Effect of Ar in the source gas on the microstructure and optoelectronic properties of microcrystalline silicon films deposited by plasma-enhanced CVD

Hydrogenated microcrystalline silicon films have been prepared by plasma-enhanced chemical vapor deposition technique using silane diluted in H2 or H2 + Ar. The microstructures for silicon films have been evaluated by Raman scattering spectroscopy, X-ray diffraction and Fourier-transform infrared spectroscopy. Optical characterization has been done by UV–vis spectroscopy. It is found that the addition of Ar in diluent gases efficiently improves the deposition rate and crystallinity due to an enhanced dissociation of the source gas and the energy of deexcitation of Ar* released within the growth zone. Meanwhile, the enhanced crystallinity and the reducing of hydrogen ion bombardment with increasing Ar dilution lead to the polymerization and also a bad passivation of the hydrogen which cause the widening of the optical gap and increase of defect states in the μc-Si films. The absorption coefficient and dark conductivity are found to decrease basically with increasing Ar dilution corresponding to the widening optical gap and more defects. That the activation energy increases with increasing Ar dilution or decreasing hydrogen dilution is due to the fact that more defect states lead to a pulling down of the Fermi level.