Spectroscopic ellipsometric investigation of graphene and thin carbon films from the point of view of depolarization effects

A spectroscopic ellipsometric study of single-, double- and five-layer graphene transferred to thick SiO2 support layers is presented. Depolarization measurements showed significant peaks. To understand the nature of this depolarization, a sample series consisting of SiO2 support layers of different thicknesses covered with thin pulsed laser deposited carbon layers is also studied. Our investigations show that depolarization originates both from the measurement conditions and from the sample properties, and becomes significant due to the presence of the support layer. Our findings reveal that the observable depolarization peaks diminish with the increase of absorption and thickness of the layer covering the support layer. Since the support layer is generally used to increase the sensitivity of ellipsometry based on the interference enhancement method, we study the influence of depolarization on the results of ellipsometric evaluation. It is shown that neglecting depolarization during the analysis can cause significant inaccuracy in the deduced thickness and optical properties of graphene. This difference decreases with increasing layer number, i.e. with increasing graphene thickness. This effect is also shown for thicker test carbon layer series.