Our DVD-RAM/2.6GB was just commercialized in this year. Today, our R/D activities are aimed toward the next generation of DVD-RAM media and even the generation beyond that. We are striving to achieve higher recording density, a higher data transfer rate and higher reliability (cyclability). Through our study, we have found that a nitride interface layer was very effective for increasing the performance of phase-change optical disks. In this paper, we will present a new disk structure having Ge-N layers at the interface between the Ge-Sb-Te active layer and the ZnS-SiO2 protective layers. The effects of the nitride layer and the disk performance will also be described.

Fast microscopic phase change phenomena are observed in real time by measuring reflectivity and transmissivity of chalcogenide thin films. The method for calculating the complex index of refraction from reflectivity and transmissivity in microscopic area is also developed. By comparing the calculated reflectivity and transmissivity with the measured ones, the dynamic phase change phenomena of the film during and after the irradiation can be guessed. Phase change of TeOx film, Te-Ge-Sn-Au film and GeTe film is observed. For the TeOx film, a model for crystallizing process is proposed.

We have realized a V-grooved optical disc that achieves an increase in recording density over conventional optical discs. This report describes the results of our research : namely, its fabrication process and optical read-out method.The features of the V-grooved optical disc system are:1.2mm, NA must be limited to a maximum of 0.6.

Two marked effects are obtained by forming a Ge-N interface layer on either side of Ge-Sb-Te recording layer. One effect is a suppression of atomic diffusion between Ge-Sb-Te layer and protective layers, ZnS-SiO2 representatively, which leads to a significant improvement in overwrite cyclability, and the other is the acceleration of crystallization process which leads to higher speed optical disks. A rapid-cooling type experimental disk with Ge-N layers on both sides of the Ge-Sb-Te recording layer proved to be capable of exceeding 1O cycle overwrites and a recording data rate 40 Mbps at linear velocity 12 m/s. The recording conditions: bit length O.28,am and track pitch O.6/2m (LIG method) using laser source with a wavelength 658 nm and a numerical aperture 0.6 correspond to a capacity 4.7 GB! ct120 mm.