The effect of oblique-angle sputtering on large area deposition: a unidirectional ultrathin Au plasmonic film growth design.

Growing ultrathin nanogranular (NG) metallic films with continuously varying thickness is of great interest for studying regions of criticality and scaling behaviors in the vicinity of quantum phase transitions. In the present work, an ultrathin gold plasmonic NG film was grown on a sapphire substrate by RF magnetron sputtering with an intentional deposition gradient to create a linearly variable thickness ranging from 5 to 13 nm. The aim is to accurately study the electronic phase transition from the quantum tunneling regime to the metallic conduction one. The film structural characterization was performed by means of high-resolution transmission electron microscopy, atomic force microscopy, as well as x-ray diffraction and reflectivity techniques, which indicate the Volmer-Weber film growth mode. The optical and electrical measurements show a transition from dielectric-isolated gold NPs towards a continuous metallic network when t becomes larger than a critical value of t = 7.8 nm. Our results show that the onset of the percolation region occurs when a localized surface plasma resonance transforms to display a Drude component, indicative of free charge carriers. We demonstrate that, by using a continuously varying thickness, criteria for metallicity can be unambiguously identified. The onset of metallicity is clearly distinguished by the Drude damping factor and by discontinuities in the plasma frequencies as functions of thickness.