Crystal Growth - PowerPoint Presentation

Slide1

Crystal Growth

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.

T

here

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.

Slide2

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.

Some uses of CrystalsSlide3
Slide4

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. Slide5

Solid Growth

- Solid-to-Solid phase transformation

Liquid Growth - Liquid to Solid phase transformation Vapour Growth- Vapour to Solid phase transformation 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, Slide6

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.

Slide7

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) àSolid (Freezing)(b) Gas (

Vapor

)

à

Solid (Condensation)

(c) Solution

à

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

.Slide8

What can be done to limit the number

of nucleation sites?

Several techniques are used separately orin combination to induce nucleation of thesolid phase at a slow and controlled rate :(a) Slow Cooling of Melts(b) Temperature Gradients(c) Introduction of Seed CrystalsSlide9

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 insystems where melting points are very high and/or incongruentmelting occurs.• Crystals grown in this way are often rather small. Thus, thismethod is frequently used in research, but usually notappropriate for applications where large crystals are needed.Slide10

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.Slide11

Congruent and Incongruent

Melting in

Binary and Ternary SystemsThe 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.