Defect formation mechanism in a-Si:H during PECVD at substrate temperatue below 250 deg C is considered to be breaking of weak bonds in the Urbach tail. To break weak bonds, an extra energy is necessary.This energy is supplied by the reaction energy of SiH_3 precursor at the growing surface incorporating SiH_2 into the network.the defect density is experimentally shown to be proportional to a product of the energy supply frequency,i.e., SiH_2 density,and the weak bond density which is obtained by the Urbach energy.By analysis using the configurational coordinate diagram the energy level of the broken weak bond is determined to be 0.2 eV above the valuence band mobility edge. There is similarity of the defect for mation mechanism during deposition to that of the Staebler-Wronski effect.
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