Yukichi Tatsumi

Crystallization of Amorphous Si and Ge Whiskers

Crystallization processes of amorphous Si (a-Si) and Ge (a-Ge) whiskers have been investigated by using differential thermal analysis (DTA) and X-ray diffraction. On the crystallization of the a-Si and a-Ge whiskers, the DTA peak temperature is 907 and 527°C at a heating rate of 20°C/min, the activation energy is about 5 and 2.3 eV, and the exothermic heat is about 4 and 3 kcal/mol, respectively. It is found that the energy difference between the amorphous and crystalline states is much smaller than the energy barrier for the crystallization in both cases of Si and Ge. Furthermore, from the observations on the crystallization of amorphous films, it can be concluded that the a-Si whisker is more stable than other amorphous semiconductor films.

Characteristics of Whisker Growth in Amorphous Silicon

The growth of amorphous silicon whiskers grown by thermal decomposition of silane (SiH4) in the temperature range between 500 and 800°C in an argon atmosphere has been investigated in detail. A thin gold film (about 200 A) on the substrate seems to act as the most effective catalyzer for the whisker growth. The time rate of axial growth at the beginning of each growth curve seems to be proportional to the partial pressure of SiH4. Radial growth proceeds at a constant rate and remains constant even after termination of the extension in length. The two mechanisms of elongation and thickening are controlled by independent activation processes whose activation energies are 0.73 eV and 2.1 eV respectively.

Electrical and magnetic properties of the high-Tc superconductors (Y1−xMx)Ba2Cu3Oy and Y(Ba1−xMx)2Cu3Oy (M=Mg, Ca, Sr, Ba)

The superconducting state of YBa2Cu3Oy substituted at the II a group was investigated by X-ray powder diffraction, iodometric titration, electrical resistivity and magnetization measurements. The Tc is shifted to low temperature by the substitution of Y or Ba from a heavy atom to light atom at x=0.1, and depends on the ionic radius. In the results obtained from the magnetic hysteresis loop, it is interesting that in the case of the substitution of Ca for x=0.1, the magnetization for Y is larger than that for Ba with decreasing temperature, and the magnetization in the case of Ba for Y is rapidly lowered at x=0.1.

Visual Observation of Whisker Growth in Amorphous Silicon

Amorphous silicon whiskers have been grown in an atmosphere composed of SiH 4 (5%) and argon (95%) at around 650°C. Time-lapse sequence of the growth of whiskers is presented in this report. By using an optical system, it has been observed that the growth proceeds at their tips.

Effects of Substitution by Group Ia and IIIa Elements on Superconducting Properties of YBa2Cu3Oy.

Physical properties of (Y1-x Mx )Ba2Cu3Oy and Y(Ba1-x Mx )2Cu3Oy (M=K, Rb, Cs, Sc and La) were studied. It is shown that the temperature difference ΔT between the onset and the zero of the resistivities due to the superconducting transition depends on the valence and ionic radius of substituting elements. The maximum magnetization of the hysteresis loop is analyzed by the T4 dependence based on the two-fluid model. It is determined that the critical temperatures T c due to these substitutions are the same or slightly higher than that of YBa2Cu3Oy . The substitution of group IIa elements seems to be effective for pair-breaking.

Hydrogen-Absorption Effect on YBa2Cu3Ox.

Hydrogenated samples of YBa2Cu3Ox ? Hy were prepared by allowing an oxide superconductor YBa2Cu3Ox to absorb hydrogen. From magnetization measurements, it was shown that the critical temperature increased from 91.8 K to 94.2 K for 0?y?0.3 and decreased from 94.0 K to 91.9 K for 0.4?y?1.4. The iodometric titration indicated that an average valence number of copper decreased linearly with increasing hydrogen content. The hydrogenated samples displayed superconductivity above 90 K even when the average valence number of copper was below 2.0. Consequently it was concluded that the oxygen content remained at its initial value of 6.98 even though the oxide superconductor had absorbed hydrogen.

Oxidation mechanism of amorphous silicon in air

The pure amorphous silicon (a-Si) films without cracks or pores were prepared by electron beam evaporation, and the oxidation of the resultant films has been studied by electron diffraction and infrared absorption spectroscopy in order to pursue the progress of oxidation. The oxidation proceeds in any depths throughout the film, indicating that the oxygen diffuses through the Si network. The oxidation is found to occur as a result of the breaking of a Si-Si bond by an oxygen, which is inserted between the two Si atoms, making a bridge between them. The amount of oxygen atoms incorporated in a-Si film increases in proportion to the logarithm of the time of exposure to air in a similar way as the oxidation of the c-Si surface. The occurrence of the oxidation is attributed to the presence of dangling bonds in a-Si films.

Influence of Growth Conditions on Amorphous Germanium Films Prepared by Vacuum-Evaporation

Amorphous germanium films under various growth conditions are investigated by X-ray diffraction and the method of densimetry. The density decreases continuously with decreasing deposition rate, and when the deposition rate ranges from 1.7 ×10 -3 to 5.1 nm/s, the density changes from 2.77 to 4.63 g/cm 3 . Furthermore, the densities are 2.91 g/cm 3 at a pressure of 0.12 Pa and ∼4.3 g/cm 3 at ∼10 -4 Pa under the same deposition rate of 0.38 nm/s. It can be concluded that the most possible origin for the low density is the existing residual gas.

Structure of the Natural Oxide of Amorphous Silicon

The structure of the natural oxide, which is produced in air at room temperature, of amorphous silicon has been investigated using high-energy electron diffraction. From a least-sequares analysis of the reduced radial distribution functions, three atomic distances of the oxide were found to be 1.63, 2.85 and 3.2 A besides Si-Si bond length of 2.37 A. The structure of the oxide can not be the SiO4-tetrahedron, since the first and second distances cannot be explained by a tetrahedron arrangement. As the first and second distances can be considered the Si-O bond and O-O atomic distances, respectively, the average O-Si-O angle is about 122°, larger than that of the SiO4-tetrahedron.

Densimetry of Amorphous Silicon Films by Using a Quartz Oscillator

By using a quartz oscillator with one side of the quartz crystal optically polished, both the weight and thickness of the same film can be measured. With this method a film can be deposited on one part of one side of a quartz oscillator; thus, it is necessary to estimate the frequency constant N by Ag and Au films. As a result, N was found to be 1.86±0.10×105 Hzcm, a 12% increase over the nominal frequency constant of an AT-cut quartz oscillator. The density of an amorphous silicon (a-Si) film deposited on an Au film was determined to be 1.71±0.09 g/cm3; the density was 27% less than the density of crystal Si. From the result of high-resolution electron micrographs, it is clear that the cause for the low density is not due to such defects as cracks, pores or voids over 2 nm.