An expanding thermal plasma for deposition of a-Si:H

A remote argon/hydrogen plasma is used to deposit amorphous hydrogenated silicon. The plasma is generated in a DC thermal arc (typical operating conditions 0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 10³ m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H₂:SiH₄=55:10:6 scc/s. The electron temperature in the jet is low (typ. 0.3 eV): silane radicals are thought to be produced mainly by hydrogen abstraction, but also by a sequence of dissociative charge exchange and consecutive dissociative recombination. In-situ ellipsometry yields refractive indices of 3.6-4.2 at 632.8 nm and growth rates of 10-20 nm/s. FTIR analysis yields a hydrogen content of 9-25 at.% and refractive indices of 2.7-3.3 in the infrared. The SiH density decreases with increasing hydrogen content, whereas the SiH₂ density increases. Above 11 at.%, the majority of hydrogen is bonded in the SiH₂ configuration. The optical bandgap remains constant at approximately 1.72 eV. The photoconductivity is of the order 10^-6 (Ωcm)^-1 and the photoresponse 10⁶.