Crystal growing technologies
3photon has established extremely close collaboration with laser crystal, non—linear crystal, photorefractive crystal and scintillating crystal growers. These partnerships allow 3photon to pick the highest quality material and combine it with long years of expertise in polishing and coating crystals to produce perfect product.
Most of 3photon crystals are grown via Flux Growth or Czochralski methods and immediately processed in polishing and coating facilities. Tightly controlled manufacturing chain is 3photon key advantage in manufacturing top-tier crystals.
Czochralski method
The Czochralski method (also known as Pulling technique) is the process of growing single crystals of semiconductors, metals, salts or synthetic gemstones. In this method, an oriented single-crystal seed attached to a cooled pulling shaft is initially dipped into the molten semiconductor charge contained in a heated cylindrical crucible and is subsequently withdrawn while reducing the power supplied to the crucible. In this way, a large oriented crystalline ingot is grown from its melt. Depending on the material to be grown, the crucible can be made of quartz, aluminum, graphite, or metals like platinum, rhodium, or iridium, while the heating can be provided by radiofrequency coupling or by a resistive heater set around the crucible.
Advantages:
> Relatively high growth rate
> No direct contact between the crucible walls and the crystal which helps to produce unstressed single crystal
> Grow large single crystals
Disadvantages:
> Tensions during the cooling down, necessity of warming up of the grown crystal, especially large
> Possible segregation of constituents
Flux growth method
While Czochralski method has greatly improved the quality of Silicon single crystals, it is worth mentioning that Si crystal is composed of one single element which can be grown from a melt. Because technologies evolve and their needs increases, it is important to upgrade the quality of crystals that are made from more complicated structures that are not possible to grow from a melt. This is where flux growth method comes into place. To put it simply, one must dissolve the preferred substance in a flux (low melting point solvent) and the solution is then treated in high temperatures. After that, seed crystal is dipped into the solution and slow cooling process begins to form the crystal.
Advantages:
> Simplified growing equipment because flux process operates at atmospheric pressure
> Crystals are grown without incorporating water molecules. Because of that, grown crystals can be used in infrared without worrying too much about water absorption near 3.3 microns.
> Thermal strain-free grown crystals
Disadvantages:
> Difficult to control nucleation and many crystals may form during the process
> Crystals may be grown to limited size thus limiting the applications such as inelastic neutron scattering
High pressure vertical Bridgman method (HPVBM)
High pressure vertical Bridgman method is mostly used to grow II-VI compound crystals. These II-VI crystals are known to be challenging to grow because of their physical and chemical properties such as being chemically corrosive in liquid and vapor state. For this method, a classical vertical Bridgman furnace must be upgraded with high-pressure chamber. Using graphite as a crucible is one of the most popular options when growing II-VI crystals because of its’ porosity. The porosity feature reduces gaseous impurities inside the crystal. However, this is a double-edged sword – for example, CdZnTe melt consists of volatile components, which are continuously lost from the vapor phase because of the porous graphite walls. It is possible to suppress the loss with applying external gas pressure, usually 10-150 Atm of Argon.
Advantages:
> HPVBM crystals yield high electrical resistivity, long term stability and good polarization properties
Disadvantages:
> Impossible to grow even stoichiometric composition along the entire length of an ingot
> Because one of the components from II-VI compound has lower diffusion coefficient in inert gas, usually the composition of the grown crystal shifts towards that component
> Macroscopic defects like cracking and pipes are unavoidable
High pressure vertical zone melting (HPVZM)
A method that is quite similar to Bridgman, but with a few upgrades. As the name suggests, this method melts the crystal zone by zone. Basically, a moving molten zone is moving through the material. As this zone moves, it recrystallizes the molten interface. This also helps to purify the crystal because impurities travel to one end of the ingot. For growing II-VI compound crystals, high pressure chamber is required to control the composition.
Advantages:
> Possibility to control composition during the growth of the crystal
> Much higher ingot volume when compared to HPVBM, approximately two times – allows to obtain over 70% of the ingot with stoichiometric composition
> Occupies less space because all the heating elements are positioned vertically
Disadvantages:
> Macroscopic defects like bubbles are still unavoidable because of permanent melt dissociation. At least 0.05% vol is guaranteed to be bubble defects
> Larger risk to crack the crucible because it may be more difficult to load and unload materials
Edge defined, film-fed growth (EFG) method
Edge defined, film-fed growth (EFG) method is used to mostly grow sapphire crystals. This technique saves a lot of time and expenses because it is possible to grow crystals in desired shapes, such as plates, rods and tubes. That means one does not need to grow the crystal in block and then polish and cut it to fulfill the needs. In EFG, crystals are grown from melt film that is formed on the top of a die. Then, within the capillary channel the melt rises to the crystallization front.
Advantages:
> Crystal-shaping technique, no need for additional manufacturing processes
> Compared to flame fusion method for growing sapphire, EFG saves a lot more of the material and is less complex
> Large diameter sapphire crystals can be grown (diameters of 80 mm for rods)
> Possibility to grow several crystals at a time
Disadvantages:
> Defects such as grain boundaries are more noticeable in pulled orientation of the crystals
> Sliping (twinning) may occur along the plane with highest atomic density
Kyropoulos method
A technique very similar to Czochralski. One main difference is the pulling method – in Kyropoulos method only the crystal neck is pulled, thus growing a crystal shaped like a pear. A crystal is grown in such steps: first, the desired crystal material is melted, make the seed crystal interface with the melt, pull and rotate the seed so that the interface crystallizes and by continuous lifting a crystal is grown.
Advantages:
> Precise control of the crystal cooling rate reduces thermal stress
> Has a lower defect density when compared to Czochralski method
> Possibility to grow larger crystals than Czochralski
> Technical simplicity
Disadvantages:
> Unstable speed of growth because of heat exchange changes limits the growth rate
