Limitations of the integrated sub-band-gap absorption for determining the density of defects in amorphous silicon

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Abstract
We discuss the procedure of calculating the density of states in the gap for amorphous silicon by integrating the excess sub-band-gap absorption. We show that in general the proportionality coefficient between the density of states and the integrated sub-band-gap absorption is not unique. The sum rule has been used improperly since integration of the excess subgap absorption is terminated either at a fixed energy or at an energy which does not include all optical transitions to the conduction band. We estimate the errors that can arise from this procedure.