Optical anisotropy of shaped oriented cobalt nanoparticles by generalized spectroscopic ellipsometry

Abstract

We report on the application of generalized spectroscopic ellipsometry to the study of oriented prolate ellipsoidal cobalt nanoparticles embedded in a silica thin layer. The elongation of the cobalt particles with ellipsoidal form has been obtained by irradiation of spherical cobalt particles with swift heavy ions. Such a nanostructured medium constitutes an absorbing uniaxial medium with axis oriented 50\ifmmode^\circ\else\textdegree\fi{} from the normal. The rotating polarizer ellipsometer with three elements has been extended to generalized ellipsometry and has been used to determine the anisotropic optical responses of the nanostructured layers. The technique we have developed is based on the acquisition of various spectra for different positions of the elements of the instrument and numerical extraction from these data of generalized ellipsometric parameters. The analysis of the sample is processed in three steps: development of an a priori model and simulation of the optical responses, acquisition of the experimental data and extraction of the generalized ellipsometric parameters, and, finally, numerical fitting of the model. Calculations of the anisotropic electromagnetic response are based on the Berreman formalism. The sample is represented by a stack of three sublayers on a silicon substrate: a top and bottom sublayer made of pure silica and an intermediate sublayer made of a mix of anisotropic cobalt particles and silica. The mix is represented by the generalized Maxwell-Garnett model that gives the effective dielectric constant of the nanostructured medium when this medium is supposed to be made of particles with a single kind of shape in a host medium. To improve our model, a different generalized Maxwell-Garnett formula has been developed to take two different kinds of shapes into account. This approximation has especially been used to represent a medium made of both spherical and ellipsoidal particles. It has been shown that the good agreement between calculated and experimental data can be improved using this different approach of generalized Maxwell-Garnett formula. Despite the complexity of the implanted anisotropic sample, our experimental method and models have lead, on the one hand, to the understanding of this complex anisotropic sample and, on the other hand, to the confident determination of various meaningful parameters that compose this optical response, such as the thicknesses of the sublayers, volume fraction, orientation, and shape factor of the cobalt particles.