Effect of the dopant content on the physical properties of Y 2O 3–ZrO 2 and CaO–ZrO 2 thin films produced by evaporation and sputtering techniques

Abstract

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 ZrO 2 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 Y 2O 3 (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 ZrO 2–Y 2O 3 system obtained by co-evaporation. Residual stresses were highly compressive in ZrO 2–Y 2O 3 (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 ZrO 2 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 ZrO 2 films. Bulk like refractive index with nearly unity packing density have been obtained for the samples containing 4–7 mol% Y 2O 3 deposited by the sputtering method. The results suggest that the physical constant of ZrO 2-based films are principally determined by the crystallographic form rather than by the nature or amount of the added cation.