ABSTRACT
There are three ways of forming undoped or n-and p-type semiconducting layers for devices or IC fabrication on III-V semiconductor wafers. Diffusion is the oldest process, but as explained elsewhere (Chapter 10), it is not commonly used in III-V processing. Ion implantation is a low-cost process and will be discussed in detail in Chapter 9. The third method, epitaxial growth, provides the means to fabricate devices not possible with diffusion or ion implantation and improve device quality and performance at the same time. Epitaxy is the growth of single-crystalline layers on top of single-crystal substrates, preserving the single-crystal nature of the substrate through the grown layer. Epitaxy can be divided into two broad categories: heteroepitaxy, where the grown layer is a different material from the substrate, for example, GaAs on sapphire; and epitaxy or autoepitaxy, where the material is of the same or similar crystal structure but may have a different chemical composition, for example, AlGaAs on GaAs. The mechanism and process for heteroepitaxy are not straightforward and ways of making the transition from the substrate to the epilayer must be developed. Also, the substrate must be inert to the growth environment and closely matched in thermal expansion, otherwise excess stress can build upon cooling after growth. Further discussion will be limited to epitaxy and heteroepitaxy of similar materials.
There are three main techniques for growing epitaxial layers. These are molecular beam epitaxy (MBE), vapor-phase epitaxy (VPE), and liquid-phase epitaxy (LPE). These methods differ in the delivery of the reactant materials to the substrate. MBE has the most direct transport of materials in vacuum and can produce layers of immense variety and complexity. VPE has many variants, MOCVD (also called metal-organic vapor-phase epitaxy [MOVPE]) being the most common for III-V materials. LPE is the oldest technique and will be discussed first.
