An Overview of SiC Epitaxial Growth
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and low-pressure CVD SiC epitaxial growth on SiC substrates, typically at substrate temperatures ranging from 1200°C to 1600°C. Examples of less commonly employed single-crystal growth techniques include metalorganic CVD, 10 " rapid thermal CVD, molecular-beam epitaxy,12 and liquid-phase epitaxy.13 Most previous work employed APC VD with inductively heated graphite susceptors that were either uncoated or coated with polycrystalline SiC in horizontal and vertical cool-wall crystal growth reactors. The susceptor's SiC coating is intended to minimize the high temperature reaction with the required H 2 carrier (coreactant) gas, which dramatically impacts morphology and dopant incorporation during SiC epitaxial layer growth as discussed in a later section. During CVD, depletion of precursor(s) along the flow path can result in thickness and doping nonuniformities. Procedures to minimize these epitaxial film nonuniformities include increasing the carrier gas flow (decreased residence time of precursor), reducing the deposition pressure, and tilting the susceptor with respect to the gas flow. Researchers have recently reported encouraging results in both thickness and doping uniformities using commercially available III-V growth systems that were redesigned for the increased temperature operation required for CVD SiC epitaxial growth. One such commercial system14 establishes and maintains laminar gas flow by rotating the disc-shaped susceptor at speeds sufficient for creating a molecular drag pump action in addition to implementing both low-pressure and large gas velocities.15"17 Another commercial system18 mainly employs a specially designed inner reactor sleeve and low-pressure operation for establishing laminar gas flow, whereas substrate rotation is utilized for increased epitaxial layer thickness uniformities.19 Aspects of SiC CVD Precursors Typical CVD techniques for SiC epi-
taxial layer growth employ separate sources of Si and C, each with different pyrolytic cracking efficiencies, and requiring substrate temperatures greater than 1300°C. Silane is commonly the Si source, while various choices of hydrocarbons have been used as the C sources; both are introduced into the reactor with a high purity H2 carrier gas (actually coreactant). Propane has been the most popular among the hydrocarbon choices probably because of its association with the first successful demonstration of large-area epitaxial SiC on Si substrates,20 while methane, ethylene, and acetylene have been used less extensively. Methane is of interest because of the increased purity available from commercial sources compared to that of propane, although the larger thermal stability results in a smaller C source cracking efficiency. The conventional Si and C source molecules, called multiple-source precursors, have dominated the literature in reports of successfully reproducible CVD growth of epitaxial SiC. Single-source CVD SiC precursors have also been studied (albeit much less extensively) because they offer several potential advantages over multiple-
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