Chalcogenide chemical vapour deposition

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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 (notably silicon) as well as a large number of compounds including carbides, nitrides, oxides. CVD can be used to synthesize chalcogenide glasses. [1]

Chalcogenide class of chemical compounds

A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term chalcogenide is more commonly reserved for sulfides, selenides, tellurides, and polonides, rather than oxides. Many metal ores exist as chalcogenides. Photoconductive chalcogenide glasses are used in xerography. Some pigments and catalysts are also based on chalcogenides. The metal dichalcogenide MoS2 is a common solid lubricant.

Sulfide salt or other derivative of hydrogen sulfide or organic compound having the structure RSR (R ≠ H)

Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. Sulfide also refers to chemical compounds large families of inorganic and organic compounds, e.g. lead sulfide and dimethyl sulfide. Hydrogen sulfide (H2S) and bisulfide (SH−) are the conjugate acids of sulfide.

A selenide is a chemical compound containing a selenium anion with oxidation number of −2 (Se2−), much as sulfur does in a sulfide. The chemistry of the selenides and sulfides is similar. Similar to sulfide, in aqueous solution, the selenide ion, Se2−, is prevalent only in very basic conditions. In neutral conditions, hydrogen selenide ion, HSe−, is most common. In acid conditions, hydrogen selenide, H2Se, is formed.

Contents

Sulfide based thin films

The fabrication of chalcogenide thin films is a topic of research. [2] For example, routes to germanium disulfide films could entail germanium chloride and hydrogen sulphide:

Germanium disulfide chemical compound

Germanium disulfide or Germanium(IV) sulfide is the inorganic compound with the formula GeS2. It is a white high-melting crystalline solid. The compound is a 3-dimensional polymer, in contrast to silicon disulfide, which is a one-dimensional polymer. The Ge-S distance is 2.19 Å.

GeCl4 (g) + 2 H2S(g) → GeS2(s) + 4 HCl (g)

Alternatively via plasma enhanced CVD there is the reaction GeH4/H2S. [3] [4]

Germanium sulfide CVD setup GeS CVD setup.png
Germanium sulfide CVD setup

Telluride based thin films

Phase change random access memory (PCRAM) has attracted considerable interest as a candidate for non-volatile devices for higher density and operation speed. [6] [7] The ternary Ge2Sb2Te5 (GST) compound is widely regarded as the most viable and practical phase change family of materials for this application. [8] CVD techniques have been applied to deposit GST materials in sub micron cell pores. [9] Challenges include the need to control device to device variability and undesirable changes in the phase change material that can be induced by the fabrication procedure. A confined cell structure where the phase change material is formed inside a contact via is expected to be essential for the next generation PCRAM device because it requires lower switching power. [10] This structure however requires more complex deposition of the active chalcogenide into a cell pore. CVD techniques could provide better performance and enable the production of thin films with superior quality compared to those obtained by sputtering, especially in terms of conformality, coverage, and stoichiometry control, and allows implementation of phase-change films in nanoelectronic devices. In addition, CVD deposition is well known to provide higher purity materials and provides the scope for new phase change materials with optimized properties to be deposited.

The CVD apparatus for Ge-Sb-Te thin film deposition is shown schematically to the right.

Schematic diagram of CVD system used for Ge-Sb-Te thin film deposition GST CVD setup.tif
Schematic diagram of CVD system used for Ge-Sb-Te thin film deposition

Related Research Articles

Chemical vapor deposition chemical process used in the semiconductor industry to produce thin films

Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.

Epitaxy crystal growth process

Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with a well-defined orientation with respect to the crystalline substrate. The new layers formed are called the epitaxial film or epitaxial layer. The relative orientation of the epitaxial layer to the crystalline substrate is defined in terms of the orientation of the crystal lattice of each material. For epitaxial growth, the new layer will be crystalline and will all have a single orientation relative to the substrate; amorphous growth or multicrystalline growth with random crystal orientation does not meet this criteria.

Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM and C-RAM or CRAM is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In the older generation of PCM, heat produced by the passage of an electric current through a heating element generally made of TiN was 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. 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.

Metalorganic vapour-phase epitaxy Method of producing thin fils (polycrystalline and single crystal)

Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. It is a highly complex process for growing crystalline layers to create complex semiconductor multilayer structures. In contrast to molecular-beam epitaxy (MBE), the growth of crystals is by chemical reaction and not physical deposition. This takes place not in vacuum, but from the gas phase at moderate pressures. As such, this technique is preferred for the formation of devices incorporating thermodynamically metastable alloys, and it has become a major process in the manufacture of optoelectronics. It was invented in 1968 at North American Aviation Science Center by Harold M. Manasevit.

Atomic layer deposition

Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. Through the repeated exposure to separate precursors, a thin film is slowly deposited. ALD is a key process in the fabrication of semiconductor devices, and part of the set of tools available for the synthesis of nanomaterials.

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.

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, also known as stannous selenide, is an inorganic compound with the formula (SnSe), where Tin has a +2 oxidation state. Tin(II) selenide is a narrow band-gap (IV-VI) semiconductor and has received considerable interest for applications including low-cost photovoltaics and memory-switching devices. Tin(II) selenide is a typical layered metal chalcogenide; that is, it includes a Group 16 anion (Se2−) and an electropositive element (Sn2+), and it is arranged in a layered structure.

Bismuth ferrite (BiFeO3, also commonly referred to as BFO in materials science) is an inorganic chemical compound with perovskite structure and one of the most promising multiferroic materials. The room-temperature phase of BiFeO3 is classed as rhombohedral belonging to the space group R3c. It is synthesized in bulk and thin film form and both its antiferromagnetic (G type ordering) Néel temperature (approximately 653 K ) and ferroelectric Curie temperature are well above room temperature (approximately 1100K). Ferroelectric polarization occurs along the pseudocubic direction ( ) with a magnitude of 90–95 μC/cm2.

Isobutylgermane chemical compound

Isobutylgermane (IBGe, Chemical formula: (CH3)2CHCH2GeH3, is an organogermanium compound. It is a colourless, volatile liquid that is used in MOVPE (Metalorganic Vapor Phase Epitaxy) as an alternative to germane. IBGe is used in the deposition of Ge films and Ge-containing thin semiconductor films such as SiGe in strained silicon application, and GeSbTe in NAND Flash applications.

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. This technology bears some similarities to conductive-bridging RAM (CBRAM), and phase-change memory (PCM).

Indium(III) selenide is a compound of indium and selenium. It has potential for use in photovoltaic devices and it has been the subject of extensive research. The two most common phases, α and β, have a layered structure, while γ is a "defect wurtzite structure." In all, there are five known forms (α, β, γ, δ, κ). The α- β phase transition is accompanied by a change in electrical conductivity. The band-gap of γ-In2Se3 is approximately 1.9 eV. The crystalline form of a sample can depend on the method of production, for example thin films of pure γ-In2Se3 have been produced from trimethylindium, InMe3, and hydrogen selenide, H2Se, using MOCVD techniques.

Copper indium gallium selenide solar cells direct bandgap semiconductor useful for the manufacture of solar cells

A copper indium gallium selenide solar cell is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper, indium, gallium and selenium on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials.

Chemical vapor deposition of ruthenium is a method to deposit thin layers of ruthenium on substrates by Chemical vapor deposition (CVD).

Tantalum(V) ethoxide chemical compound

Tantalum(V) ethoxide is a metalorganic compound with formula Ta2(OC2H5)10, often abbreviated as Ta2(OEt)10. It is a colorless solid that dissolves in some organic solvents but hydrolyzes readily. It is used to prepare tantalum oxide films.

The glass forming ability of gallium(III) sulfide and lanthanum sulfide was discovered in 1976 by Loireau-Lozac’h, Guittard, and Flahut. This family of chalcogenide glasses, referred to as gallium lanthanum sulfide (Ga-La-S) glasses, have a wide region of glass formation centred about the 70Ga2S3:30La2S3 composition and can readily accept other modifiers into their structure. This means that Ga-La-S can be compositionally adjusted to give a wide variety of optical and physical properties. Optically, Ga-La-S has a high refractive index, a transmission window covering most of the visible wavelengths and extending to about 10 µm and a low maximum phonon energy, approx. 450 cm−1. Thermally, the refractive index of Ga-La-S glasses has a strong temperature dependence and low thermal conductivity, which results in strong thermal lensing. However, the high glass transition temperature of Ga-La-S makes it resistant to thermal damage, it has good chemical durability and unlike many chalcogenides which are based on arsenic, its glass components are non-toxic. A clear advantage over other chalcogenides is its high lanthanum content which allows excellent rare-earth solubility and dispersion of the ions in the glass matrix for active devices. Ga-La-S can exist in both glassy and crystalline phases, in a glassy phase, it is a semiconductor with a bandgap of 2.6 eV corresponding to a wavelength of 475 nm; consequently Ga-La-S glass takes a deep orange colour. As with all chalcogenides the phase of the bulk is determined by two key factors; the material composition and the rate at which the molten material is cooled. These variables can be controlled to manipulate the final phase of the material.

Low-energy plasma-enhanced chemical vapor deposition

Low-Energy Plasma-Enhanced Chemical Vapor Deposition (LEPECVD) is a plasma-enhanced chemical vapor deposition technique used for the epitaxial deposition of thin semiconductor films. A remote low energy, high density DC argon plasma is employed to efficiently decompose the gas phase precursors while leaving the epitaxial layer undamaged, resulting in high quality epilayers and high deposition rates.

References

  1. ↑ D. W. Hewak, D. Brady, R. J. Curry, G. Elliott, C. C. Huang, M. Hughes, K. Knight, A. Mairaj, M. N. Petrovich, R. Simpson, C. Sproat, "Chalcogenide glasses for photonics device applications", Book section in Photonic glasses and glass-ceramics (Ed. Ganapathy Senthil Murugan) ISBN   978-81-308-0375-3, 2010
  2. ↑ P. J. Melling, "Alternative Methods of Preparing Chalcogenide Glasses", Ceramic Bulletin, 63, 1427–1429, 1984.
  3. ↑ E. Sleeckx, P. Nagels, R. Callaerts and M. Vanroy, "Plasma-enhanced CVD of amorphous GexS1−x and GexSe1−x films", J. de Physique IV, 3, 419–426, 1993.
  4. ↑ Huang, C. C.; Hewak, D. W. (2004). "High-purity germanium-sulphide glass for optoelectronic applications synthesised by chemical vapour deposition". Electronics Letters. 40 (14): 863–865. doi:10.1049/el:20045141.
  5. ↑ C. C. Huang, C. C. Wu, K. Knight, D. W. Hewak, J. Non-Cryst. Solids, 356, 281–285 (2010)
  6. ↑ M. H. R. Lankhorst, B. W. S. M. M. Ketelaars and R. A. M. Wolters, Natural Materials, 4 (2005) 347–352.
  7. ↑ C. W. Jeong; S. J. Ahn; Y. N. Hwang; Y. J. Song; J. H. Oh; S. Y. Lee; S. H. Lee; K. C. Ryoo; J. H. Park; J. H. Park; J. M. Shin; F. Yeung; W. C. Jeong; J. I. Kim; G. H. Koh; G. T. Jeong; H. S. Jeong; K. Kim (2006). "Highly reliable ring-type contact for high-density phase change memory". Japanese Journal of Applied Physics. 45 (4B): 3233–3237. Bibcode:2006JaJAP..45.3233J. doi:10.1143/JJAP.45.3233.
  8. ↑ R. Bez and F. Pellizzer, "Progress and Perspective of Phase-Change Memory" Archived 2014-01-04 at the Wayback Machine , E*PCOS 2007, Zermatt, Switzerland, September 2007.
  9. ↑ J. Bae, H. Shin, D. Im, H. G. An, J. Lee, S. Cho, D. Ahn, Y. Kim, H. Horii, M. Kang, Y. Ha, S. Park, U. I. Chung, J. T. Moon, and W. S. Lee, "Recent Progress of Phase Change Random Access Memory (PRAM)" Archived 2014-01-04 at the Wayback Machine , E*PCOS 2008, Prague, Czech Republic, September 2008.
  10. ↑ Y. S. Park, K. J. Choi, N. Y. Lee, S. M. Yoon, S. Y. Lee, S. O. Ryu, and B. G. Yu, Jpn. J. Appl. Phys., 45 (2006) L516–L518.
  11. ↑ C. C. Huang, B. Gholipour, J. Y. Ou, K. Knight, D. W. Hewak, Electronics Letters, 47, 288–289 (2011)
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