Nobuo Haneji

Possible observation of Pb0 and Pb1 centers at irradiated (100)Si/SiO2 interface from electrical measurements

By the use of ac conductance measurements, we have observed two distinct defects with different capture cross sections at the x‐ray irradiated (100)Si/SiO2 interface. The capture cross section for one defect is a strong function of energy, while the other one is basically independent of energy. There is a strong possibility that these two interface defects correspond to the Pb0 and Pb1 centers seen in the electron spin resonance experiments. The fact that we do not observe the same behavior on (111) samples lends support to this hypothesis.

Possibility of Kosterlitz-Thouless effect at the resistive transition of high Tc oxide superconductors

Abstract The intrinsic resistive transition of an oxide superconductor is found to follow the relation of a Kosterlitz-Thouless transition of binding-unbinding of vortex pairs.

Electron Cyclotron Resonance-Reactive Ion Beam Etching of InP by Cyclic Injection of CH4/H2/Ar and O2.

Electron cyclotron resonance-reactive ion beam etching (ECR-RIBE) is very useful for fabricating semiconductor photonic devices and integrated circuits (PICs). The mixture gas of CH4/H2 is used for etching InP, but carbon polymer films deposited on the surface during the etching process suppress the etching and degrade the surface quality. Thus, the polymer films must be ashed off in O2 plasma. The authors have introduced the cyclic injection of CH4/H2/Ar and O2 to ECR-RIBE and have demonstrated that it is very useful for the etching of InP. Concerning optimized etching conditions for achieving a high etching rate and smooth etched surface, it has been shown that the etching time and the ashing time should be the same in one cycle, and that the interval times should be as short as possible. Good vertical anisotropic characteristics have been achieved by cyclic etching using a three-layer resist with a Ti layer. From photoluminescence measurements, it has also been verified that this cyclic ECR-RIBE process did not cause any serious damage.

Observation of Giant Electrorefractive Effect in Five-Layer Asymmetric Coupled Quantum Wells (FACQWs)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We fabricated a waveguide phase modulator with a core layer of GaAs/AlGaAs multiple FACQWs, and a giant electrorefractive (ER) effect in the FACQW was measured for the first time. The ER sensitivity Δn/ΔF measured by the Fabry-Perot resonance method was as large as 1.7 ×10-4 cm/kV at around an electric field of 40 kV/cm. This giant sensitivity is in fair agreement with the theory. This result shows that the FACQW is promising for realizing ultrawide-band, ultrafast and low-voltage optical modulators and switches.

Fabrication and Optical Characterization of Five-Layer Asymmetric Coupled Quantum Well (FACQW)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We succeeded in fabricating GaAs/AlGaAs FACQW with monolayer accuracy by the molecular beam epitaxy (MBE) method by monitoring reflection high-energy electron diffraction (RHEED) specular beam intensity oscillation. Photoabsorption current measurements of the FACQW sample showed good agreement with theoretical results, and a potential for much lower voltage operation. In addition, we studied the growth sequences of GaAs/AlGaAs QWs in the migration-enhanced epitaxy (MEE) method in order to fabricate the FACQW with steeper and flatter heterointerfaces. The sequence of supplying materials for Al0.3Ga0.7As growth, on which there is no report, was modified and optimized, and the QWs of higher quality were obtained at a growth temperature of 490°C using the optimized sequence. The results of photoluminescence measurements show that the MEE method modified as mentioned above is a promising growth technique for the fabrication of FACQWs of higher quality.

Silicon Dioxide Film with Low Dielectric Constants using Liquid-Phase Deposition

We propose a method which is advantageous for forming F bonds in the Si–O network without any external energy assistance. This method can be used to grow SiO2 films in supersaturated fluorosilicic acid (H2SiF6) at room temperature. The dissociation reaction between H2SiF6 and H2O leads to the formation of silica and the deposition of SiO2 films on the surface of substrates. Because the reaction involves an aqueous acid which contains fluorine, F bonds are formed in the films naturally. The percentage of F atoms increased from 3.5 to 4.75% according to X-ray photoelectron sectroscopy (XPS) survey, and the F1S peak changed from 687.72 to 687.18 eV with an increase in concentration from 2.25 to 3.25 mol/l, respectively. In addition, due to the effect of F bonds inside the film, the dielectric constant also decreased from 3.7 to about 3. Atomic force microscopy (AFM) images showed that the surface roughness increased with increasing H2SiF6 concentration due to the attack of F; however, the maximum surface roghness was less than 0.5 nm.

Anomalous effect in La2-xSrxCuO4 of doping level x = 1/4n

Anomalous electric response of La 2-x Sr x CuO 4 thin film at Sr doping levels of x 1/4 and 1/16 is observed when charge moves along the c axis. The capacitance C t of the Pd/La 2-x Sr x CuO 4 /SrTiO 3 /Pd structure increases sharply, exceeding the capacitance C STO of the Pd/SrTiO 3 /Pd structure. Oscillatory negative voltage is observed in 4-terminal resistivity measurements of La 2-x Sr x CuO 4 thin film using Pd electrodes at small current. These phenomena are explained by a model of the spatial ordered state in which CuO 2 layers of La 2-x Sr x CuO 4 is replete with O hole pairs.

Electron Cyclotron Resonance-Reactive Ion Etching of III-V Semiconductors by Cyclic Injection of CH4/H2/Ar and O2 with Constant Ar Flow

Electron cyclotron resonance-reactive ion etching (ECR-RIE) is very useful for fabricating semiconductor photonic devices and integrated circuits (PICs). The mixture gas of CH4/H2 is used for etching III-V semiconductors, but the carbon polymer film deposited on the surface during the etching process presents some problems. Thus, the polymer film must be ashed off using an O2 plasma. We introduced the cyclic injection of CH4/H2/Ar and O2 to ECR-RIE, and demonstrated that it was very useful for etching of InP. However, compound semiconductors containing Al (e.g., AlGaAs and InAlAs) react with oxygen and an alumina layer is formed, which cannot be etched by CH4/H2 etching. Therefore, we used a new cyclic etching process with constant Ar flow in the chamber to remove this alumina layer by Ar ion etching, and obtained good results for etching rate and surface morphology for the compound semiconductors containing Al. We also proposed a suitable combination of three cyclic etching procedures (continuous etching, cyclic etching without constant Ar flow and cyclic etching with constant Ar flow) for etching the multilayer heterostructure of III-V semiconductors including InP and/or compound semiconductors containing Al.

Anomalous ac dielectric response and carrier state of c-axis oriented La2−xSrxCuO4 films

Abstract In order to study the “fractional Hall effect model” of the carrier state of high temperature superconductors, we measured the x dependence of the capacitance C t of multilayer structure Pd/La 2− x Sr x CuO 4 /(100)SrTiO 3 /Pd. A strong correspondence is found between the x - C t relationship and the filling factor ν dependence of the system energy in the fractional quantum Hall effect.

Anodic oxidation of Si in oxygen/chlorine plasma

By adding a few percent of chlorine to oxygen plasma, the anodization rate of Si was enhanced; for example, the rate was doubled for oxygen containing 3-percent chlorine. With a chlorine concentration of 1.5 percent, the density of trap states at the Si-SiO<inf>2</inf>interface was reduced from 7×10¹¹/cm².eV to 5×10¹¹/cm².eV at the midgap of Si; after annealing at 800°C in argon for 60 min, it was reduced to 8 × 10¹⁰/cm².eV, and did not return to the original value after heating the specimen to 800°C. The density and capture cross section of traps in plasma-anodic oxide were also measured using the constant-current avalanche-injection method. The number of electron traps with small cross sections in plasmaanodic SiO<inf>2</inf>films was reduced by annealing them at 800°C in argon, but SiO<inf>2</inf>films which were anodized in oxygen/chlorine plasma showed an increase of trap density under the same annealing condition.