Anisotropic optical properties of indium tin oxide thin films prepared by ion beam sputtering under oblique angle deposition

Unraveling the anisotropic optical properties of nanometer-scale structured thin films is of great interest for both fundamental and applicative perspectives, but can be challenging and is therefore generally overlooked, especially for transparent and conducting materials. In this paper, porous slanted columnar thin films of indium tin oxide are prepared under Oblique Angle Deposition (OAD) by Ion Beam Sputtering (IBS) using argon and xenon as process ions. Their anisotropic optical properties are investigated both in the Visible-NIR range (400 nm to 1.7 mu m) and the Infrared range (1.7 to 30 mu m) by means of Mueller Matrix Spectroscopic Ellipsometry. The optical models and their results are confronted to and confirmed by High Resolution Transmission Electron Microscopy (HRTEM) studies. In the Visible-NIR range, the resulting picture is that of a ripple-like structure with greater optical index in the direction perpendicular to the slanting plane. This configuration is quite original for OAD films, for which the optical index is usually greater in the direction parallel to the columns, and appears to be a specific footprint of IBS due to the enhanced diffusion of the species during deposition. In the Infrared range, the strong optical anisotropy observed for the Xe-deposited films is explained by the anisotropic free-carrier scattering inside the nanoscopic sub-columnar structure evidenced by HRTEM, yielding to a biaxial Drude model. This mechanism is highly original for OAD films, and may be of a great help to elucidate the anisotropic optical properties of other OAD metallic systems, for which such a sub-columnar structure appears to be a general trend. More generally, this work proposes an original and thorough method combining advanced optical modeling and transmission electron microscopy studies to unravel the anisotropic optical properties of nanostructured thin films.