Crystal Growth Techniques

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 the characteristic arrangement of a crystalline Bravais lattice. The growth typically follows an initial stage of either homogeneous or heterogeneous (surface catalyzed) nucleation, unless a “seed” crystal, purposely added to start the growth, was already present.

The action of crystal growth yields a crystalline solid whose atoms or molecules are typically close packed, with fixed positions in space relative to each other. The crystalline state of matter is characterized by a distinct structural rigidity and virtual resistance to deformation (i.e. changes of shape and/or volume). Most crystalline solids have high values both of Young’s modulus and of the shear modulus ofelasticity. This contrasts with most liquids or fluids, which have a low shear modulus, and typically exhibit the capacity for macroscopic viscous flow.

Slow cooling bottom growth (without seed)

Solution growth techniques are often applied to fabricate high-quality single crystals which cannot be grown from their own melts. Depending on the particular crystal class aqueous solutions or flux melts (high-temperature solutions) are usually employed.

The crystal growth occurs either by spontaneous nucleation in the solution volume, at container walls and at seed rods or with the help of a seed crystal dipped into a supersaturated solution. For bulk crystal growth we use the slow cooling method. Due to local bottom cooling and the use of a suitable temperature gradient, the crystal growth occurs usually at the bottom of the crucible.

For a suitable homogenization of the solution the accelerated crucible rotation technique (ACRT) is applied during the crystallization process. This warrants a more homogeneous distribution of substituents/dopants in the entire crystal volume and the minimization of flux inclusion.

Besides single-crystalline material for commercial applications we obtain seed crystals for methods that require seeds in order to grow large single crystals (see TSSG technique).

The “Top seeded solution growth (TSSG)” technique (with seed)

Crystallization may  be accomplished with the help of seed crystals by dipping these crystals into a supersaturated solution. The procedure of the TSSG technique corresponds to that of the well-known Czochralski technique¹ or the Nacken-Kyropoulos technique² for melt-grown material.

The TSSG technique may be applied in two ways –  the vertical temperature gradient transport method (using nutrient material) and the slow cooling method (in a supersaturated solution). The former is primarily used for compounds containing substituents or dopants while the latter is commonly used for pure compounds.

For an effective homogenization of the solution and a controlled growth process, optimized hydrodynamic conditions can be achieved by suitable seed rotation. Additionally, if the temperature gradient transport method is applied, it is necessary to pull the seed from the solution.

This method allows the growth of large, high-quality single crystals. Exposure of these crystals to any tension in the solidified solution is avoided by pulling them from the solution before the cooling process.

Usually the yield and quality of commercially applicable material obtained by TSSG crystallization is higher than that of spontaneously nucleated crystals grown by the bottom growth technique.

Continuous pulling of the seed crystal (located on the melt surface) during the growing process
Growth of a seed crystal dipped into the melt completely

Liquid phase Epitaxy

Liquid phase epitaxy (LPE) is an excellent method to deposit micrometer-thick films with high crystalline perfection. The epitaxial growth of dissolved solutes occurs in supersaturated solutions on preferably lattice-matched single-crystalline substrates.

For this, static or rotating substrates are immersed in supersaturated solutions until the desired film thickness is reached. Then the sample is withdrawn from the solution and remnants are removed by spin-down rotation.

Advantages of the LPE technology in contrast to deposition methods from the gas phase:

• Fast film growth with high crystalline perfection across the whole film thicknesses for layers between 1 and 500 mm thickness

• Formation of thermodynamic stable phases exhibiting ideal or nearly ideal stoichiometry

• Growth of extremely flat surfaces

• Robust and low-cost equipment

Disadvantages:

• Not suitable for film thicknesses considerably smaller than 1 µm

• Availability of lattice-matched substrates which have to be inert in the used solutions