Optical response of columnar TiO₂:Co on Si substrate

Vertically arranged columnar TiO₂:Co structure has been deposited on Si (100) substrate using the metal organic chemical vapor deposition (MOCVD) technique. SEM images confirm the columnar structure of the TiO₂:Co layer on Si (100). Concentration of the Co dopant has been defined by means of the energy dispersive x-ray (EDX) spectra. Co concentrations are varied at 0.18, 0.33, and 0.68 at. % using pure TiO₂ as the reference. X-ray diffraction (XRD) spectra show that the TiO₂:Co layer have a rutile phase. Further analysis of this structure has been performed by using room temperature spectroscopic ellipsometry (SE) measurements in the energy range of 0.5 - 6.5 eV. This optical response shows the unique feature of significantly high density fringe especially below the band gap energy region (lower than 3.3 eV) in contrast to the conventional TiO₂ thin film structure. This unique feature is interpreted as a dominant contribution of the vertical interfaces at columnar boundaries towards reflected light from the sample. The evolution of Ψ and Δ will be discussed as the function of Co concentration and some optical properties will be extracted after reasonably well data fitting by multilayer modelling. Furthermore, by comparing the SE data from different measurement orientations, it has been shown that TiO₂:Co tends to be anisotropic. This study enables us to further evaluate structural properties through optical responses given by SE measurement.