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Courtesy of Martin Luther University,
Halle, Germany
The Bridgman technique is used to produce various semiconductor materials
in crystalline form. Growing semiconductor crystals in outer space is
influenced by the strength and direction of the residual gravity vector.
Drs. P. Reinshaus and W. Seifert have applied FIDAP to study the growth
of BiSbTe3 mixed crystals under normal and microgravity conditions
(financial support by: Deutsches Zentrum für Luft- und Raumfahrt,
grant number 50WM9329-7).
Modeling Microgravity
To mimic the actual processing conditions used in space, the researchers
applied boundary conditions in the model corresponding to furnace temperature
profiles measured in experiments conducted aboard the space station Mir
in February, 1997. They then studied the effects of gravity level on the
buoyancy driven flow in the Bridgman ampule and the resulting changes
in radial and axial solute segregation. They also compared horizontal
and vertical orientations of the ampule, and determined the influence
of small changes in tilt angle for normal gravity vertical Bridgman growth.
The three-dimensional FIDAP model included fluid flow, heat transfer,
species transport and solute segregation across the melt interface. A
transient hydrodynamic moving-boundary problem was solved to simulate
thermosolutal convection and component distribution in the melt close
to real processing conditions. A typical mesh included about 85,000 finite
elements.
Asymmetric Growth Conditions
Key findings of the study include the fact that experimentally unavoidable
tilt angles as small as 0.5° cause non-axisymmetric growth conditions.
The fluid flow caused by natural convection was found to be more vigorous
in the horizontal orientation due to larger density differences in the
axial direction. Solutal forces are relatively small on a global basis,
but they influence the velocity field locally near the melt interface.
See the references for further details.
In the future, FIDAP's prediction of the axial and radial solute segregation
profiles will be compared with the values from the experiments aboard
Mir.
References:
A. Bachran, P. Reinshaus, W. Seifert: Cryst.Res.Technol. (33) 1, 27-36 (1998)
W. Seifert, P. Reinshaus, A. Bachran: Cryst.Res.Technol. (33) 6, 899-910 (1998)
W. Seifert, P. Reinshaus: Cryst. Res. Technol. (33) 8 (1998), accepted
P. Reinshaus, W. Seifert, W. Heiliger, A. Bachran: 46th Int. Astronautical
Congress, abstracts IAF-98-J.3.06, Melbourne, Australia 1998
Velocity vectors (left) show a double cell flow pattern,
driven by natural convection induced by the Bridgman temperature
profile. The tellurium distribution (right) shows enrichment along the
centerline at the melt surface.
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