Chalcogenide glass (hard "ch" as in "chemistry") is a glass containing one or more chalcogenide elements (not counting oxygen). The name chalcogenide originates from the Greek word "chalcos" meaning ore and "gen" meaning formation, thus the term chalcogenide is generally considered to mean ore former. These are three elements in Group 16 in the periodic table: sulfur, selenium and tellurium. Such glasses are covalently bonded materials and may be classified as network solids; in effect, the entire glass matrix acts as an infinitely bonded molecule. Although Polonium is chemically a chalcogenide as well, it is not used in chalcogenide glasses because of its strong radioactivity and high price. Oxygen is also a group 16 element, but it is not considered a chalcogenide. Though oxide materials are the oldest known glass forming systems it has become more traditional to treat them separately from more recently discovered chalcogenide compounds. Scientifically oxide materials behave rather differently from other chalcogenides, in particular their widely different band gaps contribute to very dissimilar optical and electrical properties. Chalcogenides can exist naturally as minerals; two of the most well-known being FeS2 (pyrite) and AuTe2 (calaverite). In fact AuTe2 was the chief reason for the name given to the gold-rush town 'Telluride' in Southwest Colorado.
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. The semiconducting properties of chalcogenide glasses were revealed in 1955 by B.T. Kolomiets and N.A. Gorunova from Ioffe Institute, USSR. This discovery initiated numerous researches and applications of this new class of semiconducting materials.
Modern chalcogenide compounds like AgInSbTe and GeSbTe, widely used in rewritable optical disks and phase-change memory devices, are fragile glass-formers; by applying heat, 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.
The model of a binary glass forming chalcogenide is considered to be analogous to silica; there are two group 16 chalcogen elements bonded to a single group 14 element. Another common class of chalcogenides have glass forming regions where three chalcogens are bonded to two group 15 elements. Most stable binary chalcogenide glasses are compounds of a chalcogen and a group 14 or 15 element. This allows a wide range of atomic ratios. Ternary glasses allow a larger variety of atoms to be incorporated into the glass structure; thus giving greater engineering capacity. Although chalcogenides can exist over a wide range of compositions, not all of which exist in glassy form, it is often 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 sulphide on its own is not a known glass former however it readily bonds with sodium or lanthanum sulphides to form a glass, gallium lanthanum sulphide (GLS). Amorphous chalcogenide materials can be broadly classed by the type of atoms to which they bond to form amorphous systems. One of the more well-known chalcogenide glasses is based on arsenic trisulphide, an example of a stable binary glass which preferentially exists in a glassy phase. In contrast, compounds based on heavier chalcogenides, for example tellurium based materials are more likely to exist as a crystal.
The modern technological applications of chalcogenide glasses are widespread. Examples 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 ions. Many 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 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 common such materials are InSe, SbSe, SbTe, InSbSe, InSbTe, GeSbSe, GeSbTeSe and AgInSbSeTe.
Electrical switching in chalcogenide semiconductors emerged in the 1960s, when the amorphous chalcogenide Te48As30Si12Ge10 was found to exhibit sharp, reversible transitions in electrical resistance above a threshold voltage. The switching mechanism would appear initiated by fast purely electronic processes. If current is allowed to persist in the non-crystalline material, it heats up and changes to crystalline form. This is equivalent to information being written on it. A crystalline region may be melted by exposure to a brief, intense pulse of heat. Subsequent rapid cooling then sends the melted region back through the glass transition. Conversely, a lower-intensity heat pulse of longer duration will crystallize an amorphous region.
Attempts to induce the glassy–crystal transformation of chalcogenides by electrical means form the basis of phase-change random-access memory (PC-RAM). This emerging technology is on the brink of commercial application by ECD Ovonics. For write operations, an electric current supplies the heat pulse. The read process is performed at sub-threshold voltages by utilizing the relatively large difference in electrical resistance between the glassy and crystalline states. Examples of such phase change materials are GeSbTe and AgInSbTe.
Although the electronic structural transitions relevant to both optical discs and PC-RAM were featured strongly, contributions from ions were not considered—even though amorphous chalcogenides can have significant ionic conductivities. At Euromat 2005, however, it was shown that ionic transport can also be useful for data storage in a solid chalcogenide electrolyte. At the nanoscale, this electrolyte consists of crystalline metallic islands of silver selenide (Ag2Se) dispersed in an amorphous semiconducting matrix of germanium selenide (Ge2Se3).
All of these technologies present exciting opportunities that are not restricted to memory, but include cognitive computing and reconfigurable logic circuits. It is too early to tell which technology will be selected for which application. But scientific interest alone should drive the continuing research. For example, the migration of dissolved ions is required in the electrolytic case, but could limit the performance of a phase-change device. Diffusion of both electrons and ions participate in electromigration—widely studied as a degradation mechanism of the electrical conductors used in modern integrated circuits. Thus, a unified approach to the study of chalcogenides, assessing the collective roles of atoms, ions and electrons, may prove essential for both device performance and reliability.
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