Katsumi Kawahara

Phase-Change Optical Disk Having a Nitride Interface Layer

A thin nitride layer formed at the interface of a Ge–Sb–Te recording layer and a ZnS–SiO2 protective layer successfully suppresses the phenomenon that reflectivity or signal amplitude becomes markedly small due to repeated overwrites. Based on secondary ion mass spectrometry (SIMS) observations, the 5-nm-thick interface layer was found to restrain sulfur atoms in the ZnS–SiO2 layer from diffusing into the Ge–Sb–Te layer and from changing the optical characteristics of the layer. Among several nitride materials, germanium nitride (Ge–N) sputtered film is found to have the most suitable properties as an interface layer: high barrier effect and good adhesiveness with Ge–Sb–Te and ZnS–SiO2 layers. The optical disk having the Ge–N interface layer achieves more than 5×105 cycles of overwrites with almost no changes in signal amplitude, reflectivity and jitter based on DVD-RAM specifications. The disk shows no degradation such as cracking, peeling, and corrosion after exposure to accelerated environmental conditions of 90°C and 80% RH for 200 h.

Effect of Dielectric Material Films on Crystallization Characteristics of Ge2Sb2Te5 Phase-Change Memory Film

Reduction of the film thickness of phase-change film and the adoption of GeN- or ZrO2-based dielectric films are both effective in achieving good thermal stability in phase-change optical disks. It was experimentally confirmed that, at a heating rate of 10 °C/min, the crystallization temperature Tx of the Ge2Sb2Te5 amorphous film when sandwiched by ZnS–SiO2 films markedly increases from 162 to 197 °C, while the thickness of the Ge2Sb2Te5 film decreases from 10 to 3 nm. Tx also slightly increases when ZnS–SiO2 films are substituted for GeN-based films (from 162 to 165 °C) and ZrO2-based films (from 162 to 167 °C). At the same time, the activation energy of crystallization is 2.4 eV for both GeN- and ZrO2-based films, and is higher than 2.2 eV for ZnS–SiO2 films.