Ultra-smooth ultrathin silver films deposited on acid treated Silicon substrates

Abstract

Fabrication of smooth noble metal ultrathin films is crucial to many optical and electronic devices. However, the metals’ adatom-adatom cohesive force is usually stronger than the adatom-substrate adhesive force leading to a Vollmer-Weber, island-like, growth mode. This phenomenon imposes limitations on the surface smoothness and minimum (percolation) thickness necessary to obtain a smooth film. Here, we demonstrate a facile method to fabricate ultra-smooth ultrathin silver (Ag) films on silicon via physical vapor deposition. By removing the oxide layer on silicon substrates using a hydrofluoric acid treatment, Ag atoms bind strongly to the energetically favorable silicon atoms leading to smooth Ag films. We compare the results for Ag deposited on HF treated and untreated Si substrates for different Ag thicknesses. Our results show that HF acid treatment and annealing lead to a significant reduction in the surface roughness (∼0.5 nm), narrower peak-to-valley height distribution, and higher Kurtosis. Continuous Ag films were obtained down to Ag thickness of 5 nm. We expect our results to play a crucial role in minimizing electronic and optical losses for optoelectronic, plasmonic and optical metamaterial devices.