Growth of co-sputtered titanium-strontium films: influence of strontium concentration and ion bombardment

Strontium-containing titanium films were synthesized by non-reactive magnetron sputtering from elemental targets. Three series of depositions were carried out at (1) different relative Sr-to-Ti target powers, (2) different sputter-gas pressures, and (3) different bias voltages. Large amounts of oxygen were detected in films with high Sr concentrations as well as in films exhibiting a porous morphology. By the addition of Sr, amorphous regions appeared in the coatings with a corresponding reduction of the crystalline volume fraction. The amorphous phase fraction increased with increasing Sr content. Upon increasing the Sr contents to 7.2 at.% and higher, completely X-ray amorphous coatings were obtained. However, from high-resolution transmission electron microscopy imaging of the film containing 7.2 at.% Sr, Ti nanocrystals of the order ~ 6 nm embedded in an amorphous matrix were observed. Furthermore, the absolute value of the floating potential increased with decreasing sputter-gas pressure, resulting in a more energetic ion bombardment of the growing films. Strong resputtering of strontium resulted in very low Sr concentrations in films deposited at floating potential and low gas pressures or with an applied bias voltage. Films deposited at floating potential and high gas pressures exhibited a porous columnar morphology and had low degrees of crystallinity with correspondingly small grain sizes (~ 9 nm), whereas denser and more crystalline films with larger grains (~ 18 nm) were obtained from depositions at floating potential and low gas pressures and from depositions with an applied bias voltage. Film hardness scaled linearly with film electron density, as evidenced from nanoindentation and X-ray reflectivity measurements.