Laser-heated pedestal growth
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Laser-heated pedestal growth (LHPG) or laser floating zone (LFZ) is a
crystal growth A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists of the ...
technique. A narrow region of a crystal is melted with a powerful CO2 or YAG laser. The laser and hence the floating zone, is moved along the crystal. The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it. This technique for growing
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
s from the melt (liquid/solid
phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...
) is used in materials research.


Advantages

The main advantages of this technique are the high pulling rates (60 times greater than the conventional Czochralski technique) and the possibility of growing materials with very high melting points. In addition, LHPG is a
crucible A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles were historically usually made from clay, they can be made from any material that withstands te ...
-free technique, which allows
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 boundaries.RIWD. "Re ...
s to be grown with high purity and low stress. The geometric shape of the crystals (the technique can produce small diameters), and the low production cost, make the single-crystal fibers (SCF) produced by LHPG suitable substitutes for bulk crystals in many devices, especially those that use high
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
materials. However, single-crystal fibers must have equal or superior optical and structural qualities compared to bulk crystals to substitute for them in technological devices. This can be achieved by carefully controlling the growth conditions.


Optical elements

Until 1980, laser-heated crystal growth used only two laser beams focused over the source material. This condition generated a high radial thermal gradient in the molten zone, making the process unstable. Increasing the number of beams to four did not solve the problem, although it improved the growth process. An improvement to the laser-heated crystal growth technique was made by Fejer ''et al.'', who incorporated a special optical component known as a '' reflaxicon'', consisting of an inner cone surrounded by a larger coaxial cone section, both with reflecting surfaces. This optical element converts the cylindrical laser beam into a larger diameter hollow cylinder surface. This optical component allows radial distribution of the laser energy over the molten zone, reducing radial thermal gradients. The axial
temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degre ...
in this technique can go as high as 10000 °C/cm, which is very high when compared to traditional crystal growth techniques (10–100 °C/cm).


Convection speed

A feature of the LHPG technique is its high
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...
speed in the liquid phase due to Marangoni convection.The video presented in the following reference shows the liquid phase convection during
lithium niobate Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...
(LiNbO3) fiber pulling using a very small piece of
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
wire inside the liquid that is allowed to spin. It is possible to see that it spins very fast. Even when it appears to be standing still, it is in fact spinning fast on its axis.


See also

*
Crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...
*
Crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...
*
Crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...
and engineering aspects * Fractional crystallization *
Micro-pulling-down The micro-pulling-down (μ-PD) method is a crystal growth technique based on continuous transport of the melted substance through micro-channel(s) made in a crucible bottom. Continuous solidification of the melt is progressed on a liquid/solid inte ...
*
Nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
*
Protocrystalline A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form. The term is typically associated with silicon films in optical applications such as solar cells. Applications Silicon sol ...
*
Recrystallization (metallurgy) In materials science, recrystallization is a process by which deformed grains are replaced by a new set of defect-free grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanie ...
*
Seed crystal A seed crystal is a small piece of single crystal or polycrystal material from which a large crystal of typically the same material is grown in a laboratory. Used to replicate material, the use of seed crystal to promote growth avoids the other ...


References

{{reflist, 30em Crystals Crystallography Materials science Mineralogy Methods of crystal growth