Determination of the absorption edge of a thin film from transmission measurements

An accurate formula that calculates the transmission of a thin absorbing film on a thick nonabsorbing substrate in terms of the interfacial intensity reflection coefficients is derived. Some special cases of the transmission, namely, its value at the maximum and the minimum and at midpoints of the interference pattern, are shown to yield simple formulas to determine the absorption coefficient as well as an internal check procedure that checks the quality of the spectrum. These methods are shown to yield results for the absorption coefficient that are consistent with one another in films of evaporated amorphous germanium, i.e., they give rise to a smoothly varying absorption edge. The transmission of a nonabsorbing, thin oxide layer on top of the film is also calculated and shown to make a negligible contribution to the nonoxidized film transmission.