Optimization of device quality nano-nano crystalline n-type silicon thin films deposited using VHF-PECVD

Deposition of phosphorous doped hydrogenated nanocrystalline silicon films (nc-Si:H) by VHF PECVD (60 MHz) at low temperature and low chamber pressure is reported. Substrate temperature was kept at 230°C and the chamber pressure was maintained at 0.25 Torr. Deposition has been carried out with different phosphine (PH₃) concentration from 0.25 to 1.5 % of the total chamber pressure to study the influence of PH₃ doping on the film properties. Argon was used as the diluent for silane without using Hydrogen. Structural characterization and surface morphology has been studied by X-ray diffraction and scanning electron microscopy. Optical band gap of the materials has been deduced from the UV-Visible optical absorption spectroscopy and was found to vary from 1.55 to 1.98 eV. Phosphorous doped nc-Si:H films showed a wide range of conductivity from 1.8 × 10^-3Ω^-1cm^-1 to 4.7 × 10^-8Ω^-1cm^-1 with the highest value of 1.8 × 10^-3Ω^-1cm^-1 for films deposited from PH₃ concentration of 0.75 %. The highest deposition rate achieved was ∼14.8 Å/s for the films deposited with PH₃ concentration of 1.5%. (author)