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Crystal Growth
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How do single crystals differ from polycrystalline samples?
Atomic arrays that are periodic in three dimensions, with repeated
distances are called single crystals. Single crystal specimens maintain
translational symmetry over macroscopic distances (crystal
dimensions are typically 0.1 mm – 10 cm).
Why would one go to the effort of growing a single crystal?
Structure determination and intrinsic property measurements are
preferably, sometimes exclusively, carried out on single crystals.
For certain applications, most notably those which rely on optical
and/or electronic properties (laser crystals, semiconductors, etc.),
single crystals are necessary. There would be no electronic industry,
no photonic industry, no fiber optic communications without single
crystal materials.
Growth of single crystals and their characterization towards device
fabrication have assumed great impetus (motivation) due to their
importance for both academic as well as applied research.
Some uses of Crystals
• Nonlinear optical crystals are very important for laser frequency
conversion. For example
• Potassium dihydrogen phosphate (KDP) is suitable for higher
harmonic generation of huge laser systems for fusion
experiments because it can be grown to larger sizes and also
KDP has a high laser damage threshold.
• Potassium titanyl phosphate (KTP) is a useful nonlinear optical
crystal to get efficient green light by the frequency doubling of
Nd:YAG laser. It has high optical nonlinearity, large
temperature and angular allowance and it is non hygroscopic
and mechanically hard.
METHODS OF Crystal Growth
• It is clearly more difficult to prepare single
crystal than poly-crystalline material and extra
effort is justified because of the outstanding
advantages of single crystals.
• Growth of crystal ranges from a small
inexpensive technique to a complex
sophisticated
expensive
process
and
crystallization time ranges from minutes,
hours, days and to months.
 Single crystals may be produced by the
transport of crystal constituents in the solid,
liquid or vapour phase. On the basis of this,
crystal growth may be classified into three
categories as follows,
Solid Growth -
Solid-to-Solid phase transformation
Liquid Growth - Liquid to Solid phase transformation
Vapour Growth- Vapour to Solid phase transformation
 An efficient process is the one, which produces
crystals adequate for their use at minimum cost.
 Better choice of the growth method is essential
because it suggests the possible impurity and other
defect concentrations.
 Choosing the best method to grow a given material
depends on material characteristics.
 Liquid growth includes both melt and solution
growth.
 A survey of the methods of growth suggests that
 Almost 80% of the single crystals are grown from the
melt
 Roughly 5% from vapour, 5% from low temperature
solution, 5% from high temperature solution, and 3%
from the solid and only 2% by hydrothermal
methods.
What factors control the size and purity of single
crystals?
Nucleation and Growth. If nucleation rates are slow and
growth is rapid, large crystals will result. On the other hand, if
nucleation is rapid, relative to growth, small crystals or even
polycrystalline samples will result.
What can be done to increase the growth rates?
In order to attain the rapid growth rates needed to grow
macroscopic crystals, diffusion coefficients must be large.
Hence, crystal growth typically occurs via formation of a solid
from another state of matter :
(a) Liquid (Melt) Solid (Freezing)
(b) Gas (Vapor) Solid (Condensation)
(c) Solution Solid (Precipitation)
• It should be noted that defect concentrations tend to
increase as the growth rate increases.
Consequently the highest quality crystals need to be grown
slowly.
What can be done to limit the number
of nucleation sites?
Several techniques are used separately or
in combination to induce nucleation of the
solid phase at a slow and controlled rate :
(a) Slow Cooling of Melts
(b) Temperature Gradients
(c) Introduction of Seed Crystals
Slow cooling of the melt
• With congruently melting materials (those which maintain the same
composition on melting), one simply melts a mixture of the desired
composition then cools slowly (typically 2-10 °C/h) through the melting
point.
• More difficult with incongruently melting materials, knowledge of the
phase diagram is needed.
• Very often, the phase diagram is not known. Consequently, there is no
guarantee that crystals will have the intended stoichiometry.
• Molten salt fluxes are often used to facilitate crystal growth in
systems where melting points are very high and/or incongruent
melting occurs.
• Crystals grown in this way are often rather small. Thus, this
method is frequently used in research, but usually not
appropriate for applications where large crystals are needed.
Growth from melt
Melt growth is undoubtedly the best method for growing large
single crystals of high perfection relatively rapidly and has been
extensively used for metals, semiconductors ionic crystals and a
few organic compounds. In semiconductors and laser host crystals,
impurities (dopants) can be deliberately added and
homogeneously dispersed in a large percentage of the grown
crystals. The method has been developed largely in electronics,
optics and synthetic gemstone industries.
Melt growth requires that the material melts congruently (i.e., it
does not decompose below or near its melting point) and has
Q1`
a manageable vapour pressure at its melting point.
The rate of growth of crystals by this method is mostly used in
commercial purposes. However, many hydrates and anhydrated
salts, organic salts and virtually all biological materials cannot be
grown from this method. The melt grown can be further
subdivided into various techniques.
Congruent and Incongruent Melting in
Binary and Ternary Systems
• The thermal behavior of intermediate
compounds is of three basic types: congruent
melting, incongruent melting, or dissociation.
• An intermediate compound is a combination
of the two end members of a binary or
ternary phase diagram that forms a different
component between the two solids.
• Congruency of melting is important in the
determination of phase analysis diagrams
and in drawing crystallization paths.