Influence of aluminum interlayer on optical properties of very thin silver thin film

Abstract

Generally, Ag thin films on glass substrates grow three-dimensionally because of the high Ag/glass interface free energy. However, flatter growth has been reported to be achieved by introducing an appropriate interlayer between the substrate and Ag film. With flatter growth, enhanced optical properties are expected. In the present study, very thin Ag films were deposited on an Al interlayer and their optical properties over a wide wavelength range from the visible-light region to the infrared region were investigated. Ag/Al films and Ag films were formed on substrates via resistance-heating vacuum evaporation. The Al interlayer thickness was fixed at 1 nm, and the Ag thickness was varied from 6 to 15 nm. The surface morphology was observed by atomic force microscopy, and the sheet resistance was measured by the four-point-probe method. The visible-light transmittance was evaluated using a spectrophotometer, and the infrared reflectance was evaluated using Fourier transform infrared spectroscopy. The surface roughness of a 6 nm-thick Ag film was found to be reduced from 1.0 nm to 0.6 nm by the introduction of the interlayer. In the absence of the interlayer, the sheet resistance was too high to be measured; however a sheet resistance of 16.2 Ω/sq. was obtained when the interlayer was present. Similar effects were observed for a 10 nm-thick Ag film. Ag films with thicknesses of 6 and 10 nm showed improved visible-light transmittance and infrared reflectance upon introduction of the interlayer. However, when the thickness of the Ag film was 15 nm, no difference in the sheet resistance or optical properties was observed between samples with and without the interlayer. As a consequence, we found that introducing an Al interlayer is effective for two-dimensional-like growth of Ag films, leading to enhanced visible-light transmittance and infrared reflectance.