the liquid or gaseous substances , such as hydrogen or trichlorosilane , enntered in the gas - phase deposition of silicon may contain n - type doping impurities which can be removed by adduct formation with silicon , titanium or tin halides . the impurities can be liberated from the adducts by thermal treatment and finally removed . the halides left behind are capable of again forming adducts and are again used to remove the impurities . the process can consequently be operated cyclically and is remarkable for the low quantity of chemicals required and for its ecological harmlessness .