Laser-heated pedestal growth
Encyclopedia
Laser-heated pedestal growth (LHPG) is a crystal growth
technique. The technique can be viewed as a miniature floating zone, where the heat source is replaced by a powerful CO2 or YAG laser. Among all the modern techniques for growing crystal
s from the melt (liquid/solid phase transition
), it has become one of the most powerful for materials research.
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
-free technique, which allows single crystal
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
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.
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. The largest 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
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).
Another interesting feature of the LHPG technique is its high convection
speed in the liquid phase due to Marangoni convection. The video presented in the following reference shows the liquid phase convection during lithium niobate
(LiNbO3) fiber pulling using a very small piece of platinum
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.
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 in the addition of new atoms, ions, or polymer strings into...
technique. The technique can be viewed as a miniature floating zone, where the heat source is replaced by a powerful CO2 or YAG laser. Among all the modern techniques for growing crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s from the melt (liquid/solid phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
), it has become one of the most powerful for materials research.
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
Crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
-free technique, which allows single crystal
Single crystal
A single crystal 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...
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
Melting point
The melting point of a solid 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 depends on pressure and is usually specified at standard atmospheric pressure...
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.
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. The largest 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
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 degrees per unit length...
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).
Another interesting feature of the LHPG technique is its high convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
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
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
(LiNbO3) fiber pulling using a very small piece of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
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 structureCrystal structureIn mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
- CrystalliteCrystalliteCrystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...
- CrystallizationCrystallizationCrystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
and engineering aspects - Fractional crystallizationFractional crystallization (chemistry)In chemistry, fractional crystallization is a method of refining substances based on differences in solubility. If a mixture of two or more substances in solution is allowed to crystallize, for example by allowing the temperature of the solution to decrease, the precipitate will contain more of...
- Micro-pulling-downMicro-pulling-down-Basics:The micro-pulling-down method is a crystal growth technique based on continuous transport of the melted substance through micro-channel made in a crucible bottom. Continuous solidification of the melt is progressed on a liquid/solid interface positioned under the crucible...
- NucleationNucleationNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
- ProtocrystallineProtocrystallineA 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.-Silicon solar cells:...
- Recrystallization (metallurgy)Recrystallization (metallurgy)Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous...
- Seed crystalSeed crystalA seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...