Optical constants and thickness determination of very thin amorphous semiconductor films

This contribution addresses the relevant question of retrieving, from transmittance data, the optical constants, and thickness of very thin semiconductor and dielectric films. The retrieval process looks for a thickness that, subject to the physical input of the problem, minimizes the difference between the measured and the theoretical spectra. This is a highly underdetermined problem but, the use of approximate-though simple-functional dependencies of the index of refraction and of the absorption coefficient on photon energy, used as an a priori information, allows surmounting the ill posedness of the problem. The method is illustrated with the analysis of transmittance data of very thin amorphous silicon films. The method enables retrieval of physically meaningful solutions for films as thin as 300 A. The estimated parameters agree well with known data or with optical parameters measured by independent methods. The limitations of the adopted model and the shortcomings of the optimization algorithm are presented and discussed. (C) 2002 American Institute of Physics.