Influence of helium dilution of silane on microstructure and opto-electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by HW-CVD

We report influence of helium dilution of silane in hot wire chemical vapor deposition for hydrogenated nano-crystalline silicon films. Structural properties of these films have been investigated by using Raman spectroscopy, low angle x-ray diffraction, Fourier transform infra-red spectroscopy and non-contact atomic force microscopy. Optical characterization has been performed by UV–visible spectroscopy. It has been observed that in contrast to conventional plasma enhanced chemical vapor deposition, the addition of helium with silane in hot wire chemical vapor deposition has an adverse effect on the crystallinity and the material properties. Hydrogen content in the films was found <2.2 at.% whereas the bandgap remain as high as 2 eV or more. Increase in Urbach energy and defect density also suggests the deterioration effect of helium on material properties. The possible reasons for the deterioration of crystallinity and the material properties have been discussed.

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► nc-Si:H films synthesized using HW-CVD method from silane and helium gas mixture without hydrogen.
► Volume fraction of crystallites and its size in the films decreases with increase in He dilution of SiH4.
► Increase in Urbach energy and defect density with increase in He dilution of SiH4.
► Increasing He dilution, hydrogen bonding in the films shifts from SiH2 and (SiH2)n complexes to SiH.
► Hydrogen content films were found to be <2.2 at.% but the bandgap remains as high as 2.0 eV or even more.