Takuo Tamura

Local structure and mean-square relative displacement in SiO2 and GeO2 polymorphs

Extended X-ray absorption fine structure (EXAFS) spectra near the Si and Ge K-edge for SiO_2 and GeO_2 polymorphs were measured in transmission mode with synchrotron radiation at the Photon Factory, Tsukuba. The local structures and mean-square relative displacements were determined in \alpha-tridymite, \alpha-quartz and stishovite. In stishovite, Si is octahedrally coordinated and the four coplanar Si—O bonds [1.755 (8) A] are shorter than the other two axial bonds [1.813 (15) A]. The high-temperature phase tridymite [1.597 (3) A] has a smaller local bond distance than \alpha-quartz [1.618 (5) A]. The temperature variation of the local structural parameters for quartz-type GeO_2 (q-GeO_2) and rutile-type GeO_2 (r-GeO_2) have been determined in the temperature range 7–1000 K. The harmonic effective interatomic potential V(u)=\alpha{u}^2/2 was evaluated from the contribution to the thermal vibration, where u is the deviation of the bond distance from the location of the potential minimum. The potential coefficient \alpha for the Ge—O bond of the tetrahedron in q-GeO_2 is 24.6 eV A−2. The potential coefficients \alpha for the four coplanar Ge—O bonds and the two axial bonds of the octahedron in r-GeO_2 are 12.9 and 14.9 eV A−2, respectively. The potential coefficient \alpha for the second-nearest Ge—Ge distance in q-GeO_2 is 9.57 eV A−2. The potential coefficients \alpha for the second- and third-nearest Ge—Ge distances in r-GeO_2 are 11.6 and 7.18 eV A−2, respectively. The effective interatomic potential is largely influenced by the local structure, particularly by the coordination numbers. The phonon dispersion relations for q-GeO_2 and r-GeO_2 were estimated along [100] by calculating the dynamical matrix using the potential coefficients \alpha for the Ge—O and Ge—Ge motions. The quartz-type structure has a more complex structure with a wide gap between 103 and 141 meV and a highest energy of 149 meV, whereas the rutile-type structure has a continuous distribution and a highest energy of 126 meV.

Local structure of (Ca, Sr)2 (Mg, Co, Zn) Si2O7 melilite solid-solution with modulated structure

The local structure around Co, Zn and Sr atoms in incommensurately modulated, melilite-type X2T1 T22O7 (X=Ca and Sr, T1=Mg, Co and Zn, T2=Si) solid-solutions has been investigated by EXAFS analyses. The modulated structure was confirmed in Ca2-xSrxCoSi2O7 solid-solutions with X=0.0 to 0.6 and for both Ca2Mg1-YCoYSi2O7 and Ca2Mg1-YZnYSi2O7 solid-solutions over the whole compositional range at room temperature.The actual bond-distances determined by the EXAFS method for the T1 site (Co-, Zn-O) in the modulated structure are longer than the mean bond-distances obtained from the X-ray diffraction method. This is attributable to the libration of the T1 tetrahedra. In the Ca1-XSrXCoSi2O7 solid-solution both the Sr-O and Co-O distances by the EXAFS method for the X-site increase from Ca end-member to Sr end-member. These increases are respectively 0.8% and 0.6%. This means the local expansions of the tetrahedral sheets and of the XO polyhedra are well matched. In the modulated Ca2Co1-YMgYSi2O7 and Ca2Zn1-YMgYSi2O7 solid-solutions, the actual Co-O and Zn-O distances for the T1-sites are nearly constant in the whole compositional range. The compositional variations of the local structure around the cations in the solid-solution are different for the X and T1 sites.It is concluded that the local geometric restriction for the size of substituted cation in X site is larger than that in T1 site. The dimension of the tetrahedral sheet puts restriction on the size of the cations situated at the interlayer X sites. In other words, the different behavior of the local geometric restriction between the X and T1 sites is an important feature of the melilite structure and is also related to the modulated structure.

Crystal Growth of Laser Annealed Polycrystalline Silicon as a Function of Hydrogen Content of Precursors

The influence of hydrogen in a precursor on excimer laser crystallization behavior was investigated. The crystal orientation and lattice constant of polycrystalline silicon films were analyzed by X-ray diffraction measurement. An intensity ratio of 111 to 220 was used as an index of the (111) preferred orientation. A randomly oriented film with an index of 2 at lower energy density changed to the highly (111) oriented phase with an index of more than 15 at higher energy density. It was observed in the PE-CVD films that increasing the hydrogen in the precursor films decreased the orientation index. The lattice constant of the laser-annealed PE-CVD silicon was found to be larger than that of the PVD silicon and to decrease with an increase in laser energy density. The network structure of as-deposited PVD film with less hydrogen content was denser than that of as-deposited PE-CVD. The network structure of the precursor strongly affected the crystal growth, and the structure of the ELC poly-Si was still affected by the precursors, even though the hydrogen content decreased after laser annealing.

Development of an X-Ray Diffractometer for Thin Film Analyses Applying a Compact X-ray Generator with High Loaded Power Rate to a Small Focus.

An X-ray diffractometer newly designed for thin film analyses is consisted of a compact X-ray generator with high loaded power rate to a small focus, a curved position sensitive detector and K-axis type 4-circle goniometer. The diffractometer enables measurements of thin film samples for electronic devices, e.g. rapid pole figure measurements of thin film samples and depth-sensitive measurements of multilayer thin films. A preffered orientation analysis of Co-Nb-Zr films for VTR core and a crystallinity analysis of multilayer films for circuit board were performed by using this apparatus.