Crystallographic studies on high-speed phase-change materials used for rewritable optical recording disks

Optical recording disks, which exploit the reversible phase-change phenomenon between amorphous and crystal states of a material, are now in extensive use, particularly in videodisc recorders for home use. The most commonly used recording materials are GeTe-Sb2Te3 pseudobinary compounds and Sb-Te binary compounds with a small quantity of added In, Ag, and/or Ge. These compounds, which have superior capabilities for holding data at room temperature for extended periods and high rewrite speeds, all tend to have structures in which one lattice site is occupied by two or more elements at random, and the lattices are all cubic or pseudocubic at the high temperatures at which recording and erasure are performed. In addition, with rising temperature, the amplitude of atomic thermal vibration increases exponentially. It is believed that these two characteristics of these materials that are suitable for phase-change optical disks, i.e., an isotropic structure and intense thermal vibration at high temperatures, play very important roles in the virtually instantaneous transformation from the amorphous to the crystal phase. We describe here the relationship between the structural features and the high-speed phase-change mechanism.