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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
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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
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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
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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
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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.
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