Stanley Mroczkowski Yale University Department of Engineering and Applied Science New Haven. CT 06520
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Needs and Opportunities in Crystal Growth
Modern technoloev ... is based larnelv .. . on materials such as semiwnductors; ferrites; magnetic garnets; solid state lasers; piezoelectric, ultra\,iolet, and infrared sensitive crystals; and crystalline films for mirruelectronics and computer induxries. 1\11 this involves research i n rrystal grt,wth. There are thrre major stages involved in this research.'l'he first is the provision of pure materiali and impruved equipment. The second is the production crystals, first experimentally and then ior wrhnical wiring-with research inru methuds of produrtiun, effwt oirrvstallization renditions. etr. The third is the study of properties and use of the crystals in devices. From this we get some feedback to the first two stages. A survey is presented of the scientific basis for single crystals production. Some of the theoretical and experimental advances in this area, together with the most important ways of producing single crystals, are discussed. The future prospects for semiconductors, magnetic lasers, nonlinear optics, piezoelectrics, and other crystals are surveyed. The lmmrtance of Crvstals to Socletv Crystals aro the unacknowledged pillars un which our advanced technology rests. Thc manufarture of crystals and the devices that they ha\v made pnssil~lci i now a large and cxpanding i n d ~ ~ s t r ienterprise. al A multitude of objects that we use daily, hut rarely pause to consider, would not he available today were it nut for an industry centered on growing single crvstals. Must watrhes contain "iewels" made of synthetic rubies. Radios, televisions, record players, hearing aids, and automobile ienition svstems all contain solid-state components. T h e ;hiquito& chip-a small single crystal with a complex history-is the emblem of the information of communication networks that serve as the nerves and brain of our comolex social ornanism. The applications of crystals include (1)power regulation for cities, (2) management of bank accounts and credit cards, (3) telephone communications, (4) direction of air and rail traffic and reservations, (5) diagnosis of disease. The everyday h ~ ~ r i n eof s srechnulugisally de\,eloped countries is pnrfuundly dependent un cryswls; yet very feu, people are aware uf either rhe urilitv s u l s~or the- induaries .--. ~ ~d r n*~ ~ ~ that"oroduce them. This consumer ignorance, a natural result of the packaging of crystal-incorporating products, ought to be dispelled without delay. The crystal industry is today worth millions of dollars, without it modern society could not function. d~~~
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What a r e the Aspects of Crystal Growth and Who is the Driving Force? The facets of crvstal erowth on which I would like to focus " are the physical and human origins of solid-state technology. The earliest transistors relied on the unique properties of germanium. Germanium belongs to Group i~oi' thk periodic table-each germanium atom has four electrons in its outermost shell. Pure germanium is a poor conductor of electricity a t room temperature, but if we add trace impurities such as arsenic or boron, the conductivity can be significantly altered.
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Presented at the ACS National Meeting, March 25, 1980,as part of the State-of-the-ArtSymposium on Solid State Chemistry in the Undergraduate Curriculum sponsored by the Division of Chemical Education.
This change in conductivity arises as a result of increasing either free electrons or free holes in the crystal lattice. Condurtivity in arsenic-doped germanium is enhanced hy inrreasing the number of frec clectrons; such material, where current consists of moving negative charges, ii called N -Itor . negative) type. In horon-doped germanium it i.: t h ~motiun #,t' elertrm-that make uo the rurrent. It . "holesM-ahsent -.-.- ~ and similarly conducting materials are calledp- (for positive) type. An abrupt change in doping from arsenic to boron results in a P-N junction diode. These junctions allow current to pass in only one direction and so convert alternating to direct current. If a crystal is properly doped, i.e. PNP, and suitable leads are attached it can act as a transistor-the simplest amolifier. The invention of the transistor required a highly pure single cwstal as raw material. Purity is essential in order to prevent random impurities from contributing to or impeding the conductivity properties we wish to create and control. Single crystals are required because polycrystalline boundaries scatter the conducting holes and electrons and thereby reduce their mobilitv. The transistor required the innovation of techniques for preparing materials with less than n k w parts oer billim of unwanted hourities alonc with special techniques for preparing highly regular rrystals. Almost all of the terhnuloriral breakthroughs since the first transistor have entailed materials disco;ery as they have increased the demand for more refined and regular crystals. Crystal growth technology has also been confronted with additional challenges due to the growing and intrinsic complexity of the materials scientists use. In the early days of semiconductors, germanium was supplanted by silicon. With silicon as the base crystal, doping can be controlled more effectively, and coating with a thm oxide film (one of the many steps in the processing of material for transistors) is also facilitated. The advent of silicon crystal growth brought about a burst of electronic development by greatly extending transistor capabilities, which in turn has issued in the innovations of contemporary integrated circuitry. It is worth remembering that both silicon and germanium have to be made as single crystals since they do not exist in a pure element form in nature. Production of high quality silicon crystals has required solutions to a number of difficult material oroblems. -Purification techniques frequently involve moving molten zones through a solid ingot, and growth techniques often entail "pulling" crystals from a melt. Clearly silicon, with a melting point of 1412'C, is a more difficult material to grow and purify than germanium, which has a melting point of 931°C. Using the crude parameter of melting points, we can see that today's demand for good quality yttrium garnet (with a melting point of 1980°C) presents us with a new problem: Who are the scientists whoassume the respmsil)ility oisolvinp the prohlems assuciared with crystals needed for the tcchndogy uf today and of the future? To solve these problems it will be necessary to train scientists in new fields. Before the transistor era, there was no materials science as such. Crystallographers, chemists, physicists, metallurgists, or whoever had the inclination, assumed the role of crystal grower. Today, the solid state chemists assume the responsibility for growing the needed ~~~~~
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Volume 57, Number 8. August 1980 1 537
cmtals. Their main resionsibilities lie in three areas: (1)the d&overy of new materials with useful properties; (2) basic research into the relations between chemical bonding and structural and electronic properties of solids; (3) and the improvement of proeessine techniques and characterization. Let ;la more . look ~ -- Eloselv ~ ~ ~into these areas. The discovery of new ma&ls and their applications is not a tooic that I can cover both com~rehensiveivand in detail. I will restrict my ronjideration, then, to the materials and cmstals with which I am most inmiliar, i.e., sem~conductora, a i d magnetic and optical crystals. Recent developments in semiconductor compounds have extended device capabilities beyond those of the silicon transistor. Compounds such as GaAs, Gap, and GaAsP have increased the range of large band gap materials. A whole series of new crystals called III-V's have been created that call for improved physical measurement methods to handle the stoichiometric control of volatile arsenic and phosphorous. GaP ohosohorous is a ~romisinematerial for use in lieht e m i t k g diodes ( L E D S ) : ~handgap is such that i t radiates in the green region of the spectrum, a wavelength to which the human eye is especially sensitive. Gallium aluminum arsenide is used in heterojunction semiconductor lasers where its material properties provide both carrier and optical confinement to yield an efficient lasing structure. Foisuch devices, as many as 6 separate layers in the form of thin epitaxial single crystals (on the order of microns in thickness) with controlled doping may he necessary. T o produce one of these devices demands extreme processing sophistication. Today it is not uncommon to he able to hold impurities down to less than a few parts per billion. The preparation of devices such as semiconductor lasers requires an in-depth understanding of crystal growth and materials science. Success in such an endeavor also often depends on the materials scientist being gifted with the patience of Job. Magnetic crystals are another new material currently being developed. Discoveries made in the '50s and '60s have led to the control of properties in insulating maenetic materials. Devices such as magnetic memories werethen obtainable without single crystals. The polycrystalline ferrites are the best example. e hey are formed into2 mm diameter cores for computer memories. These magnetic core memories are quite large on the scale of modem integrated circuits. They also suffer the disadvantages of inconveniently large power consumption and heat generation. Semiconductor memories have been developed that do not have these difficulties and, in addition, have the advantage of allowing shorter access times. At about the same time, another important discovery was made in the magnetic field-iron garnets. Iron garnets are of technological importance for use in magnetic memories, magneti-optical modulators, and microwave delay circuits. Yttrium iron garnets usually have a noncongruent melting point, i.e., the components that make up the compound do not melt at the same temperature. Consequently, uniform crystallization from a melt is not oossible. Instead..thevare . mown by spontaneous nucleation during slow cooling of nutrients in a flux solution of lead oxide. lead fluoride, and horon oxide. With the flux techniques, however, it is difficult to grow large crystals except by using a large amount of melt in a platinum crucible. It is also hard to avoid inclusion, strain, dislocation, and lead diffusion into the garnet structure. The production of large, high quality crystals of iron garnet by the fl ux technique has serious problems, especially when the raw material is very pure. Nearly instantaneous nucleation occurs in the purest solvents (flux). The exoandine field of magnetic activitv. ~resentlv involves . exploration ol'miniaturized information storage in magnetic domains within sin& crvstals oi maenetic earnets. This dcvelopment holds b&h the possibilit;of minimizing the size of computer memories and of improved overall computer
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538 / Journal of ChemicalEducation
operation. Magnetic domains in these platelets of iron garnet can be manipulated to perform logical operations. The specific requirements for these garnets are (1) Uniform platelets (25 pm thick); (2) low saturation magnetization, 4 ap, = 200 Gauss; (3) low coercivity; and (4) preferred magnetic axis normal to the plate surface. All of these traits are combined in yttrium iron garnet crystals. If a small magnetic field is applied to a yttrium iron garnet crystal the magnetic fields regroup. At a critical field strength they form cylindrical domains which can be used in a shift register and other logic performing devices. Bubble technology, as this manipulation of magnetic domains is called, has recently moved from the laboratory into development and pilot production. Nature was kind in the garnet problem in two respects. First, the flux system from which the garnets for huhhle applications are grown can be supercooled much more effectively than previous experience had led us to expect. This good fortune gave rise to the "liquid phase epitaxial (LPE) technique" in which a magnetic film is produced by dipping a nonmagnetic substrate in the flux. The kinetics of growth by the LPE technique are now very well understood. Consequently, we can tailor the garnet composition and film thickness to the specific requirements of bubble applications. Secondly, we were fortunate in finding asuitable nonmagnetic substrate. The LPE technique can he of use only if we have a large size, defect-free suhstrate material that is nonmagnetic and with a lattice constant very closely matched to that of the film deposited on it. Gadolinium gallium garnet (GGG) proved to he such a material. GGG, which unlike iron garnet that melts incongruently, can he grown by the pulling technique from a melt. Thanks to fully automated, computer-controlled pulling equipment, large defect-free GGG crystals can he grown in sizable quantities. Experience in the controlled growth of thin film garnets can he transferred to the growth of thin films of optical materials for integrated optics. If huhhle garnets find large scale applications, they might well become one of the largest volume activities in the future. Presently, the optical field is flourishing especially in electronic devices. New materials and crystal innovations are playing a key role in this area. The progress in optical devices has followed on the development of new single crystal materials such as yttrium aluminum garnets (YAG) doped with neodymium ions. This synthetic material is used in lasers. YAG has a high melting point, which is critical for laser applications since much of the energy is dissipated as heat, and is doped with an ion that lases a t low energy. In the host material an ion of radius and charge similar to the deposit must be a part of thestructure in order to provide alocation for the impurity. In the neodymium-doped YAG crystal, neodymium displaces aluminum. In several optical devices used in comhinatioo with lasers, single crystals play important roles. The frequency or amplitude of laser light can he modulated by an electric field as a way of impressing information on the laser beam. To accomplish this we use crystals with large electro-optic effects in which an applied field changes the index of refraction and thus produces a.change in the polarization of the light as it passes through the crystal. Laser light of one frequency is modulated to a variety of other frequencies, again with coherence intact. A by-product of the search for crystals whose optical properties are susceptible to the influence of an electric field is the discovery of new crystals in which mechanical strain produces large electronic or optical effects. Crystals like lithium tantalate, which produces an electronic charge in response to pressure, and tellurium dioxide, which modulates light in response to sound waves, are the result of recent solid state chemistry. The potential of optical communications systems has generated material activities directed toward the preparation of low-cost waveguide fibers and thin light-guiding films. A
whole new technology of "integrated optics" appears to be hovering on the horizon. Perhaps optical functions will soon he performed on a single chip in the same wav as electronic by solid-state integrated circuits. functions are now Glancinn- back over the variety of new materials and applications, we can see that materials science is of necessity a highly interdisciplinary field. I wish now to turn to the second ioh of the solid state c h e m i s t t h a t of explaining the conuecaons between chemical bonding. and electronic nronerties. In the earlv ~ structural. .. . . days of materials science (- 1950),advances were frequently made bv the "brute force" method. The brute force method was "tr; it first; if it works, let's figure out why afterwards." Bv means of the brute force method and exhaustive testing of many materials, technologists developed crystals with outstanding characteristics for specific applications. When a suitable material was found a growth technique was perfected that would yield crystals of the desired size and quality. Growth and testing were continued until all reasonable combinations had been tried, and the one best fitted to the particular function was chosen. Then, for each type of compound, there was usually one (or a t most a few) that was considered superior. The less immediately impressive compounds were shelved and seldom mentioned in the literature. By this trial and error svstem, cwstal mowers were slowly learning how to ~ r e d i c thiprop&&s t of new materials. Today the brute force method has largely been replaced by that determine efforts to understand the physical chemical honding, molecular structures, and electronic properties. We owe much to the Edisonian ingenuity, labor, and sweat of the early researchers, hut as materials science becomes more comolex we must improve the nredictabilitv of our approach. ~ i t h o u better t prddictive models t h e t a s of testine new materials will exceed the resources available. The tl&d major responsibility of the solid state chemist is to improve processing techniques. In addition to innovation in the techniques of production this area includes improvement of our description of materials, i.e., the problem of characterizing materials. Progress in processing technology has always been essential to progress in materials and crystals. Before WWII, natural quartz (primarily from Bohemia and Brazil) was used for technologicalapplications. During the war the dcmand for high qualit). qunrtz, empl~yedin sonar and radar equipment, naturally increased. Sinw the Nazi's cuntrdled the sources, research in this country to devrlop synthetic quartz was stimulawd. Now quartz represent&the mnst ahllndant man-made crvstal with u,orld rmdurtinn close to 1.5 million pounds/vear. Another &stnl, whose processing has been facilitated by better theon.tical understanding is the diamond. Of the several types of industrially produced diamonds, the clear ones are most desirable for jewelry, but the best for technoloeical applications (particularly for hot injection lasers and heat sinks in high power electron microscopes) are the so called type 2 diamonds. The scarcity of natural type 2 diamonds placed a limit on their technological exploitation and so stimulated research on diamond svnthesis. Diamonds now are -~ - produced in huge machines that generate temperatures up to 1500°C and pressures up to 150,000 atm, simultaneously. Successful material work also involved detailed characterization. Data for some of the characteristics can be obtained by well-known procedures, but for others there are no generally useful approaches. Consider, for instance, the determination of stoichiometry. As a rule, solids at high temperature are non-stoicbiometric. Much of the existing thermodynamic and transport data base has been estahlished assuming idealized line composition. As a result, the data are inadequate for describing and understanding high temperature phenomena such as composition variation and its consequences during crystal growth processes. Intemmetallic compounds such as CdS and LiNh03 are usually expected to have only fractional percentage deviations in their compositional ration. This is smaller than can he detected by chemical analysis. In
the LiNbO- instances. indirect measinements must be taken of compositional uniformity suited to a specific material. In sume rases it is essential that the index of refraction be extremely uniform throughout the crystal. The index in LiNhOBdepends on the ratio of Li02 and Nhz05 in the melt from which the crystals are grown. I t has been found by G. E. Peterson and coworkers at Bell Labs that a wide compositional ranee occurs for LiNbOs. If the crvstal is erown from a stoichiAetric melt with a 5'tempera&e fluctuation lasting only 1sec. the comnasitional variation will oroduce refractive index striations. his is a serious defect in'the material to be used in an optical harmonic generator. We have found similar results in our work a t Yale on Bi12Si020.A band of noticeahly darker material follows each temperature change of less than a degree Kelvin. The crystal growth process has been altered by this small change in growth conditions. To produce usefui homogeneous crystals of Bi12Si020. i t is necessmy to insure long-term stability of erowth oarameters within auite narrow tderances The combination of magnetic resonance characterization and special growth methods has resulted in the preparation of crystals suffic~entlyhomogeneous for optical devices. Other techniques for determining crystal characteristics, such as X-Ray and neutron diffraction, electron microscopy, and low enernv electron diffraction have achieved high - levels of sophis&ation. In principle, it is possible to use any physical measurements for characterization, hut meaningful cnntrol of properties only occurs when measurements are closely linked to preparation procedures. The best of all possible worlds would be to have measurements available of materials that are in critical stages of nreoaration. The ideal situation would be if measurements . . and evaluations on some critical material preparation steps could be done in situ. ex.. the aualitv . .of oxide durine the oxidation of silicon. such monitoring would enable direct control of processing techniques.
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Predictions and Directions for the Future Here I would like to consider the futures open to crystal technology. Perhaps the best way to do this is to reflect on the past. In 1945, there was no well-defined direction to the development of materials science. What is more important, the transistor had not yet been invented. A reasonable man might have decided to invest his monev in imnrovine the vacuum tube. Some wiser men decided that solidstate science was an important area to support. We see now that thev were correct .. in &ognizing the pnssible relevance nf this stiligrowing body nf knowledpe. With this cautionary lesson from the past we can speculate freely on the possible futures. I have a list of things I foresee in my synthetic crystal ball. The guidance for the future would be low voltage, high-efficiency, low cost, and clean. 1 ) Light Sources. Light emitting diodes (LED'S)could conceivably replace light bulbs at lower cost, lower voltage, and higher efficiency. 2) Optical Communication. The communications industry may be modernized by optical informationtransmission using laser light as a carrier. A given channel of laser light is able in theory of converting mueh more information than radio or microwaves because it is mueh higher in frequency. Modulation can therefore be done at higher frequencies and the information flux boosted. 3) lmproued Computers. W e already see desk calculators everywhere. However, we can use yet another advance in this ares. IAproved capacity, speed, reliability, and east reduction would make practical artificial intelligence, large scale predictions of weather, etc. 4) Prosthetic Deuiees for the Handicapped. The field of biochemistry involves the development of tiny radio transmitters that are swallowed or surgically embedded in tissue. This would enable monitoring of vital diagnostic information. 5 ) Low Pollution Tromportation. A long-lifeinexpensive catalyst for use in the internal combustion engine. 6)Law Pollution and New Energy Sources. Magnetohydrodynamic generation fuel cells, new batteries, solar energy and fusion generation Volume 57, Number 8, August 1980 / 539
all involve material and single crystal applications. 7) High Temperature Superconductors. A superconductor that would work at temperatures well above those of liquid hydrogen or nitrogen could transform power transmission. 8) X-Roy Lasers. X-ray Lasers would he of enormous use in numerous scientific fields. X-ray holograms for instance would phatograph the atomic structure of a crystal. 9) New Energy Storage. Materials with large pyroelectrie coefficients, electrolytes, molten salt materials with electronic and ionic conductivity in the range above 500° are all under investigation and are significant to the problem of meeting future energy needs. 10) Recycling, substitution, waste disposal. A challenge which should be welcomed hq all of us working in research. I n the more distant future I expect to see: 1) O ~ g a n i cSernieonduc~orsSemiconduetnr hehavior ha3 long been observed in organic materials. Work in t h ~area s may illuminateand be dluminated hy work on hiologicnl electrochemical systems, such as nerve cells and energy transd&ing membranes. 2) Liquid Crystals. Liquid crystals are like solids in some directions hut disordered like liquids in others. These are currently under investigation, and it is not unreasonable to expect that they will possess unsuspected useful properties. 3) Super-Thin Film. The surfaces of solids are actively being investigated. Better understandingof the chemically unsatisfied bonds in a surface could lead to specific control of the growth of layers of single crystals perhaps to the level of super-thin films only a few atoms in thickness. This would open manifold possibilities for new electronic devices and catalysts.
In order for these ideas to be realized, we must think toward the development of the solid state sciences. First, it should he mentioned that solid state chemistry and solid state physics may become more applications oriented, and research and develonment mav move more toward materials. This is not -. only necessary for materials scietye, hut also for those areas of society dependent on solid state technology to fmd the right direction. We must operate in close concert with physicists, device eneineers, and svstem engineers. Moreover, we should expect thk discoveries of newphenomena and increasing general soohistication in chemistry. second; another important requirement is a close and continuous interaction between the solid state chemist and the potential user-perhaps a physicist measuring electronic properties or a builder of devices (a philosophy very much intact in our department a t Yale). Much time and energy can be saved and much additional insight gained when a variety of investigative techniques are brought to hear on the same problem; and even more is gained if research is correlated with thn variables involved in the material prooerties. Third, as an experimentalist. l pers&dly believe that all inout should w m e from close interdisciplinary output and wi'th very close cooperation with theoreGcians studying the fundamental science of crystal growth, such as the Monte Carlo method and statistical mechanics. Both techniques can he anolied to computer simulation techniques to predict the rateb;fgrowth, shape, and perfection of a real crystal. Let me give an example of what I mean by a "real crystal!' (1) A suliiitate theorist is predicting some interesting properties of a new c r y t a l and they must be verified experimentally. If the crystal is pmr in quality, the predicted effects may be missed and the solid state theorist will not know if it were his.fault. The eenrration of a "real crvstal" in rhe sense that allows the exact confrontation between it is highly theory and practice. (2) If you have a complicated system such as the garnet, where you have 160 atoms per unit cell, it is difficult for the theorist to predict all the possible effects which might he incorporated within the crystal due to comnlicated structure. However, if the solid state chemist can design a crystal which will fit the same category as the garnet, hut with a much less complicated unit cell of, let us say only 12 atoms per unit cell, thL will allow rhesolid state theoretician to oredict all the oossible effectswhich one might expect from a much simplified structure of 12 atoms per unit cell as compared with 160 atoms per unit cell. -
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Conclusion In conclusion, we come to a question of "who should perform the crystal growth and material science of the future" and perhaps even more important is "who should pay for it?" This particular question is obvious only if society agrees that technology is good and if society wants technology to advance a t its present rate or kept a t its present level. Once this is clear, the answer is ohvious-It should be paid for by industry and government. At the moment, it is believed that private industry undersoends on solid state chemistrv and materials science, espe&dlyconsidering how many of its problems and opportunities involve material limitations. Historically, academia and industry had a productive relationship, each helping to support the other mission. However, the links between academia and industry weakened in the last two decades due to 3 principle factors underlining this decline: (1) the separation of university research from perceived industrial needs; (2) the decreased interest among university graduates in research careers: (3) and industrv's diminishine role in basic research. As a res&bf these andbther factors,-there has been a marked decline in the ties between academia and industries which is a very dangerous trend that has to be reversed. Academia a t the oresent has a numher of reasons for motivating the reassess&g of their ties with industry-a growing research interest in solving critical domestic problems and a renewed application for the role of industry in such problem solving. It seems that this might be a new platform on which new university-industry relations can he established in the form of research support, as a potential employer for advanced deeree students. as a source of oart-time facultv. and most importantly as focus on majo; continuing edication programs. Existine harriers to enhancing the universitv-industrv onnection sh&ld he abolished, or aileast modified to alimit where hoth sectors could mutually profit. The government should play a significant role in bringing the academic and private sectors together via such agencies as N.B.S., NASA, the Department of Commerce, the Department of Energy, and the National Science Foundation, ;o mention only ;few. The government should also play an interactive role: darn collection and mst.siment, standards and rerhnioues research la la National Hureau of Standards). .. government in-house defense, and fundamental material research on a scale to-nrovide imoetus for controlline officers. The universities'iole should continue to he the training eround for solid state chemists. We would like to see more chemical disciplines involved in that training. We know that the attention to solids is nil in our underaraduate program. Graduate students are ignorant of contakinations in-solid state chemistrv. Many of our problems and opportunities are uniquely chekical, and chem
