Ken-ichi Ogata

ZnO and ZnMgO growth on a-plane sapphire by molecular beam epitaxy

The growth of ZnO and ZnMgO alloy on a-plane sapphire substrates using an Epiquest-MBE system with an RF oxygen plasma cell was studied. Prior to growth, thermal treatment at 680/spl deg/C followed by oxygen plasma irradiation at 500/spl deg/C was performed for the improvement of the surface roughness of sapphire substrates. Then, ZnO layers with thickness of 0.6 /spl mu/m were grown in the range of 400-700/spl deg/C. Two factors were found for obtaining high quality layers. One is the use of a low temperature (250/spl deg/C) ZnO buffer layer and successive thermal treatment at high temperature (750/spl deg/C), which suppressed the formation of twin structure of subsequent layers. The other is the choice of growth temperature. The root mean square (RMS) roughness measured by atomic force microscopy (AFM) was minimized to 0.42 nm when growth temperature was 500/spl deg/C.

Improvements of ZnO qualities grown by metal-organic vapor phase epitaxy using a molecular beam epitaxy grown ZnO layer as a substrate

Zinc oxide (ZnO) of high quality was homoepitaxially grown by metal-organic vapor phase epitaxy (MOVPE) on molecular beam epitaxy (MBE)-grown ZnO layers after the pretreatment of the underlying MBE-ZnO at 1000°C in N2 which resulted in an atomically flat surface. In photoluminescence at 15 K, the 3 meV line width of the emission from donor-bound-excitons (D0X) and the observation of the fourth phonon replica of the emission from free-excitons (EX) have demonstrated the high potential of MOVPE growth of ZnO toward optical applications.

Implant isolation of ZnO

We study ion-irradiation-induced electrical isolation in n-type single-crystal ZnO epilayers. Emphasis is given to improving the thermal stability of isolation and obtaining a better understanding of the isolation mechanism. Results show that an increase in the dose of 2 MeV 16O ions (up to ∼2 orders of magnitude above the threshold isolation dose) and irradiation temperature (up to 350 °C) has a relatively minor effect on the thermal stability of electrical isolation, which is limited to temperatures of ∼300–400 °C. An analysis of the temperature dependence of sheet resistance suggests that effective levels associated with irradiation-produced defects are rather shallow (<50 meV). For the case of implantation with keV Cr, Fe, or Ni ions, the evolution of sheet resistance with annealing temperature is consistent with defect-induced isolation, with a relatively minor effect of Cr, Fe, or Ni impurities on the thermal stability of isolation. Results also reveal a negligible ion-beam flux effect in the case o...

Piezoelectric Carrier Confinement by Lattice Mismatch at ZnO/Zn0.6Mg0.4O Heterointerface

This paper describes a strong piezoelectric carrier confinement at a ZnO/Zn0.6Mg0.4O heterointerface grown on an a-plane sapphire substrate by molecular beam epitaxy. A ZnO/Zn0.6Mg0.4O double-heterojunction structure was grown without intentional doping and the formation of a deep potential well for electrons was confirmed using cathodoluminescence and transmittance spectra. Photoluminescence spectra at 4.5 K consisted of an intense near-band-edge emission at 3.359 eV and a broad and weak peak on the low-energy side. The results of Hall effect measurement revealed that the conduction is n-type with a high carrier concentration of ?1.2?1013 cm-2. The mobilities are ?170 cm2/V?s at 300 K and ?400 cm2/V?s at 77 K without deterioration at lower temperatures; these values are much higher than those of a thick single-layer ZnO film grown on an a-plane sapphire substrate. We attribute these optical and electrical properties to the formation of two-dimensional electron gas at the ZnO/Zn0.6Mg0.4O heterointerface by the piezoelectric polarization in a strained ZnO well.

Ion-Sensitive Characteristics of an Electrolyte-Solution-Gate ZnO/ZnMgO Heterojunction Field-Effect Transistor as a Biosensing Transducer

Characteristics of an ion-sensitive ZnO/ZnMgO heterojunction field-effect transistor (HFET) with an amine-modified single-crystalline O-polar ZnMgO gate electrode are discussed to develop the application to biosensing transducers. The ion-sensitivity was based on the proton transfer to/from the amino groups on the gate electrode, the amine-modification of which was performed using a silanization technique by immersing the HFET into an aminosilane based solution. Stable operation in electrolyte solution in accordance with the standard FET theory with small hysteresis and small leakage current was confirmed, and the amperometric operation revealed a high pH sensitivity of -20 µA/pH with a reproducible result. A potential application of the ion-sensitive HFET to amperometric biosensing transducers was also demonstrated by immobilizing enzyme molecules of glucose oxidase on the amine-modified gate electrode.

Characteristics of Enzyme-Based ZnO/Zn0.7Mg0.3O Heterojunction Field-Effect Transistor as Glucose Sensor

The characteristics of an enzyme-based ZnO/Zn0.7Mg0.3O heterojunction field-effect transistor (HFET) for glucose sensing are reported. The enzyme glucose oxidase (GOD) was immobilized on an amine-modified gate electrode of the ZnO/Zn0.7Mg0.3O HFET, and glucose sensing was performed by detecting biocatalytically yielded protons using an ion-sensitive function of the gate electrode. The chemical bonding states of the amine-modified and GOD-immobilized surfaces were analyzed by X-ray photoemission spectroscopy, and the enzyme activity of GOD was examined by a colorimetric method. In agreement with the promising results of these experiments, this enzyme-based HFET exhibited stable sensing performance with a linear response in a wide range of glucose concentrations from 0 to 4 mgcm-3 and at a short time constant less than 20 s.

Characteristics of Polycrystalline ZnO-Based Electrolyte-Solution-Gate Field-Effect Transistors Fabricated on Glass Substrates

Characteristics of ion-sensitive operation of electrolyte-solution-gate field-effect transistors (ESG-FETs) are reported with implications for the development of healthcare chips. ZnO-based polycrystalline films grown on glass substrates by sputtering were used for the ESG-FETs with the modification of the gate electrode by amino groups. The equilibrium proton transfer to/from the amino groups was found to change the gate electrode potential at a constant rate of 58 mV/pH, which successfully modulated the current flow in transistors in accordance with a conventional FET theory. A typical pH sensitivity of -0.8 µA/pH with a small time-constant of 5 s was obtained for the ESG-FET operation with 3×5 mm2 gate area.

Fabrication of ZnO nanorods on O-polar ZnO layers grown by molecular beam epitaxy and electrical characterization using conductive atomic force microscopy

Using the aqueous solution with microwave-assisted heating, ZnO nanorods were fabricated on an O-polar single crystal molecular beam epitaxial grown ZnO layer. Film-like ZnO due to aggregation of nanorods was observed when heated at 95 °C with a precursor concentration of 100 mM. On the other hand, hexagonal ZnO nanorods with well-defined orientation were achieved by lowering the reaction temperature down to 60 °C with a precursor concentration of 10 mM. The top surface of the hexagonal ZnO nanorods has a roughness of a few nm, revealed by means of atomic force microscopy (AFM). Conductive-AFM measurement indicates Schottky rectifying behavior in Au-coated cantilever/ZnO nanorods. The current–voltage characteristics are discussed in connection with the loading force of AFM measurements.

X-ray photoelectron spectroscopy characterization of aminosilane anchored to ZnO nanorod arrays grown by an aqueous solution method with microwave-assisted heating

An intermediate for immobilizing biofunctional molecules, 3-aminopropyltrimethoxysilane (APTMS) was anchored to ZnO nanorod arrays which had been grown by an aqueous solution method with microwave-assisted heating on a-plane sapphire substrates. X-ray photoelectron spectroscopy analysis revealed that the quantity of APTMS anchored on the ZnO nanorod arrays was more than that on a flat ZnO layer. Also, annealing of the APTMS anchored to ZnO nanorods at 500°C showed that the amino groups were disappeared while Si atoms remained after the annealing probably due to strong Si–O bonding.

Anomalous elastic–plastic transition of MgO under shock compression

The particle velocity profiles of an MgO single crystal under shock compression were measured up to a pressure of 35 GPa by a velocity interferometer system for any reflector using a LiF window combined with a powder gun. The Hugoniot-elastic limit (HEL) of MgO along the ⟨100⟩ direction was 2.9–4.3 GPa and the initial elastic waves contain some oscillations. Along the ⟨110⟩ direction, the HEL was higher than along the ⟨100⟩ direction and anomalous, two-step-structure elastic waves were observed. These characteristics are discussed based on the slip system of the MgO crystal.