Germanium telluride

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Germanium telluride
Germanium(II)-tellurid.png
Unit cell of germanium telluride.
Identifiers
3D model (JSmol)
ChemSpider
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PubChem CID
  • InChI=1S/GeTe2/c2-1-3 Yes check.svgY
    Key: GPMBECJIPQBCKI-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/GeTe2/c2-1-3
    Key: GPMBECJIPQBCKI-UHFFFAOYAU
  • [Ge]=[Te]
Properties
GeTe
Molar mass 200.21 g/mol
Appearancesolid
Density 6.14 g/cm3
Melting point 725 °C (1,337 °F; 998 K)
Band gap 0.6 eV [1]
5
Structure
Rhombohedral, hR6
R3m, No. 160
a = 4.1719 Å, c = 10.710 Å [2]
161.430 Å3
Related compounds
Other anions
Germanium monoxide
Germanium monosulfide
Germanium monoselenide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Germanium telluride (GeTe) is a chemical compound of germanium and tellurium and is a component of chalcogenide glass. It shows semimetallic conduction and ferroelectric behaviour. [3]

Contents

Germanium telluride exists in three major crystalline forms, room-temperature α (rhombohedral) and γ (orthorhombic) structures and high-temperature β (cubic, rocksalt-type) phase; α phase being most phase for pure GeTe below the ferroelectric Curie temperature of approximately 670 K (746 °F; 397 °C). [4] [5]

Doped germanium telluride is a low temperature superconductor. [6]

Phase Transition

Solid GeTe can transform between amorphous and crystalline states. The crystalline state has a low resistivity (semiconducting at room temperature) and the amorphous state has a high resistivity. [7] The difference in resistivity can be up to six orders of magnitude depending on the film quality, GeTe compositions, and nucleation site formation. [7] [8] The drastic changes in the properties of the material have been exploited in data storage applications. The phase transitions of GeTe can be fast, reversible and repeatable, with drastic property changes, making GeTe a promising candidate in applications like radio frequency (RF) switching and direct current (DC) switching. [8] Research on mechanisms that relate the phase transition and radio frequency (RF) switching is underway, with a promising future in optimization for telecommunication applications. [8] Although both solid states can exist at room temperatures, the transition requires a specific heating and cooling process known as the thermal actuation method. [8] To achieve the amorphous state the solid is heated up beyond the melting temperature with a high current pulse in a short amount of time and rapidly quenched or cooled down. Crystallization happens when the GeTe is heated to a crystallization temperature lower than the melting temperature with a relatively longer and lower current pulse, and a slow quenching process with the current gradually reduced. [8] Both direct and indirect heating can induce phase changes. [8] Joule heating approach is the common direct heating method and indirect heating can be accomplished by a separate layer of dielectric material added to the RF switch. [8] The crystal structure of GeTe is rhombohedrally distorted rock salt-type structure that forms a face-centered cubic (FCC) sublattice at room temperature. [8]

Synthesis

Single-crystalline GeTe nanowires and nanohelices

Semiconducting GeTe nanowires (NW) and nanohelices (NH) are synthesized via vapor transport method, with metal nanoparticle catalysts. GeTe was evaporated and carried by Ar gas at optimum temperature, pressure, time, and gas flow rate to the downstream collecting/grow site (SiO2 surface coated with colloidal gold nanoparticles). High temperature over 500 °C produces thicker nanowires and crystalline chunks. Au is essential to the growth of NW and NH and is suggested to the metal catalyst of the reaction. This method gives rise to NW and NH with a 1:1 ratio of Ge and Te. NW produced by this method average about 65 nm in diameter and up to 50 μm in length. NHs averages to 135 nm in helix diameter. [9]

Nanocrystal (quantum size effect)

The synthesis described above has not reached the sized required to exhibit quantum size effect. Nanostructures that reach the quantum regime exhibit a different set of phenomena unseen at a larger scale, for example, spontaneous polar ordering and the splitting of diffraction spots. The synthesis of GeTe nanocrystals of average size of 8, 17, and 100 nm involves divalent Ge(II) chloride – 1,4 dioxane complex and bis[bis(trimethylsilyl)amino]Ge (II) and trioctylphosphine-tellurium in a solvent such as 1,2-dichlorobenzene or phenyl ether. Ge(II) reduction kinetics has been thought to determine the GeTe formation. Large the Ge(II) reduction rate may lead to the increase in particle nucleation rate, resulting in the reduction of particle diameter. [10]

Applications

Memory storage

GeTe has been heavily used in non-volatile optical data storage such as CDs, DVDs, and Blu-ray and may replace dynamic and flash random access memories. In 1987, Yamada et al. explored the phase changing properties of GeTe and Sb2Te3 for optical storage. The short crystallization time, cyclability and high optical contrast made these material better options than Te81Ge15Sb2S2 which has a slow transition time. [8]

RF switching

The high contrast in resistivity between the amorphous and crystalline states and the ability to reverse the transition repeatedly make GeTe a good candidate for RF switching. RF requires a thin layer of GeTe film to be deposited on the surface of the substrate. Seed layer structure, precursor composition, deposition temperature, pressure, gas flow rates, precursor bubbling temperatures and the substrates all play a role in the film properties. [8]

Related Research Articles

<span class="mw-page-title-main">Melting</span> Material phase change

Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which increases the substance's temperature to the melting point. At the melting point, the ordering of ions or molecules in the solid breaks down to a less ordered state, and the solid melts to become a liquid.

Phase-change memory is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In PCM, heat produced by the passage of an electric current through a heating element generally made of titanium nitride is used to either quickly heat and quench the glass, making it amorphous, or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state. PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies with the same capability.

Chalcogenide glass is a glass containing one or more chalcogens. 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.

<span class="mw-page-title-main">Single crystal</span> Material with a continuous, unbroken crystal lattice

In materials science, a single crystal is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The absence of the defects associated with grain boundaries can give monocrystals unique properties, particularly mechanical, optical and electrical, which can also be anisotropic, depending on the type of crystallographic structure. These properties, in addition to making some gems precious, are industrially used in technological applications, especially in optics and electronics.

<span class="mw-page-title-main">Barium titanate</span> Chemical compound

Barium titanate (BTO) is an inorganic compound with chemical formula BaTiO3. Barium titanate appears white as a powder and is transparent when prepared as large crystals. It is a ferroelectric, pyroelectric, and piezoelectric ceramic material that exhibits the photorefractive effect. It is used in capacitors, electromechanical transducers and nonlinear optics.

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 overwrite capability up to 106 cycles. It is suitable for land-groove recording formats. It is often used in rewritable DVDs. New phase-change memories are possible using n-doped GeSbTe semiconductor. The melting point of the alloy is about 600 °C (900 K) and the crystallization temperature is between 100 and 150 °C.

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. It is a quaternary compound of silver, indium, antimony, and tellurium.

<span class="mw-page-title-main">Polyamorphism</span> Ability of a substance to exist in more than one distinct amorphous state

Polyamorphism is the ability of a substance to exist in several different amorphous modifications. It is analogous to the polymorphism of crystalline materials. Many amorphous substances can exist with different amorphous characteristics. However, polyamorphism requires two distinct amorphous states with a clear, discontinuous (first-order) phase transition between them. When such a transition occurs between two stable liquid states, a polyamorphic transition may also be referred to as a liquid–liquid phase transition.

<span class="mw-page-title-main">Hafnium(IV) oxide</span> Chemical compound

Hafnium(IV) oxide is the inorganic compound with the formula HfO
2. Also known as hafnium dioxide or hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of 5.3~5.7 eV. Hafnium dioxide is an intermediate in some processes that give hafnium metal.

<span class="mw-page-title-main">Indium(III) oxide</span> Chemical compound

Indium(III) oxide (In2O3) is a chemical compound, an amphoteric oxide of indium.

<span class="mw-page-title-main">Tin selenide</span> Chemical compound

Tin selenide, also known as stannous selenide, is an inorganic compound with the formula SnSe. Tin(II) selenide is a typical layered metal chalcogenide as it includes a group 16 anion (Se2−) and an electropositive element (Sn2+), and is arranged in a layered structure. Tin(II) selenide is a narrow band-gap (IV-VI) semiconductor structurally analogous to black phosphorus. It has received considerable interest for applications including low-cost photovoltaics, and memory-switching devices.

Germanium dioxide, also called germanium(IV) oxide, germania, and salt of germanium, is an inorganic compound with the chemical formula GeO2. It is the main commercial source of germanium. It also forms as a passivation layer on pure germanium in contact with atmospheric oxygen.

Combined with certain metallic species, amorphous films can crystallize in a process known as metal-induced crystallization (MIC). The effect was discovered in 1969, when amorphous germanium (a-Ge) films crystallized at surprisingly low temperatures when in contact with Al, Ag, Cu, or Sn. The effect was also verified in amorphous silicon (a-Si) films, as well as in amorphous carbon and various metal-oxide films.

<span class="mw-page-title-main">Glass transition</span> Reversible transition in amorphous materials

The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials from a hard and relatively brittle "glassy" state into a viscous or rubbery state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass. The reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification.

<span class="mw-page-title-main">Allotropes of boron</span> Materials made only out of boron

Boron can be prepared in several crystalline and amorphous forms. Well known crystalline forms are α-rhombohedral (α-R), β-rhombohedral (β-R), and β-tetragonal (β-T). In special circumstances, boron can also be synthesized in the form of its α-tetragonal (α-T) and γ-orthorhombic (γ) allotropes. Two amorphous forms, one a finely divided powder and the other a glassy solid, are also known. Although at least 14 more allotropes have been reported, these other forms are based on tenuous evidence or have not been experimentally confirmed, or are thought to represent mixed allotropes, or boron frameworks stabilized by impurities. Whereas the β-rhombohedral phase is the most stable and the others are metastable, the transformation rate is negligible at room temperature, and thus all five phases can exist at ambient conditions. Amorphous powder boron and polycrystalline β-rhombohedral boron are the most common forms. The latter allotrope is a very hard grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel.

<span class="mw-page-title-main">Tauc plot</span> Method for determining the band gap of a material

A Tauc plot is used to determine the optical bandgap, or Tauc bandgap, of either disordered or amorphous semiconductors.

<span class="mw-page-title-main">Antimony telluride</span> Chemical compound

Antimony telluride is an inorganic compound with the chemical formula Sb2Te3. As is true of other pnictogen chalcogenide layered materials, it is a grey crystalline solid with layered structure. Layers consist of two atomic sheets of antimony and three atomic sheets of tellurium and are held together by weak van der Waals forces. Sb2Te3 is a narrow-gap semiconductor with a band gap 0.21 eV; it is also a topological insulator, and thus exhibits thickness-dependent physical properties.

Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. Polymers can crystallize upon cooling from melting, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer. The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10 and 80%, with crystallized polymers often called "semi-crystalline". The properties of semi-crystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains.

Allotropes of silicon are structurally varied forms of silicon.

Arsenic(III) telluride is an inorganic compound with the chemical formula As2Te3. It exists in two forms, the monoclinic α phase which transforms under high pressure to a rhombohedral β phase. The compound is a semiconductor, with most current carried by holes. Arsenic telluride has been examined for its use in nonlinear optics.

References

  1. ↑ R. Tsu; et al. (1968). "Optical and Electrical Properties and Band Structure of GeTe and SnTe". Phys. Rev. 172 (3): 779–788. Bibcode:1968PhRv..172..779T. doi:10.1103/PhysRev.172.779.
  2. ↑ Bauer Pereira, Paula; Sergueev, Ilya; Gorsse, Stéphane; Dadda, Jayaram; Müller, Eckhard; Hermann, Raphaël P. (2013). "Lattice dynamics and structure of Ge Te, Sn Te and Pb Te". Physica Status Solidi B. 250 (7): 1300–1307. Bibcode:2013PSSBR.250.1300B. doi:10.1002/pssb.201248412.
  3. ↑ A. I. Lebedev; I. A. Sluchinskaya; V. N. Demin; I. H. Munro (1997). "Influence of Se, Pb and Mn impurities on the ferroelectric phase transition in GeTe studied by EXAFS". Phase Transitions. 60 (2): 67. doi:10.1080/01411599708220051. Archived from the original on 2016-03-03. Retrieved 2006-05-20.
  4. ↑ E. I. Givargizov; A.M. Mel'nikova (2002). Growth of Crystals. Birkhäuser. p. 12. ISBN   0-306-18121-5.
  5. ↑ Pawley, G.; Cochran, W.; Cowley, R.; Dolling, G. (1966). "Diatomic Ferroelectrics". Physical Review Letters. 17 (14): 753. Bibcode:1966PhRvL..17..753P. doi:10.1103/PhysRevLett.17.753.
  6. ↑ Hein, R.; Gibson, J.; Mazelsky, R.; Miller, R.; Hulm, J. (1964). "Superconductivity in Germanium Telluride". Physical Review Letters. 12 (12): 320. Bibcode:1964PhRvL..12..320H. doi:10.1103/PhysRevLett.12.320.
  7. 1 2 A H Gwin; R A Coutu Jr. (2015). Teherani, Ferechteh H; Look, David C; Rogers, David J (eds.). "Electronic control of Germanium Telluride (GeTe) phase transition for electronic memory applications". Proceedings. Oxide-based Materials and Devices VI. 9364: 93640G. doi:10.1117/12.2079359. S2CID   122243829.
  8. 1 2 3 4 5 6 7 8 9 10 P. Mahanta; M. Munna; R. A. Coutu Jr. (2018). "Performance Comparison of Phase Change Materials and Metal-Insulator Transition Materials for Direct Current and Radio Frequency Switching Applications". Technologies. 6 (2): 48. doi: 10.3390/technologies6020048 .
  9. ↑ D. Yu; J. Wu; Q. Gu; H. Park (2006). "Germanium Telluride Nanowires and Nanohelices with Memory-Switching Behavior". J. Am. Chem. Soc. 128 (25): 8148–9. doi:10.1021/ja0625071. PMID   16787074.
  10. ↑ M. J. Polking; H. Zheng; R. Ramesh; A. P. Alivisatos (2011). "Controlled Synthesis and Size-Dependent Polarization Domain Structure of Colloidal Germanium Telluride Nanocrystals". J. Am. Chem. Soc. 133 (7): 2044–7. doi:10.1021/ja108309s. PMID   21280629.
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