Mass spectrometric studies of impurities in silane and their effects on the electronic properties of hydrogenated amorphous silicon

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Abstract
The concentration of volatile impurities in silane (SiH4) and disilane (Si2H6) used to prepare hydrogenated amorphous silicon (a‐Si:H) thin films by glow‐discharge deposition has been measured by modulated molecular beam mass spectrometry with a sensitivity of 1 ppm. From dark conductivity measurements on these films the location of the Fermi energy EF was determined. The Fermi levels of a‐Si:H films were progressively lower with increasing concentrations of chlorine‐containing molecules (mostly HCl and SiH3Cl) in the silane, indicating that these species can act as weak p‐type dopants in a‐Si:H. Oxygen‐containing species (mostly SiH3OSiH3) were also detected in silane. It is clear from these results that large variations in electronic properties of glow discharge a‐Si:H can be attributed to differences in purity of silane used in the deposition.