INTRODUCTION:

Chalcogenide Glasses

A chalcogenide glass is a glass containing one or more chalcogenide elements. These are Group 16 in the periodic table e.g. sulphur, selenium or 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.

The classical chalcogenide glasses (mainly sulphur-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.

A CD-RW (CD). Amorphous chalcogenide materials form the basis of re-writable CD and DVD solid-state memory Chalcogenide glasses are based on the chalcogen elements S, Se, and Te. These glasses are formed by the addition of other elements such as Ge, As, Sb, Ga, etc. They are low-phonon-energy materials and are generally transparent from the visible up to the infrared. Chalcogenide glasses can be doped by rare earth elements, such as Er, Nd, Pr, etc., and hence numerous applications of active optical devices have been proposed. Since chalcogenide-glass fibres transmit in the IR, there are numerous potential applications in the civil, medical, and military areas. Passive applications utilize chalcogenide fibres as a light conduit from one location to another point without changing the optical properties, other than those due to scattering, absorption, and reflection. These glasses are optically highly nonlinear and could therefore be useful for all-optical switching (AOS).Chalcogenide glasses are sensitive to the absorption of electromagnetic radiation and show a variety of photoinduce deffects as a result of illumination. Various model shave been put forward to explain these effects, which can be used to fabricate diffractive, wave guide and fibre structures. Next-generation devices for telecommunication and related applications will rely on the development of materials which possess optimized physical properties that are compatible with packaging requirements for systems in planar or fibre form. This allows suitable integration to existing fibre-based applications, and hence requires appropriate consideration as to material choice, stability, and long-term aging behaviour.

Applications

The modern technological applications of chalcogenide glasses are widespread. Examples include infrared detectors, mouldable infrared optics such as lenses, and infrared optical fibres, 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 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 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 ( Se) dispersed in an amorphous semiconducting matrix of germanium selenide ( ).

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.

CONCLUSION:

Chalcogenide glasses are low-phonon-energy materials and are generally transparent from the visible to the infrared. Doping chalcogenide glasses by rare earth elements has opened up numerous applications of active optical devices of their large nonlinearities, chalcogenide glasses are promising candidates for AOS applications. The structure of chalcogenide glasses cannot be described by means of a continuous random network, but can be rather layer like, as for example in As2S3, and chain-like, as in pure S or Se. Flexibility of their structures, as a result of the van der Waal’s bonding between layer sallows for easily accommodation of changes in their structures. Various structural techniques such as NIR Raman spectroscopy, RBS, WRS, and EXAF Shave been used to probe different structural units present in chalcogenide glasses. Different techniques have been employed to determine the density of localized states in the gap and it is now generally believed that on top of a featureless distribution of states in the tails, a structured density of defect states exists, attributed to VAPs. It could be said that well-defined states exist in the gap of chalcogenide glasses. The absorption coefficient, α, of films has been measured using several techniques, such as with a conventionalspectrophotometer in the visible region and PDS for wavelengths beyond the band edge. The results of PDS measurements have shown that as- deposited films have losses below 0.1 dB cm−1 across the telecommunication band. The optical gap obtained from the analysis of the data shows that chalcogenide glasses have optical gaps∼2–3 eV. Various photoinduced effects, such as photodarkening, the metal-photodissolution effect and PA, have been used to fabricate devices such as gratings, waveguides, Bragg gratings, etc. Doping chalcogenide glasses with rare-earth elements has allowed the possibility of using these glasses for active applications such as amplifiers and lasers. Since chalcogenide glass fibres transmit in the IR, there are numerous potential applications in the civil, medical and military areas. Chalcogenide fibres are well-suited for chemical-sensor applications, such as fibre-optic chemical sensor systems for quantitative remote detection and identification as well as detecting chemicals in mixtures. Different techniques have been used to measure the optical constants of chalcogenide glasses and films, such as optical transmission and reflection, ellipsometry, prism coupling, etc. While, up to now, evaporated and sputtered films have been used for producing films of chalcogenide glasses, pulsed laser-deposition of these films has proved useful. These PLD films have a stoichiometry similar to their parent materials and do not need annealing after deposition. They have been used in fabric atingmany devices, such as waveguides, fibre Bragg gratings, nonlinear directional couplers, etc. using light-induced photostructural changes. Results of loss measurement sin laser-written waveguides show that losses lower than 0.3 dB cm−1at 1,550nm are found for a typical chalcogenide glass such as .

REFERENCE:

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5. Frumar, M.; Frumarova, B.; Wagner, T. (2011). "4.07: Amorphous and Glassy Semiconducting Chalcogenides".

Received on 05.01.2013 Accepted on 12.01.2013

Research J. Science and Tech 5(1): Jan.-Mar.2013 page 213-215