Gallium(III) arsenide 144143 225213348 2008-07-12T14:21:47Z 72.93.68.89 {{Chembox new | Name = Gallium arsenide | ImageFile = Gallium_arsenide.jpg | ImageFile1 = Gallium-arsenide-unit-cell-3D-balls.png | IUPACName = Gallium arsenide | Section1 = {{Chembox Identifiers | CASNo = 1303-00-0 | SMILES = Ga#As }} | Section2 = {{Chembox Properties | Formula = GaAs | MolarMass = 144.645 g/mol | Appearance = Gray cubic crystals | Solubility =< 0.1 g/100 ml (20°C) | MeltingPt = 1238°C (1511 K) | BoilingPt = °C (? K) }} | Section3 = {{Chembox Structure | MolShape = Linear | CrystalStruct = [[Cubic crystal system|Zinc Blende]] | Dipole = }} | Section7 = {{Chembox Hazards | ExternalMSDS = [http://www.wafertech.co.uk/msds/msds_gaas.html External MSDS] | MainHazards = Carcinogenic | {{NFPA 704}} | NFPA-H = 3 | NFPA-F = 1 | NFPA-R = 1 | NFPA-O = }} }} '''Gallium arsenide''' ('''GaAs''') is a [[chemical compound|compound]] of two elements, [[gallium]] and [[arsenic]]. It is an important [[semiconductor]] and is used to make devices such as [[microwave]] frequency [[integrated circuit]]s (ie, [[Monolithic Microwave Integrated Circuit|MMIC]]s), [[infrared]] [[light-emitting diode]]s, [[laser diode]]s and [[solar cells]]. ==Preparation and chemistry== Gallium arsenide can be prepared from the elements and a number of industrial processes use this, for example<ref name = "Moss">S. J. Moss, A. Ledwith (1987) ''The Chemistry of the Semiconductor Industry'', Springer, ISBN 0216920051</ref>: *the crystal growth using a horizontal zone furnace ([[Bridgman-Stockbarger technique]]) where Ga and Arsenic vapour react and deposit on a seed crystal at the cooler end of the furnace. *LEC (liquid encapsulated [[Czochralski process|Czochralski]]) growth Alternative methods for producing films of GaAs include<ref>Lesley Smart, Elaine A. Moore, (2005), ''Solid State Chemistry: An Introduction'' ,CRC, ISBN 0748775161</ref><ref name = "Moss"/> : *[[Chemical vapor deposition|VPE]] reaction of gaseous gallium metal and [[arsenic trichloride]] :2Ga + 2AsCl<sub>3</sub> → 2GaAs + 3Cl<sub>2</sub> *[[MOCVD]] reaction of [[trimethylgallium]] and [[arsine]]: :Ga(CH<sub>3</sub>)<sub>3</sub> + AsH<sub>3</sub> → GaAs + CH<sub>4</sub> Wet etching of GaAs industrially uses an oxidising agent e.g. [[hydrogen peroxide]] or [[bromine]] water<ref>M. R. Brozel, G. E. Stillman (1996)''Properties of Gallium Arsenide'', IEE Inspec, ISBN 085296885X</ref>, and the same strategy has been described in a patent relating to processing scrap components containing GaAs where the Ga<sup>3+</sup> is complexed with a [[hydroxamic acid]], "HA"<ref> Oxidative dissolution of gallium arsenide and separation of gallium from arsenic, United States Patent 4759917, Coleman, J. P.,Monzyk, B. F (1988)</ref>e.g.: :GaAs + H<sub>2</sub>O<sub>2</sub> + "HA" → "GaA" complex + [[arsenic acid|H<sub>3</sub>AsO<sub>4</sub>]] + 4H<sub>2</sub>O Oxidation of GaAs occurs in air and degrades performance of the semiconductor, the surface can be passivated by depositing a cubic [[gallium(II) sulfide]] layer using a tert-butyl gallium sulfide compound such as (<sup>t</sup>BuGaS)<sub>7</sub><ref>Chemical vapor deposition from single organometallic precursors, A. R. Barron, M. B. Power, A. N. MacInnes, A. F.Hepp, P. P. Jenkins, US Patent 5300320 (1994)</ref> ==Applications== ===GaAs advantages=== GaAs has some electronic properties which are superior to those of [[silicon]]. It has a higher [[saturated electron velocity]] and higher [[electron mobility]], allowing transistors made from it to function at frequencies in excess of 250 GHz. Also, GaAs devices generate less [[noise (physics)|noise]] than silicon devices when operated at high frequencies. They can also be operated at higher power levels than the equivalent silicon device because they have higher [[breakdown voltage]]s. These properties recommend GaAs circuitry in [[mobile phone]]s, [[communications satellite|satellite]] communications, microwave point-to-point links, and some [[radar]] systems. It is used in the manufacture of [[Gunn diode]]s for generation of microwaves. Another advantage of GaAs is that it has a [[direct band gap]], which means that it can be used to emit light efficiently. Silicon has an [[indirect bandgap]] and so is very poor at emitting light. (Nonetheless, recent advances may make silicon [[LED]]s and [[laser]]s possible).<!--read about comma splice--> Due to its high switching speed, GaAs would seem to be ideal for computer applications, and for some time in the 1980s many thought that the microelectronics market would switch from silicon to GaAs. The first attempted changes were implemented by the [[supercomputer]] vendors [[Cray Computer|Cray Computer Corporation]], [[Convex Computer|Convex]], and [[Alliant Computer Systems|Alliant]] in an attempt to stay ahead of the ever-improving [[CMOS]] microprocessor. Cray eventually built one GaAs-based machine in the early 1990s, the [[Cray-3]], but the effort was not adequately capitalized, and the company filed for bankruptcy in 1995. Complex layered structures of gallium arsenide in combination with [[aluminium arsenide]] (AlAs) or the alloy [[Aluminium gallium arsenide|Al<sub>x</sub>Ga<sub>1-x</sub>As]] can be grown using [[molecular beam epitaxy]] (MBE) or using [[metalorganic vapour phase epitaxy]] (MOVPE). Because GaAs and AlAs have almost the same [[lattice constant]], the layers have very little induced [[Strain (chemistry)|strain]], which allows them to be grown almost arbitrarily thick. ===Silicon's advantages=== Silicon has three major advantages over GaAs for integrated circuit manufacture. First, silicon is abundant and cheap to process. Si is highly abundant in the Earth's crust, in the form of [[silicate]] minerals. The economy of scale available to the silicon industry has also reduced the adoption of GaAs. The second major advantage of Si is the existence of [[silicon dioxide]]&mdash;one of the best [[Electrical insulation|insulator]]s. Silicon dioxide can easily be incorporated onto silicon circuits, and such layers are adherent to the underlying Si. GaAs does not form a stable adherent insulating layer. The third, and perhaps most important, advantage of silicon is that it possesses a much higher [[Electron hole|hole]] mobility. This high mobility allows the fabrication of higher-speed P-channel [[field effect transistor]]s, which are required for [[CMOS]] logic. Because they lack a fast CMOS structure, GaAs logic circuits have much higher power consumption, which has made them unable to compete with silicon logic circuits. ====Solar cells and detectors==== Another important application of GaAs is for high efficiency [[solar cell]]s. In [[1970]], the first GaAs heterostructure solar cells were created by [[Zhores Alferov]] and his team in the [[USSR]].<ref>Alferov, Zh. I., V. M. Andreev, M. B. Kagan, I. I. Protasov, and V. G. Trofim, 1970, ‘‘Solar-energy converters based on p-n Al<sub>x</sub>Ga<sub>1-x</sub>As-GaAs heterojunctions,’’ Fiz. Tekh. Poluprovodn. 4, 2378 (Sov. Phys. Semicond. 4, 2047 (1971))]</ref><ref>[http://www.im.isu.edu.tw/seminar/2005.11.16.pdf Nanotechnology in energy applications], pdf, p.24</ref><ref>[http://nobelprize.org/nobel_prizes/physics/laureates/2000/alferov-lecture.pdf Nobel Lecture] by [[Zhores Alferov]], pdf, p.6</ref> In the early 1980s, the efficiency of the best GaAs solar cells surpassed that of silicon solar cells, and in the 1990s GaAs solar cells took over from silicon as the cell type most commonly used for [[Photovoltaic array]]s for satellite applications. Later, dual- and triple-junction solar cells based on GaAs with [[germanium]] and [[indium gallium phosphide]] layers were developed as the basis of a triple junction solar cell which held a record efficiency of over 32% and can operate also with light as concentrated as 2,000 suns. This kind of solar cell powers the [[Rover (space exploration)|rovers]] [[Spirit rover|Spirit]] and [[Opportunity rover|Opportunity]], which are exploring [[Mars]]' surface. Also many [[Solar car racing|solar car]]s utilize GaAs in solar arrays. Complex designs of Al<sub>x</sub>Ga<sub>1-x</sub>As-GaAs devices can be sensitive to infrared radiation ([[QWIP]]). GaAs diodes can be used for the detection of x-rays.<ref>[http://ppewww.physics.gla.ac.uk/preprints/97/05/psd1/psd1.html Glasgow University report on CERN detector]</ref> ====Light emission devices==== GaAs has been used to produce (near-infrared) laser diodes since the early 1960s.<ref>R. C. Miller, F. M. Ryan and P. R. Emtage, “Uniaxial Strain Effects in Gallium Arsenide Laser Diodes,” 7th Intl. Conf. Phys. Semicond., Academic Press, Paris, 1964.</ref><!--someone find an earlier reference--> [[Single crystal]]s of gallium arsenide can be manufactured by the [[Bridgeman technique]], as the [[Czochralski process]] is difficult for this material due to its mechanical properties. However, an encapsulated Czochralski method is used to produce ultra-high purity GaAs for semi-insulators. GaAs is often used a substrate material for the epitaxial growth of other III-V semiconductors including: InGaAs and GaInNAs. ==Safety== The toxicological properties of gallium arsenide have not been thoroughly investigated. On one hand, due to its arsenic content, it is considered highly [[toxic]] and [[carcinogenic]]. On the other hand, the crystal is stable enough that ingested pieces may be passed with negligible absorption by the body. When ground into very fine particles, such as in wafer-polishing processes, the high surface area enables more reaction with water releasing some arsine and/or dissolved arsenic. The environment, health and safety aspects of gallium arsenide sources (such as [[trimethylgallium]] and [[arsine]]) and industrial hygiene monitoring studies of [[metalorganic]] precursors have been reported recently in a review.<ref>Environment, health and safety issues for sources used in MOVPE growth of compound semiconductors; D V Shenai-Khatkhate, R Goyette, R L DiCarlo and G Dripps, Journal of Crystal Growth, vol. 1-4, pp. 816-821 (2004); {{doi|10.1016/j.jcrysgro.2004.09.007}}</ref> ==See also== * [[Electronics]] * [[Field effect transistor]] * [[Heterostructure emitter bipolar transistor]] * [[Integrated circuit]] * [[Semiconductor]] * [[Semiconductor devices]] * [[solar cell]] * [[magnetic semiconductor]] * [[Photomixing]] ===Related materials=== * [[Aluminium arsenide]] * [[Indium arsenide]] * [[Gallium antimonide]] * [[Gallium phosphide]] * [[Aluminium gallium arsenide]] * [[Indium gallium arsenide]] * [[Gallium arsenide phosphide]] * [[Gallium nitride]] * [[MOVPE]] * [[Trimethylgallium]] * [[Arsine]] ==References== {{reflist}} ==External links== * [http://www.atsdr.cdc.gov/HEC/CSEM/arsenic/ Case Studies in Environmental Medicine: Arsenic Toxicity] * [http://www.ioffe.ru/SVA/NSM/Semicond/GaAs/index.html Extensive site on the physical properties of Gallium arsenide] * [http://www.logitech.uk.com/gallium_arsenide.asp Facts and figures on processing Gallium Arsenide] *[http://www.semiconductor-today.com Semiconductor Today: Online resource covering compound semiconductors and advanced silicon materials and devices] [[Category:Arsenides]] [[Category:Compound semiconductors]] [[Category:Inorganic compounds]] [[Category:Gallium compounds]] [[Category:IARC Group 1 carcinogens]] [[Category:Optoelectronics]] [[Category:Semiconductor materials]] [[Category:III-V compounds]] [[ar:زرنيخيد غاليوم ثلاثي]] [[de:Galliumarsenid]] [[es:Arseniuro de galio]] [[fr:Arséniure de gallium]] [[hi:गैलिअम आर्सेनाइड]] [[it:Arseniuro di gallio]] [[he:גליום ארסניד]] [[hu:Gallium-arzenid]] [[nl:Galliumarsenide]] [[ja:ガリウムヒ素]] [[pl:Arsenek galu]] [[pt:Arsenieto de gálio]] [[ru:Арсенид галлия]] [[fi:Galliumarsenidi]] [[sv:Galliumarsenid]] [[uk:Арсенід галію]] [[zh:砷化鎵]]