Akio Tsuchino

Rewritable Triple-Layer Phase-Change Optical Disk Providing 100 Gbyte Capacity

A rewritable phase-change optical disk providing a large capacity of 100 Gbyte on a 120 mm disk was first demonstrated using the multilayer Blu-ray DiscTM (BD-XL) format. The doubled capacity of this optical disk compared with that of a conventional dual-layer disk was achieved firstly by stacking triple recording layers and secondly by increasing the recording capacity per layer from 25 to 33.4 Gbyte at 33.6%. The high transmittances of 50% (middle layer) and 60% (front layer) were achieved by thinning a Ge–Sb–Te phase-change film to 7.5 and 6 nm and also by thinning a Ag-alloy film to 9 and 7 nm, respectively. An additional TiO2-based film formed on the Ag-alloy film was effective in improving the transmittance at 3%, compared with the structure using a conventional TiO2 film. Furthermore, a transmittance-balanced structure was adopted for these layers in order to stabilize the recording-reading properties. To improve cyclability, ZrO2–Cr2O3-based interface films were provided on both sides of the phase-change film for the middle and front layers. The increase in recording capacity per layer was achieved by reducing the minimum mark length from 0.149 to 0.112 µm. Since the optical changes degrade with the reductions in the mark lengths and thicknesses of the Ge–Sb–Te and Ag-alloy films, a phase-change material with a GeTe-rich composition on a GeTe–Sb2Te3 pseudo-binary line was adopted for every layer to compensate it. It was confirmed that the sample disk successfully satisfies all the requirements of the BD-XL format.

System of laser pump and synchrotron radiation probe microdiffraction to investigate optical recording process.

We have developed a system of laser-pump and synchrotron radiation probe microdiffraction to investigate the phase-change process on a nanosecond time scale of Ge2Sb2Te5 film embedded in multi-layer structures, which corresponds to real optical recording media. The measurements were achieved by combining (i) the pump-laser system with a pulse width of 300 ps, (ii) a highly brilliant focused microbeam with wide peak-energy width (ΔE∕E ~ 2%) made by focusing helical undulator radiation without monochromatization, and (iii) a precise sample rotation stage to make repetitive measurements. We successfully detected a very weak time-resolved diffraction signal by using this system from 100-nm-thick Ge2Sb2Te5 phase-change layers. This enabled us to find the dependence of the crystal-amorphous phase change process of the Ge2Sb2Te5 layers on laser power.

Crystallization properties of Ge2Bi2Te5 and Ge10Sb90 amorphous nanoparticles subjected to pulsed laser irradiation

Laser-induced crystallization time (Δtc) was studied for Ge2Bi2Te5 and Ge10Sb90 amorphous nanoparticles with diameters of ~20–50 nm and a height of 10 nm, using 10-nm-thick blanket films as references. Δtc of Ge10Sb90 nanoparticles (12 µs) was more than 500 times that of the blanket film (24 ns), whereas that of Ge2Bi2Te5 nanoparticles (80 ns) was close to that of the blanket film (56 ns). These results suggest a significant effect of the characteristic crystallization processes, i.e., the nucleation-dominant (Ge2Bi2Te5) or growth-dominant (Ge10Sb90) crystallization, on Δtc of nanoparticles.