The origin of intragrain defects in polycrystalline silicon films grown by ion-assisted deposition (IAD) on aluminum-induced crystallization seed layers on glass is investigated. The microstructure of these polycrystalline Si films is bimodal, with near defect-free regions of (001) orientation along the growth direction and highly defective regions containing smaller grains of (111) orientation. In the defective regions, the dominant structural defects are twins in the seed layer and stacking faults in the IAD-grown epitaxial layer, both lying on (111) planes. The stacking faults originate at the seed layer surface due to surface imperfections, indicating that the quality of the seed layer surface plays an important role for the quality of the epitaxial Si film. We find a clear correlation between the structural crystal quality and defect-related radiative transitions at sub-bandgap wavelengths. Two dominant defect levels ({approx} 0.20 eV and {approx} 0.29 eV below the conduction band edge) are observed and identified as impurity-related.