The Kyropoulos method, also known as the KY method or Kyropoulos technique, is a method of bulk

crystal growth Crystal growth is a major stage of a crystallization, crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of the crystalline lattice. The growth typically follows an ini ...

used to obtain

single crystal In materials science, a single crystal (or single-crystal solid or monocrystalline 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 boundary, grain bound ...

s. The largest application of the Kyropoulos method is to grow large

boules Boules (, ), or ''jeu de boules'', is a collective name for a wide range of games similar to bowls and bocce in which the objective is to throw or roll heavy balls as closely as possible to a small target ball, called the ''jack''. 'Boules' its ...

of single crystal sapphire used to produce substrates for the manufacture

gallium nitride Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV af ...

-based LEDs, and as a durable optical material.Dobrovinskaya, Elena R., Leonid A. Lytvynov, and Valerian Pishchik. Sapphire: material, manufacturing, applications. Springer Science & Business Media, 2009.

History

The method is named for , who proposed the technique in 1926 as a method to grow brittle alkali halide and alkali earth metal crystals for precision optics. The method was a response to the limited boule sizes attainable by the Czochralski and Verneuil methods at the time. The Kyropoulos method was applied to sapphire crystal growth in the 1970s in the

Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

The method

The feedstock is melted in a

crucible A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay, they can be made from any material that withstands temperat ...

. (For sapphire crystal growth, the feedstock is high-purity

aluminum oxide Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...

—only a few parts per million of impurities—which is then heated above 2100 °C in a

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...

crucible.) A precisely oriented seed crystal is dipped into the molten material. The seed crystal is slowly pulled upwards and may be rotated simultaneously. By precisely controlling the temperature gradients, rate of pulling and rate of temperature decrease, it is possible to produce a large, single-crystal, roughly cylindrical ingot from the melt. In contrast with the Czochralski method, the Kyropoulos technique crystallizes the entire feedstock volume into the boule. The size and

aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...

of the crucible is close to that of the final crystal, and the crystal grows downward into the crucible, rather than being pulled up and out of the crucible as in the Czochralski method. The upward pulling of the seed is at a much slower rate than the downward growth of the crystal, and serves primarily to shape the meniscus of the solid-liquid interface via

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...

. The growth rate is controlled by slowly decreasing the temperature of the furnace until the entire melt has solidified. Hanging the seed from a weight sensor can provide

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...

to determine the growth rate, although precise measurements are complicated by the changing and imperfect shape of the crystal diameter, the unknown convex shape of the solid-liquid interface, and these features' interaction with buoyant forces and

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

within the melt. The Kyropoulos method is characterized by smaller temperature gradients at the crystallization front than the Czochralski method. Like the Czochralski method, the crystal grows free of any external mechanical shaping forces, and thus has few lattice defects and low internal stress. This process can be performed in an inert atmosphere, such as

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

, or under high

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

Advantages

The major advantages include technical simplicity of the process and possibility to grow crystals with large sizes (≥30 cm). The method also shows low dislocation density.

Disadvantages

The most significant disadvantage of the method is an unstable speed of growth which happens due to heat exchange changes incurred by a growing boule size and which are difficult to predict. Due to this problem the crystals are typically grown at very slow speed in order to avoid unnecessary internal defects.

Application

Currently the method is used by several companies around the world to produce

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...

for the

electronics Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...

and optics industries.

Crystal sizes

The sizes of sapphire crystals grown by the Kyropoulos method have increased dramatically since the 1980s. In the mid-2000s sapphire crystals up to 30 kg were developed which could yield 150 mm diameter substrates. By 2017, the largest reported sapphire grown by the Kyropoulos method was 350 kg, and could produce 300 mm diameter substrates. Because of sapphire's

anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

, the orientation of the cylindrical axis of the boules grown by the Kyropoulos method is perpendicular to the orientation required for deposition of GaN on the LED substrates. This means that cores must be drilled through the sides of the boule before being sliced into wafers. This means the as-grown boules have a significantly larger diameter than the resulting wafers. As of 2017 the leading manufacturers of blue and white LEDs used 150 mm diameter sapphire substrates, with some manufacturers still using 100 mm, and 2 inch substrates.

References

External links

Crystal growth technique summaries
{{DEFAULTSORT:Kyropoulos Process Semiconductor growth Industrial processes Crystals Methods of crystal growth