Helium deposition with

Chemical vapor deposition (CVD) has been used to obtain deposits of nitrogen-containing carbon on the surface of other carbon materials. A helium stream with an added aromatic compound such as benzene [28] or pyridine... 

For example, helium gas is found in uncombined form in underground deposits with natural gas. 

Other elements that appear in nature in the uncombined state are the elements in Group 8 helium, neon, argon, krypton, xenon, and radon. Because the atoms of these elements do not combine readily with those of other elements, we call them the noble gases. For example, helium gas is found in uncombined form in underground deposits with natural gas. 

Hafnia can also be deposited by the pyrolysis of a metallo-organic such as hafnium acetylacetonate, ( 511702)3, at 400-750°C, or hafnium trifluoro-acetylacetonate, Hf(C5H402F3)4, at 500-550°C with helium and oxygen as carrier gases. 

Both reactions may take place simultaneously at a deposition temperature of 450°C and at atmospheric pressure. Reaction (1) can also take place in a plasma at 15-300 mTorr and in a temperature range of200-300°C with a silane-to-oxygen ratio of 10/1 in a flow of argon or helium. These two reactions are used to produce doped silica by adding a doping gas such as diborane (B2H ) or phosphine (PH3) to the stream. 

He is found in natural gas deposits principally because alpha particles are produced during natural radioactive decay processes. These alpha particles are 4 He nuclei they obtain two electrons from the surrounding material to become helium atoms. This gaseous helium then accumulates with the natural gas trapped beneath the earth. Although other noble gases are produced by radioactive decay—notably 40 Ar—they are not produced in the large quantities that helium is. 

The use of appropriate ligands for a given metal can permit one to obtain high purity deposits. The Tjballyl and T -cyclopentadienyl ligands coordinated to palladium seem to be especially attractive for the deposition of pure palladium on silica surfaces. It has been proposed that [Pd(T hC3H5)(T -C5H5)] under a helium atmosphere could easily react with a silica surface to produce a surface allyl palladium species and cyclopentadiene [57] (9.9) ... 

As is well known, when hydrocarbons transported in an inert gas such as helium are heated, carbon or pyrolytic graphite will deposit on the walls. In the HTCVD, the ethylene helps make the particles stable and, in doing so, carbon is transported into the chamber together with the silicon. It is a sort of symbiosis in the transport between the silicon and carbon. 

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