Monocrystals

Monocrystals, or single crystals, are solids in which the crystal lattice is continuous and unbroken throughout the entire sample, with no grain boundaries. This distinguishes them from polycrystals, which consist of many crystallites oriented differently, and from amorphous solids, which lack long-range order.

Because the atomic arrangement is continuous, many physical properties are anisotropic, meaning they vary with crystallographic direction. Mechanical strength, thermal expansion, and optical indices can depend on orientation. Impurities and defects such as dislocations reduce crystal perfection and performance.

Common growth methods include the Czochralski process, where a seed crystal is rotated and pulled from a melt to form a boule; the Bridgman-Stockbarger technique, which cools a molten sample through a temperature gradient to promote single-crystal solidification; flux growth, which uses a solvent to dissolve the material and promote slow crystallization; and hydrothermal growth, used for certain oxides at high pressure. Epitaxial methods grow thin single-crystal layers on a substrate.

Natural monocrystals occur in geology and are valued for hardness and optical properties (diamond, quartz, sapphire). Synthetic monocrystals are produced for technology, including silicon wafers, gallium arsenide for electronics, and various optical crystals such as lithium niobate, lithium tantalate, and sapphire. Applications span microelectronics, photovoltaics, LEDs, lasers, nonlinear optics, and X-ray instrumentation.

Quality is described by defect density, mosaic spread, and inclusion content. Large, defect-free crystals are challenging to grow and costly, which limits size and availability for some applications. The orientation of the seed determines the crystal directionality and is important for device performance.