Erasing Characteristics and Crystallization Process of GeSbTe Phase-Change Optical Disk Media at Very Fast Linear Velocities

The erasing characteristics of phase-change optical disk media with a recording layer made of either Ge2Sb2Te5 or Ge19Sb25Te56 alloys were investigated at very high linear velocities ranging from 10 m/s to 60 m/s. We used transmission electron microscope to observe incompletely erased marks, and used computer simulation to treat laser beam heating process in terms of time and temperature. Following results were obtained. (1) The crystallization times, in which the amorphous marks can be erased completely, for Ge19Sb25Te56 and Ge2Sb2Te5 recording films are 55 ns and less than 33 ns respectively. (2) The erasing ratio of Ge19Sb25Te56 disk degrades at high linear velocity (higher than 40 m/s). (3) The erasing ratio degradation is caused by decreasing the number of crystals formed in the marks. (4) When the annealing time τ50 is less than 55 ns, nucleation process becomes bottleneck for crystallization of Ge19Sb25Te56 recording films, and the nucleation time for each grain in the marks is different from one another.