Crystalline structure dependence on optical properties of silver thin film over time

Silver and gold are the most commonly used materials in optics and plasmonics. Silver has the lowest optical losses in visible and near infrared wavelength range. Many plasmonic devices were fabricated using silver, e.g., spasers, sensors, waveguides, etc. However, there is a big disadvantage. It was many times previously reported that silver thin film optical properties rapidly degraded as it was exposed to atmosphere. This phenomenon was described by different mechanisms: rapid silver oxidation, sulfur or oxygen sorption, formation of silver compounds with chlorine, sulfur and oxygen. In this work, we present our research of optical properties degradation of the silver thin films measured by spectroscopic ellipsometer and film surface chemical composition evolution using Auger electron spectroscopy from the moment of silver film deposition (exposure to atmosphere from vacuum chamber) to the period of a month in a consistent manner. High quality scanning electron microscopy images of researched structures are also presented. Measurements are made for the following silver thin films structures: polycrystalline structure with a crystallite size < 500 nm and crystallite size > 2 μm, and single-crystalline structure on different substrates. All thin films are fabricated immediately before measurements by means of high vacuum electron beam evaporator. Silicon, fused silica and sapphire substrates are used as they are most commonly mentioned in the previous articles and are suitable for plasmonic devices fabrication. The present research could help to optimize silver film deposition technology in terms of its protection from initial optical properties degradation. In addition, a common degradation mechanism of a silver thin film with different crystalline structures is presented.