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Czochralski Method

Czochralski Method
Course

Crystallography: Morphological Properties of crystals

22 Documents
Students shared 22 documents in this course
Academic year: 2021/2022
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University of Mysore

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Czochralski Method

The technique of pulling from the melt first practiced by Czochralski in the year 1917. It is refered to as Crystal pulling method which likely to produce the highest quality crystal of most perfect and homogeneous. The crystals are free from physical constraints imposed by the crucible. Crystal pulling is applicable only to materials which melts congruently or nearly congruently. Crystal pulling is usually faster than Bridgman-Stockberger growth, so that less contaminatit on occur. Crystal pulling makes more demands on the equipment than Bridgman-Stockberger growth. Pulled crystals are more expensive than Bridgman-Stockberger crystals. Pulling is therefore used when the greater perfection attainable is needed.

The criteria that must be fulfilled for successful pulling are:

  1. The crystal should melt congruently without decomposition.

  2. The crystal should not be reactive with the crucible or the atmosphere present during pulling. In a closed apparatus inert, oxidizing or reducing atmosphere may be provided.

  3. The melting temperature should be attainable with the heaters available and should be below the melting temperature of the crucible.

  4. It should be possible to establish a combination of pulling rate and thermal gradients where single crystal material can be formed.

The principal advantage of pulling is that growth can be achieved on the seed under conditions of very good control. Good control results from the fact that the seed and grown crystal are visible during growth. Growth in any given direction is usually easy to obtain when oriented seeds are available. When a seed is not available, spontaneous nucleated crystallization is usually started on a wire results in polycrystalline mass. Once seeds are available it is preferable to start growth on a seed of comparatively small diameter

and then to increase the diameter of the crystal slowly by lowering temperatures until the desired diameter is obtained. Initial nucleation is usually better controlled on small area seeds.

Czochralski first applied the pulling technique for low melting point metals such as tin, lead and zinc.

Over the years the techniques has been used for congruently melting compounds of all classes but perhaps its widest application is to semiconductors. This technique is an important commercial technique in present day semiconductor technology.

For Czocharalski growth the following equipment is required.

  1. A means of heating the melt and controlling melt temperature and gradients.

  2. A means of containing the melt - crucible.

  3. A means of holding, rotating and withdrawing the seed.

  4. A means of controlling the atmosphere if air is not a compatible atmosphere.

The main heating means are radio-frequency (RF) heating and resistance heating. Radio frequency heating requires either that the melt or crucible be conductive enough to couple to the radio frequency field or that a susceptor be used to couple to the field and that the crucible and melt be heated by the susceptor. A common frequency is 450 kc (c=261Hz) and a useful power to heat a 50 cm quartz crucible contained in a graphite susceptor to the melting point of Ge (937oC) is 5kW.

manually lowered and raised for growth to begin at an appropriate level.

Thermal stability during growth is essential to the production of good quality crystals. Changes in gradient lead to changes in instantaneous growth rate as the interface seeks the melting point isotherm. Thus it is important to achieve the best possible temperature control. Similar changes in conditions occur as the melt level falls in the crucible and as the crystal diameter and length change. In addition to these changes there are short term fluctuations in temperature of the melt caused by non-uniform thermal convection. It would seem that many of the imperfection associated with pulled crystals have their origin in the short term thermal fluctuations that are almost always present in such system.

The basic method is simple. A typical routing is the following.

a) The melt is raised to a temperature a few degrees above the melting point.

b) The seed crystal, rotating slowly is brought slowly into contact with the melt surface, and lowering is stopped. If the melt temperature was its optimum value, the seed melts slowly and at a decreasing rate so that the seed remains in contact with the melt. If the seed does not remain in contact, the melt temperature was too high. If visible crystal growth occurs, the temperature was too low and must be increased until all the grown material and a bit of the seed have melted.

c) After a short pause – typically 1 to minutes, pulling is commenced at a slow rate. New crystals should grow with a diameter which is slightly less than the seed diameter.

d) The growth rate and rotation rates are increased to their final values. The diameter of the growing crystal should decrease. If a dislocation free crystal is required, a long narrow neck is grown.

e) After completion of the neck, the melt temperature is lowered slowly. The diameter of the crystal increases.

f) Growth at a constant diameter is maintained until the desired length of the crystal has grown.

g) Growth is then terminated. In some cases this is done by sharply increasing the pulling rate, so that the crystal breaks contact with the melt.

In order to maintain a constant diameter it is usual to increase or decrease the power input to the melt. An increases in the power decreases the crystal diameter.

Crucibles are usually cylindrical, sometimes with a small taper, the open end is of slightly larger diameter than the closed one. The base of the crucible may be flat over most of the diameter, with a curved rather than abrupt junction with the cylindrical portion.

Crucible almost invariably have ratios of height to diameter in the range 0 to 1 with a marked preference for the range 0 to 1.

The Czochralski process lends itself to produce crystals with length- to-diameter ratios which greatly exceeds unity. And the diameter of the crystals rarely exceeds half the crucible diameter.

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Czochralski Method

Course: Crystallography: Morphological Properties of crystals

22 Documents
Students shared 22 documents in this course

University: University of Mysore

Was this document helpful?
Czochralski Method
The technique of pulling from the melt first practiced by Czochralski
in the year 1917. It is refered to as Crystal pulling method which likely
to produce the highest quality crystal of most perfect and
homogeneous. The crystals are free from physical constraints
imposed by the crucible. Crystal pulling is applicable only to materials
which melts congruently or nearly congruently. Crystal pulling is
usually faster than Bridgman-Stockberger growth, so that less
contaminatit on occur. Crystal pulling makes more demands on the
equipment than Bridgman-Stockberger growth. Pulled crystals are
more expensive than Bridgman-Stockberger crystals. Pulling is
therefore used when the greater perfection attainable is needed.
The criteria that must be fulfilled for successful pulling are:
1. The crystal should melt congruently without decomposition.
2. The crystal should not be reactive with the crucible or the
atmosphere present during pulling. In a closed apparatus inert,
oxidizing or reducing atmosphere may be provided.
3. The melting temperature should be attainable with the heaters
available and should be below the melting temperature of the
crucible.
4. It should be possible to establish a combination of pulling rate
and thermal gradients where single crystal material can be formed.
The principal advantage of pulling is that growth can be achieved on
the seed under conditions of very good control. Good control results
from the fact that the seed and grown crystal are visible during
growth. Growth in any given direction is usually easy to obtain when
oriented seeds are available. When a seed is not available,
spontaneous nucleated crystallization is usually started on a wire
results in polycrystalline mass. Once seeds are available it is
preferable to start growth on a seed of comparatively small diameter

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