Phase-change erasable optical recording uses a focused laser beam as a heat source to reversibly switch a micron-sized area in a thin film between the amorphous and crystalline states. A bit of information is stored as an amorphous spot in a crystalline background, and the state of the bit is determined by the differing optical properties of the amorphous and crystalline phases. This concept was first demonstrated in 1971 and then, after about a decade of exploratory work, the field accelerated throughout the 1980s at several research laboratories. Currently the subject of number of reviews, the field of phase-change materials promises to broaden and intensify in the 1990s.
The active layer, where the storage occurs, is typically a tellurium-based alloy with a variety of solute species. Early work studied the recording properties of single-layered films, but it has been clearly shown that multilayered films, where the active layer is sandwiched between two or more dielectric layers, have superior recording properties and resistance to irreversible damage caused by laser heating. The dielectric layers (typically SiO2, Si3N4, or ZnS) provide barriers to active-layer oxidation and contamination, help prevent the hole formation associated with ablative write-once storage methods, and act as crucibles and heat sinks which contain the molten spot and influence its cooling properties, respectively. A typical multilayer structure is shown in the cross-sectional transmission electron micrograph of Figure 1.