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.
- Introduction
- Correct
positioning of crystalization front
- The
importance of the temperature field
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.
 The figure in
the right shows a sketch of the Cambridge Instruments CI358 puller.
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 so called contact-, wetting-, or
meniscus-equilibrium 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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