Structural effects due to the incorporation of Ar atoms in the lattice of ZrO2 thin films prepared by ion beam assisted deposition

Abstract

Two sets of ZrO2 thin films have been prepared at room temperature by ion beam induced chemical vapour deposition and subsequently annealed up to 1323 K. The two sets of samples have been prepared by using either O2+ or mixtures of (O2++Ar+) ions for the decomposition of a volatile metallorganic precursor of zirconium. The structure and microstructure of these two sets of samples have been determined by means of X-ray diffraction, Fourier transform infrared spectroscopy and positron beam analysis (PBA). The samples were very compact and dense and had a very low-surface roughness. After annealing in air at T⩾573 K both sets of films were transparent and showed similar refraction indexes.

For the (O2++Ar+)-ZrO2 thin films it is shown by X-ray photoelectron spectroscopy and Rutherford back scattering that a certain amount of incorporated Ar (5–6 at.%) remains incorporated within the oxide lattice. No changes were detected in the amount of incorporated Ar even after annealing at T=773 K. For higher annealing temperatures (T>1073 K), the amount of Ar starts to decrease, and at T=1223 K only residual amounts of Ar (<0.4%) remain within the lattice. It has been found that as far as Ar atoms remain incorporated within the ZrO2 network, the (O2+–Ar+)-ZrO2 films present a cubic/tetragonal phase. When the amount of “embedded” Ar decreases, the crystalline phase reverts to monoclinic, the majority phase observed for the (O2+)-ZrO2 films after any annealing treatments.

The microstructure of the films after different annealing treatments has been investigated by PBA. The presence of Ar ions and the initial amorphous state of the layers were detected by this technique. An increase of the open volume was observed after annealing up to 773 K in both sets of samples. For higher annealing temperatures the samples showed a progressive crystallisation resulting in a decrease of the open volume. During this sintering the samples without embedded Ar present a higher concentration of open volume defects. After the release of Ar occurs (T⩾1223 K) both samples approach to a similar defect free state. The incorporation of Ar within the ZrO2 thin film structure, is proposed as the main factor contributing to the stabilisation of the cubic/tetragonal phase of ZrO2 at room temperature.