Structural and optical properties of thin zirconium oxide films prepared by reactive direct current magnetron sputtering

Thin films of zirconium oxide have been deposited onto glass and Si(100) substrates at room temperature by reactive dc magnetron sputtering of a metallic Zr target in an argon–oxygen atmosphere. The films were characterized by Rutherford backscattering spectroscopy, x-ray diffraction, x-ray reflectometry, atomic force microscopy, and optical spectroscopy to investigate the variation of stoichiometry, structure, density, and optical properties upon increasing oxygen partial pressure. The Zr target shows a well-defined hysteresis upon varying the oxygen pressure as determined by quartz crystal microbalance measurements. Rutherford backscattering experiments reveal that the films are metallic with a small amount of oxygen incorporation at oxygen flows well below 2.7 sccm. Stoichiometric ZrO2 films, however, are formed above 2.8 sccm O2 flow (oxidic mode). Grazing angle incidence x-ray diffraction studies show that crystalline ZrO2 films with monoclinic structure grow in this oxygen flow regime. X-ray reflectivity studies determine a constant density of 6.5 g/cm3 and a deposition rate of approximately 1.5 nm/s in the metallic mode. The transition to the oxidic mode is accompanied by a decrease of film density and a reduction in deposition rate to below 0.2 nm/s for a constant cathode current of 900 mA. In this oxygen flow regime a density of 5.2 g/cm3 is determined, which is approximately 90% of the bulk density of monoclinic ZrO2. With increasing O2 flow in the oxidic sputtering regime the surface roughness of the films increases as is also confirmed by atomic force microscopy. For these O2 flow rates fully transparent ZrO2 films are grown. From measurements of the optical transmittance and reflectance we have determined the optical constants such as the band gap Eg, refractive index n, and extinction coefficient k as well as the film thickness. While the refractive index of the films decreases upon increasing O2 flow, the band gap Eg increases simultaneously from 4.52 to 4.67 eV.