Some aspects on the saturation level and the origin of metastable defects in a-Si:H (solar cells)

Using the method of keV-electron irradiation to create metastable (Staebler-Wronski-like) defects in hydrogenated amorphous silicon, a saturation level of about 10¹⁸ spins/cm³ has been found for high-dose irradiation (up to 300 J/cm²). The saturation effect which becomes perceptible for electron doses exceeding Q_el=50 J/cm² has been investigated by electron spin resonance and photoconductivity spectroscopy (constant photocurrent mode) measurements. Annealing experiments were used to exemplify the identity of the defects induced by light and by keV-electron irradiation. No differences concerning the defect creation and saturation could be found between glow discharge and a-Si:H and magnetron-sputtered a-Si:H films, the latter containing extremely few oxygen impurities (less than 3*10¹⁸ oxygen atoms per cm³). From this fact it is concluded that oxygen does not play an important role in the defect creation process.