Seed crystal
A seed crystal is a small piece of single crystal material from which a large crystal of, usually, the same material is to be grown. The large crystal can be grown by dipping the seed into a supersaturated solution, into molten material that is then cooled, or by growth on the seed face by passing vapour of the material to be grown over it.

Examples where the a seed crystal is used to grow large boules or ingots of a single crystal include the semiconductor industry where methods such as the Czochralski process or Bridgeman technique are employed.
Single crystal
A single crystal, also called monocrystal, is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The opposite of a single crystal sample is a polycrystalline sample, which is made up of a number of smaller crystals known as crystallites. Because of a variety of entropic effects on the microstructure of solids, including the distorting effects of impurities and the mobility of crystallographic defects and dislocations, single crystals of meaningful size are exceedingly rare in nature, and can also be difficult to produce in the laboratory under controlled conditions.

Because grain boundaries can have significant effects on the physical and electrical properties of a material, single crystals are of interest to industry, and have important industrial applications. The most notable of these is the use of single crystal silicon in the fabrication of semiconductors. On the quantum scale that microprocessors operate on, the presence of grain boundaries would have a significant impact on the functionality of field effect transistors by altering local electrical properties. Therefore, microprocessor fabricators have invested heavily in facilities to produce large single crystals of silicon.
Fabrication of single crystals usually involves the building of a crystal layer by layer of atoms. Techniques to produce large single crystals (boules) include slowly drawing a rotating "seed crystal" in a molten bath of feeder material (as in the Czochralski process and the Bridgeman technique). Some thin film deposition techniques can be used for epitaxy, forming a new layer of material with the same structure on the surface of an existing single crystal.
Monocrystals are often made by Czochralski process, controlled crystallization from the melted material.

Uses
Monocrystals of silicon and other semiconductors are important for manufacture of integrated circuits.
Monocrystals of sapphire and other materials are used for lasers and nonlinear optics.
Monocrystals of fluorite are sometimes used in the objective lenses of apochromatic refracting telescopes.
Monocrystals of metals, especially superalloys, are used for their special mechanical properties. Turbine blades of some gas turbines are made of single crystal cast superalloy.

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