Katsuhiko Tani

Structural Analysis of Ag–In–Sb–Te Phase-Change Material

The Ag–In–Sb–Te system is widely used for phase-change discs such as compact disc rewritable (CD-RW) and rewritable digital versatile disc (DVD+RW). To clarify the effect of Ag and In in the Ag–In–Sb–Te system, we studied the local structure of Ag–In–Sb–Te phase-change material by extended X-ray absorption fine structure (EXAFS). The results suggest that the existence of Ag contributes to the thermal stability of amorphous marks and existence of In contributes to high-speed phase-change.

EXAFS study of Sb-Te alloy films

The local structures of three phases; stable compound Sb2Te3, metastable crystalline c-SbTe and amorphous a-SbTe films having the atomic ratio Sb/Te=3: in Sb-Te system have been studied by EXAFS. The c-SbTe has a partly similar local structure to crystalline Sb2Te3. The a-SbTe film has a local structure of NaCl-type, which is topologically analogous to the crystalline form. The amorphous phase has shorter bond distances 2.86A (around Sb-site) and 2.83A (around Te-site) than the corresponding distances 2.89A and 2.87A in the crystalline phase. This unbalanced bond distances between the Sb-site and Te-site implies that site-disordering occurs. Shortening of bond distances in the amorphous phase is due to the relaxation of locally distorted crystalline structure.

Surface Structure Analysis of Polyacrylate Thin Films

The surface contact angles of poly(fluoroalkyl acrylate)s and poly(fluoroalkyl methacrylate)s vary with the cooling mode used after heat treatments. Grazing angle X-ray reflectivity measurement was applied to characterize the surface roughness of polyacrylate thin films on silicon wafers. Surface and interface roughnesses of polyacrylate thin films with large contact angles were measured as 0.7–1.2 nm, which are smaller than roughnesses (1.2–1.8 nm) with small contact angles. It was found that the ordering of side chains in polyacrylate thin films causes the surface to change the contact angles.

In-House Measurements of XAFS by Means of Total Reflection

We have developed a double channel-cut monochromator composed of two channel-cut Si(111) crystals, and obtained highly parallel and highly monochromatized X-ray beam. Reflectivity of an As2Se3 film on an Al substrate was measured as a function of photon glancing angle around the critical one of total reflection. The complex refractive index was determined so that the simulated Fresnel reflectivity fits the measured values. Since the imaginary part of the refractive index is correlated with the linear absorption coefficient, we can obtain XAFS spectrum by means of reflectivity measurements in house.

Degradation of photoresponse time of a-Si:H and correlation with NI measured by low frequency C-V method

Abstract We investigated the degradation of photo conductivity (σ p) and photoresponse time (τ d) in undoped a-Si:H, and found a degradation mode in which τ d first decreased and then increased with increase in the light soaking time. We also obtained the correlation between σ p, τ d and NI measured by low frequency C-V method. All the results can be consistently explained by taking the electron traps into consideration.

Characterization of Ultra-Fast Phase Change Optical Disk with GaSb Material

We studied the crystallization mechanism of ultra-fast phase change optical disks with recording layers made of GaSb material for digital versatile disk (DVD) systems. The results of a static recording test and an amorphous mark formation simulation suggest that GaSb maintains a high crystal growth rate even at temperatures 150 degrees lower than the materials melting point. Disks with recording layers made of this material have a write speed margin ranging from DVD 3× to 8× or more.

Multiple scattering approach to K-edge XANES of Sb-Te systems.

The Sb-Te systems are important for optical memories. The local structures of these Sb-Te systems are crucial to understand the properties. Here we study three different Sb-Te systems, Sb2Te3, c- and a-SbTe (c; crystalline, a; amorphous) by use of XANES analyses. The present calculations for Sb2Te3 system show quite good agreement with the observed spectra. In contrast to Sb2Te3 system, both c- and a-SbTe systems, in which a ratio of Sb to Te is 3, are metastable; both of the structures have not been known yet. We thus investigate these local structures by use of the multiple scattering approach, and propose a possible model for each of c- and a-SbTe.

Sb K-edge absorption fine structure of Sb2Te3

In order to study the local structure of some antimony compounds, Sb K-edge XAFS spectra were measured for Sb2Te3 and InSb. The measurements were performed in the transmission mode at BL-01 of SPring-8. Though crystal structure of antimony telluride Sb2Te3 is known by X-ray diffraction analysis, the EXAFS analyses give controversial result on the Sb-Te distance and the coordination number of Sb2Te3. Comparing the observed XANES spectra for Sb2Te3 with FEFF8 calculation, we can explain the controversial results which are also supported by the observed X-ray powder diffraction.

X-ray absorption fine structure of V2O5 and LixV2O5

Vanadium K-edge XAFS measurements of five compounds were carried out. The compounds studied were crystalline V2O5 and four types of LixV2O5 with different insertion levels x, which show different EMFs (2.0–3.4 V). The EXAFS data analysis shows no remarkable difference in the V—O distances in the five compounds, showing that the structure of the VO5 square pyramids changes only slightly as lithium atoms are intercalated into the V2O5. In the XANES spectra, three peaks are observed for crystalline V2O5 and LixV2O5 (3.4 V). When the EMF is decreased, the intensity difference between the first and second peaks disappears and the third peak shifts to lower energy. The XANES data were analysed using full multiple-scattering calculations. In the calculated spectra the intensity of the first peak increases as the V2O5 intercalates lithium atoms.

Reverse Monte Carlo modeling of amorphous structures in phase-change In0.21Sb0.79 thin film.

The structures of phase-change In(0.21)Sb(0.79) thin film in the amorphous phase were modeled using the reverse Monte Carlo (RMC) method, making simultaneous use of measured x-ray diffraction and x-ray absorption fine structure data. The experimental data that we used do not contain the crystalline phase, which can be observed in the diffraction pattern. Three kinds of initial configurations--a simple cubic lattice, a rhombohedral A7 structure and a dense randomly packed hard sphere form--were used in attempts to reproduce the measured data. The former configuration is thought to be the most probable structure for the crystalline In(0.21)Sb(0.79) thin film. For the latter configuration we could not reproduce the structure of the amorphous In(0.21)Sb(0.79) thin film under the present RMC conditions. We obtained probable structure models for the amorphous In(0.21)Sb(0.79) thin film. We found that the models obtained possess some traces of crystallinity.