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The basic idea in float zone (FZ) crystal growth is to move a liquid zone through the
material. If properly seeded, a single crystal may result. |
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The method was first used for purification (zone melting),
taking advantage of the small segregation coefficients of many impurities.
The impurities contained in the feed material would then prefer to remain in the melt and thus could be swept to the end
of the feed stock. |
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If properly done, the newly crystallizing material could be obtained as a single crystal. Again, it was
the Siemens AG that pioneered the use of FZ crystal growth for the production of
high-quality Si single crystals. |
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Since the melt never comes into contact with anything but vacuum (or inert gases), there is no incorporation
of impurities that the melt picks up by dissolving the crucible material as in the CZ
crystal growth method. This is especially true for oxygen, which can not be avoided in CZ crystal growth. FZ
crystals therefore are always used when very low oxygen concentrations are important. |
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The problem of FZ crystal growth is clear, however, if one looks at the drawing: How do we keep
the liquid Si from just collapsing? If it would only be held in place by surface tension,
the maximum diameter of crystals possible in this way would be about 20 mm - not very useful. There are, fortunately,
other stabilizing mechanisms, and drawing the liquid zone through a "hole" - as indicated - also helps. Still,
for large diameter crystals the difficulties grow rapidly and FZ crystal growth is rarely (if at all) used for diameters
larger than 150 mm. |
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© H. Föll (Electronic Materials - Script)
Czochralski Crystal Growth Process 
With frame