Introduction

CHEMICAL VAPOR DEPOSITION (CVD) is the deposition of thin, solid films from gas-phase precursors. Growth usually occurs through heterogeneous reactions catalyzed by a heated surface, although homogeneous reactions also can occur in the gas phase. Gas-phase, or parasitic, reactions should be avoided, because they deplete the precursor process and may produce deleterious solid particles, or "dust."

Epitaxial deposition is a process that produces single-crystal films with the same crystallographic orientation as the underlying substrate. Epitaxial growth from a CVD process is sometimes referred to as vapor-phase epitaxy (VPE). Other epitaxial processes that have been developed since 1960 include molecular-beam epitaxy (MBE) and liquid-phase epitaxy.

The vapor-phase methods that are discussed in this article have several advantages, when compared with liquid-phase epitaxy. One is the flexibility of depositing films with distinctly different compositions than the substrates. Even disparate chemistries can be used to deposit epitaxial films, as long as the lattice constant is matched sufficiently to the substrate. An example is the growth of gallium arsenide on silicon. A second advantage is the availability of high-purity gaseous precursors. The refinement of liquid sources is much more difficult, and it limits the quality of the resultant films. Vapor-phase techniques permit the growth of multilayered structures with excellent compositional and dimensional control, as well as abrupt interfaces. A third advantage is that selected gas-phase processes can be scaled to large areas and batch processing for commercial production.

The ability to grow thin, epitaxial films and multilayers on a variety of substrates has led to the development of new devices and technologies. Silicon epitaxial films are used routinely in the manufacture of high-performance bipolar and complementary metal-oxide semiconductor (CMOS) integrated circuits. The deposition of compound semiconductors with direct bandgaps has led to the production of millimeter and microwave devices, as well as optoelectronic devices, such as lasers, light-emitting diodes, and high-efficiency photovoltaic cells. The ability to grow thin, multilayered structures with very fine dimensional and compositional control has made possible quantum-well devices and strained-layer superlattices. The development of devices with novel capabilities and higher performance levels will certainly continue as the control and understanding of the growth process matures.

This article describes vapor-phase growth techniques that are applied to the epitaxial deposition of semiconductor films. The growth of Group III-V compounds via the metal-organic CVD (MOCVD) method is emphasized, because that method has become the most widely used and commercially important process for depositing these materials. The advantages of MOCVD, when compared with techniques such as MBE and VPE, are defined. The thermodynamic and kinetic processes responsible for epitaxial growth also are presented.

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