Optical properties and electron spectroscopy characterization of AlxTiyOz thin films

Abstract

Al x Ti y O z thin films (i.e., mixture of Al2O3 and TiO2) with different Al/Ti ratios have been characterized by several electron spectroscopies. The Auger parameter of Ti, determined by x-ray photoelectron spectroscopy, increases with the percentage of Ti in the films. The same trend is found for the evolution of the refractive index in the visible region n0, that, as measured by UV-Vis absorption spectroscopy, varies from 1.5 to 2.5 as a function of the percentage of Ti. Within the same range of compositions, the Auger parameter of Ti changes by 1.2 eV. Such behavior permits an empirical use of the Auger parameter to assess optical parameters of this type of films. On the other hand, reflection electron energy loss spectroscopy has provided information on the optical properties of the films in the far-ultraviolet region (5<ℏω<80 eV). From the analysis of the loss spectra, the refractive index n(ℏω), the extinction coefficient k(ℏω), and the absorption coefficient α(ℏω) have been obtained as a function of the energy ℏω. Differences in the magnitudes of these functions at given energies as well as in the elemental dipolar transitions have been found as a function of the composition of the thin films. Thus, for example, for an energy ℏω∼8 eV, the dependence of n(ℏω) on the titanium percentage is the opposite to that found in the visible region (ℏω=2.5 eV). The films show significant absorption [α(ℏω)≠0] for ℏω>5 eV, which produces a modulation in n(ℏω), that depends on the film composition. As a consequence, it is concluded that the correlation between the Auger parameter and the refractive index in the visible region does not hold when other energies are considered.