Single crystal

A huge KDP crystal grown from a seed crystal in a supersaturated aqueous solution at LLNL which is to be cut into slices and used on the National Ignition Facility for frequency doubling and tripling.

A single crystal usually refers to a grain boundary free monocrystalline solid

Background and uses

A single crystal solid is a material 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 an amorphous structure where the atomic position is limited to short range order only. In between the two extremes exist polycrystalline and paracrystalline phases, which are 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 are also difficult to produce in the laboratory, though can be made - under controlled conditions (see also recrystallisation).

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.

Semiconductor industry use

One notable use 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.

Optics

This section requires expansion.

A single-crystal quartz bar grown by the hydrothermal method

• 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.^{[citation needed]}

Materials engineering

Another application of single crystal solids is in materials science - in the production of high strength materials, e.g. turbine blades;^[1] here, the absence of grain boundaries gives a significant increase in physical strength.

Electrical conductors

Single crystal copper has better conductivity than polycrystalline copper - As of 2009 no single crystal copper is manufactured industrially, but methods of producing very large individual crystal sizes for copper conductors are exploited for high performance electrical applications. These can be considered meta-single crystals with only a few crystals per metre of length.

Other uses

The detailed study of the Crystal structure of a material by Bragg diffraction techniques is much easier with monocrystals. They may be grown for this purpose, even when the material is otherwise only needed in polycrystalline form.

Manufacture

In the case of silicon and metal single crystal fabrication the techniques used involved high controlled and therfore relatively slow crystallization.

Specific techniques to produce large single crystals (aka boules) include the Czochralski process and the Bridgeman technique). Other less exotic methods of crystallization may be used, depending on the physical properties of the substance - including hydrothermal synthesis, sublimation, or simply solvent based crystallization.

A different technology to create single crystalline materials is called epitaxy. As of 2009 this process is used to deposit very thin (micrometre to nanometer scale) layers of the same or different materials on the surface of an existing single crystal. Applications of this technique lie in the areas of semiconductor production, with potential uses in other nanotechnological fields and catalysis.

See also

• Crystal structure
• Crystallite - the domain in a polycrystalline material that consists of a single crystal
• Crystallisation and related topics:
  • Engineering aspects of crystallisation
  • Fractional crystallization (chemistry)
  • Laser Heated Pedestal Growth
  • Micro-Pulling-Down
  • Recrystallization
  • Seed crystal

References

Further reading

• "Small Molecule Crystallization" (PDF) at Illinois Institute of Technology website