Structure and properties of crystalline titanium oxide layers deposited by reactive pulse magnetron sputtering

Abstract

Crystalline TiO2 layers could be deposited on glass and silicon substrates by reactive pulse magnetron sputtering without additional substrate heating using a long-term stable process with high deposition rate. XRD investigations have revealed that the variation of pulse mode between unipolar and bipolar allows the deposition of layers with anatase and rutile structures, respectively. The SEM micrographs of anatase layers show faceted crystallites on the surface as well as in the fracture. In comparison the rutile layers have a smoother surface and a more glassy like fracture. Cross-section TEM investigations on samples with anatase or rutile phase have shown that the growth of the layers starts with an amorphous sub-layer. With increasing distance from the substrate the nucleation of first crystallites can be recognized. The lateral size of the columnar grains ranges between 100 and 200 nm for anatase and rutile films. In contrast, the defect density of the rutile phase is drastically higher than in films with anatase structure. Spectroscopic ellipsometry measurements have shown that no absorption is present in the visible spectrum range. The refractive index at 550 nm is approximately 2.55 for anatase and 2.68 for rutile phase, respectively. The hardness and Young's modulus measured by nanoindentation is approximately 8 and 170 GPa for anatase and 17 and 260 GPa for rutile layers. Layers with an anatase structure are well suited for photocatalytic applications using the hydrophilic, self-cleaning and antifogging properties. Rutile layers can be used as optical layers with high refractive index and as protective layers with good mechanical properties.