Ultrasmooth metal nanolayers for plasmonic applications: surface roughness and specific resistivity

The future of plasmonic devices depends on effective reduction of losses of surface plasmon-polariton waves propagating along metal-dielectric interfaces. Energy dissipation is caused by resistive heating at the skin-deep-thick outer layer of metal and scattering of surface waves on rough metal-dielectric interfaces. Fabrication of noble metal nanolayers with a smooth surface still remains a challenge. In this paper, Ag layers of 10, 30, and 50 nm thickness deposited directly on fused-silica substrates and with a 1 nm wetting layer of Ge, Ti, and Ni are examined using an atomic-force microscope and four-probe resistivity measurements. In the case of all three wetting layers, the specific resistivity of silver film decreases as the thickness increases. The smallest, equal 0.4 nm root mean squared roughness of Ag surface of 10 nm thickness is achieved for Ge interlayer; however, due to Ge segregation the specific resistivity of silver film in Ag/Ge/SiO₂ structures is about twice higher than that in Ag/Ti/SiO₂ and Ag/Ni/SiO₂ sandwiches.