Crystals, Synthetic. Methods Used in the Production of Technologically Important Crystalline Materials

A crystal is an immobilized arrangement of atoms, ions or molecules packed in a regular array. Although naturally occurring crystals can be things of beauty, it is rare to find a fault-free specimen of any considerable size. Crystals can now be synthesized as gems, but more often they are manufactured for utility.

This discussion on synthetic crystals will outline some of the techniques that are used in the manufacture of technologically important crystalline materials other than semiconductors, which are treated elsewhere in this encyclopedia.

One of the first applications of crystalline materials was in specialist optics. In 1828 William Nicol of Edinburgh developed a prism based on calcite (CaCO₃) crystals which could polarize light. Two such prisms were used in a polaroscope (an instrument for measuring the concentration of optically active chemicals) and the design remained unchanged until the 1980s.

At first, piezoelectricity (a voltage causing mechanical deformation and vice versa in a crystal) was a curiosity but it assumed critical importance in 1917 when Paul Langevin assembled a mosaic of thin quartz crystals as a passive sonar array for submarine detection. Quartz crystals became very important as radio engineers struggled to ‘‘lock’’ the frequencies of transmitters so that more users could be accommodated in the limited radio-frequency spectrum.

We can assume that by the end of World War I, the demand for certain crystals outstripped the supply of suitable naturally occurring materials. Fortunately, techniques for manufacture were already to hand and others have been developed throughout the twentieth century. These have been driven by technological advances, such as the requirements of the semiconductor industry. The demand for synthetic ruby grew enormously after it was used in the first lasers in 1960.

For the purposes of classification the plethora of techniques can be listed as: growth from a melt, growth from solution, growth involving high temperature, growth involving high pressure, and growth involving both high temperature and pressure. However, before discussing these we should remember that nature does not like a state of order and that is exactly what a crystal is.

Therefore, crystals will only grow if there is a net benefit, such as a significant decrease in energy. The physical chemistry of the crystal formation is concerned with phase diagrams and with equilibria. The latter dictates that growth must normally be a slow process if defect-free crystals are to be obtained.

Although the French chemist, Edmond Fremy, had been producing commercial quality gemstones in 1877, the first major advance was the production of ruby using the Verneuil process (developed by French chemist Auguste Victor Louis Verneuil) in 1902. In this technique a seed crystal is held under an oxy-hydrogen flame and the base material (alumina, Al₂O₃ powder with an addition of chromia, Cr₂O₃ as a contaminant to give color) is fed through the flame.

Other materials such as sapphire, rutile (titanium dioxide, a gem that is also used as a white pigment in paints, paper, plastics, sunscreens, and cosmetics), spinel (a mineral that can be used to imitate blue sapphire or aquamarine; also has good optical qualities for laser applications), and strontium titanate (has no natural counterpart, but can imitate diamond; and as a ceramic has been widely used for various electronic applications) were later made using this method.

Synthetic gems have the same chemical composition and crystal structure as natural gems, but lack irregularities or tiny inclusion imperfections that give real gems flaws that produce their unique appeal.

The Czochralski method published by Jan Czochralski in 1918 involves dipping a seed crystal into a container of molten base material. The seed is rotated as it is slowly withdrawn and the material adhering to it assumes the same crystal orientation. This process has been used to make ruby, sapphire, spinel, and yttrium-aluminum-garnet (YAG), an important material for lasers.

The Czochralski technique was later to become the cornerstone of the silicon industry and bars of single-crystal silicon up to 250 millimeters in diameter are now in regular production. Czochralski growth of III-V semiconductors such as gallium phosphide presents considerable problems because of the volatility of phosphorous. This has been overcome by adding boron oxide to the melt; which is immiscible and floats on top, rather like oil on water, inhibiting evaporation.

Growth from aqueous solution is historically much older than any of the other techniques, but it remains very important in the manufacture of certain crystals; for example, Rochelle Salt (potassium-sodium tartrate) is a much more effective piezoelectric material than quartz and in 1917 Alexander Nicolson at Bell Laboratories demonstrated that it could be used for sonar applications.

In single-crystal form, potassium dihydrogen phosphate (KDP) is a nonlinear optical material used for doubling, tripling and quadrupling the frequency of the output from high-power lasers. In 1998 the world’s largest KDP crystal (250 kilograms) was grown at the Lawrence Livermore National Laboratory for use in the Megajoule Laser Fusion Experiment.

The same growth techniques can be applied in nonaqueous systems. In the particular case of growth of crystals of materials that have an extremely high melting point the solvent is chosen for its flux properties; that is, its ability to induce growth at significantly lower temperatures. Thus aluminum oxide crystals have been grown in a flux of lead fluoride at 840°C. Once the system has cooled down the flux is dissolved in nitric acid to reveal alumina crystals.

Aqueous growth under high pressure (hydrothermal growth) is a useful means of growing otherwise difficult crystals. It is currently the preferred technique for the production of synthetic quartz.

There have been many attempts to produce synthetic diamond but the crystallization of carbon requires both high temperature and high pressure. It was not until materials such as tungsten carbide became available in 1930 that work could commence on high-pressure containment systems. The use of self-sealing techniques, such as that developed by Percy W. Bridgman, were also essential.

Diamonds were produced by Baltzar von Platen and colleagues at the ASEA Laboratory in Stockholm in 1953, but the work was kept secret. Thus in 1955 Francis Bundy, Tracy Hall, Herbert Strong and Robert Wentorf at General Electric were able to claim the first commercial production of synthetic diamond. Today, films of diamond can be produced using less severe conditions.

This has developed as an offshoot of a technique called vapor phase epitaxy, which is very important in silicon device manufacture and involves the growth of crystalline layers by chemical vapor deposition (CVD). For example gaseous silane (SiH₄) or silicon trichoride (SiHCl3) is carried through a reaction chamber in a stream of hydrogen. At the center of the reactor is a graphite block held at approximately 1200°C with one or more silicon substrates resting on top. The silane breaks down on the hot surface to yield silicon and hydrogen and the deposited material has the same the crystal orientation as the substrate.

This summary lists some of the methods for growing technologically important crystals. Improvements continue, but total innovations are rare. One of the few exceptions has been molecular beam epitaxy (MBE), where crystals are grown under extreme high vacuum conditions. It is costly, but yields tailor-made materials called ‘‘superlattice crystals’’ which have considerable promise for the future. Perhaps the next step is routine crystal fabrication under zero gravity conditions in space.