ZnO and Al doped ZnO thin layers (< 100 nm) have been deposited through RF magnetron sputtering on quartz and Si substrates. Structural (XRD), compositional (SNMS), morphological (SEM) and optical/electrical (Spectroscopic Ellipsometry) characteristics of the samples have been studied in order to deduce the effect of dopant level variation. A combination of classical Tanguy + Drude + Sellmeier terms are used to analyze the dielectric function of ZnO and Al: ZnO. It has been found that increasing Al contents leads to smaller crystalline size, band gap shift and a gradual reduction of the refractive index and increase of the broadening of this transition. The electrical parameters extracted from the Drude model suggest a limit for the aluminium doping efficiency. (C) 2016 Elsevier B.V. All rights reserved.