with a small hysteresis width and a low gate leakage current was realized. The immunosensor exhibited a proportional sensitivity to the logarithmic human IgG concentration in the range of 0.35–23 µmol·dm−3 with a rapid response time of ∼15 s.

Rectification effects in ZnO-based transparent self-switching nano-diodes

We report on the fabrication and characterization of the self-switching nano-diodes using zinc oxide films grown on glass substrates for use as transparent rectifiers. Diode-like characteristics were clearly observed. The current clearly increased with increases in the channel length and the turn-on voltage sifted toward zero volt with decreasing the channel width. Overall, we found that the rectifying characteristics can be controlled by the device geometry. The rectifying characteristics were further investigated by reducing the electron concentration. Although the current value decreased as a result of the to reduction in the electron concentration, it was found that rectifying characteristics, in particular the turn-on voltage, can be controlled by the electron concentration.

Pb-doped bisrcacuo superconducting thin films grown by halide-CVD

Abstract Effects of Pb-doping in superconducting BiSrCaCuO films have been experimentally studied by using a halide-CVD from BiCl3, PbCl2, SrI2, CaI2 and CuI powders. The Pb/(Bi + Pb) ratio in oxide films was successfully controlled by the temperature of PbCl2. The Pb-doping enhanced the high-Tc phase to increase the critical temperature of the superconducting transition. The optimal Pb concentration was determined to be about 0.15 for the 2223 high-Tc phase.

Flexible zinc oxide thin-film transistors using oxide buffer layers on polyethylene napthalate substrates

We report on the fabrication and characterization of flexible ZnO thin-film transistors (TFTs) using multilayer oxide buffer layers. ZnO-TFTs on polyethylene napthalate (PEN) substrates were fabricated by pulsed laser deposition (PLD) at room temperature (RT). ZnO films were characterized by Hall effect measurements. The electron mobility and the carrier density were 52.4 cm²/Vs and 3.2×10¹⁷ cm⁻³. Top-gate-type TFTs were fabricated from a thin film of HfO<inf>2</inf> dielectric as a high-k material gate insulator. A transconductance, g<inf>m</inf>, of 1.7 mS/mm, an on/off-current ratio of 2.4×10⁶ and a threshold voltage, V<inf>th</inf>, of −1.2 V were achieved for a ZnO TFT with a SiO<inf>2</inf>/TiO<inf>2</inf> buffer layer.

Nonlinear electron transport in InAs/AlGaSb three-terminal ballistic junctions

We have fabricated and characterized an InAs/AlGaSb three-terminal ballistic junction device. The fabricated device exhibited nonlinear electron transport properties because of ballistic motion of electrons in this structure that is comparable to the electron mean free path. When the left branch is biased to a finite voltage Vand the right to a voltage of -V (push-pull fashion), negative voltages appeared at the floating central branch regardless of the polarity of the input voltages. In the case of the central branch grounded in push-pull fashion, the clear current rectification effect also observed in the current flow of the central branch at 4.2K to even at 300K.