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Toward a Microgravity Research Strategy (Appendix C) Toward a Microgravity Research Strategy C Electronic Materials STATUS The field of electronic materials encompasses the understanding and control of microstructure and composition to achieve the electrical and optical properties required by high-performance computer and communications systems. Most work today is performed in silicon- and gallium arsenidebased systems in which device microstructures must be fabricated over progressively smaller dimensions. Thus the major research questions center on the measurement and manipulation of materials to realize device and circuit microstructures down to the level of atomic monolayers. Another emerging area is the use of micromachining methods to fabricate semiconductors for such applications as sensors and detectors. In these areas, materials preparation and synthesis techniques revolve mainly around thin-film deposition and patterning. None of these techniques benefits from the study of phenomena under microgravity conditions because of REPORT MENU the inherently small dimensions, reduced pressures, and relative insensitivity to NOTICE gravitational acceleration disturbances. MEMBERSHIP SUMMARY However, the preparation of bulk semiconductor and optical crystals has CHAPTER 1 an important second-order influence on the control of microelectronic materials CHAPTER 2 properties. Bulk crystals serve as substrates for epitaxial-film deposition and/or CHAPTER 3 growth and in limited applications for the direct processing of devices and CHAPTER 4 integrated circuits. The major research questions here include contamination and APPENDIX A gettering effects, compositional uniformity, wafer surface flatness and/or defects, APPENDIX B and electrically active, structural micro-defects. In the industrial growth of bulk APPENDIX C APPENDIX D crystals, compositional control is mostly achieved through control over APPENDIX E segregation and convection phenomena at the growth interface. For these APPENDIX F phenomena, gravitational acceleration exerts a major influence through interaction with fluid density gradients caused by inhomogeneities of temperature or composition. Moreover, studies in microgravity have the potential to elucidate the nature of these phenomena at gE, so that they can be controlled more effectively on Earth. Another important influence of gravitational acceleration in the crystal growth of bulk electronic materials is in containerless processing. Molten floating file:///C|/SSB_old_web/cmgr92appendc.htm (1 of 6) [6/18/2004 11:09:57 AM]
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Toward a Microgravity Research Strategy (Appendix C) zones have been used for many years to prepare single crystals, for which container reaction and contamination are major concerns. However, on Earth, the vertical height of a molten floating zone is limited by the ability of the melt surface tension to counterbalance the hydrostatic pressure effects of gravity. In microgravity, the length of a static molten column is limited only by shape instability criteria. Thus there is greater latitude to manipulate molten floating zones for bulk solidification in microgravity than on Earth. MAJOR RESEARCH ACCOMPLISHMENTS Only the U.S. microgravity research program is reviewed here. So far, the work comprises only a few hundred hours of experimental time, as opposed to much greater experience with ground-based experiments. Also, the use of space for vacuum processing by employing an orbiting molecular wake shield is not considered here, because there are large uncertainties about the gas pressure and dynamics in such an environment and because laboratory conditions on Earth have continued to improve. The materials selected for study to date have been mostly model systems or have been scaled to small dimensions to accommodate on-orbit power limitations. Thus, when combined with the limited flight experiment opportunities, little new information has been acquired in this area that is directly relevant to the wider set of research priorities in electronic materials. However, the work reported here does contribute to our basic understanding of bulk crystal growth processes, both through theoretical studies of heat and mass transport as well as the experimental study of model systems. The paradigm for these studies is use of the microgravity environment to decouple the interferences of gravitationally driven fluid flows from the effects produced by other forces. As of January 1991, there were seven ground-based investigations and four flight experiments related to electronic materials in the U.S. microgravity research program. The principal results can be summarized according to the growth method: melt, vapor, or solution growth. Melt Growth Melt growth experiments have focused on solid solution systems (PbSnTe, HgCdTe) in which density gradients due to both temperature and concentration can interact to drive flow. Up to now, experiments have revealed solute segregation and convection phenomena by means of comparison between Earth-based and space-based conditions as well as by comparison with theoretical models. In Pb(1 - x)Sn(x)Te, the rejection of solute on solidification leads to a dynamically unstable density gradient, and complete mixing was observed in the melt under low-gravity conditions. However, acceleration levels file:///C|/SSB_old_web/cmgr92appendc.htm (2 of 6) [6/18/2004 11:09:57 AM]
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Toward a Microgravity Research Strategy (Appendix C) were not measured during the growth. Also, the apparatus suffered thermal control problems. In addition, it would have been most useful for the solid-melt interface to have been delineated in order to aid analysis. In Hg(1 - x)Cd(x)Te, for which the rejected solute raises the density and lowers the melting point, no space experiments have been flown; however, extensive ground-based experiments and analysis have demonstrated diffusion-controlled axial segregation for small sample sizes, but large lateral concentration differences. The reasons for this variation are not entirely understood, and low-gravity experiments would give major insight into successful growth conditions for compositional control. For these experiments to be successful, the thermal, compositional, interfacial, and accelerational environments must be controlled and monitored more carefully than they have been in previous experiments. There has been a trend toward the use of heteroepitaxial growth of solid- solution semiconductor systems on simple elementary or binary compound substrates. For example, HgCdTe is now grown as epitaxial films on CdTe substrates. During epitaxial growth, forced convection is used very successfully to improve compositional control, and there is no need to resort to the complex control of bulk melt thermal and solutal parameters; hence, it is unnecessary to use the microgravity environment to study or enhance the crystal growth of these solid solution materials. Vapor Growth Vapor growth of the HgI2,GeSe, and HgCdTe systems has been studied in detail. In HgI2 growth, space experiments have shown that less dislocation motion occurred than in ground-based growth, presumably due to the smaller self- deformation at lower accelerations. The electrical properties of the space-grown crystals improved; both electron and hole mobilities increased by a factor of two, and lifetime increased by a similar factor. However, the results could not be reproduced because these properties change with time when the crystals are not properly coated. The enhancement in properties by growth at low accelerations seems real, but the causes of the improvement are not well understood, especially since subsequent mechanical and optical handling are also major factors affecting the performance of this material. Until the measurement and handling protocols arc better determined, it does not appear as if more space experiments will lead to more useful information. In vapor growth of GeSe crystals, space experiments have shown that reduced nucleation and larger crystals can be achieved in closed-tube configurations. Comparison to theory showed that when gas-phase reactions were eliminated, diffusion-limited transport was accomplished in the vapor phase. However, the improved size, surface morphology, and homogeneity of the space- grown crystals are of limited interest since there are no applications for such bulk file:///C|/SSB_old_web/cmgr92appendc.htm (3 of 6) [6/18/2004 11:09:57 AM]
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Toward a Microgravity Research Strategy (Appendix C) crystals. Similar experiments with HgCdTe have been started on the ground by using the information gained from the GeSe system; improvements in both chemical homogeneity and crystalline quality have been realized, and the gas- phase chemistry has been analyzed theoretically. The HgCdTe system would benefit from space experiments yielding results for comparison. It also has been proposed that epitaxial vapor growth experiments be conducted with the HgCdTe/CdTe system. Caution should be exercised here since, as for epitaxial growth from melts, forced convection produces excellent results and it is not clear that low gravity offers any advantage. Solution Growth Solution growth experiments have been conducted for triglycine sulfate (TGS) in space. This material is used for pyroelectric detection of infrared radiation, and the sensitivity of the material is reduced by crystalline imperfections. However, the origins of the loss are not understood. The space experiments were designed to observe fluid flow effects during growth. This experiment was more significant from the point of view of residual fluid motions observed in space than for any improvements in crystalline quality or performance. The experiment is also too complex; the solution is sulfuric acid and must be contained under low-acceleration conditions. Further work in this system has been discontinued because of the small scientific payoffs for the large investments. In essence, aqueous solution growth systems are models for which fluid motion can be directly correlated to crystal growth phenomena. However, there are far better systems that are easier to handle and characterize than TGS; protein crystal growth is one of them. Otherwise, solution growth has no known value to microelectronic materials. RESEARCH PROSPECTS AND OPPORTUNITIES The results to date are not of widespread interest to the microelectronic research community because the research topics do not address important contemporary issues in microelectronics. Typically, bulk crystals are used as substrates upon which epitaxial films are grown. These epitaxial films are the most widely used material for fabricating microelectronic devices and circuits. There is a weak relationship between the properties of the bulk substrate and the epitaxial film. However, the major research issues relate to control of the atomic layer required in epitaxial film growth. Therefore, bulk crystals, such as those studied in microgravity, are not of primary interest. Opportunities consist of using model systems to explore the fluid phenomena that occur in crystal growth and that result in measurable changes in crystal composition, microstructure, or electrical properties. These measurements file:///C|/SSB_old_web/cmgr92appendc.htm (4 of 6) [6/18/2004 11:09:57 AM]
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Toward a Microgravity Research Strategy (Appendix C) can be compared with theoretical models of the fluid flow and segregation phenomena in order to enhance our fundamental understanding of these basic processes. However, the choice of the model system should be dictated by scientific principles and not by the expediency of its relationship to any commercial class of materials. Therefore, experiments should be supported and evaluated on the principle of increased understanding of processes. The microgravity program should emphasize processing science and deemphasize categorization of activities by disciplinary areas, such as microelectronics, for which the contributions must be judged against a far more comprehensive scientific base. Last update 7/13/00 at 1:56 pm Site managed by Anne Simmons, Space Studies Board file:///C|/SSB_old_web/cmgr92appendc.htm (5 of 6) [6/18/2004 11:09:57 AM]