## Optical Properties of Sub-Stoichiometric Tantalum Oxide Thin Films from Spectroscopic Ellipsometry

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Abstract
06-204 ANDRE SANTAROSA FERLAUTO FERLAUTO, A.S.(Federal University of ABC); Palhares, J.H.(Federal University of Minas Gerais); Medeiros-Ribeiro, G.(Federal University of Minas Gerais); Radtke, C.(Federal University of Rio Grande do Sul); Ribeiro, G.M.(Universidade Federal de Minas Gerais); Tantalum oxide is a dielectric material with a high index of refraction that has been studied for optical applications and as dielectric layer in memory devices. Standard low temperature thin film deposition techniques lead to $Ta{O}_{x}$ films with an amorphous structure that usually have a stoichiometry similar to the crystalline $T{a}_{2}{O}_{5}$ phase ($x\equiv O}{Ta}\simeq 2.5$); however, $x$ can be intentionally controlled by, for example, tuning the oxygen flow during reactive sputtering. Sub-stoichiometric $Ta{O}_{x}$ films are formed by a solid solution of clusters of metallic Ta atoms within the $T{a}_{2}{O}_{5}$ matrix. Interest in this material was boosted with the rise of resistive memory devices in which switching between two resistive states occurs via formation of conductive filaments (CFs) within the oxide film. Oxygen vacancy migration is responsible for CFs formation; however, understanding of such mechanism is yet incomplete. Experimental and theoretical studies that indicate that the CFs are associated to metallic Ta clusters and make clear that CF formation depends on the oxygen content in the films. In this work, we have investigated $Ta{O}_{x}$ thin films with varying oxygen concentration deposited by reactive sputtering. Its main novel aspect is the determination of $Ta{O}_{x}$ optical functions from a rigorous spectroscopic ellipsometry analysis. We have found that the complex dielectric functions of $Ta{O}_{x}$ with $x<2.5$ can be modeled by assuming a composite material having a Drude-like metallic phase embedded in insulating matrix, thus providing the volume fraction of the metallic phase. This model is valid for a wide range of O concentration, which suggests that even in materials with small deviations from stoichiometry, oxygen deficiency results in the clustering of mettalic Ta. Our findings provide further evidence for the understanding of CF filament formation in $Ta{O}_{x}$ based devices. Acknowledgements: This work was supported by CNPq and Fapemig.
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