Yasuhiro Satoh

Local Structure Analysis and Interface Layer Effect of Phase-Change Recording Material Using Actual Media

The influences of the interface layer on crystal structure, the local atomic arrangement, and the electronic and chemical structure of a GeBiTe (GBT) phase-change recording material have been investigated using X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), and hard X-ray photoelectron spectroscopy (HX-PES) methods using actual rewritable high-speed HD DVD media without special sample processing. XRD results showed that the crystal structure of laser-crystallized GBT alloy in the actual HD DVD media is the same as that of GeSbTe (GST) alloy, which has a NaCl-type structure. No differences between samples with and without interface layers were found. The lattice constant of GBT is larger than that of GST. Bi increases the lattice constant of GST with respect to the Bi substitution ratio of Sb. According to HX-PES, the DOS of in the recording film amorphous state with an interface layer is closer to that of the crystalline state than the recording film without an interface layer. From XAFS results, clear differences between amorphous (Amo.) and crystalline states (Cry.) were observed. The interatomic distance of amorphous recording material is independent of the existence of an interface layer. On the other hand, the coordination number varied slightly due to the presence of the interface layer. Therefore, the electronic state of the recording layer changes because of the interface layer, although the local structure changes only slightly except for the coordination number. Combining these results, we conclude that the interface layer changes the electronic state of the recording layer and promotes crystallization, but only affects the local structure of the atomic arrangement slightly.

High-Speed Deposition of New Low-Refractive-Index Dielectric Film "SiOC" for Rewritable HD DVD Media

A low-refractive-index film plays an important role in attaining a high density of rewritable HD DVD optical recording media, for which the blue-violet laser of 405 nm wavelength incident on a 0.6-mm-thick substrate is used. We have developed a new low-refractive-index material, SiOC, for the medium, which can be sputter-deposited at much higher rates than the conventional SiO2 film. The SiOC film is formed using a SiC target with an Ar and O2 gas mixture using either a RF or DC power supply. The film exhibited similar optical constants as SiO2, and DC sputtering in particular showed very low substrate temperature rise. The resultant deposition rate per sputter power is about three times higher than the rate of deposition using a SiO2 target. The obtained thin film consists of Si, O, and C. Although the film includes a substantial amount of carbon, it showed almost the same optical characteristics as SiO2. We demonstrated that the single-layer HD DVD rewritable medium with 20 GB capacity using SiOC film had satisfactory recording characteristics. We believe that this media can be manufactured at a cost similar to that of conventional rewritable DVDs in mass production.

XAFS study of phase-change recording material using actual media

The relation of the local structure of the phase-change recording material and the interface layer has not been clarified while we already reported that the interface layer affects the electronic state of recording material by using the HX-PES method. It is necessary to understand more detailed physical phenomenon for crystallization mechanism of recording layer in order to develop the high-speed and higher density rewritable optical recording media. The influence of the interface layer to the local structure for atomic arrangement of a GeBiTe phase-change material was investigated by using XAFS on the actual rewritable HD DVD media. The XAFS signal and EXAFS oscillation from the actual media is obtained nondestructively. It has been shown that the interface layer influence slightly the local atomic arrangement of the recording layer, while the electronic state of recording layer is changed by the presence of the interface layer.

Development of 40 GB dual-layer rewritable HD DVD media

We have developed a single-side dual-layer rewritable HD DVD media having a larger capacity of 40 GB (20 GB per layer) for the optical system with the NA of 0.65 and the wavelength of 405 nm, and achieved the good recording characteristics. For both layers, the same track pitch of 0.34 μm as HD DVD-RAM was used. We applied more accurate thermal analysis to the dual-layer rewritable media. It was expected that the effect of the cross-erase was very small. By optimizing thermal balance of dual-layer media, the good recording characteristics were obtained for both layers with enough tilt margins. The feasibility of the dual-layer rewritable media of 40 GB user data capacity was shown for HD DVD system.