Nanostructures and defects in silicon–hydrogen alloys prepared by argon dilution

Nanostructural heterogeneity of silicon–hydrogen (Si:H) alloy films deposited in a conventional radio frequency plasma-enhanced chemical vapour deposition unit from silane argon mixture has been studied by small-angle X-ray scattering (SAXS). The densities of defect states of Si:H films have been estimated by dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS) and the modulated photocurrent (MPC) method. From the structural and defect studies we identify two regions of Ar dilution, where the structure of the films are distinctly different. Up to 90% Ar dilution, the nanostructural as well as the large-scale (>30 nm) structural heterogeneities in the amorphous Si:H (a-Si:H) network decrease. A lowering of the bulk defect density has also been observed in this Ar dilution region. Increasing Ar dilution to greater than 90% of the mixture, the a-Si:H films show some dense regions embedded in the amorphous matrix. The high- and low-density amorphous structures within the films can explain the experimental results obtained from SAXS, DBP and PDS. A negligible conduction band tail, as observed from MPC result, suggests the formation of high degree of crystallinity in the film deposited with 99% Ar dilution and higher rf power density (80 mW/cm2).