Theoretical Model for Determining the Thickness and Optical Constants of Transparent Conducting Oxide Thin Films From the Measured Reflectance Spectra

Thickness and optical constants of transparent conducting oxide (TCO) of metal oxide thin films have been commonly estimated by invoking theoretical models based on measured optical reflectance or transmittance. One of such widely employed models is reflectance-based Kushev model. However, the relatively higher value of the absorption coefficient at the absorption edge of TCOs of metal oxide thin films results in a large deviation of their thickness values estimated by this model. To address the underlying problems, in the present work, we have improved the Kushev model. Specifically, for minimizing the error in the estimated thickness values from the proposed model, we have employed t test for discarding outliers of a set of thickness values of a given thin film, which were calculated from nearby extrema of a reflectance curve. Then the thickness calculated from model was validated by comparing the value of ZnO thin transparent films extracted from ellipsometric measurement. Our model shows excellent agreement with the thickness values of ZnO films from ellipsometric measurement with only 1.2% deviation. Furthermore, observed values were also compared with the values from already established Cauchy model. The perfectly matching of the simulated reflectance spectra from the present model with the measured reflectance and excellent agreement of thickness values derived from the ellipsometric measurements with the values extracted from the model may infer us that the present model is suitable for fairly accurately estimating film thicknesses of TCO thin films.