GeSbTe

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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.

Contents

During writing, the material is erased, initialized into its crystalline state, with low-intensity laser irradiation. The material heats up to its crystallization temperature, but not its melting point, and crystallizes. The information is written at the crystalline phase, by heating spots of it with short (<10 ns), high-intensity laser pulses; the material melts locally and is quickly cooled, remaining in the amorphous phase. As the amorphous phase has lower reflectivity than the crystalline phase, data can be recorded as dark spots on the crystalline background. Recently, novel liquid organogermanium precursors, such as isobutylgermane [1] [2] [3] (IBGe) and tetrakis(dimethylamino)germane [4] [5] (TDMAGe) were developed and used in conjunction with the metalorganics of antimony and tellurium, such as tris-dimethylamino antimony (TDMASb) and di-isopropyl telluride (DIPTe) respectively, to grow GeSbTe and other chalcogenide films of very high purity by metalorganic chemical vapor deposition (MOCVD). Dimethylamino germanium trichloride [6] (DMAGeC) is also reported as the chloride containing and superior dimethylaminogermanium precursor for Ge deposition by MOCVD.

Material properties

Phase diagram of the GeSbTe ternary alloy system GeSbTe phase diagram.jpg
Phase diagram of the GeSbTe ternary alloy system

GeSbTe is a ternary compound of germanium, antimony, and tellurium, with composition GeTe-Sb2Te3. In the GeSbTe system, there is a pseudo-line as shown upon which most of the alloys lie. Moving down this pseudo-line, it can be seen that as we go from Sb2Te3 to GeTe, the melting point and glass transition temperature of the materials increase, crystallization speed decreases and data retention increases. Hence, in order to get high data transfer rate, we need to use material with fast crystallization speed such as Sb2Te3. This material is not stable because of its low activation energy. On the other hand, materials with good amorphous stability like GeTe has slow crystallization speed because of its high activation energy. In its stable state, crystalline GeSbTe has two possible configurations: hexagonal and a metastable face-centered cubic (FCC) lattice. When it is rapidly crystallized however, it was found to have a distorted rocksalt structure. GeSbTe has a glass transition temperature of around 100 °C. [7] GeSbTe also has many vacancy defects in the lattice, of 20 to 25% depending on the specific GeSbTe compound. Hence, Te has an extra lone pair of electrons, which are important for many of the characteristics of GeSbTe. Crystal defects are also common in GeSbTe and due to these defects, an Urbach tail in the band structure is formed in these compounds. GeSbTe is generally p type and there are many electronic states in the band gap accounting for acceptor and donor like traps. GeSbTe has two stable states, crystalline and amorphous. The phase change mechanism from high resistance amorphous phase to low resistance crystalline phase in nano-timescale and threshold switching are two of the most important characteristic of GeSbTe.

Applications in phase-change memory

The unique characteristic that makes phase-change memory useful as a memory is the ability to effect a reversible phase change when heated or cooled, switching between stable amorphous and crystalline states. These alloys have high resistance in the amorphous state ‘0’ and are semimetals in the crystalline state ‘1’. In amorphous state, the atoms have short-range atomic order and low free electron density. The alloy also has high resistivity and activation energy. This distinguishes it from the crystalline state having low resistivity and activation energy, long-range atomic order and high free electron density. When used in phase-change memory, use of a short, high amplitude electric pulse such that the material reaches melting point and rapidly quenched changes the material from crystalline phase to amorphous phase is widely termed as RESET current and use of a relatively longer, low amplitude electric pulse such that the material reaches only the crystallization point and given time to crystallize allowing phase change from amorphous to crystalline is known as SET current.

The early devices were slow, power consuming and broke down easily due to the large currents. Therefore, it did not succeed as SRAM and flash memory took over. In the 1980s though, the discovery of germanium-antimony-tellurium (GeSbTe) meant that phase-change memory now needed less time and power to function. This resulted in the success of the rewriteable optical disk and created renewed interest in the phase-change memory. The advances in lithography also meant that previously excessive programming current has now become much smaller as the volume of GeSbTe that changes phase is reduced.

Phase-change memory has many near ideal memory qualities such as non-volatility, fast switching speed, high endurance of more than 1013 read –write cycles, non-destructive read, direct overwriting and long data retention time of more than 10 years. The one advantage that distinguishes it from other next generation non-volatile memory like magnetic random access memory (MRAM) is the unique scaling advantage of having better performance with smaller sizes. The limit to which phase-change memory can be scaled is hence limited by lithography at least until 45 nm. Thus, it offers the biggest potential of achieving ultra-high memory density cells that can be commercialized.

Though phase-change memory offers much promise, there are still certain technical problems that need to be solved before it can reach ultra-high density and commercialized. The most important challenge for phase-change memory is to reduce the programming current to the level that is compatible with the minimum MOS transistor drive current for high-density integration. Currently, the programming current in phase-change memory is substantially high. This high current limits the memory density of the phase-change memory cells as the current supplied by the transistor is not sufficient due to their high current requirement. Hence, the unique scaling advantage of phase-change memory cannot be fully utilized.

A picture showing the typical structure of a phase-change memory device Diagram showing the typical structure of a phase-change memory device.jpg
A picture showing the typical structure of a phase-change memory device

The typical phase-change memory device design is shown. It has layers including the top electrode, GST, the GeSbTe layer, BEC, the bottom electrode and the dielectric layers. The programmable volume is the GeSbTe volume that is in contact with the bottom electrode. This is the part that can be scaled down with lithography. The thermal time constant of the device is also important. The thermal time constant must be fast enough for GeSbTe to cool rapidly into the amorphous state during RESET but slow enough to allow crystallization to occur during SET state. The thermal time constant depends on the design and material the cell is built. To read, a low current pulse is applied to the device. A small current ensures the material does not heat up. Information stored is read out by measuring the resistance of the device.

Threshold switching

Threshold switching occurs when GeSbTe goes from a high resistive state to a conductive state at the threshold field of about 56 V/um. [8] This can be seen from the current-voltage (IV) plot, where current is very low in the amorphous state at low voltage until threshold voltage is reached. Current increases rapidly after the voltage snapback. The material is now in the amorphous "ON" state, where the material is still amorphous, but in a pseudo-crystalline electric state. In crystalline state, the IV characteristics is ohmic. There had been debate on whether threshold switching was an electrical or thermal process. There were suggestions that the exponential increase in current at threshold voltage must have been due to generation of carriers that vary exponentially with voltage such as impact ionization or tunneling. [9]

A graph showing the RESET current pulse with high amplitude and short duration and SET current with lower amplitude and longer duration RESET SET.jpg
A graph showing the RESET current pulse with high amplitude and short duration and SET current with lower amplitude and longer duration

Nano-timescale phase change

Recently, much research has focused on the material analysis of the phase-change material in an attempt to explain the high speed phase change of GeSbTe. Using EXAFS, it was found that the most matching model for crystalline GeSbTe is a distorted rocksalt lattice and for amorphous a tetrahedral structure. The small change in configuration from distorted rocksalt to tetrahedral suggests that nano-timescale phase change is possible [10] as the major covalent bonds are intact and only the weaker bonds are broken.

Using the most possible crystalline and amorphous local structures for GeSbTe, the fact that density of crystalline GeSbTe is less than 10% larger than amorphous GeSbTe, and the fact that free energies of both amorphous and crystalline GeSbTe have to be around the same magnitude, it was hypothesized from density functional theory simulations [11] that the most stable amorphous state was the spinel structure, where Ge occupies tetrahedral positions and Sb and Te occupy octahedral positions, as the ground state energy was the lowest of all the possible configurations. By means of Car-Parrinello molecular dynamics simulations this conjecture have been theoretically confirmed. [12]

Nucleation-domination versus growth-domination

Another similar material is AgInSbTe. It offers higher linear density, but has lower overwrite cycles by 1-2 orders of magnitude. It is used in groove-only recording formats, often in rewritable CDs. AgInSbTe is known as a growth-dominated material while GeSbTe is known as a nucleation-dominated material. In GeSbTe, the nucleation process of crystallization is long with many small crystalline nuclei being formed before a short growth process where the numerous small crystals are joined together. In AgInSbTe, there are only a few nuclei formed in the nucleation stage and these nuclei grow bigger in the longer growth stage such that they eventually form one crystal. [13]

Related Research Articles

Melting 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.

Tellurium Chemical element, symbol Te and atomic number 52

Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally found in native form as elemental crystals. Tellurium is far more common in the Universe as a whole than on Earth. Its extreme rarity in the Earth's crust, comparable to that of platinum, is due partly to its formation of a volatile hydride that caused tellurium to be lost to space as a gas during the hot nebular formation of Earth.

A metalloid is a type of chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid and no complete agreement on which elements are metalloids. Despite the lack of specificity, the term remains in use in the literature of chemistry.

Freezing phase transition in which a liquid turns into a solid due to a decrease in thermal energy

Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid or the liquid content of a substance, usually due to cooling.

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.

Crystallization Process by which a solid with a highly organised atomic or molecular structure forms

Crystallization or crystallisation is the process by which a solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some of the ways by which crystals form are precipitating from a solution, freezing, or more rarely deposition directly from a gas. Attributes of the resulting crystal depend largely on factors such as temperature, air pressure, and in the case of liquid crystals, time of fluid evaporation.

Chalcogenide glass is a glass containing one or more chalcogens. Such glasses are covalently bonded materials and may be classified as covalent network solids. 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.

Amorphous ice is an amorphous solid form of water. Common ice is a crystalline material wherein the molecules are regularly arranged in a hexagonal lattice, whereas amorphous ice has a lack of long-range order in its molecular arrangement. Amorphous ice is produced either by rapid cooling of liquid water, or by compressing ordinary ice at low temperatures.

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.

Polyamorphism 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.

Germanium telluride Chemical compound

Germanium telluride (GeTe) is a chemical compound of germanium and tellurium and is a component of chalcogenide glasses. It shows semimetallic conduction and ferroelectric behaviour.

Tin selenide 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.

Strain engineering refers to a general strategy employed in semiconductor manufacturing to enhance device performance. Performance benefits are achieved by modulating strain in the transistor channel, which enhances electron mobility and thereby conductivity through the channel.

Resistive random-access memory is a type of non-volatile (NV) random-access (RAM) computer memory that works by changing the resistance across a dielectric solid-state material, often referred to as a memristor.

Antimony telluride 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.

Crystalline silicon

Crystalline silicon (c-Si) is the crystalline forms of silicon, either polycrystalline silicon, or monocrystalline silicon. Crystalline silicon is the dominant semiconducting material used in photovoltaic technology for the production of solar cells. These cells are assembled into solar panels as part of a photovoltaic system to generate solar power from sunlight.

Chalcogenide chemical vapour deposition is a proposed technology for depositing thin films of chalcogenides, i.e. materials derived from sulfides, selenides, and tellurides. Conventional CVD can be used to deposit films of most metals, many non-metallic elements as well as a large number of compounds including carbides, nitrides, oxides. CVD can be used to synthesize chalcogenide glasses.

Hoffman nucleation theory is a theory developed by John D. Hoffman and coworkers in the 1970s and 80s that attempts to describe the crystallization of a polymer in terms of the kinetics and thermodynamics of polymer surface nucleation. The theory introduces a model where a surface of completely crystalline polymer is created and introduces surface energy parameters to describe the process. Hoffman nucleation theory is more of a starting point for polymer crystallization theory and is better known for its fundamental roles in the Hoffman–Weeks lamellar thickening and Lauritzen–Hoffman growth theory.

Lithium aluminium germanium phosphate Chemical compound

Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is Li
1+xAl
xGe
2-x(PO
4)
3. LAGP belongs to the NASICON family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries. Typical values of ionic conductivity in LAGP at room temperature are in the range of 10–5 - 10–4 S/cm, even if the actual value of conductivity is strongly affected by stoichiometry, microstructure, and synthesis conditions. Compared to lithium aluminium titanium phosphate (LATP), which is another phosphate-based lithium solid conductor, the absence of titanium in LAGP improves its stability towards lithium metal. In addition, phosphate-based solid electrolytes have superior stability against moisture and oxygen compared to sulfide-based electrolytes like Li
10GeP
2S
12 (LGPS) and can be handled safely in air, thus simplifying the manufacture process. Since the best performances are encountered when the stoichiometric value of x is 0.5, the acronym LAGP usually indicates the particular composition of Li
1.5Al
0.5Ge
1.5(PO
4)
3, which is also the typically used material in battery applications.

References

  1. ↑ Deo V. Shenai, Ronald L. DiCarlo, Michael B. Power, Artashes Amamchyan, Randall J. Goyette, Egbert Woelk; Dicarlo; Power; Amamchyan; Goyette; Woelk (2007). "Safer alternative liquid germanium precursors for MOVPE". Journal of Crystal Growth. 298: 172–175. Bibcode:2007JCrGr.298..172S. doi:10.1016/j.jcrysgro.2006.10.194.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  2. ↑ Bosi, M.; Attolini, G.; Ferrari, C.; Frigeri, C.; Rimada Herrera, J.C.; Gombia, E.; Pelosi, C.; Peng, R.W. (2008). "MOVPE growth of homoepitaxial germanium". Journal of Crystal Growth. 310 (14): 3282. Bibcode:2008JCrGr.310.3282B. doi:10.1016/j.jcrysgro.2008.04.009.
  3. ↑ Attolini, G.; Bosi, M.; Musayeva, N.; Pelosi, C.; Ferrari, C.; Arumainathan, S.; Timò, G. (2008). "Homo and hetero epitaxy of Germanium using isobutylgermane". Thin Solid Films. 517 (1): 404–406. Bibcode:2008TSF...517..404A. doi:10.1016/j.tsf.2008.08.137.
  4. ↑ M. Longo, O. Salicio, C. Wiemer, R. Fallica, A. Molle, M. Fanciulli, C. Giesen, B. Seitzinger,P.K. Baumann, M. Heuken, S. Rushworth; Salicio; Wiemer; Fallica; Molle; Fanciulli; Giesen; Seitzinger; Baumann; Heuken; Rushworth (2008). "Growth study of GexSbyTez deposited by MOCVD under nitrogen for non‐volatile memory applications". Journal of Crystal Growth. 310 (23): 5053–5057. Bibcode:2008JCrGr.310.5053L. doi:10.1016/j.jcrysgro.2008.07.054.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ↑ A. Abrutis, V. Plausinaitiene, M. Skapas, C. Wiemer, O. Salicio, A. Pirovano, E. Varesi, S. Rushworth, W. Gawelda, J. Siegel; Plausinaitiene; Skapas; Wiemer; Salicio; Pirovano; Varesi; Rushworth; Gawelda; Siegel (2008). "Hot‐Wire Chemical Vapor Deposition of Chalcogenide Materials for Phase Change Memory Applications". Chemistry of Materials. 20 (11): 3557. doi:10.1021/cm8004584. hdl: 10261/93002 .{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ↑ X. Shi; M. Schaekers; F. Leys; R. Loo; M. Caymax; R. Brus; C. Zhao; B. Lamare; E. Woelk; D. Shenai (2006). "Germanium Precursors for Ge and SiGe Deposition". ECS Transactions. 3: 849. doi:10.1149/1.2355880. S2CID   110550188.
  7. ↑ Morales-Sánchez, E.; Prokhorov, E. F.; Mendoza-Galván, A.; González-Hernández, J. (2002-01-15). "Determination of the glass transition and nucleation temperatures in Ge2Sb2Te5 sputtered films". Journal of Applied Physics. AIP Publishing. 91 (2): 697–702. Bibcode:2002JAP....91..697M. doi:10.1063/1.1427146. ISSN   0021-8979.
  8. ↑ Krebs, Daniel; Raoux, Simone; Rettner, Charles T.; Burr, Geoffrey W.; Salinga, Martin; Wuttig, Matthias (2009). "Threshold field of phase change memory materials measured using phase change bridge devices". Applied Physics Letters. 95 (8): 082101. Bibcode:2009ApPhL..95h2101K. doi:10.1063/1.3210792.
  9. ↑ Pirovano, A.; Lacaita, A.L.; Benvenuti, A.; Pellizzer, F.; Bez, R. (2004). "Electronic Switching in Phase-Change Memories". IEEE Transactions on Electron Devices. Institute of Electrical and Electronics Engineers (IEEE). 51 (3): 452–459. Bibcode:2004ITED...51..452P. doi:10.1109/ted.2003.823243. ISSN   0018-9383. S2CID   43106563.
  10. ↑ Kolobov, Alexander V.; Fons, Paul; Frenkel, Anatoly I.; Ankudinov, Alexei L.; Tominaga, Junji; Uruga, Tomoya (2004-09-12). "Understanding the phase-change mechanism of rewritable optical media". Nature Materials. Springer Nature. 3 (10): 703–708. Bibcode:2004NatMa...3..703K. doi:10.1038/nmat1215. ISSN   1476-1122. PMID   15359344. S2CID   677085.
  11. ↑ Wuttig, Matthias; Lüsebrink, Daniel; Wamwangi, Daniel; Wełnic, Wojciech; Gilleßen, Michael; Dronskowski, Richard (2006-12-17). "The role of vacancies and local distortions in the design of new phase-change materials". Nature Materials. Springer Nature. 6 (2): 122–128. doi:10.1038/nmat1807. ISSN   1476-1122. PMID   17173032.
  12. ↑ Caravati, Sebastiano; Bernasconi, Marco; Kühne, Thomas D.; Krack, Matthias; Parrinello, Michele (2007). "Coexistence of tetrahedral- and octahedral-like sites in amorphous phase change materials". Applied Physics Letters. 91 (17): 171906. arXiv: 0708.1302 . Bibcode:2007ApPhL..91q1906C. doi:10.1063/1.2801626. S2CID   119628572.
  13. ↑ Coombs, J. H.; Jongenelis, A. P. J. M.; van Es‐Spiekman, W.; Jacobs, B. A. J. (1995-10-15). "Laser‐induced crystallization phenomena in GeTe‐based alloys. I. Characterization of nucleation and growth". Journal of Applied Physics. AIP Publishing. 78 (8): 4906–4917. Bibcode:1995JAP....78.4906C. doi:10.1063/1.359779. ISSN   0021-8979.
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