The use of phase-change chalcogenide alloy films to store data electrically and optically was first reported in 1968 and in 1972, respectively. Early phase-change memory devices used tellurium-rich, multi-component chalcogenide alloys with a typical composition of Te81Ge15Sb2S2. Both the optical and electrical memory devices were programmed by application of an energy pulse of appropriate magnitude and duration. A short pulse of energy was used to melt the material, which was then allowed to cool quickly enough to “freeze in” the glassy, structurally disordered state. To reverse the process, somewhat lower amplitude, longer-duration pulse was used to heat a previously vitrified region of the alloy to a temperature below the melting point, at which crystallization could occur rapidly. Differences in electrical resistivity and the optical constants between the amorphous and polycrystalline phases were used to store data.
During the 1970s and 1980s, significant research efforts by many industrial and academic groups were focused on understanding the fundamental properties of chalcogenide alloy amorphous semiconductors. Prototype optical memory disks and electronic memory device arrays also were announced, beginning in the early 1970s. Rapidly crystallizing chalcogenide alloys were later reported by several optical memory research groups. These new material compositions, derived from the Ge-Te-Sb ternary system, did not phase segregate upon crystallization like the earlier Te-rich alloys, but instead exhibited congruent crystallization with no large-scale atomic motion.
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