The single-crystal GaAs wafers are used as substrates for the growth of very thin layers of the same or other III-V compounds having the desired electronic or optical properties. The crystal structure of the grown layer matches that of the substrate. Epitaxy is the process of growing thin films of crystals, in which the substrate determines the crystallinity and orientation of the grown layer. A variety of epitaxial growth techniques are used in III-IV display and device production. The two most common techniques are Vapor Phase Epitaxy (VPE) and Liquid Phase Epitaxy (LPE).
VPE uses a heated stream of gaseous elements or compounds that interact at the surface of the substrate to form the crystalline layer. VPE is primarily used in LED epitaxy. In LPE, the crystalline layer is formed by exposing the substrate to a heated metallic solution saturated with the desired layer components. This method is primarily used in microwave IC epitaxy.
In addition to VPE and LPE, vacuum epitaxy in the form of molecular beam epitaxy (MBE) has developed as an extraordinarily versatile technique. MBE of GaAs consists of an ultra-high vacuum system containing sources for atomic or molecular beams of Ga and As and a heated substrate wafer. The molecular beam sources are usually containers for liquid Ga or solid As. The sources have an orifice that faces the substrate wafer. When the container is heated, atoms of Ga or molecules of As effuse from the orifice. For GaAs, growth usually takes place with a substrate temperature above 450 ºC.
VPE is the primary method in use and is discussed in this section. There are two major techniques of VPE, based on two different chemistries:
- The III-halogens (GaCl3) and V-halogens (AsCl3) or V-hydrogen (AsH3 and PH3).
- The III metal-organics and V-hydrogen, such as Ga3(CH3) and AsH3.
The thermochemistries of these techniques are very different. The halogen reactions are usually "hot" or "cold", in which the III-halogen is generated in a hot zone by reaction of the III element with HCl, and then diffuses to the cold zone, where it reacts with the V species to form III-V material. The metal-organic chemistry is a hot wall process in which the III metal-organic compound cracks or pyrolyzes away the organic group, and the remaining III and hydride V react to form III-V.Back to Top