Corrections to the constant photoconductivity method for determining defect densities, with application to amorphous silicon

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Abstract
The constant photoconductivity measurement (CPM) technique has been widely used to determine the density of defects in thin films of materials where only a small absorption coefficient exists for these defects, particularly in determining the dangling-bond density in hydrogenated amorphous silicon. Interpretation and modeling in the literature assume that CPM gives an accurate value for the density of defects present. The limitations of this method are examined as a function of several defect parameters, and particularly as a function of the Fermi energy, in models consisting of one level, a Gaussian distribution of levels, two independent levels, and the levels typical of a multivalent defect, applying the results of the latter to actual data on optical degradation kinetics in amorphous silicon. It is concluded that a prerequisite for reliably relating CPM-determined densities to actual defect densities is a knowledge of the relative location of the equilibrium Fermi level and the energy level of the defect.