How does float zoning produce nearly perfect single crystals of silicon?

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zone melting Figure 1: Schematic representation of (A) normal freezing, (B) zone refining

Float zoning is a specialized zone melting technique designed to produce ultrapure silicon by eliminating the risk of contamination from a container. In this process, a vertical silicon rod is held by end clamps, and a short molten zone is created using induction heating.

The molten zone is held in place by its own surface tension, which prevents the liquid from collapsing and keeps it from contacting any container walls that could introduce impurities. As this molten zone is moved slowly along the length of the rod, it redistributes impurities and allows the silicon to recrystallize. By avoiding container contact and utilizing the principles of zone refining, this method successfully produces nearly perfect single crystals of ultrapure silicon for use in transistor manufacturing and other semiconductor applications.1