Minoru Tachiki

First Demonstration of Rectifying Property of P-I-N Heterojunctions Fabricated by Tri-Layered Semiconducting Oxides

We have fabricated semiconducting oxide p-i-n diodes in order to explore minority carrier injection effects in oxide materials. The diodes comprised p-La0.85Sr0.15MnO3/i-SrTiO3/n-La0.05Sr0.95TiO3. All oxide layers were grown epitaxially by the eclipse pulsed laser deposition method on (100) SrTiO3 substrates. The diodes showed good rectifying properties, and from the flat-band capacitance measurement, we obtained a diffusion potential of about 0.2 V for these diodes in agreement with the calculated results. In the lower temperature regime, the space-charge-limited-current property was pronounced, arising from residual traps inside the i-SrTiO3 layer. The backward current was mostly due to tunneling through the i-SrTiO3 layer, showing no meaningful expansion of depletion regions in both p- and n-oxide layers.

Control wettability of the hydrogen-terminated diamond surface and the oxidized diamond surface using an atomic force microscope

Abstract The force–distance curve indicates the difference in wettability between the hydrogen-terminated (H-terminated) diamond surface and the AFM-field-assisted local oxidized (O-terminated) area. Using a hydrophilic (silicon with native oxide) tip and a hydrophobic (coated with gold) tip as AFM tips, the adhesion force mapping measurement shows that the H-terminated surface is hydrophobic and that the O-terminated surface is hydrophilic. After heating in vacuum, the difference in the adhesion force between two surfaces decreased. This means that water adsorbed on the tip and sample surfaces affects the adhesion force. As an alternative measurement, the contact angle measurement of the H-terminated surface and the chemically oxidized surface was performed. It is proved that the oxidized surface is more hydrophilic than the H-terminated surface and that its surface energy is derived from surface polarity such as that involved in hydrogen bonding and electric dipole which is twofold that the H-terminated surface.

High-frequency performance of diamond field-effect transistor

The microwave performance of a diamond metal-semiconductor field-effect transistor (MESFET) is reported for the first time. MESFETs with a gate length of 2-3 /spl mu/m and a source-gate spacing of 0.1 /spl mu/m were fabricated on the hydrogen-terminated surface of an undoped diamond film grown by microwave plasma chemical vapor deposition (CVD) utilizing a self-aligned gate fabrication process. A maximum transconductance of 70 mS/mm was obtained on a 2 /spl mu/m gate MESFET at V/sub GS/=-1.5 V and V/sub DS/=-5 V,for which a cutoff frequency f/sub T/ and a maximum oscillating frequency f/sub max/ of 2.2 GHz and 7 GHz were obtained, respectively.

Room-Temperature Heteroepitaxial Growth of NiO Thin Films using Pulsed Laser Deposition

Room-temperature growth of NiO thin films using the pulsed laser deposition (PLD) technique was investigated. Epitaxial growth of NiO thin films on (100)MgO substrates was not obtained when we simply placed a shadow mask between the substrate and target (eclipse PLD method), but was obtained when we used the conventional PLD method under the optimum ambient pressure condition. However, by applying a magnetic field from the substrate side during growth, NiO epitaxy was achieved even for the eclipse PLD method. This is due to the enhanced ionization and energization of flying species by the magnetic field application.

Nanofabrication on Hydrogen-Terminated Diamond Surfaces by Atomic Force Microscope Probe-Induced Oxidation.

Field-assisted local oxidation on a hydrogen-terminated (001) diamond surface was performed using an atomic force microscope (AFM). Anodic oxidation by a surface water meniscus layer is suggested to account for this oxidation process. Through the oxygenated area, Fowler-Nordheim (F-N) tunneling current was observed. The difference in electron affinity between the hydrogen-terminated surface and the oxygenated area was confirmed by scanning electron microscopic (SEM) observations.

Cl− sensitive biosensor used electrolyte-solution-gate diamond FETs

We have investigated the electrolyte-solution-gate field effect transisitors (SGFETs) used hydrogen terminated (H-terminated) or partially oxygen terminated (O-terminated) polycrystalline diamond surface in the Cl- and Br- ionic solutions. The H-terminated channel SGFETs are insensitive to pH values in electrolyte solutions. The threshold voltages of the diamond SGFETs shift according to the density of Cl- and Br- ions about 30 mV/decade. One of the attractive biomedical applications for the Cl- sensitive SGFETs is the detection of chloride density in blood or in sweat especially in the case of cystic fibrosis. The sensitivities of Cl- and Br- ions have been lost on the partially O-terminated diamond surface. These phenomena can be explained by the polarity of surface change on the H-terminated and the O-terminated surface.

Ozone-treated channel diamond field-effect transistors

Diamond field-effect transistors (FETs) whose channel is partially oxidized and highly resistive are fabricated by ozone treatment. These FETs are operated in electrolyte solutions. From XPS analyses, it is evident that hydrogen-terminated (H-terminated) diamond is partially oxygen-terminated (O-terminated) by ozone treatment. The quantification of surface oxygen in ozone-treated diamond is carried out. The quantification shows that the surface oxygen increases with an increase in ozone treatment time indicating the control of oxygen coverage. The partially O-terminated diamond surface channel is much less conductive compared with the H-terminated diamond. The ozone-treated FETs were operated stably even though the channel of the FETs becomes highly resistive. For the sensing of particular ions or molecules by the immobilization of sensing components, the control of surface termination is necessary.

Manipulation of Laser Ablation Plume by Magnetic Field Application

Time- and wavelength-resolved observation of a laser ablation plume from a strontium titanate target was performed under a dipole magnetic field applied from the substrate side. In contrast to the conventional method, the present method gave rise to increasing ionization of the growth species in the plume in the vicinity of the substrate. Using this method, NiO films could be epitaxially grown on (100)MgO substrates even at room temperature. When we placed a shadow mask between the target and the substrate (eclipse method), the deposition rate was increased to by 2.6-fold that under zero magnetic field.

Potential applications of surface channel diamond field-effect transistors

Abstract In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm −1 . High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f T and maximum frequency of oscillation f max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f T and 30 GHz of f max .

Effect of iodide ions on the hydrogen-terminated and partially oxygen-terminated diamond surface

Abstract The effect of I − ions on the threshold voltages of the electrolyte-solution-gate diamond field-effect transistors (SGFETs) in KI solution is investigated. The threshold voltages of hydrogen-terminated (H-terminated) diamond SGFETs shift in the KI concentration range of 10 −6 –10 −1 M in aqueous solutions. The sensitivity of the H-terminated diamond surface to I − ions is higher than that to Cl − or Br − ions. However, the sensitivity to I − ions of the partially oxygen-terminated (O-terminated) diamond surface drastically decreases with ozone treatment. The mechanisms of these phenomena can be explained by the surface charge and the adsorbability of I − ions on the H-terminated and O-terminated diamond surfaces.