Vapourtransport

Vapour transport, often referred to as chemical vapour transport (CVT), is a materials processing technique in which a solid material is volatilised in a heated source zone and transported through the vapour phase to a cooler deposition region, where it recondenses as a solid. The transport is typically facilitated by a volatile transport agent that forms gaseous compounds with the material, enabling mobility at temperatures lower than those required for direct sublimation. In a typical setup, a sealed quartz ampoule contains the source material and a transporting agent (such as iodine, chlorine, or hydrogen halides) and is heated at one end to a higher temperature than the other end, creating a temperature gradient that drives the movement of volatile species.

Mechanistically, the material reacts with the transport agent to form a volatile compound, which migrates from the source to the deposition zone and releases the material by decomposition or reaction, regenerating the transporting agent. The rate of transport depends on gas-phase equilibria, temperature gradient, and the presence of any carrier gas.

Applications include the growth of high-purity single crystals and thin films of semiconductors and chalcogenides, as well as purification of materials through repeated transport cycles. Advantages include relatively low-temperature operation and the ability to transport materials with low vapour pressures, while drawbacks include potential halogen contamination, toxicity of transport agents, and sensitivity to gradient control.

Vapour transport is related to, but distinct from, chemical vapour deposition (CVD) and physical vapour transport (PVT). It is commonly employed when selective crystal growth and purification are desired.