Structural and optical properties of sputtered ZnO films

Abstract

ZnO thin films were deposited by r.f. magnetron sputtering from zinc oxide target. The composition (O/Zn atomic ratio) determined by Rutherford backscattering spectrometry depends narrowly on the sputtering parameters. The O/Zn atomic ratio is found to increase with the oxygen partial and the total pressures; however substoichiometric films were obtained at low pressures and high sputtering powers in the non-reactive deposition process. X-ray diffraction measurements show that all the films are crystallized in the würtzite form and present a preferred orientation along the [002] direction. The crystallinity is found to increase with the kinetic energy of the sputtered particles. The films contain compressive stresses originating mainly from the contribution of the intrinsic component. A post deposition heat treatment is essential to produce relaxation of the stresses. Scanning electron micrographs show that the ZnO deposits have a columnar structure: the compactness of the films is dependent on the sputtering conditions. The EXAFS measurements show that Zn atoms have a tetrahedral environment with a zinc–oxygen distance of 1.95Å. Infrared investigations confirm these findings and show that zinc atoms stay tetrahedrally coordinated even though the O/Zn atomic ratio changes from 0.95 to 1.06. The optical constants (refractive index and absorption coefficient) vary also in a wide range. Their variations were related to the composition and structure evolution.