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Basics of the ProcessIn this section some well known facts about Czochralski crystal growth are presented. They seem to be required for a basic understanding of the difficulties encountered in diameter control.IntroductionThe main idea of the Czochralski-process was developed in the year 1918 by the Polish scientist Jan Czochralski (1885-1953). Henceforth, this process is called the standard process. Over the years, it has often been modified in order to embrace higher requirements. Two important variants of the standard process are the liquid encapsulated (LEC) and the vapour pressure controlled (VCz) process.
A crucible consisting of boron-nitride contains the molten gallium-arsenide (GaAs). A pulling rod holds the crystal which is pulled up carefully while rotating. To make dislocations grow out of the crystal the radius increases slowly in the area of the shoulder. Because of the surface tension the crystallization front, which
is the phase interface between solid and liquid GaAs, resides a bit above
the melt level. The liquid GaAs does not wet the crystal completely. In
fact it contacts the solid crystal under a certain angle The area of the melt below the crystal which is raised above the melt
level is called meniscus. The crystallization front has the temperature
of the melting point of GaAs (1511 Kelvin). The position of the crystallization
front raised above the melt is very important for the properties of the
growth process. If it is raised to far above the melt the crystal radius
decreases, otherwise it increases. Furthermore, the shape of the front should
be slightly convex (seen from the melt) to ensure dislocations growing
out.
Correct positioning of the crystallization frontThe main problem in diameter control is the unstable character of the crystallization front position which leads to a cylindrical crystal. If the diameter decreases, then the amount of heat which flows through the crystal decreases, too. As a result, the temperature in the area around the crystallization front increases; this forces the decrease of the crystal-diameter, and so on. Conversely, an increasing diameter leads to a greater heat loss through the crystal, the temperature decreases, and because of this the diameter increases. On the other side: An increasing diameter results in an increasing interface surface and as a consequence the amount of latent heat goes up. So, the latter effect slightly stabilizes the region!
Heat convection in the melt can be influenced by rotating both the crystal and the crucible about their common vertical axis. Furthermore, the crucible can be raised within the main heater's temperature field such as to achieve optimal heat input into the system. Both liquid and solid GaAs have a very high arsenic vapour pressure.
Therefore, GaAs tends to dissociation. To prevent evaporation of arsenic
from the melt it can be encapsulated under a layer of liquid boron-oxide.
Simultaneously, the process takes place under heightened pressure which
results in the need of a pressure chamber. A standard-Czochralski
process modified in such a way is called liquid encapsulated
Czochralski (LEC). By this invention it became possible to grow GaAs
or in general III-V-semiconductors with satisfying results using the Czochralski technique.
Importance of the temperature field
In order to produce crystals with minimum dislocation densities it is necessary
to have a nearly linear temperature field within the crystal. This means that partial derivatives of the temperature greater than first order are equal to zero. This condition can be approximately achieved by minimizing
the radial and axial temperature gradients. Particularly, the radial temperature
gradient has to be reduced to ensure an axial heat flow through the crystal which is
as linear as possible. Using the conventional LEC-process it is not possible
to achieve this aim when growing large diameter crystals. The temperature
profile within the crystal is very disadvantageous in this case, so the
amount of dislocations increases in a untenable way.
A minimization of the temperature gradients can be achieved by using new insulation materials for the heat-shield. Moreover, an additional gas proof chamber is used the temperature of which is about 1000 Kelvin. However, these steps, which indeed result in lower temperature gradients, diminish the compositional stability of the crystal: Arsenic begins to sublimate from the crystal, which starts to degenerate. To avoid this terrible effect an arsenic-source is used to inject arsenic into the atmosphere. Therefore, the crystal grows in thermodynamical equilibrium - it gets a mirroring surface. This modified LEC process is called the vapour pressure controlled Czochralski (VCz). It provides a key for growing large diameter GaAs-crystals with low dislocation densities using the Czochralski technique. It has to be remarked that it is -of course- possible to grow large diameter crystals with conventional LEC methods (or different process concepts), but the great amount of dislocations within such crystals forbids this type of processing for opto-electronic or epitaxial components.
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The figure in the right shows a sketch of the Cambridge Instruments
CI358 puller used at the 
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the so called contact-, wetting-, or meniscus-equilibrium angle.
