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In the case of weak absorption, the spectral reflectance of a thin film on a nonabsorbing substrate can be used to determine the spectral dependence of the absorption constant. For this purpose, a relatively simple method is described evaluating the envelopes of the interference extrema. This method as well as a Kramers-Kronig analysis were applied to epitaxial, polycrystalline, and amorphous silicon films in the energy range 0.5-5.5 eV. With decreasing crystal quality, changes in the spectral dependence of the optical constants of these films are observed. This can be understood in terms of a decreasing long range order. * Electrochemical Society Active Member.

The transistor parameters of the ESFI\circledRSOS field-effect transistors strongly depend on the properties of the thin epitaxial silicon film on the sapphire substrate: thickness (<0.8 µm), crystal quality, doping concentration and profile, interface states and lateral homogeneity. The crystal quality is measured by means of the optical absorption (E=2.0 to 2.7 eV) obtained by the reflectance interference method similar to the film thickness evaluation. An appropriately defined optical absorption factor characterizes the crystal quality very well and was gauged by other more direct methods like ion backscattering and electron diffraction. The quality of the film is good near the silicon surface (similar to that of bulk silicon), but rather poor near the silicon/substrate interface due to heteroepitaxial growth, as it is observed by step-by-step etching and measuring the correlated absorption constant. The method of controlled depletion was improved by computer evaluation and was used as a tool to study the doping and mobility profiles with FET's after the process.

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