Crystal Growing Technologies
3photon has established exceptionally close collaborations with growers of laser crystals, non-linear crystals, photorefractive crystals, and scintillating crystals. These partnerships enable 3photon to select the highest quality materials and leverage its many years of expertise in polishing and coating to produce flawless products.
Most 3photon crystals are grown using Flux Growth or Czochralski methods and are immediately processed in our polishing and coating facilities. A tightly controlled manufacturing process is 3photon’s key advantage in producing top-tier crystals.
Czochralski method
The Czochralski method (also known as the Pulling technique) is a process used for growing single crystals of semiconductors, metals, salts, or synthetic gemstones. In this method, a single-crystal seed, oriented in a specific direction and attached to a cooled pulling shaft, is initially dipped into the molten semiconductor charge contained in a heated cylindrical crucible. The seed is then gradually withdrawn while the power supplied to the crucible is reduced. This process allows a large, oriented crystalline ingot to grow from the melt. Depending on the material being grown, the crucible can be made from quartz, aluminium, graphite, or metals such as platinum, rhodium, or iridium. Heating can be provided through radiofrequency coupling or a resistive heater placed 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
> Ability to grow large single crystals
Disadvantages:
> Tensions during cooling, require gradual warming of the grown crystal, especially larger ones
> Possible segregation of constituents
Flux growth method
While the Czochralski method has greatly improved the quality of silicon single crystals, it is worth mentioning that silicon crystals consist of a single element, which can be grown from a melt. As technologies evolve and their demands increase, it becomes essential to enhance the quality of crystals made from more complex structures that cannot be grown from a melt. This is where the flux growth method comes into play. Simply put, the desired substance is dissolved in a flux (a low-melting-point solvent) and the solution is then treated at high temperatures. Afterwards, a seed crystal is dipped into the solution, and a slow cooling process begins, allowing the crystal to form.
Advantages:
> Simplified growing equipment, as the flux process operates at atmospheric pressure
> Crystals are grown without incorporating water molecules, which makes them suitable for infrared applications without significant concerns about water absorption near 3.3 microns
> Thermally strain-free crystals
Disadvantages:
> It can be difficult to control nucleation, leading to the formation of multiple crystals during the process
> Crystal growth may be limited in size, restricting applications such as inelastic neutron scattering
High pressure vertical Bridgman method (HPVBM)
The high-pressure vertical Bridgman method is primarily used to grow II-VI compound crystals. These II-VI crystals are notoriously difficult to grow due to their physical and chemical properties, such as being chemically corrosive in both liquid and vapour states. To use this method, a standard vertical Bridgman furnace must be upgraded with a high-pressure chamber. Graphite is one of the most commonly used materials for crucibles in the growth of II-VI crystals due to its porosity. This porosity helps to reduce gaseous impurities within the crystal. However, it also poses challenges—such as in the case of CdZnTe melts, where volatile components are continuously lost through the porous graphite walls. This loss can be mitigated by applying external gas pressure, typically 10-150 atm of Argon.
Advantages:
> HPVBM crystals exhibit high electrical resistivity, long-term stability, and good polarization properties
Disadvantages:
> It is impossible to grow a perfectly stoichiometric composition along the entire length of an ingot
> Because one component of the II-VI compound has a lower diffusion coefficient in an inert gas, the crystal’s composition tends to shift toward that component during growth
> Macroscopic defects, such as cracking and pipes, are unavoidable
High pressure vertical zone melting (HPVZM)
This method is quite similar to Bridgman but with a few upgrades. As the name suggests, the crystal is melted zone by zone. Essentially, a molten zone moves through the material, and as it progresses, it recrystallizes the molten interface. This process also helps purify the crystal, as impurities travel to one end of the ingot. For growing II-VI compound crystals, a high-pressure chamber is required to control the composition.
Advantages:
> Allows control of the composition during crystal growth
> Produces a much larger ingot volume compared to HPVBM, approximately double – enabling over 70% of the ingot to have a stoichiometric composition
> Occupies less space, as all heating elements are positioned vertically
Disadvantages:
> Macroscopic defects like bubbles are unavoidable due to permanent melt dissociation; at least 0.05% of the volume will have bubble defect
> There is a higher risk of cracking the crucible, as loading and unloading materials may be more challenging
Edge defined, film-fed growth (EFG) method
The Edge-defined, film-fed growth (EFG) method is primarily used to grow sapphire crystals. This technique saves both time and cost because it allows crystals to be grown in specific shapes, such as plates, rods, and tubes. As a result, there is no need to grow a crystal as a block and then cut and polish it to meet specific requirements. In EFG, crystals are grown from a melt film that forms on top of a die. The melt rises through a capillary channel to reach the crystallization front.
Advantages:
> Crystal-shaping technique, eliminating the need for additional manufacturing processes
> Compared to the flame fusion method for growing sapphire, EFG saves more material and is less complex
> Large – diameter sapphire crystals can be grown (up to 80 mm in diameter for rods)
> Multiple crystals can be grown simultaneously
Disadvantages:
> Defects such as grain boundaries are more noticeable in the pulled orientation of the crystals
> Slipping (twinning) may occur along the plane with the highest atomic density
Kyropoulos method
A technique very similar to the Czochralski method. The main difference lies in the pulling technique – in the Kyropoulos method, only the crystal neck is pulled, resulting in a crystal shaped like a pear. The crystal growth process involves the following steps: first, the desired crystal material is melted; the seed crystal is then introduced to the melt; the seed is pulled and rotated, allowing crystallization at the interface; finally, through continuous lifting, the crystal is grown.
Advantages:
> Precise control of the crystal cooling rate reduces thermal stress
> Lower defect density compared to the Czochralski method
> Capability to grow larger crystals than with the Czochralski method
> Technically simple
Disadvantages:
> Unstable growth speed due to changes in heat exchange, which limits the growth rate
