Vapor-phase epitaxy

Vapor-phase epitaxy (VPE)
Silicon is most commonly deposited from silicon tetrachloride in hydrogen at approximately 1200 °C: 
SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g) 
This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative growth rates (etching) may occur if too much hydrogen chloride byproduct is present. (In fact, hydrogen chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with the deposition reaction:
SiCl4(g) + Si(s) ↔ 2SiCl2(g) 
Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way:
SiH4 → Si + 2H2 
This reaction does not inadvertently etch the wafer, and takes place at lower temperatures than deposition from silicon tetrachloride. However, it will form a polycrystalline film unless tightly controlled, and it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide.VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE and metalorganic VPE.

Further information: 

1. A. Polman et. al., J. Appl. Phys., Vol. 75, No. 6, 15 March 1994
2. S. Mirabella et. al., Appl. Phys. Lett. 86, 121905 ~2005
3. Jaeger, Richard C. (2002). "Film Deposition". Introduction to Microelectronic Fabrication. Upper Saddle River: Prentice Hall
4. Article Vapor-phase epitaxy from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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