Chalcogenide glass
Encyclopedia
A chalcogenide glass is a glass
containing one or more chalcogen
ide 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 solid
s. 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 . 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.
, and infrared optical fiber
s, 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 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.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
containing one or more chalcogen
Chalcogen
The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...
ide elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
. These are Group 16 in the periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
e.g. sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
or tellurium. Such glasses are covalently bonded materials and may be classified as network solid
Network solid
A network solid or covalent network solid is a chemical compound in which the atoms are bonded by covalent bonds in a continuous network. In a network solid there are no individual molecules and the entire crystal may be considered a macromolecule...
s. 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 . This discovery initiated the numerous researches and applications of this new class of semiconducting materials.
Modern chalcogenide compounds like GeSbTe
GeSbTe
GeSbTe, Germanium-Antimony-Tellurium or GST is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications. Its recrystallization time is 20 nanoseconds, allowing bitrates of up to 35 Mbit/s to be written, and direct...
, widely used in rewritable optical disks and PRAM
Phase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...
devices, are fragile glass-formers, therefore they are able to crystallize in about 100 ns.
Applications
The modern technological applications of chalcogenide glasses are widespread. Examples include infrared detectors, mouldable infrared optics such as lensesLens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
, and infrared optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
s, with the main advantage being that these materials transmit across a wide range of the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. The physical properties of chalcogenide glasses (high refractive index, low phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
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
Phase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...
. Examples of such phase change materials are GeSbTe
GeSbTe
GeSbTe, Germanium-Antimony-Tellurium or GST is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications. Its recrystallization time is 20 nanoseconds, allowing bitrates of up to 35 Mbit/s to be written, and direct...
and AgInSbTe
AgInSbTe
AgInSbTe, or Silver-Indium-Antimony-Tellurium, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications...
. In optical discs, the phase change layer is usually sandwiched between dielectric layers of ZnS
Zinc sulfide
Zinc sulfide is a inorganic compound with the formula ZnS. ZnS is the main form of zinc in nature, where it mainly occurs as the mineral sphalerite...
-SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, 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
GeSbTe
GeSbTe, Germanium-Antimony-Tellurium or GST is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications. Its recrystallization time is 20 nanoseconds, allowing bitrates of up to 35 Mbit/s to be written, and direct...
and AgInSbTe
AgInSbTe
AgInSbTe, or Silver-Indium-Antimony-Tellurium, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications...
.
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.
External links
- The Register phase change memory
- Optical switching