Table of contents

Excess noise in amorphous Selenium avalanche photodiodes (a-Se APD) has been measured in a frequency range from 3 kHz to 30 kHz. The deduced excess noise factors, including dependences on photocurrent, frequency, applied electric field and the a-Se layer's thickness, agreed with McIntyre's theoretical values.

It had been reported that lead alloying causes the disappearance of PD in Ge-S films. However, it was confirmed that LESR continues to exist in spite of the disappearance of the PD. For Ge-S films without lead, the LESR was observed by irradiation, the intensity of which is not so strong as that giving rise to the PD. It also turned out that the density of defects, including neutral and charged ones at room temperature, remains constant after strong irradiation at low temperature. From these results, it has become apparent that in Ge-S system the PD does not originate from bond-breaking.

Si dangling bonds and Ge dangling bonds in a-Si_1-xGe_x:H are separately estimated both by low-energy optical absorption measurements using a constant photocurrent method (CPM) and by ESR measurements. The ratio of the density of Si dangling bonds to that of Ge dangling bonds decreases with increasing germanium content, as expected. However, the ratio deduced from CPM is far larger than that deduced from ESR. Origins of the difference are discusssed.

Tellurium precipitation in CdTe is found to be induced by photoirradiation with energy higher than the energy gap at 240 W/cm². It is suggested that this photoinduced precipitation is related with the strong electron-phonon interactions, possibly self-trapped excitons. This irreducible tellurium precipitation may cause a serious problem for the life of semiconductor devices.

Carbonization of Si surfaces using hydrocarbon gas molecular beams was investigated. In the case of carbonizing bare Si surfaces with C₂H₂, single-crystalline 3C-SiC layers were obtained only in the narrow range of a substrate temperature near 780°C. Among various surface treatments, the existence of a surface oxide layer and temperature rise during carbonization is proved to be effective in reproducibly obtaining single-crystalline 3C-SiC layers.

Selective area growth of InP or InGaAs single layers and their superlattices has been studied. Increasing substrate temperature suppresses the formation of polycrystal grains on stripe masks. No deposition of the polycrystals occurs on 40-µm-wide masks above 510°C for InP and above 540°C for InGaAs. Secondary electron microscopy and cathode-luminescence (CL) spectroscopy of the superlattice reveal that layer thicknesses and CL peak wavelengths are uniform across the unmasked area, except in the vicinity of mask edges. The mechanism of selective growth is also discussed.

Y-based superconductors were prepared by sintering the mixtures of calcined YBa₂Cu₃O_y powder and NaNO₃. NaNO₃ has been found to have high oxygen potential in a YBa₂Cu₃O_y superconductor. With this potential, the critical temperature of YBa₂Cu₃O_y+4.5 wt.% NaNO₃ sintered in air was almost the same as that of a pure YBa₂Cu₃O_y sintered in oxygen. NaNO₃ or its decomposition products seemed to induce liquid phase sintering to result in larger grains and denser structure. With these structural changes, the critical current density was found to increase with NaNO₃.

To produce polycrystalline superconductor ceramics with potentially higher critical currents by eliminating grain boundary weak links caused by grain orientation misfit, we use a method combining grain shape alignment and magnetic field alignment by orthogonal alignment forces. Continuing this work, we report here X-ray diffraction data demonstrating the first preparation of an ∼80µm thick Bi₂Sr₂Ca_0.7Yb_0.3Cu₂O_x film having substantial biaxial grain alignment; a very high degree of c-axis alignment is combined with >30% preferential <100> axis alignment within the a-b plane (relative to a random <100> axis distribution in that plane). While films with this composition are not good superconductors, they demonstrate the feasibility of the method.

The superconducting transition temperature (T_c) and lattice constants of "1212" lead cuprate, (Pb/Cu)Sr₂(Ln/Ca)Cu₂O_z (Ln=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu and Y), were investigated. (Pb_0.75Cu_0.25)Sr₂(Ln_0.4Ca_0.6)Cu₂O_z ceramics were prepared by sintering at 1000°C for 1 h in oxygen. Obtained ceramics were annealed at 800°C for 24 h in oxygen and subsequently quenched. Linear correlation between lattice constants and ionic radii of Ln was observed in the "1212" phases. The T_c value increased with decreasing ionic radii of Ln. The Ln=Lu sample exhibited a T_c (zero resistivity) of 61 K in our oxperiments.

Polyhedral single crystals of oxide superconductor YBa₂Cu₃O_x have been grown under the condition where the solid phase Y₂BaCuO₅ and liquid phase coexist. The crystals obtained possess well-developed habits and have a thickness of up to 3 mm along the c-axis, when the solid-melt mixture is kept at 1050-1010°C and then cooled to 990-970°C. The annealed crystals show a sharp superconductive transition at 91 K. The temperature dependence of resistivity is found to be metallic both in the ab- and c-directions. A marked anisotropy in the broadening of the resistive transition is observed under magnetic fields parallel or perpendicular to the c-axis.

Hydrogenation of Nd_1.85Ce_0.15CuO₄ has been carried out at a low temperature of 110°C in 1 atm H₂. The contents of hydrogen and oxygen of H-doped samples are estimated by pulse H-NMR measurements and thermogravimetric analysis. The T_c values of H-doped samples are independent of the hydrogen content, whereas the volume fraction of superconductivity is significantly decreased with the increase in hydrogen content. These results are explained by the local disappearance of superconductivity in the neighborhood of a doped hydrogen atom.

The surface morphology of Tl₂Ba₂Ca₂Cu₃O_y thin films prepared by the thallium diffusion process was studied. Ba-Ca-Cu-O rf-sputtered films were annealed with a powder mixture of Tl₂O₃, BaCuO₂, Ca₂CuO₃ and CuO at a nominal composition of Tl₁Ba₁Ca₃Cu₃O_y. The films obtained by two-step annealing (at 900°C for 8 h and at 930°C for 8 min) had uniform appearance over a rather large area and exhibited superconductivity of T_c (zero resistivity)=117 K.

Superconducting Bi₂Sr₂Ca_n-1Cu_nO_y (n=1∼5) thin films have been prepared in situ by molecular beam epitaxy with highly condensed ozone as oxidizing gas. All films including thermally metastable phases of n=4 and 5 were obtained by sequential shutter control, and even the n=1 phase film showed superconductivity. The surface of the n=1 phase film was observed by reflection high-energy electron diffraction, and the modulated structure was clearly observed.

The effect of microwave-plasma on the decomposition and the oxidation of Ba(THD)₂ is studied for estimating the possibility of low-temperature chemical vapor deposition of oxide superconducting thin film. Pseudo in situ auger electron spectroscopy clarifies that carbon-free film is grown with oxygen-plasma at over 400°C. Pseudo in situ electron diffraction analysis indicated that the film grown at over 400°C is crystalline BaO. These results support the possibility that oxide superconducting film can grow at around 400°C by chemical vapor deposition using THD complexes.

An artificial step-edge junction for developing the high T_c weak links is presented. The step-edge junction is fabricated by using the step edge of the double layer of the epitaxial YBaCuO films. The dc superconducting quantum interference device (SQUID) made of the junction shows a clear voltage-vs-flux (V-Φ) relation up to T=81.3 K. The voltage modulation depth ΔV of the V-Φ relation is ΔV=1.6 µV at T=77 K. A preliminary experiment shows that the magnetic-flux noise of the SQUID at T=77 K is, at most, Φ_n=4.4×10^-4Φ₀/√Hz at f=10 Hz. These results show the promising properties of the step-edge junction for the weak link of high T_c superconductors.

The development of compositional inhomogeneities in sputtered Co-22 at.% Cr films is studied as a function of substrate temperature using the spin-echo ⁵⁹Co-NMR technique. It is observed that a highly Co-rich component grows as the substrate temperature increases from 30°C to 400°C. The Cr content of the Co-rich part in the film deposited at 400°C is estimated to be as low as 5-7 at.%. The results are consistent with the appearance of a clearly striped compositional microstructure which suggests compositional separation.

The tilt angle behavior of the smectic C phase was studied in several binary systems. The concentration dependence of the tilt angle deviated from the linear relationship. The degree of the deviation was influenced by the difference in the molecular structure between the two components. The tilt angles were found to correlate to the ratio of the S_C-S_A transition temperature to the S_A-N (or I) transition temperature.

A planar growth with a growth direction parallel to a 5-fold symmetrical axis was found in a melt-quenched Al₇₂Pd₂₅Cr₃ icosahedral alloy. An interpretation is given on the basis of a stability criterion that the plane perpendicular to the 5-fold axis is that of stable growth in an icosahedral phase (i-phase) and the solid-liquid interface instability occurs during solidification. It leads to the formation of cellular growth along the 5-fold axis and dendritic morphology. We find that decagonal phase (D-phase) grains nucleate from the i-phase and grow with the relationship that one of the 5-fold directions of the i-phase which have an angle of 63.43° between each other coincides with the 10-fold direction of the D-phase.

Pulsed laser deposition in ultrahigh vacuum (UHV) was applied to the epitaxial growth of CeO₂ film on Si(001). Although the direct deposition of CeO₂(001) on Si(001) was unsuccessful, the desired epitaxy was achieved by inserting the growth of a SrTiO₃ layer. The formation of a CeO₂(001)//SrTiO₃(001)//Si(001) layered structure was verified by reflection high-energy electron diffraction analysis of the growing surface at a temperature between 650 and 700°C in UHV. In addition to lattice matching, chemical interaction at the growing surface had a decisive effect on the epitaxy and orientation of growing ceramic lattices.

Hydrogen peroxide (H₂O₂) was used as the oxidant in a sequential surface-chemical-reaction-limited growth system to obtain high-quality thin films of Al₂O₃. It is observed that trimethylaluminum (TMA) reacts with H₂O₂ readily at temperatures as low as room temperature, resulting in the identical growth of Al₂O₃ wherever the reactants reach. The films are highly insulating and ideally uniform.

The field reversed configuration (FRC) plasma had been thought to have poloidal field only, but in this experiment, toroidal field was observed. We report magnetic probe measurements on our FIX machine, and the existence of toroidal field in the FRC plasma during its translation. The magnitude of the toroidal field is 100∼150 G at the peak, and the direction of the toroidal field is counterclockwise looking toward the θ-pinch region, irrespective of the direction of the external axial field. The obtained toroidal field has a direction opposite to the toroidal field in FRX-C/LSM.

The carbon K-edge excitation spectra of solid C₆₀ have been obtained using electron energy-loss spectroscopy. In the near-edge region, in addition to the π^* and σ^* resonance bands well known for graphite and amorphous carbon, a shoulder is found at 289 eV, suggesting the presence of a peak in the density of unoccupied π^* states. Another feature discernible to the solid C₆₀ is the width of the σ^* resonance band; it is about 15 eV, which is an intermediate value between those for graphite and amorphous carbon.