A proposal for epitaxial thin film growth in outer space
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INTRODUCTION
THE utilization of space for materials processing has been limited to the use of the microgravity aspects of space, tJ'21 Recently, however, studies have led to the indication that another component of space, the vacuum component, can be significantly utilized for materials processing, t3'4j especially in the area of thin-film growth. Specific thin-film growth, described as epitaxial thin-film growth, has risen to importance recently due to the major promise of enhanced thin-film properties in new and unique materials systems. 15'6]In this article, we describe both the benefits of the new thin-film epitaxial technology as well as the major benefits that space ultra-vacuum can bring to that technology in application of epitaxial thin film in space. Epitaxial growth of thin-film materials has presently been exercised mainly through molecular beam epitaxy (MBE) ITJor chemical beam epitaxy (CBE). t81Epitaxy is the growth of thin crystalline films in which the substrate determines the crystallinity and orientation of the grown layer. This growth is accomplished in an atom-by-atom, layer-bylayer manner. Molecular beam epitaxy is the growth of thin film materials by the reaction of one or more thermal molecular beams with a crystalline surface under ultra-high vacuum conditions. Chemical beam epitaxy is the reaction of one or more gaseous beams with a crystalline surface under high vacuum conditions. Note that in both instances, high vacuum is of prominence in the growth of high quality, defect-free, single crystal films. The importance of high vacuum is readily seen in Figure 1 which describes the MBE/CBE process. Of detriment to the growth of high purity, high quality thin films is contamination from the background vacuum environment, contamination from the beam sources themselves, and segregation of impurities from the bulk. The minimization of this contamination as well as minimization of interdiffusion and desorption within the film/substrate region will result in high quality, high purity thin-film growth. Such growth is currently attempted in rather sophisticated and large ultra-high vacuum systems which generally incorALEX IGNATIEV, Director, and C.W. CHU, Past Director, are with Space Vacuum Epitaxy Center, University of Houston, Houston, TX 77004. This paper is based on a p r e s e n t a t i o n made in the s y m p o s i u m "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee. METALLURGICAL TRANSACTIONS A
porate multiple-growth chambers, analysis chambers, and interconnecting ultra-high vacuum tubes within which samples are transported from one chamber to the other (Figure 2). Such complexities are required for the practical growth of epitaxial thin films. Applications of these types of films are many and can be broken down into areas of semiconducting materials and devices, metallic materials
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