Chalcogenide glass

A chalcogenide glass (hard "ch" as in "chemistry") is a glass containing one or more chalcogenide elements. These are Group 16 in the periodic table e.g. sulfur, selenium or tellurium. Such glasses are covalently bonded materials and may be classified as network solids. In effect, the entire glass matrix acts like an infinitely bonded molecule. The classical chalcogenide glasses are strong glass-formers (mainly sulphur based ones) such as systems As-S, Ge-S possess glasses within large concentration regions. Glass forming abilities decrease with increasing molar weight of constituent elements i.e. S>Se>Te. Semiconducting properties of chalcogenide glasses were revealed in 1955 by B.T. Kolomiets and N.A. Gorunova from Ioffe Institute, USSR ^[1]. This discovery initiated the numerous researches and applications of this new class of semiconducting materials.

Modern chalcogenide compounds like GeSbTe, widely used in rewritable optical disks and PRAM devices, are fragile glass-formers, therefore they are able to crystallize in about 100 ns.

Applications

A CD-RW (CD). Amorphous chalcogenide materials form the basis of re-writable CD and DVD solid-state memory technology.^[2]

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 and other active devices especially if doped with rare earth ions. 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 ^[2] 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.^{[citation needed]} Other less common such materials are InSe, SbSe, SbTe, InSbSe, InSbTe, GeSbSe, GeSbTeSe, and AgInSbSeTe.^[3]

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.^[4][5][6]

References

1. ^ B.T. Kolomiets, Physica Status Solidi, vol. 7, p. 359, p. 713, 1964.
2. ^ ^{a b} Greer, A. Lindsay; Mathur, N (2005). "Materials science: Changing face of the chameleon". Nature 437 (7063): 1246–1247. Bibcode 2005Natur.437.1246G. doi:10.1038/4371246a. PMID 16251941. 
3. ^ US Patent 6511788
4. ^ Ovshinsky, S.R., Phys. Rev. Lett., Vol. 21, p. 1450 (1968); Jpn. J. Appl. Phys., Vol. 43, p. 4695 (2004)
5. ^ Adler, D. et al., J. Appl. Phys., Vol. 51, p. 3289(1980)
6. ^ Vezzoli, G. C., Walsh, P. J., Doremus, L. W., J. Non-Cryst. Solids, Vol. 18, p. 333(1975)

External links

• The Register phase change memory
• Optical switching