Floating Zone Crystal Growth

A similar technique can be used to grow crystals without wall contact by melting a narrow zone in the solid that can be contained by the surface tension of the melt. A dimensionless group called the Bond number relates the hydrostatic pressure in the floating zone to the capillary or surface tension pressure containing the melt. The Bond number Bo is defined as 

As one might imagine, the analysis of the composition after repeated zone passes is not straightforward. The governing equation is 

For n = l, the C i = Cq and Equation 13.15 reduces to Equation 13.12. A number of analytical solutions to Equation 13.15 have been found that involve double and triple series (see Pfann, Zone Melting). Milliken was able to simplify the solution to a single series. 

The initial concentration after n zone passes for different k values is shovm in Table 13.1. It may be seen that the number of passes needed to achieve high purity goes up quickly with increasing k as would be expected. Often the material to be purified is placed in a long 

Zone purification after n passes for fc = 0.2. Not shown is the portion L-l where the rejected material piles up at the end. 

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Float zone crystal growth, silicon purification via, 22 496 Float zone refining, of silicon, 22 492-493 Flocculant addition point, 11 639 Flocculant chemistry, selection of, 11 638 Flocculants, 8 709-710 16 659. See also Flocculating agents dispersants contrasted, 8 687 organic, 11 627-631... 

The primary application for floating-zone melting is crystal growth rather than purification. Semiconductor-grade siUcon is not purified by zone refining siUcon chlorides are distilled and then reduced with hydrogen. 

A. Muehlbauer, A. Muiznieks, J. Virbulis. Analysis of the dopant segregation effects at the floating zone growth of large silicon crystals. J Cryst Growth 750 372, 1997. 

Crystal Growth of Borides 6.7.4.2. Liquid-Phase Methods B.7.4.2.2. Floating-Zone Technique. 

Overview of Unit Operations. To maximize the electron or hole (carrier) mobility and thus device speed, ICs are built in single-crystal substrates. Methods of bulk crystal growth are therefore needed. The most common of these methods are the Czochralski and float-zone techniques. The Czochralski technique is a crystal-pulling or melt-growth method, whereas the float-zone technique involves localized melting of a sintered bar of the material, followed by cooling and, thus, crystallization. 

Figure 1. Schematic diagrams of several commonly used systems for melt crystal growth of electronic materials (a) vertical Bridgman, (b) Czochralski, and (c) small-scale floating-zone systems.

Transitions from steady-state to time-dependent surface-tension-driven motions are well known also and are important in meniscus-defined crystal growth systems. For example, the experiments of Preisser et al. (51) indicate the development of an azimuthal traveling wave on the axisymmetric base flow in a small-scale floating zone. 

In addition, the structure and properties of point defects at low temperatures and at high temperatures may be different (29). The observation of extrinsic-type dislocation loops in dislocation-free, float-zone Si indicate that self-interstitials must have been present in appreciable concentrations at high temperature during or after crystal growth (30, 31). However, it is unclear whether these self-interstitials were present at thermal equilibrium or were introduced during crystal growth by nonequilibrium processes. 

The next step is to produce nearly perfect single-crystal boules of silicon from the ultrapure polycrystalline silicon. Many techniques have been developed to accomplish this, and they all rely on a similar set of concepts that describe the transport process, thermodynamically controlled solubility, and kinetics [8]. Three important methods are the vertical Bridgman-Stockbarger, Czochralski, and floating zone processes, fully described in Fundamentals of Crystal Growth by Rosenberger [9]. 

Bulk Crystal Growth Czochralski Bridgman Float zone... 

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