Makoto Harigaya

Completely Erasable Phase Change Optical Disc II: Application of Ag–In–Sb–Te Mixed-Phase System for Rewritable Compact Disc Compatible with CD-Velocity and Double CD-Velocity

Aiming to obtain a rewritable compact disc (CD-rewritable) with high performance at both CD-velocity (CD1X) and double CD-velocity (CD2X), the phase change optical disc with the Ag-In-Sb-Te system has been developed. In the one-pass overwriting mode with eight-to-fourteen modulation, high carrier-to-noise ratio, wide power margin and high erase ratio have been obtained at both velocities. As for stability, we acquired a satisfactory result in the read-out test of 106 cycles with read power of 1.5 mW. We conclude that the phase change optical disc with the Ag-In-Sb-Te system is suitable for use at both CD1X and CD2X. Furthermore, the phase change process of the Ag-In-Sb-Te system has also been investigated. The chemical structure of the Ag-In-Sb-Te system in its ordered state was found to be (Ag-Sb-Te)x(In1-ySby)1-x.

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.

Compact Disc Erasable (CD-E) with Ag–In–Sb–Te Phase-Change Recording Material

The compact disc erasable (CD-E) disc has been developed with Ag–In–Sb–Te phase-change recording material. The disc has been designed for recording at a speed twice that of conventional CDs. The CD-E disc has high modulation, high sensitivity, wide power margin after 1000 overwrite cycles and high reliability. It is readable using CD-ROM players and recordable using CD-R recorders with minimum modification.

High-Density and High-Data-Transfer-Rate Optical Disk with Blue Laser Diode and Ag-In-Sb-Te Phase-Change Material

We studied phase-change recording using a 405 nm blue laser diode, an objective lens of numerical aperture (NA)=0.6 and Ag–In–Sb–Te phase-change material. We report land and groove recording on a conventional disk structure with a 0.6-mm-thick and 120-mm-diameter PC substrate. We confirmed the possibility of achieving a high density, 12 GB capacity and a high data-transfer rate of 30 Mbps, corresponding to that of a read-only digital versatile disk (DVD-ROM) under the conditions of 6.5 m/s recording speed, 0.18 µm/bit linear density (minimum mark length 0.27 µm) and 0.42 µm trackpitch.

In-Sb Phase-Change Material for 16× Rewritable Digital Versatile Disk

We investigated Sb-based In-Sb as a material for high-speed recording that is capable of maintaining a highly stable amorphous phase. The crystalline phase of In-Sb is unstable at approximately its eutectic composition. After an aging test, InSb was segregated, and its reflectivity was reduced. For an Sb-rich In-Sb, we confirmed that its crystalline phase is stable and its crystallization speed is high. We demonstrated the feasibility of 16× digital versatile disk (DVD) recording using 2T-period multiple-pulse strategy.

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.

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.

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.