User:KDS4444/Physical vapor transport

Physical vapor transport is a crystal growth method for creating crystals of silicon carbide in varying degrees of perfection for uses as abrasives in manufacturing or in the creation of wafers for semiconductor components. It is accomplished by placing a portion of powdered or solid SiC-- called a source-- at the bottom of a furnace/ crucible in either a vacuum or in the presence of a noble gas ambient (argon and helium are common ambients) and heating this source until it vaporizes (at a temperature between 1600 and 2700C). Once the vapor has become supersaturated within the furnace, it will tend to rise and migrate to the cooler upper portions of the furnace where it comes into contact with a monocrystaline seed of SiC. There it will begin to deposit in a regular pattern eventually growing into a larger boule of SiC which can then be used in a semiconductor. There are three processes or techniques which are considered versions of the physical vapor transport method: the Acheson process, the Lely process, and the modified Lely process.

Acheson process
For full article see Acheson process In the Acheson process, which was developed in the 1890s, a trough-shaped furnace holding a combination of silica, carbon, sawdust, and salt is heated to around 2700C. When this mixture cools, the end product is a collection of heavily contaminated or "dirty" crystallites and polycrystalline agglomerates in varying degrees of perfection. These crystals are suitable for use as substrates for chemical vapor deposition growth techniques and liquid phase epitaxy, and are well-suited suited as grit for coated abrasives.

Lely process
For full article see Lely process The Acheson process could not produce crystals suitable for use in semiconductors. The Lely process, developed in 1955, could. In this process, powdered SiC is placed in a graphite "sleeve" with a temperature gradient the applied across this sleeve. As the powdered SiC vaporizes and then cools along this sleeve, it nucleates creating crystals of various sizes and polytypes. These crystals are consistently of very high quality and are usually completely free of undesirable micropipes. However, due to irregularity in the shape of the platelet and the small size of the crystals produced (<20mm) this method, in its original form, is not suitable for mass-production of SiC crystals.

Modified Lely process
In the modified Lely process, a seed crystal is introduced to encourage the growth of crystals on its periphery in a more or less lock-step pattern (usually along the c-axis. The pressure under which this takes place is less than 20 torr and the temperature gradient between the source and seed is 20-35C per centimeter.  This process overcomes the limitations of the previous processes and is suitable for the mass-production of very high quality SiC crystals.