There are two main methods for growing silicon crystals for solar cells - Czochralski (CZ) and float zone (FZ). In CZ growth, a silicon seed crystal is dipped into molten silicon and slowly pulled to form a cylindrical ingot. FZ growth uses a floating molten zone to recrystallize a silicon rod. FZ crystals have higher purity, lifetime and efficiency but require higher quality feedstock. CZ is more commonly used due to lower costs. Improvements aim to increase growth rate, reduce energy use, and cut more wafers from each ingot to lower costs.

1. Crystals
There are numerous factors during crystal growth which affect both the size and quality of the
crystal. The most important in terms of organic compounds are purity, the solvent chosen for
recrystallization, the number of nucleation sites, mechanical agitation to the system, and time.
 Solvent: some generalizations
Typically, it is good to choose a solvent in which your compound is moderately soluble.
If the solute is too soluble, crystal size will tend to be small. Avoid solvents in which
your compounds form saturated solutions, because again, saturated solutions typically
yield crystals which are too small.
 Exceptions: When a compound is soluble in most liquids, then slow cooling of saturated
solutions can yield good single crystals. Be sure that the flask and solvent are dust free!
 Nucleation
The fewer "nucleation" sites at which crystals begin to grow results in fewer crystals,
generally of large size, which is desirable! Conversely, many nucleation sites result in a
smaller average crystal size, and it is not desirable. Dust in the laboratory, and
microscopic paper fillings (scraping cpd. from paper filters), are "thalidimide" to baby
crystals.
Minimize dust and extraneous particulate matter in the solution of the compund
and the growing vessel.
 Mechanics
Mechanical disturbance of the crystal growing vessel can result in ruining all of the above
efforts. Let the crystals grow with minimum disturbance. This means:
- don't try to grow crystals next to your vacuum pump
- don't check the progress of your crystals on a daily basis
 TIME!!
Crystals fully recognize that patience is a virtue and will reward those who practice
it.
Techniques
"Crystal growing is an art." Not.
The techniques chosen depends less on "art" and "intuition" than on the chemical properties of
the compound of interest:
 Is the compound hygroscopic?
 What are the compound's solubility properties?
 Is the compound air or light sensitive?
 What about decomposition?
 etc. etc. etc.

2.  Slow Evaporation
Simplest way ro grow compounds and works well for compounds that are not sensitive to
ambient conditions in the laboratory.
- Prepare a solution of the compound in a suitable solvent (saturated or nearly saturated)
- Filter the solution through a clean glass frit into a clean vessel* and cover, but not
tightly.
- Gently put the container in a quiet, out of the way place and allow the solvent to
evaporate slowly.
- This method works best when there is ample material to allow for at least a few milliters
of solvent.
 Slow Cooling
This method works well for solute-solvent systems which are less than moderately
soluble and the solvent's boiling point is less than 100oC.
- Working with a saturated solution: No Dust/Hair etc., Heat the solvent to boiling point
or slightly below it. Transfer by filtering through a warm frit into a vessel* (test tube, or
scintillation vial) and stopper tightly. Transfer vessel to flask in whcih hot water* (oil
may also be used) has been heated to a couple of degrees below the solvent boiling point.
- The water level should exceed the solvent level in the vessel but should not exceed the
height of the vessel (or you can use a thermostated oven).
 Variations on Slow Evaporation and Slow Cooling
If these techniques do not yield suitable crystals from single solvent systems, these
techniques can be expanded to binary, tertiary, and even quaternary solvent systems.
- The basic rationale is that by varying the solvent composition, growth of certain crystal
faces may be inhibited while the growth in some other direction is promoted, thus
yielding crystals of suitable morphology and size.
- Reproducibility is paramount in science! Growing crystals from multiple solvent
systems will be imprecise unless solvent compositions are recorded.... hence "art."
 Liquid Diffusion
very sucessful method for obtaining single crystals of organic compounds*
- A small amount of solution is placed in a tube, and a suitable precipitant is layered
carefully down the side of the tube onto the solution. It is very important that this second
liquid be less dense than (or visa versa*), and miscible with, the solvent.
- The volume ratio solvent:ppt will be variable, but a good place to start is 1:4 or 1:5.
- Slightly turbidity should form at the interface
- If there is no crystal growth after 24 hr, try a more concentrated solution
 Use of Seed Crystals
This method is useful when other methods provide crystals which, although of reasonable
quality, are too small. These small crystals can be used as "seed" crystals in a slow
cooling saturated solution.
- Draw the seed crystals in a pipette together with some mother liquor (do not allow the
seed crystals to dry!)
- Carefully deposit these "seed crystals" in the saturated solution.
The fewer the "seeds" the bigger the resulting crystals
 Convection
This is a less standard method, but one may attempt to grow crystals by convection by
creating a thermal gradient in the crystal growing vessel. The theory behind this method

3. is that the solution becomes more saturated in the warm part of the vessel and is
transferred to the cooler region where nucleation takes place. To create convection, one
must use either local heating ot local cooling.
- Trick to making this easy:
MeCN/Et2O hot frit
 Counterions
If your compound is ionic and is not giving suitable crystals with a given counterion,
perform a metathesis reaction to change the counterion.
- Ions of similar size tend to pack better and subsequently give better crystals
 Ionization of Neutral Compounds
If the compound of interest is neutral and contains proton donor or acceptor groups,
better crystals may be grown by first protonating or deprotonating the compound.
- The ionic form of the compound could take advantage of factors such as hydrogen
bonding to yield better crystals. Of course, this will alter the electronic properties of your
compound, but if a general conformation is what is needed from the structrue
determination, then this should not be a problem.
References
Crystals and Crystal Growing, Alan Holden and Phylis Singer, Anchor Books-Doubleday, New
York, 1960.
The Growth of Single Crystals, R. A. Laudise, Solid State Physical Electronic Series, Nick
Holonyak, Jr. Editor, Prentice-Hall, Inc., 1970.
For another excellent article on growing X-ray quality crystals, see Guide to Crystal Growth
(Texas A&M).
Crystal Growth:
References:
The Science and Art of Si Crystal Growth
Silicon Crystal Growth and Wafer Production
Crystal Growth from CAESAR
Czochralski (CZ) single silicon crystal growth
"The most common technique used for growing crystals for the development of wafers is the
Czochralski growth. There are other methods of growing crystals but the Cz is most common. The
material used in growing a single crystal silicon ingot is electronic grade silicon(EGS), which is refined
from MGS and must have 99.999999999% purity.

4. "When the polysilicon,defined as containing many crystals, is transported to the wafer
fabrication station or wafer Fab., it is placed in a fused quartz crucible that would dissolve
during the crystal growing process. After the polysilicon is loaded in the crucible, it is then
placed in an evacuated chamber that would be filled with an inert gas such as Argon, when
sealed. While the inert gas is released in the chamber, the crucible is heated to about
1500°C causing the EGS material to melt. As the polysilicon is melting, a seed crystal about
.5cm in diameter and 10cm in length is placed in a rotating shaft. The seed crystal would
then be rotated in the molten silicon to develop a silicon ingot or "boule."
"When the polysilicon(EGS) is melted to a right consistancy, the seed crystal is then
lowered into the melted material and the tip just penetrates below the surface of the molten
silicon. The shaft is then rotated counterclockwise and the crucible is rotated clockwise. As
the shaft is rotating, the crystal seed is slowly pulled away from the molten, developing an
ingot. The size of the ingots are usually 1 to 2 meters long and can achieve a maximum
diameter of 200mm. By carefully controlling the temperatures and rotating speeds of the
crucible and rod, that a precise diameter of the crystal is maintained. While the ingots are
pulled, it is cooled to form it into a solid state. The length of the ingot is determined by the
amount of molten silicon there is in the crucible.
"During crystal growing, an amount of dopant is added for the desired charge it is needed
for a device. As the silicon is melted, a specific amount of dopant is added, depending if a P
or N device is needed. In the CZ method, the impurities are usually added and dissolved in
the melt using solid impurities. For donors, elements such as phophorous and arsenic is
added. For acceptors, boron is usually the additive. By adding these dopants, they increase
the concentration for mobile carriers and that increases the conductivity of the
device." (source)

5. (source)

6. (source)
Floating Zone (FZ) single silicon crystal growth

8. Q: Methods of crystal growth
A: Crystal growing methods - University of Southampton
www.southampton.ac.uk/xray/links/crystalgrowth/grow1.ht...
Description of various crystal growing methods for use in x-ray diffraction. Read More »
A: Crystal Growth?
www.ask.com/web-question/crystal-growth
Crystals are fun to grow and many people even make a hobby out of it. The growth rate of crystals will
depend several factors, including what they are grown from Read More »

9. A: chapter 1 introduction to crystal growth methods - Shodhganga
shodhganga.inflibnet.ac.in/bitstream/10603/41/4/chapter...
In this section, various methods of crystal growth with emphasis on low temperature solution growth
technique are described. The solvent to be chosen to grow ... Read More »
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A: Temperature Affect Growth Crystals?
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Popular Q&A
Q: What Is the Link Between Purity & Crystal Growth?
A: Whenever a pure substance crystallizes under the same conditions, the same crystalline form results.
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Q: What methods are there for separating crystals?
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a particular solvent, use Read More »
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ilicon Info: Single-Crystal Ingot Growth
The single-crystal growth methods, float-zoning (FZ) and Czochralski growth (CZ), are relatively well-known,
so only some aspects pertinent to PV applications will be addressed here. The table below

11. compares the characteristics of the FZ and CZ methods. There are two principal technological
advantages of the FZ method for PV Si growth. The first is that large  values are obtained as a result of
higher purity and better microdefect control, resulting in 10% to 20% higher solar cell efficiencies. The
second is that faster growth rates and heat-up/cool-down times, along with absences of a crucible and
consumable hot-zone parts, provide a substantial economic advantage. The main technological
disadvantage of the FZ method is the requirement for a uniform, crack-free cylindrical feed rod. A cost
premium (100% or more) is associated with such poly rods. At the present time, FZ Si is used for
premium high-efficiency cell applications and CZ Si is used for higher-volume, lower-cost applications.
Schematic of CZ Growth Photo of FZ Growth Schematic of FZ Growth
Comparison of the CZ and FZ Growth Methods
Characteristic CZ FZ
Growth Speed (mm/min) 1 to 2 3 to 5
Dislocation-Free? Yes Yes
Crucible? Yes No
Consumable Material Cost High Low
Heat-Up/Cool-Down Times Long Short
Axial Resistivity Uniformity Poor Good
Oxygen Content (atoms/cm3) >1x1018 <1x1016
Carbon Content (atoms/cm3) >1x1017 <1x1016
Metallic Impurity Content Higher Lower
Bulk Minority Charge Carrier Lifetime (s) 5-100 1,000-20,000
Mechanical Strengthening 1017 Oxygen 1015 Nitrogen
Production Diameter (mm) 150-200 100-150
Operator Skill Less More
Polycrystalline Si Feed Form Any Crack-free rod

12. Electrical power requirements for these two methods are on the order of 30kWh/kg for FZ growth and 60
kWh/kg for CZ growth in the IC industry. The more cost-conscious PV industry has been achieving 35-40
kWh/kg for CZ growth, and some recent experiments indicate that levels on the order of 18 kWh/kg may
be achieved for 150-mm-dia. crystals by using improved insulation materials and lower argon-gas flow
rates (Mihalik et al., 1999). Not only were energy requirements reduced, but also argon consumption was
reduced from 3 m3/kg of Si to 1 m3/kg of Si. Also, oxygen content in the crystals was reduced by 20%,
the crystal growth rate was increased from 1.28 kg/hr to 1.56 kg/hr, and relative solar cell efficiency
increased by 5%.
In CZ Si PV technology, approximately 30% of the costs are in the crystal ingot, with 20% in wafering,
20% in cell fabrication, and 30% in module fabrication. High-speed wire saws that can wafer one or more
entire ingots in one operation have greatly improved the throughput of the wafering process. A wire saw
can produce about 500 wafers/hour compared to about 25 wafers/hour for older inside-diameter (ID) saw
technology. Furthermore, it creates shallower surface damage (10 m) than the ID saws (30 m), and
allows thinner wafers to be cut, thus increasing the number of wafers per ingot. Currently, about 20
wafers are obtained from 1 cm of ingot. Efforts are under way to obtain 35 wafers/cm. Problems with
increased breakage are seen with the thinner wafers - especially in the sawing process. At 20 wafers/cm
and a wafer thickness > 300 m, breakage is on the order of 15%. This can rise to on the order of 40%
when the wafer thickness is decreased to 200 m. It is clear that wafer handling will be an important
issue as wafers become thinner.
One clever way of dealing with the low fracture strength of thin (100) wafers from single-crystal CZ ingots
is to deliberately introduce a controlled multicrystalline structure into the growing ingot. In particular, the
tricrystalline structure described by Martinelli and Kibizov (1993) provides three grains propagating along
the length of the ingot. Each has a <110> longitudinal direction. Two of the grain boundaries are first-order
{111} twin planes, and the third is a second-order {221} twin plane. The three angles between
boundaries are thus 125.27o, 125.27o, and 109.47o. The twins block any {111} planes from crossing
entirely across the ingot, and improve the resistance to cleavage or propagation of defects that takes
place on {111} planes. Wafers from these tricrystals are observed to possess about 440 MPa fracture
strength compared to about 270 MPa for (100) single-crystal wafers and 290 MPa for multicrystalline
wafers (Endros et al., 1997). The measurements were made on wafers after etching in KOH at 100oC to
310-m thicknesses. Breakage during wire sawing of tricrystal ingots at <200 m thickness is half of that
for <100> ingots. The tricrystal ingots have been shown to maintain their structure for reasonably long
lengths (150-400 mm at the present state of technology) with minimal degradation of minority carrier
recombination properties (Wawer et al., 1997).
________________________
Endros, A.L., Einzinger, R., and Martinelli, G. (1997) 14th European Photovoltaic Solar Energy
Conference Proceedings, Barcelona, 112.
Martinelli, G. and Kibizov, R. (1993) Appl. Phys. Lett. 62, 3262.
Mihalik, G., Fickett, B., Stevenson, R., and Sabhapathy, P. (1999) Presentation at the 11th American
Conference on Crystal Growth & Epitaxy, Tucson August 1-6. To be published, J. Crystal Growth
Wawer, P., Irmscher, S., and Wagemann, H.G., (1997) 14th European Photovoltaic Solar Energy
Conference Proceedings, Barcelona, 38.