Influence of silane flow on structural, optical and electrical properties of a-Si:H thin films deposited by hot wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of hydrogenated amorphous silicon (a-Si:H) films deposited from pure silane (SiH4) using hot wire chemical vapor deposition (HW-CVD) technique are systematically studied as a function of silane flow rate FSiH4 between 5 and 30 sccm. We found that the properties are greatly affected by the silane flow rate over the range we studied. The device quality a-Si:H films with a photosensitivity >105 were deposited by HW-CVD at a deposition rate >10 Å s−1 using low silane flow rate. However, a-Si:H films deposited at higher silane flow rate and/or higher deposition rates show degradation in their structural and electrical properties. The FTIR studies indicate that the hydrogen bonding in a-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H2) and (Si–H2)n complexes when films were deposited at higher silane flow rate. The hydrogen content in the a-Si:H films increases with increase in silane flow rate and was found to be less than 10 at.%. The Raman spectra show increase in disorder and the Rayleigh scattering with increase in silane flow rate. The optical band gap also shows an increasing trend with silane flow rate. Therefore, only the hydrogen content cannot be accounted for the increase in the optical band gap. We think that the increase in the optical band gap may be due to the increase in the voids. These voids reduce the effective density of material and increase the average Si–Si distance, which is responsible for the increase in the band gap. Silane flow rate of 5 sccm, appears to be an optimum flow rate for the growth of mono-hydrogen (Si–H) bonded species having low hydrogen content (∼4.25 at%) in a-Si:H films at high deposition rate (∼12.5 Å s−1), high photosensitivity (∼105) and small structural disorder.