Chalcogenide glass

Chalcogenide glass (pronounced hard ch as in chemistry) is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent network solids. Polonium is also a chalcogen but is not used because of its strong radioactivity. Chalcogenide materials behave rather differently from oxides, in particular their lower band gaps contribute to very dissimilar optical and electrical properties. Chalcogenide glass (pronounced hard ch as in chemistry) is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent network solids. Polonium is also a chalcogen but is not used because of its strong radioactivity. Chalcogenide materials behave rather differently from oxides, in particular their lower band gaps contribute to very dissimilar optical and electrical properties. The classical chalcogenide glasses (mainly sulfur-based ones such as As-S or Ge-S) are strong glass-formers and possess glasses within large concentration regions. Glass-forming abilities decrease with increasing molar weight of constituent elements; i.e., S > Se > Te. Chalcogenide compounds such as AgInSbTe and GeSbTe are used in rewritable optical disks and phase-change memory devices. They are fragile glass-formers: by controlling heating and annealing (cooling), they can be switched between an amorphous (glassy) and a crystalline state, thereby changing their optical and electrical properties and allowing the storage of information. Most stable binary chalcogenide glasses are compounds of a chalcogen and a group 14 or 15 element and may be formed in a wide range of atomic ratios. Ternary glasses are also known. Not all chalcogenide compositions exist in glassy form, though it is possible to find materials with which these non-glass-forming compositions can be alloyed in order to form a glass. An example of this is gallium sulphide-based glasses. Gallium(III) sulphide on its own is not a known glass former; however, with sodium or lanthanum sulphides it forms a glass, gallium lanthanum sulphide (GLS). Uses include infrared detectors, mouldable infrared optics such as lenses, and infrared optical fibers, with the main advantage being that these materials transmit across a wide range of the infrared electromagnetic spectrum. The physical properties of chalcogenide glasses (high refractive index, low phonon energy, high nonlinearity) also make them ideal for incorporation into lasers, planar optics, photonic integrated circuits, and other active devices especially if doped with rare-earth element ions. Some chalcogenide glasses exhibit several non-linear optical effects such as photon-induced refraction, and electron-induced permittivity modification Some chalcogenide materials experience thermally driven amorphous-to-crystalline phase changes. This makes them useful for encoding binary information on thin films of chalcogenides and forms the basis of rewritable optical discs and non-volatile memory devices such as PRAM. Examples of such phase change materials are GeSbTe and AgInSbTe. In optical discs, the phase change layer is usually sandwiched between dielectric layers of ZnS-SiO2, sometimes with a layer of a crystallization promoting film. Other less commonly used such materials are InSe, SbSe, SbTe, InSbSe, InSbTe, GeSbSe, GeSbTeSe and AgInSbSeTe. Intel claims that its chalcogenide-based 3D XPoint memory technology achieves throughput and write durability 1,000 times higher than flash memory.