Effect of the dopant content on the physical properties of Y2O3–ZrO2 and CaO–ZrO2 thin films produced by evaporation and sputtering techniques

This paper describes the preparation and the characterization of zirconia-doped yttria (or calcia) produced by reactive thermal evaporation (RTE) and r.f. magnetron sputtering (R.F.MS) methods. The crystallographic, microstructural and optical properties of the deposited films were investigated as a function of dopant concentration in the ZrO2 matrix. X-ray diffraction (XRD) analysis permits the study of the stabilization process, lattice parameter and residual stress. XRD patterns show a gradual change in the crystalline structure from a monoclinic phase to a single cubic phase with increasing Y2O3 (or CaO) mole percentage (mol%). The cubic lattice parameter seems to increase with increasing dopant mol%. A merely indicative stress study in stabilized samples shows tensile stress in the ZrO2–Y2O3 system obtained by co-evaporation. Residual stresses were highly compressive in ZrO2–Y2O3 (or CaO) systems deposited by the sputtering technique. Scanning electron microscopy (SEM) revealed a columnar structure for the overall films. Sputtered zirconia-based films show smooth surface topography with smaller crystallite dimensions. The film refractive index, the packing density and the absorption coefficient are determined in the wavelength range 350–800 nm from the measured reflectance and transmittance at normal incidence. All doped ZrO2 films exhibit high optical transmission over most of the visible and near infrared spectrum. The refractive index decreased with increasing dopant content and agreed reasonably well with published values for ZrO2 films. Bulk like refractive index with nearly unity packing density have been obtained for the samples containing 4–7 mol% Y2O3 deposited by the sputtering method. The results suggest that the physical constant of ZrO2-based films are principally determined by the crystallographic form rather than by the nature or amount of the added cation.