Optical Transmittance for Strongly-Wedge-Shaped Semiconductor Films: Appearance of Envelope-Crossover Points in Amorphous As-Based Chalcogenide Materials

In this work, we study the influence of the geometry of a thin film on its transmission spectrum, as measured on amorphous As-based chalcogenide layers grown onto 1-mm-thick soda-lime-silica glass substrates. A new method is suggested for a comprehensive optical characterization of the film-on-substrate specimen, which is based upon some novel formulae for the normal-incidence transmittance of such a specimen. It has to be emphasized that they are not limited to the usual cases, where the refractive index, n, of the film and that of the thick transparent substrate, s, must obey: n2>>k2 and s2>>k2, respectively, where k stands for the extinction coefficient of the semiconductor. New expressions for the top and bottom envelopes of the transmission spectrum are also obtained. The geometry limitation usually found when characterizing strongly-wedge-shaped films, has been eliminated with the introduction of an appropriate parameter into the corresponding equations. The presence of crossover points in the top and bottom envelopes of the transmission spectrum, for these strongly-wedge-shaped chalcogenide samples, has been both theoretically predicted and experimentally confirmed.