The process gas mixture comprises (a) an oxygen gas carried in an inert carrier gas such as helium, argon, and xenon; (b) a silicon source gas such as silane (SiH4), disilane (Si2H6), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4), and hexachlorodisilane (Si2Cl6); and (c) a dilution gas such as hydrogen gas.