text: Homoepitaxial growth of semiconductor thin films are generally done by chemical or physical vapor deposition methods that deliver the precursors to the substrate in gaseous state. For example, silicon is most commonly deposited from silicon tetrachloride (or germanium tetrachloride) and hydrogen at approximately 1200 to 1250 °C:
  :SiCl + 2H ↔ Si + 4HCl
  where (g) and (s) represent gas and solid phases, respectively. 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. (Hydrogen chloride may be intentionally added to etch the wafer.) An additional etching reaction competes with the deposition reaction:
  :SiCl + Si ↔ 2SiCl
  Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way:
  :SiH → Si + 2H
  VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE (HVPE) and metalorganic VPE (MOVPE or MOCVD).
  The reaction chamber where this process takes place may be heated by lamps located outside the chamber. A common technique used in compound semiconductor growth is molecular beam epitaxy (MBE). In this method, a source material is heated to produce an evaporated beam of particles, which travel through a very high vacuum (10 Pa; practically free space) to the substrate and start epitaxial growth. Chemical beam epitaxy, on the other hand, is an ultra-high vacuum process that uses gas phase precursors to generate the molecular beam.
  Another widely used technique in microelectronics and nanotechnology is atomic layer epitaxy, in which precursor gases are alternatively pulsed into a chamber, leading to atomic monolayer growth by surface saturation and chemisorption.