Osamu Nittono

Microstructure and lattice distortion of anodized porous silicon layers

Abstract The lattice distortion and the microstructure of porous silicon (PS) layers, produced on p-type Si wafers with various electrical resistivities, have been investigated by means of X-ray multi-crystal diffractometry, transmission electron microscopy, infrared spectroscopy and gas adsorption method. It is shown that the lattice distortion of the PS layer is strongly influenced by hydrogen atoms chemisorbed on pore surfaces as well as by pore morphology in the layer, and that the crystalline quality of the layer is degraded secondarily as a result of oxidation on pore surfaces caused after desorption of hydrogen atoms from this layer. A hypothesis is proposed to explain consistently all the experimental results concerning crystalline properties of PS layers.

Crystal Structure of CuIn3Se5 Semiconductor Studied Using Electron and X-ray Diffractions

The crystal structure of CuIn3Se5 is investigated by complementary use of electron and X-ray diffractions. The lattice parameters are a=5.7539(3) and c=11.519(1) A and the space group is uniquely determined to be I42m. The crystal is not constructed of a rigid unit cell but by three kinds of local Se tetrahedrons, such as CuIn2Se4, CuIn3Se4 and In3Se4, which minimize the numbers of electron excess and deficient bonds. The average unit cell is represented by Cu0.8In0.4In2Se4, where Se and some In atoms fully occupy the 8i site with x≈1/4 and z≈1/8, and the 4d site, respectively, and Cu and the remaining In atoms partially occupy 2a and 2b sites, respectively. Since the space group I42m is not a subgroup of I42d which is the space group of CuInSe2, CuIn3Se5 is not related to CuInSe2 from the viewpoint of crystal symmetry. It is neither a vacancy ordered compound nor of a defect chalcopyrite structure.

Annealing Effect on Lattice Distortion in Anodized Porous Silicon Layers

The annealing effect on the lattice distortion of porous silicon (PS) layers have been studied mainly by X-ray multiple-crystal diffractometry and infrared spectroscopy. The lattice spacing of PS layers changed with increasing annealing temperature and finally became smaller than that of the unanodized Si substrate above 350°C. The lattice contraction of PS layers was found to be strongly related to the desorption of hydrogen atoms from the PS layer. It is also shown that the lattice expansion of as-prepared PS layers is attributable to hydrogen atoms chemisorbed on the pore surfaces in the PS layer.

Optical activity in the vacancy ordered III2VI3 compound semiconductor (Ga0.3In0.7)2Se3

Great optical activity is realized by a vacancy ordered III2VI3 compound (Ga0.3In0.7)2Se3 with point group 6 which is based on wurtzite structure and characterized by the screw arrangement of cation atoms along the c axis. The transition of the fundamental absorption edge is direct and the band gap is estimated to be 2.05 eV. An anomalous optical rotatory dispersion around the absorption edge is observed and the maximum rotatory power of 125°/mm is obtained at λ=620 nm. The optical activity for red light is always above 60°/mm, that is 4–6 times as large as that of α quartz. (Ga0.3In0.7)2Se3 single crystal is very useful, especially for the He–Ne laser as an optically active substance; the rotatory power reaches 103°/mm, being more than 5 times of α quartz.

Optical Anisotropy of Vacancy-Ordered Ga2Se3 Grown by Molecular Beam Epitaxy

Optical anisotropy of the vacancy-ordered Ga2Se3 grown on (100)GaP has been investigated. The electron-diffraction studies revealed that the vacancy-ordered superstructure was predominantly formed in the [011] direction when the film was grown with a VI/III ratio of 150. From the transmission spectra for light polarized toward [011] and [01], it is found that a difference in the absorption coefficients of the two polarized lights is more than 104 cm-1 at a wavelength of around 525 nm, and that the vacancy-ordered Ga2Se3 behaves like a polarizer in the selected wavelength range.

Crystal growth in c direction and crystallographic polarity in ZnO crystals

Abstract The c -axis polarity of ZnO crystals growing in the c direction was investigated for crystals grown by chemical reaction in the vapor, by VLS mechanism and by chemical vapor deposition. In the initial growth stage ZnO crystals nucleate both in the + c and - c directions in each type of growth. In growth by chemical reaction in the vapor, a higher growth rate of the crystal in the + c direction results in dominant growth of large crystals in this direction. The difference in the growth rate is attributed to a higher effective surface energy of the Zn atom surface than that of the O atom surface. On the other hand, in VLS growth long whiskers and needles grow in both c directions. This fact indicates that the difference in the growth rates between the two c directions decreases in VLS growth.

Microstructure and Crystallinity of N-Type Porous Silicon

Microstructure and crystallinity of a porous silicon (PS) layer formed on a highly doped n-type silicon substrate were examined in detail, and the luminescent nature of the PS layer was also studied qualitatively. A layered pore structure developed after about 3000 s of anodization, and pore morphology became simpler with increasing thickness of the PS layer. All luminescent PS layers were found to show spongelike structure near the surface. From the peak width of X-ray rocking curves, the crystallinity of n-type PS was found to be inferior to that of p-type PS. No systemic tendency of the lattice expansion was seen under various forming current densities. Luminescent PS layers showed slightly broader background intensity than nonluminescent ones. It was also shown that the anodization of the PS layer took place through two anodization processes: primary and secondary anodizations; secondary anodization was effective in forming luminescent parts having spongelike structure.

A high-speed X-ray topography camera for use with synchrotron radiation at the photon factory

Abstract A detailed description of the high-speed X-ray topography camera designed for the vertical wiggler beam line at the Photon Factory is presented. The camera has been successfully operated with a white beam from a normal dipole magnet beam line since June 1982. The flexibility of the camera is sufficiently large to meet various user requirements. Some of its characteristic features are as follows: it has two TV detector arms providing a view coverage of 50° upwards and 30° downwards from the horizontal plane. The goniometer can support a load of 30 kg, to mount specimen environmental conditioners such as magnets and cryostats, while it can rotate with a precision of 2″ of arc over a full circle. The entire camera is placed on a special movable carriage to enable a simple and rapid interchange between a white beam outlet and a monochromatic one. Some 20 different samples and more than 20 people were involved both in commissioning the instrument and in the preliminary experiments, using about 900 h of machine time.

X-Ray Topographic Observation of Copper Whisker Crystals

Copper whisker crystals, carefully grown from cuprous iodide by vapour reduction, were studied by means of x-ray topography and optical microscopy. About 50 whisker crystals with growth orientations [001], [110] and [111] were examined in a thickness range from 10 to 80 µ. None of them contained an axial dislocation. The same was true for kinked and spiral whisker crystals. The whisker crystals are in a high degree of perfection even at the thickness of 80 µ, although some of them contained lattice defects introduced accidentally during the growth. Results obtained indicate that copper whisker crystals do not grow by the growth mechanism, in which an axial screw dislocation plays an important role. There is a contrast change at the tip of the whisker crystal. Impurities at the tip may take part in the growth.

X-ray Topographic Studies on the Lüders Band Propagation and the Dislocation Motion in Copper Whisker Crystals

Dislocations in plastically deformed blade-like [110] copper whisker crystals with thickness of 20 to 70 µ are studied by means of X-ray transmission topograpay. Topographs are taken under the application of tensile stress. Dislocations with a semi-circular shape are always observed to spread over a long distance more than 100 µ in front of the Luders band, and they are shown to become cross-slipped and double cross-slipped dislocations. The Luders band can propagate according to the double cross slip mechanism. This may be true for the cases of [001] and [111] copper whisker crystals. Concaves on the crystal edge are the effective sites for dislocation sources. The dislocation motion is not steady and the dislocation pinning due to impurities may be responsible for the irregular dislocation motion. Results obtained indicate that the coarse copper whisker crystals are useful for the studies on the plastic deformation.