title: Gallium and gallium arsenide : supply, technology, and uses
text: ^\y^:?^ : :/mk'^kk^ JL°^ * * v •J&fr \> 4L* *;*&*> ^ - v. *^o^ - \* * ^ >*- aN V *j<J '/ ./ % "'-5 ? <+ *• • * * ^ BUREAU OF MINES / INFORMATION CIRCULAR/1988 Gallium and Gallium Arsenide: Supply, Technology, and Uses By Deborah A. Kramer UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 9208 Gallium and Gallium Arsenide: Supply, Technology, and Uses By Deborah A. Kramer UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary BUREAU OF MINES T S Ary, Director Mr Library of Congress Cataloging in Publication Data: Kramer, Deborah A. Gallium and gallium arsenide. (Information circular / United of Mines; 9208) States Department of the Interior, Bureau Bibliography: p. 25. Supt. of Docs, no.: I 28.27:9208. 1. Gallium industry. 2. Gallium of Mines. II. Title. arsenide industry. I. United States. Bureau TN295.U4 [HD9539] 622 s [338.476610675] 88-600326 CONTENTS Page Abstract 1 Introduction 2 Uses 2 Optoelectronic devices 2 Integrated circuits 3 Analog ICs 3 Digital ICs 3 Other applications 4 Properties and grades 4 Resources 5 Recovery technology 9 Gallium recovery from bauxite 9 Gallium recovery from zinc ore 10 Gallium recovery from other sources 10 Gallium purification 11 Gallium arsenide fabrication 11 Secondary recovery 15 World supply and demand 16 Production 16 Trade 16 Domestic demand 16 Structure of the industry 18 Gallium recovery, recycle, and purification 18 Australia 18 Canada 18 China 18 Czechoslovakia 18 France 19 Federal Republic of Germany 19 Hungary 20 India 20 Japan 20 Norway 20 Spain 20 Switzerland 20 U.S.S.R 20 United Kingdom 20 United States 21 High-purity arsenic production 21 Gallium arsenide ingot, wafer, and device manufacturers 21 Research and development 23 Legislation and Government programs 24 Strategic factors 24 References 25 ILLUSTRATIONS Page 1. Beja process for recovering crude gallium from Bayer liquors 9 2. de la Breteque process for recovering crude gallium from Bayer liquors 10 3. Gallium recovery from zinc ore 10 4. Gallium recovery from Musto Explorations Ltd.'s mine near St. George, UT 11 5. Elkem A/S process for recovering gallium from aluminum smelter flue dust 12 6. GaAs ingot growing in LEC furnace 13 7. LEC-grown GaAs ingot and wafers 13 8. GaAs wafer with devices 14 TABLES 1. Gallium reserves and reserve base 6 2. World alumina plant capacities and gallium potential, yearend 1987 7 3. World primary gallium production 16 4. World secondary gallium production 17 5. U.S. gallium imports for consumption, by country 17 6. Japanese gallium imports, by country 17 7. Gallium supply-demand relationships, 1977-87 18 8. Yearend gallium plant capacities 19 9. Gallium arsenide ingot, wafer, and device manufacturers 21 10. U.S. import duties for gallium, January 1, 1989 24 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT °C degree Celsius mt metric ton GHz in gigahertz inch run pet nanometer (one billionth of a meter) percent in 2 square inch ppba part per billion atomic kg kilogram ppm part per million kg/yr kilogram per year GALLIUM AND GALLIUM ARSENIDE: SUPPLYJECHNOLOGY, AND USES By Deborah A. Kramer 1 ABSTRACT As part of the Nation's growing concern with the competitiveness of U.S. 'Physical scientist, Division of Mineral Commodities, Bureau of Mines, Washington, DC. INTRODUCTION Gallium-based components can be found in a variety of products ranging from compact disk players to advanced military electronic warfare systems. Compared with silicon, a material GaAs has replaced in some of these applications, components made of GaAs can emit light, have greater resistance to radiation, and operate at a faster speed and higher temperatures. But GaAs components are more costly and more difficult to fabricate than those of silicon, so they are used only in applications where the advantage of their properties significantly outweighs then- cost disadvantage. Gallium occurs in very low concentrations in the Earth's crust, and virtually all primary gallium is recovered as a byproduct, principally from processing bauxite to alumina. Most of gallium's applications require very high purity levels, and the metal must be refined before use until it contains no more than 1 ppm of total impurities. Most gallium metal recovery and refining facilities are in Europe. Through complex processing techniques, GaAs single crystals are produced, and optoelectronic devices and integrated circuits (ICs) are fabricated. Japan and the United States lead the world in GaAs crystal and device fabrication. Considerable investments are being made to increase processing efficiency, develop new devices, and increase the applications of current GaAs-based components. USES Gallium has limited commercial applications in its metallic form. Its principal use is in the manufacture of semiconducting compounds, mainly GaAs and gallium phosphide (GaP). Over 90 pet of the gallium consumed in the United States is used for optoelectronic devices and ICs. Optoelectronic devices-light-emitting diodes (LEDs), laser diodes, photodiodes, and solar (photovoltaic) cells-take advantage of GaAs's ability to convert electrical energy to optical energy and the reverse. The principal market for optoelectronic devices is in nonmilitary applications, including communications systems and consumer electronic goods. GaAs-based integrated circuits are used primarily in defense applications, although developments in recent years have increased their use in the commercial sector. GaAs-based integrated circuits are important, particularly in defense applications, because they can send information about five times faster, can withstand more radiation, and can operate at higher temperatures than comparable silicon-based integrated circuits. OPTOELECTRONIC DEVICES An LED is a semiconductor that emits light when an electric current is passed through it. LEDs have been in commercial use for many years. The first commercial applications for LED technology were in displays for hand- held calculators and digital watches. Today, LEDs are used in visual displays in automobiles, calculators, appliances, consumer electronic equipment, and a wide variety of industrial equipment. LEDs are also used as a light source in short-distance fiber optic communications systems. LEDs consist of layers of an epitaxially grown material on a substrate. These epitaxial layers are normally gallium aluminum arsenide (GaAlAs), gallium arsenide phosphide (GaAsP), or indium gallium arsenide phosphide (InGaAsP); the substrate material is cither GaAs or GaP. The materials used to fabricate LEDs determine the color of light that is emitted. With GaP substrates, the wavelength of light can cover the spectrum from 555 nm, pure green, to about 700 nm, red. With GaAs substrates, light emitted from an LED is limited to wavelengths at the red and infrared end of the spectrum. Laser diodes operate on the same principle as LEDs, but they convert electrical energy to a coherent light output. Laser diodes, also called semiconductor lasers or injection laser diodes, principally consist of an epitaxial layer of GaAs, GaAlAs, or InGaAsP on a GaAs substrate. The two most commonly used laser diodes are GaAlAs and InGaAsP diodes. GaAlAs laser diodes operate at about 780 to 900 nm and are used in a wide variety of consumer products and in communications systems. GaAlAs diodes are used in compact disk players, nonimpact laser printers, and optical video disk players. They are also used in short-range fiber optic communications systems, satellite communications, radar transmission, and local cable transmission systems. InGaAsP laser diodes operate at longer wavelengths, 1,300 to 1,500 nm, and are primarily used for transmission of high-frequency, long-distance signals in fiber optic communication systems and in cable television supertrunks. Photodiodes, or detectors, are used to detect a light impulse generated by a source, such as an LED or laser diode, and convert it to an electrical impulse. Photodiodes are fabricated from the same materials as LEDs and are used primarily as light detectors in fiber optics systems. There are two types of gallium-based photodiodes— GaAlAs epitaxially grown on a GaAs substrate, used to detect light at short wavelengths, and InGaAsP on an indium phosphide (InP) substrate, used to detect light at longer wavelengths. Because of its ability to convert light to electrical energy, GaAs is an excellent material for solar cells. Although solar cells are not in widespread use, they have been used to power communications satellites. GaAs's advantage in this application is its electrical conversion efficiency; GaAs solar cells have been demonstrated to convert 22 pet of the available sunlight to electricity, compared with about 16 pet for silicon solar cells. Because of their higher energy efficiency, GaAs solar cells can be smaller than those constructed of silicon and still provide the same power to the satellite. Consequently, a satellite can carry a greater payload when GaAs is used as the solar cell material. GaAs is also more resistant to radiation than silicon; consequently, GaAs solar cells have a longer life in space environments (I). 2 One defense application of optoelectronic GaAs is in night vision equipment. The GaAs component converts infrared radiation to visible light, enabling soldiers to see at night. Four layers of GaAlAs are epitaxially deposited on a GaAs substrate. The substrate and two of the layers are removed, yielding a thin GaAlAs film. Fabrication of these devices is closely controlled to prevent defects in the crystal structure. Even small imperfections in the night vision device cannot be tolerated. Substitute materials are available for GaAs-based devices in many of the optoelectronic applications. Liquid crystal displays (LCDs), organic compounds that change their light reflection and refraction properties when a current is applied, are the most common substitute for LEDs. For example, LEDs have virtually been replaced by LCDs in one of their original applications-digital watches. The principal competition for gallium-based laser diodes is from InP devices; because InP devices emit light at a longer wavelength than GaAs devices, they are suitable for fiber optic communications applications. However, InP technology is at a earlier stage of development than GaAs technology; thus, InP devices are more costly. Germanium- or silicon-based devices are the primary substitutes for gallium-based photodiodes. Competing materials for solar cells are silicon, copper indium diselenide, and cadmium telluride. Although not as efficient in thin-film solar cells as GaAs, they are generally less costly. INTEGRATED CIRCUITS Although ICs currently represent a smaller share of the GaAs market than optoelectronic devices, they are considered to have potential for greater growth. Two types of ICs are produced commercially-analog and digital. Analog ICs are designed to process signals generated by radar and military electronic warfare systems, as well as those generated by satellite communications systems. Digital ICs essentially function as memory and logic elements of computers. Analog ICs Analog or microwave ICs are used principally in defense applications. Although silicon technology is preferred for signals at frequencies of 3 GHz or less, such Italic numbers in parentheses refer to items in the list of references at the end of this report. as those in television, radios, and computers, silicon operates too slowly at higher frequencies. For these higher frequencies, up to 30 GHz, GaAs microwave ICs are used. One type of GaAs IC, the monolithic microwave IC (MMIC), combines several discrete components on one chip and can perform functions that used to require bulky circuits consisting of vacuum tubes and waveguides. One application of GaAs MMICs is in phased-array radar systems. With the development of the GaAs MMIC, the size of radar components can be decreased significantly, with improved signal-to-noise ratio. In phased-array radar, the antenna elements are fixed in a matrix in a single plane, rather than in a rotating dish, and are steered electronically to allow sky-borne objects to be individually identified. Because GaAs MMICs are small, the size of the antennas can be reduced significantly, perhaps enough to enable phased-array radar systems to fit on an airplane. Current phased-array radar systems are too large for this application. Another defense application of GaAs MMICs is in expendable decoys designed to provide fighter aircraft with protection against radar-directed antiaircraft missiles. These decoys contain a small radar transmitter and receiver that, when ejected from an aircraft, begin transmitting the same frequency of radar energy as that reflected from the aircraft, although at a higher strength. Thus, a radar-directed missile would home in on the decoy, rather than the aircraft. Because of the small size of GaAs MMIC components in the radar, these decoys are only about 6 in long and 1 in in diameter. GaAs MMICs are also a component of solid-state, phased-array jammers. These jammers can be surface- mounted on aircraft to receive and jam radar signals. With GaAs MMIC technology, the need for jamming pods is eliminated, providing space to carry additional weapons. GaAs MMIC technology is still in its infancy, and many defense applications for MMICs are being developed. These military applications include space-based radar, missile seekers, "smart" munitions and other electronic warfare devices, and navigation and communications. GaAs MMIC applications may spread to the commercial sector, including direct broadcast satellite receivers and business communications equipment, such as cellular telephones. Digital ICs The first digital GaAs-based ICs were introduced into the U.S. market as "off-the-shelf products in 1984. Consequently, their use in computer systems has been limited. In most cases, GaAs components are used in the high-speed supercomputers being developed. Because of the high cost of GaAs as compared with silicon, GaAs digital ICs arc not expected to replace silicon ICs in most high-volume commercial applications, such as personal computers. The use of GaAs digital ICs will be confined to operations in which large quantities of data must be interpreted in a very short time, such as in weather forecasting and surveillance satellites. GaAs digital ICs also may be used in space-borne signal processing applications, such as those required for the implementation of Strategic Defense Initiative. OTHER APPLICATIONS Gallium is used in applications other than those in which its semiconductor properties are important. Gallium oxide is used in making some single-crystal "garnets" for special applications. As used in the electronics industry, the term "garnet" refers to compounds of mixed M 2 3 metal oxides. Gallium gadolinium garnet (GGG) is used as the substrate for a bubble memory device. The single crystals are produced by conventional means, and an epitaxial layer is added that contains rare earth oxides. These rare earth oxides provide magnetic domains, or bubbles, that can be oriented to store information and moved, by an electric field, for information readout. Memory devices .can be made with silicon materials, bubbles, or magnetic tape or disks. These three technologies compete on the bases of cost and size. Silicon devices are low cost and small, but they require power to retain information; otherwise the data are volatile and subject to loss. Although bubbles are costher and disks are bulkier than comparable silicon devices, they are nonvolatile, which makes them attractive for certain applications. Although not commonly used in most computer applications, GGG bubble memories are suited to dirty environments and environments that are subject to wide fluctuations in temperature. Commercial applications for GGG bubble memories include petrochemical data collection and plant machine control. Gallium, scandium, and gadolinium oxides are being used as another, mixed- oxide, single-crystal garnet (GSGG). GSGG has demonstrated improved efficiency as a laser host with potential inertial fusion energy applications. Small quantities of metallic gallium are used for low- melting-point alloys, for dental alloys, and as a component in some magnesium, cadmium, and titanium alloys. Gallium is also used in high-temperature thermometers and as a substitute for mercury in switches, because it has the longest liquid range of any element. Gallium has additional uses in glasses and mirror coatings. PROPERTIES AND GRADES GaAs has several properties that give it advantages over silicon in many applications. These advantages are partic- ularly prominent in optoelectronic applications. When stimulated by an electric current, GaAs gives off either visible or infrared light; silicon only gives off energy in the form of infrared radiation or heat. This makes GaAs a useful material for fabricating LEDs and laser diodes, applications for which silicon cannot be used. Both GaAs and silicon can convert light to electrical energy, which makes them useful for photodiodes and solar cells, but GaAs can convert more of the available light to electrical energy, making it more energy efficient. In IC applications, GaAs's properties make it especially appealing in defense applications. GaAs is about 10 times more resistant to radiation than silicon. This is essential in satellite operations in space, where components are exposed to damaging radiation from the sun. GaAs circuits also can operate at higher temperatures than can those of silicon-generally up to 350° C for GaAs compared with 275° C for silicon. Therefore, the need for bulky cooling equipment is reduced. Also, electrons move up to five or six times faster through GaAs than through silicon. This allows for faster operation of GaAs-based circuits, which will be of growing importance as defense applica- tions become increasingly sophisticated and require split- second decision-making capabilities. All of these properties make GaAs attractive, but GaAs has several drawbacks that limit its use only to those applications where its properties are crucial. The first of these drawbacks is its cost. Depending on the application, an ordinary GaAs wafer can cost $5/in 2 to $25/in 2 -a 3-in- diameter, undoped, semi-insulating GaAs wafer would cost about $185 to $200, compared with $12 for a 6-in-diameter silicon wafer. The second drawback is that GaAs is much more difficult to fabricate than silicon. It is much more difficult to grow a single-crystal ingot from two elements than from one, especially when the arsenic tends to diffuse out of the melt at temperatures lower than that required for GaAs crystal growth. Consequently, GaAs wafers have more imperfections in the crystal structure than silicon wafers, which may adversely affect the electronic properties of a device constructed on the wafer. GaAs also has lower production yields than silicon. From ingot to usable wafers, GaAs has an effective yield of about 15 pet. GaAs wafers are also brittle and subject to breakage during device fabrication, decreasing the effective yield still further. Purity requirements for the raw materials to produce GaAs are stringent. For optoelectronic devices, the gallium and arsenic must be at least 99.9999 pet pure; for ICs, a purity of 99.99999 pet is required. These two purity levels are referred to by several names-99.9999-pct-pure gallium is often called 6-nines, 6N, or optoelectronic (opto) grade, while 99.99999-pct-pure gallium is called 7-nines, 7N, semi-insulating (SI), or IC grade. For 7N gallium, the total of the impurities must be less than 100 ppba. In addition to the difficulty of consistently producing material with such high purity, it is difficult to analyze for the small quantity of impurities. Certain impurities can cause more problems during GaAs production than others. The impurities of most concern are calcium, carbon, copper, iron, magnesium, manganese, nickel, selenium, silicon, sulfur, tellurium, and tin. Generally these elements should be present in concentrations less than 1 ppba in both the gallium and arsenic. Lead, mercury, and zinc should be present in concentrations less than 5 ppba. Although aluminum, chlorine, and sodium are often present, each of their concentrations should be less than 10 ppba. RESOURCES Although gallium is as abundant in the Earth's crust as lead, it is widely disseminated and is rarely found in concentrations greater than 0.1 pet. Consequently, gallium is nearly always recovered as a byproduct during processing of ores or other materials to recover other metals. The principal materials in which gallium is found are bauxite, coal, phosphate ores, and sphalerite (zinc ore). Of these, gallium is currently commercially recovered during the processing of bauxite to alumina and the pro- cessing of sphalerite to zinc. Bauxite generally is considered the most likely source of byproduct gallium, because gallium occurs in virtually all bauxites and is somewhat concentrated during the extraction of alumina from bauxite via the Bayer process. The gallium content of bauxite varies depending on the individual deposit, averaging about 50 ppm for the world. Bauxites containing high quantities of gallium, 70 to 80 ppm, are found in India, Suriname, and the United States. Table 1 shows an estimate of the world gallium reserves and reserve base available from bauxite, based on bauxite reserves and reserve base and the average gallium content of the bauxite in each country (2). Although world gallium reserves of over 1 million mt are available from bauxite, much of this bauxite will not be mined for many decades, and only about 40 pet of the available gallium is recoverable with current technology. Table 2 presents a projection of the annual gallium capacity of each of the world's Bayer alumina plants, based on each plant's alumina capacity, sources of bauxite, bauxite-to-alumina ratio, and gallium content. Australia, Jamaica, the U.S.S.R., and the United States have the most potential for gallium recovery from bauxite. Because gallium is not recovered at many alumina plants throughout the world, and because most of the gallium originally contained in the bauxite does not dissolve during the alumina extraction process, large quantities of gallium are discarded in the red mud residue. Although not currently considered a gallium resource material, the red mud residue represents a large potential gallium resource. Much of the residue is contained in tailings ponds near the alumina refineries and would be an easily accessible resource. Zinc ores also represent a significant source of gallium, although not all zinc ores contain gallium. Sphalerite, a zinc sulfide mineral, generally contains detectable quantities of gallium, but little quantitative information is available to present an accurate assessment of the gallium potential of these ores. Based on the assumption that the average gallium content of sphalerite is 50 ppm, domestic sphalerite reserves of 21 million mt contain 1,050 mt of gallium. Total world reserves of 147 million mt of sphalerite may contain as much as 7,350 mt of gallium. The countries with the largest sphalerite reserves are Canada, the United States, and Australia. As with bauxite, much of this ore will not be mined for many decades and represents a long-term source of gallium. Coal fly ash and phosphate flue dusts also contain gallium, but because of the availability of gallium from bauxite and sphalerite, it is unlikely that these materials would be used as principal sources of gallium, although technology to recover gallium from these materials has been developed. Table 1. - Gallium reserves and reserve base 1 Country Bauxite reserves, icfmt Bauxite reserve base, IC^rnt Gallium content, pet Gallium, reserves,'' mt Gallium reserve base, 2 mt Australia Brazil Cameroon China Dominican Republic France Germany, Federal Republic of .... Ghana Greece Guinea Guyana Haiti Hungary India Indonesia Italy Jamaica Malaysia , Mozambique Pakistan Romania Sierra Leone Spain Suriname Turkey U.S.S.R United States Venezuela Yugoslavia Zimbabwe Other Total NAp Not applicable. 'Reserve base includes reserves. 2 Based on a 40-pct gallium recovery. 3 Rounded. 4,440,000 2,800,000 680,000 150,000 30,000 30,000 2,000 450,000 600,000 5,600,000 700,000 10,000 300,000 1,000,000 750,000 5,000 2,000,000 15,000 2,000 20,000 50,000 140,000 5,000 575,000 25,000 300,000 38,000 320,000 350,000 2,000 200,000 4,600,000 2,900,000 800,000 150,000 45,000 40,000 2,000 560,000 650,000 5,900,000 900,000 14,000 300,000 1,200,000 805,000 5,000 2,000,000 15,000 2,000 20,000 50,000 160,000 5,000 600,000 30,000 300,000 40,000 350,000 400,000 2,000  NAp 106,560 56,000 10,880 3,600 480 360 24 5,400 7,200 67,200 14,000 160 3,600 28,000 12,000 60 48,000 180 24 240 600 2,240 60 18,400 300 6,000 1,064 3,840 4,200 24 2,400 3 400,000 110,400 58,000 12,800 3,600 720 480 24 6,720 7,800 70,800 18,000 224 3,600 33,600 12,880 60 48,000 180 24 240 600 2,560 60 19,200 360 6,000 1,120 4,200 4,800 24 2,400 J 430,000 Table 2. - World alumina plant capacities and gallium potential, yearend 1987 Company name Plant location Bauxite sources Annual alumina capacity, lO'mt Annual gallium potential, mt Australia: Alcoa of Australia Ltd Do Do Nabalco Pty. Ltd Queensland Alumina Ltd Worsley Alumina Pty. Ltd Kwinana, Western Australia .. Pinjarra, Western Australia . .. Wagerup, Western Australia .. Gove, Northern Territory .... Gladstone, Queensland Worsley, Western Australia . .. Australia . . do . . do . . do . . do . . do 1,400 2,300 570 1,100 2,300 1,000 224 369 116 161 328 204 Total Australia 8,670 1,402 Saramenha, Minas Gerais . .. Pocos de Caldas, Minas Gerais Sao Luis, Maranhao Sorocabo, Sao Paulo Brazil . . do . . do . . do Brazil: Alcan Aluminio do Brazil Alcoa Aluminio S.A Aluminio do Maranhao S.A. . .. Cia Braziliera de Aluminio .... 120 180 500 400 13 19 58 44 Total Brazil 1,200 1,225 134 Canada: Alcan Basic Raw Materials Jonquiere, Quebec Province of Liaoning Province of Guizhou Province of Shanxi Province of Shangdong Province of Shaanxi Province of Hainan Brazil, Guinea, Guyana. China 73 China: Fushun Alumina Plant 250 220 200 220 130 180 35 Guiyang Alumina Plant Hejin Alumina Plant Nanding Alumina Plant Tian Alumina Plant Wenchang Alumina Plant . . do . . do . . do . . do . . do 30 28 30 18 25 Total China 1,200 100 166 Czechoslovakia: ZNSP Kovohute Praha Ziar nad Hronom Hungary, Yugoslavia. France, Guinea .. . . do 7 Gardanne La Barasse France: Aluminium Pechiney Do 700 340 62 30 Total France 1,040 92 Stade Bergheim Australia, Guinea. Australia, Sierra Leone. Australia, Guinea. . . do Greece Guinea Germany, Federal Republic of: Aluminum Oxid Stade GmbH .. Martinswerk GmbH 600 350 430 120 57 39 Vereinigte Aluminium Werke AG Do Lippewerke, Lunen Nabrewerke, Schwandorf .... iny Distommon Kimbo Linden Ajka 41 11 Total Federal Republic of Germ; Greece: Aluminium de Grece S.A.. Guinea: Friguia Societe D'Economie Guyana: Guymine 1,500 600 700 354 148 41 43 Guyana Hungary . . do . . do 35 Hungary: Hungarian Aluminium Corp. . .. Do Do 475 335 110 34 Alamasfuzito Mosonmagyarovar 24 8 Total Hunqary 920 66 Korba, Madhya Predesh .... Renukoot, Utter Pradesh .... Belgaun, Karnataka Muri, Bihar India India: Bharat Aluminium Co. Ltd 200 160 160 72 60 800 31 Hindustan Aluminium Corp. Ltd. Indian Aluminium Co Do . . do . . do . . do . . do . . do 25 25 11 Madras Aluminium Co. Ltd. . .. National Aluminium Co. Ltd. . .. Mertur, Tamil Nadu Damamjodi, Orissa 9 122 Total India .... 1,452 800 223 Ireland: Aughinish Alumina Ltd. .. Aughinish, Limerick County .. Guinea 48 Table 2. • World alumina plant capacities and gallium potential, yearend 1 987— Continued Company name Plant location Bauxite sources Annual alumina capacity, lO^mt Annual lium potential, mt Italy: Aluminio Italia Porto Marghera Eurallumina SpA Total Italy . .. Porto Vesme, Sardinia Australia, Brazil, Guinea, Yugoslavia. Australia, Guinea. Jamaica: Alpart Clarendon Alumina Production Ltd. Jamaican Joint Venture Do Total Jamaica Spur Tree Halse Hall, Clarendon Ewarton, St. Catherine Kirkvine, Manchester. Jamaica .. do .. do . . do Japan: Nippon Light Metal Co. Romania: Alumina Enterprise Do Shimizu Oradea Tulcea. Australia, Indonesia. Romania .. do. Total Romania Spain: Aluminia Espanola S.A. Suriname: Paranam Refinery Joint Venture. Turkey: Etibank Alumina Plant U.S.S.R.: Bogslav Aluminium Dnjepr Aluminium San Ciprian Paranam .. Guinea .. Suriname Seydisehir Turkey Nikolaev Alumina Novokuzneck Alumina Pavlodar Alumina Tikhvin Alumina Northern Urals Zaporashye .. Black Sea, South Ukraine Kemerovo Kazakhstan, Central Asia Leningrad U.S.S.R U.S.S.R., Guinea, Yugoslavia, Hungary, Jamaica. Guinea, Yugosla- via, Jamaica, Brazil, Guyana. U.S.S.R . . do U.S.S.R., Guinea, Yugoslavia, Brazil, Jamaica. 200 720 920 1,180 550 500 800 1,352 200 350 300 1,000 250 500 350 20 86 106 177 83 566 558 85 77 2,854 380 422 44 250 250 17 17 34 48 234 14 50 30 74 35 71 35 Ural Aluminium Urals U.S.S.R 250 35 Total U.S.S.R 3,000 330 United States: Aluminum Co. of America .... Point Comfort, TX Guinea, Suriname. 1,210 125 Kaiser Aluminum & Chemical Gramercy, LA Jamaica 725 109 Corp. Reynolds Metals Co Guinea, Brazil, Aus- tralia, Jamaica. 1,256 142 Total United States 3,191 376 Venezuela: Interamericana de Alumina C.A. Puerto Ordaz, Orinoco .... Guyana, Brazil, Suriname. 1,300 156 Yugoslavia: Energoinvest Aluminium Bacevici, Mostar, Bih Yugoslavia 280 19 Do Zvornick, Vlasneica . . do 600 42 Kombinat Aluminijuma Titograd Titograd, Montenegro .... . . do 280 19 Tvornica Lakih Metala Obravac, Dalmatia . . do 300 21 Unial Tovarna Glinice in Alumina Kidricevo, Slovenia Yugoslavia, Guinea. 140 9 1,600 110 World total 35,858 4,352 RECOVERY TECHNOLOGY GALLIUM RECOVERY FROM BAUXITE Throughout the world, alumina is recovered from bauxite by the Bayer process. In this process, alumina is extracted from bauxite through digestion with a hot caustic solution. After the slurry is cooled and solid residue is separated from the aluminum -containing liquor, the solution is seeded with alumina trihydrate crystals to precipitate the dissolved aluminum as alumina trihydrate. Alumina trihydrate is separated from the solution and calcined to produce alumina, while the caustic solution is recycled to the bauxite digestion step. Because gallium is chemically similar to aluminum, it tends to remain with aluminum during processing. When the aluminum is extracted during digestion, gallium is also extracted. Gallium is not removed from the solution during subsequent processing steps, and because the solution is recycled, gallium builds up to an equilibrium concentration of 100 to 125 ppm. When gallium recovery is desired, a bleed stream is separated from the caustic solution before it is recycled to the digestion step. Crude gallium metal, 97.0 to 99.9 pet pure (3N), is recovered by two principal processes-the Beja process and the de la Breteque process. Simplified flowsheets for these pro- cesses are shown in figures 1 and 2. In the Beja process, carbon dioxide is injected into the bleed solution to precipitate aluminum not recovered in the Bayer process as alumina trihydrate. The trihydrate is separated from the solution, and the galhum-containing solution is carbonated again. In the second carbonation, a gallium precipitate, containing between 0.3 and 1 pet gallium, is recovered. Both of the carbonation steps are carefully controlled so that about 90 pet of the aluminum is removed during the first carbonation and 90 pet of the gallium is precipitated during the second carbonation. After the gallium precipitate is separated from the solu- tion, which is recycled to the Bayer process, the precipitate is dissolved in a caustic solution to increase the gallium- to-aluminum ratio. This solution is electrolyzed to recover crude gallium as a liquid. The spent solution is recycled to the Bayer process (3). In the de la Breteque process, a bleed stream from the Bayer process is concentrated by evaporation to increase the gallium concentration. Concentrated solution is directly electrolyzed using a highly agitated mercury cathode. The agitation allows the gallium to form an amalgam with the mercury. When the gallium concen- tration reaches about 1 pet in the amalgam, it is drawn off and leached with a caustic solution. This yields a concentrated gallium solution from which crude gallium can be recovered by electrolysis (4). Because mercury losses are significant owing to the high level of cathode agitation in the de la Breteque process, a modification to the process was developed by Vereinigte Bleed stream Bayer liquor 1 CARBONATION LIQUID-SOLID SEPARATION "►Al203-3H 2 CARBONATION LIQUID-SOLID SEPARATION ± Recycle liquor to Bayer process rGallium concentrate NaOH- REDISSOLVING ELECTROLYSIS Recycle liquor to Bayer process Crude gallium Figure 1.— Beja process for recovering crude gallium from Bayer liquors. Aluminium Werke AG (VAW) of the Federal Republic of Germany. Rather than preparing a gallium-mercury amalgam by electrolysis, a sodium-mercury amalgam is prepared by electrolyzing the caustic solution with a mercury cathode. Gallium is then extracted by a cemen- tation process as the gallium in solution replaces the sodium in the amalgam. Subsequent gallium recovery follows the same steps as in the de la Breteque process. Although the de la Breteque process and the VAW modification are the most commonly used processes, sev- eral companies have developed proprietary recovery tech- niques that they claim are less costly than conventional processes. Rhonc-Poulenc S A. uses a liquid-liquid extrac- tion technique at its plant in France to recover gallium from Bayer liquors. Sumitomo Chemical Co. Ltd. of Japan uses an unidentified absorbent to extract gallium directly from Bayer liquor. 10 Bleed stream Bayer liquor EVAPORATION Concentrated gallium solution ELECTROLYSIS Recycle NaOH solution w Recycle liquor to Bayer process ^Gallium-mercury amalgam NaOH LEACHING ELECTROLYSIS ▼ Crude gallium Figure 2.— de la Breteque process for recovering crude gallium from Bayer liquors. GALLIUM RECOVERY FROM ZINC ORE Dowa Mining Co., the only company that currently recovers gallium from zinc ore, uses an electrolytic method for producing zinc. In recovering zinc by this method, a roasted zinc concentrate is leached with sulfuric acid to produce a zinc sulfate solution, which is neutralized to remove impurities. Impurities that precipitate from the zinc sulfate solution include gallium, aluminum, and iron. Leaching this residue with a caustic solution extracts the gallium, along with the aluminum and iron impurities. After the remaining residue is separated from the gallium- containing solution, the solution is neutralized to precipi- tate the metal hydroxides. The hydroxide solids are leached with hydrochloric acid to dissolve gallium and aluminum, and the gallium is separated from the alumi- num in solution by solvent extraction with ether. Distilla- tion of the ether solution yields a gallium-rich residue that still contains some iron, which is removed by treating the residue with a strong caustic solution to extract the galli- um, while leaving the iron as a solid. Iron residue is filtered from the gallium-containing solution, and crude gallium is recovered by electrolysis. A simplified flowsheet for this process is shown in figure 3. GALLIUM RECOVERY FROM OTHER SOURCES Other sources of gallium that have been investigated include phosphate flue dust, coal fly ash, aluminum smelter flue dusts, and iron oxide minerals found in Utah. The only sources that have been commercially treated to Gallium-containing residue from zinc sulfate solution purification LEACHING I •NaOH NEUTRALIZATION LIQUID-SOLID SEPARATION £ -HC1 -^Waste solution LEACHING etal hydroxides Recycle ether I HC1 SOLVENT EXTRACTION -► Waste solution DISTILLATION I Gallium residue NaOH solution 4 REDISSOLVING 4- — i LIQUID-SOLID SEPARATION k, w V ^ -NaOH ^Iron precipitate Crude gallium Figure 3.— Gallium recovery from zinc ore. recover gallium are the minerals in Utah and aluminum smelter flue dusts. In 1986, St. George Mining Corp., a subsidiary of Musto Explorations Ltd., began recovery of gallium from an abandoned copper mine near St. George, UT. Although much of the copper had been mined, the remaining iron oxide minerals contained an average of 0.042 pet gallium. Because this source material is different from bauxite or zinc ore, a new processing technique was developed, as shown in figure 4. Crushed ore is leached with sulfur dioxide gas, a sulfuric acid solution, and fluorspar to extract gallium, along with germanium, copper, iron, and zinc. After the insoluble impurities are separated, copper is cemented by the addition of iron and separated from the gallium-con- taining solution. Dissolved germanium is precipitated as a sulfide by injection of hydrogen sulfide gas into the solution, and the precipitate is removed and treated to recover germanium dioxide. The solution, containing gallium, zinc, and iron, undergoes solvent extraction, where the gallium and zinc are extracted into the organic solu- tion; the iron remains in the aqueous phase. Gallium and 11 H 2 S0 4 " SOo' Ore LEACHING Iron ■ LIQUID-SOLID SEPARATION I -Fluorspar Residue to tailings pond > CEMENTATION LIQUID-SOLID SEPARATION H S' i -^Cement copper GERMANIUM PRECIPITATION LIQUID -SOLID SEPARATION Recycle organic Precipitate to 'germanium recovery SOLVENT EXTRACTION STRIPPING NHi GALLIUM PRECIPITATION LIQUID-SOLID SEPARATION I Ferrous sulfate solution _^Zinc sulfate solution Gallium hydroxide PURIFICATION ELECTROLYSIS ▼ Crude gallium Figure 4.— Gallium recovery from Musto Explorations Ltd.'s mine near St George, UT. zinc are stripped from the organic phase, and ammonia is injected into the stripped solution to precipitate gallium hydroxide. Gallium hydroxide precipitate is separated from the zinc solution and purified, and 99.999-pct-pure (5N) gallium metal is recovered by electrolysis. The elec- trolytic technology used at Musto's plant is proprietary technology licensed from Cominco Ltd. In July 1987 Elkem A/S of Norway began producing crude gallium using aluminum smelter flue dust as a source material. Dusts generated at two smelters in Mosjoen and Tyssedal are blended to yield material with the following average concentration: Carbon, 33 pet; fluo- rine, 17 pet; oxygen, 17 pet; aluminum, 13 pet; sodium, 9 pet; iron, 6 pet; sulfur, 3 pet; calcium, 1.5 pet; and gal- lium, 0.5 pet. Leaching the flue dust with hydrochloric acid extracts the gallium. Solids are filtered from the liquid phase and mixed with portland cement before disposal. The liquid phase undergoes a series of solvent extraction stages to separate gallium from dissolved impurities. After cleaning and stripping, crude gallium is recovered by electrolysis of the water phase. A simplified flowsheet for this process is shown in figure 5. GALLIUM PURIFICATION For most applications, purity requirements for gallium are either 6N or 7N. Crude gallium is purified in essen- tially two steps-the first step produces 99.99-pct-pure (4N) gallium, and the second produces 6N to 7N metal. Many of the impurities in crude gallium occur in the surface oxide or as finely dispersed phases in the metal. Liquid gallium filtration and heating under vacuum remove these types of impurities. Metallic impurities can be reduced to less than 0.01 pet, producing 4N gallium, by sequential washing with hydrochloric acid. Another method that can be used is electrolytic refining, which involves anodic dissolution of gallium in an alkaline solu- tion, and then deposition at a liquid gallium cathode. The principal method used to produce 6N and 7N metal is gradual crystallization of molten gallium. In this pro- cess, impurities remain in the liquid phase and do not con- taminate the gallium crystal. Crystallization is repeated until gallium of the desired purity is obtained. Another method of producing high-purity gallium is to convert the gallium to a halide compound, such as gallium trichloride, which is then zone-refined. High-purity gallium is recovered by electrolysis of the halide compound. GALLIUM ARSENIDE FABRICATION GaAs single crystals are more difficult to fabricate than those of silicon. With silicon, only one material needs to be controlled, whereas with GaAs, a one-to-one ratio of gallium atoms to arsenic atoms must be maintained. At the same time, arsenic volatilizes at the temperatures needed to grow crystals. To prevent a loss of arsenic, which would result in the formation of an undesirable gallium-rich crystal, GaAs ingots are grown in an enclosed environment to contain the arsenic. Two basic methods are used to fabricate GaAs single- crystal ingots-the boat-growth, horizontal Bridgeman (HB) or gradient freeze technique, and the liquid-encapsulated Czochralski (LEC) technique. Ingots produced by the HB method are D-shaped and have a typical cross-sectional area of about 2 in 2 . By contrast, single-crystal ingots grown by the LEC method are round and are generally 3 in in diameter, with a cross-sectional area of about 7 in. In HB growth, gallium and arsenic in the proper ratio are placed in one end of a silicon dioxide (quartz) or pyrolytic boron nitride boat. A seed GaAs crystal is contained at the other end of the boat. The boat is placed in a sealed quartz tube, which is evacuated to a very low pressure. The tube is placed in a multiple-zone furnace, where the gallium and arsenic react to form GaAs. The 12 A .umlnum smelter flue dust LEACHING HC1 1 LIQDID-SOLID SEPARATION ^ r> : J 1 4 SOLVENT EXTRACTION Recycle organic 1 f STRIPPING ^ 1 r ELECTROLYSIS ^ « *i v 1 Crude r gallium Figure 5.— Elkem A/S process for recovering gallium from aluminum smelter flue dust compound is heated to 1,240° C, the melting point of GaAs. The GaAs melt is slowly cooled from the seed end, resulting in single-crystal growth. In the LEC method of crystal growth (fig. 6), carefully weighed pieces of gallium and arsenic are melted in a pressurized vessel (crystal puller). The GaAs melt is contained in a crucible con- structed of either high-purity quartz or pyrolytic boron nitride. The melt is covered with a layer of boric oxide, which retards arsenic loss from the melt by sublimation. A seed crystal is lowered through the boric oxide into the melt and slowly withdrawn as both the seed and crucible are rotating. Each of these methods produces GaAs ingots with par- ticular advantages and disadvantages. Crystals formed by the boat-growth method are particularly suitable for opto- electronic applications because their structure is highly perfect with respect to dislocations. Optoelectronic devices also require crystals with a high doping concentration, which boat-grown crystals readily provide, because the silicon dissolved from the quartz boat contributes to crystal doping. This latter benefit becomes a problem if semi- insulating GaAs crystals for ICs are being produced. The silicon impurities are called shallow donors, or N-type dopants. To compensate for these impurities, either a controlled quantity of gallium oxide can be added to the melt, or chromium, a deep acceptor or P-type dopant, can be added. Crystal growth also can be accomplished in a boron nitride container, which eliminates any contact with silicon-containing material during growth. The shape of the HB-grown ingot makes it inconvenient for subsequent wafer processing, because automated wafer processing sys- tems are designed to handle round wafers. LEC-grown ingots generally contain more crystal structure defects, i.e., they have higher dislocation densities, than HB-grown ingots. This affects the electronic properties of the device constructed on the GaAs. Dislocation densities in HB wafers normally run between 500 and 20,000 per square centimeter; those in LEC wafers can be as large as 100,000 per square centimeter. Because chips are batch processed, wafer by wafer, the larger the wafer, the more chips per wafer, and the lower the cost per chip. A 3-in-diameter LEC-grown wafer can yield more of the same-size chips than a 2- by 1.5-in HB wafer. Conversely, the capital costs for an HB system are significantly less than those for an LEC system. After the ingots are grown, the ends are cut off, and the ingots are shaped by grinding the edges. Ingots are then sliced into wafers (fig. 7). Wafers go through several stages of surface preparation, polishing, and testing before they are ready for device manufacture or epitaxial growth. Wafer preparation steps are done in a clean room and with minimal contact to avoid introducing surface contami- nants. In LEC growth, the effective yield from starting material to finished wafers is currently less than 15 pet. Pure GaAs is semi-insulating, which means that it is not a conductor of electricity. In order for GaAs to conduct electricity, a small number of atoms of another element must be incorporated into the GaAs crystal structure. This is called doping. These atoms act as electron donors or electron acceptors. Electron donor atoms have one more electron than the atoms that they are replacing, and this electron is free to move within the crystal as an electrical charge carrier. Electron acceptors have one less electron than the atoms they are replacing and behave as positively charged particles to serve as electrical charge carriers (5). To manufacture devices from GaAs wafers, the wafers must be doped with another metal or metals. Normally, this is accomplished either by ion implantation or by some type of epitaxial growth. Because GaAs is a semi- insulating substrate, no special isolation areas are required to separate each device fabricated on the chip. This results in more compact, higher density circuits, which add to GaAs's speed advantage. In ion implantation, ions of another metal are implanted into specific areas of the semi-insulating GaAs to make those areas electronically active. Areas of the chip that are to remain semi-insulating are covered with a photoresist mask before ion implantation. The process of ion implantation may be repeated several times with dif- ferent metals on different areas of the chip, depending on the type and complexity of the device being manufactured. After ion implantation, the GaAs must be annealed at about 850° C in order to activate the implanted dopants and remove crystal damage incurred during implantation. When annealing, as in crystal growth, several techniques, including encapsulation of the chip, are used to prevent arsenic losses at the elevated temperature. After doping, optoelectronic device or IC manufacture can be completed through deposition of layers of metals and insulators by various techniques. A similar technique, called ion cluster beam, is not as frequently used as ion implantation. In this technique, ions are grouped together and implanted into the wafer at 13 Figure 6.-GaAs Ingot growing in LEC furnace (Courtesy Morgan Semiconductor Div. of Ethyl Corp.) Figure 7.-LEC-grown GaAs Ingot and wafers (Courtesy Morgan Semiconductor Div. of Ethyl Corp.) 14 lower speeds than used in ion implantation. This process is reported to result in less damage to the crystal structure. The deposition of an epitaxial layer is another means of creating electronically active regions on the GaAs substrate. There are four principal methods for growing epitaxial layers-liquid-phase epitaxy (LPE), vapor-phase epitaxy (VPE), metal-organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). LPE is an earlier method of epitaxy that is generally not considered suitable for complex semiconductor production because it cannot be as precisely controlled as the other three techniques. In LPE, the substrate wafer is contained in a graphite boat within a quartz furnace tube, where it is contacted with solutions containing the metals to be deposited. Cooling the solution causes the metals to precipitate on the substrate. LPE produces relatively thick epitaxial layers, and the boundaries between layers are gradual rather than sharply defined. Two methods of VPE are used to grow epitaxial layers on a GaAs substrate-the hydride method and the chloride method. In VPE, GaAs substrates are mounted in a reactor. To make GaAsP epitaxial layers, two gaseous streams are introduced into the reactor. In the hydride process, one gas stream combines arsine (AsH 3 ) and phosphine (PH 3 ) with a hydrogen carrier gas; the other gas stream is a hydrochloric acid gas that has been passed over a gallium reservoir to form gallium trichloride, and that also is mixed with a hydrogen carrier gas. Dopants are added to the gas streams if necessary. Gallium trichloride reacts with the AsH 3 and PH 3 gases to deposit a GaAsP layer on the substrate. In the chloride process, arsenic trichloride and phosphorus trichloride gases are substituted for AsH 3 and PH 3 . VPE technology can coat multiple wafers at the same time, and the layer thickness, molecular composition, and dopant concentration can be more closely controlled than with LPE. 