It was shown that the amplitude of the interference fringes which appeared in the sub-bandgap absorption spectra that were obtained by using the constant photocurrent method differed, depending upon the direction of illumination. This suggested that surface defect absorption contributed to the constant photocurrent. It was concluded that, in order to confirm the assumption of a constant lifetime of photo-generated carriers and to eliminate the ambiguity of the absolute absorption coefficient, the reason for the difference had to be explained.

A.Asano, Y.Ichikawa, H.Sakai: Journal of Non-Crystalline Solids, 1993, 164-166[2], 905-8