Optical and Structural Properties of Ultra-thin Gold Films

TL;DR

This study demonstrates the fabrication of ultra-thin, void-free gold films on fused silica with stable structural and optical properties, highlighting the importance of interface structure on optical transmission in such films.

Contribution

It introduces a method to suppress island formation and achieve stable, ultra-thin gold films with detailed analysis of their structural and optical properties, emphasizing interface effects.

Findings

Void-free gold films as thin as 5.4 nm were realized.

Optical transmission can be modeled using bulk dielectric function with interface considerations.

Non-abrupt metal-dielectric interfaces significantly reduce optical transmission.

Abstract

Realizing laterally continuous ultra-thin gold films on transparent substrates is a challenge of significant technological importance. In the present work, formation of ultra-thin gold films on fused silica is studied, demonstrating how suppression of island formation and reduction of plasmonic absorption can be achieved by treating substrates with (3-mercaptopropyl) trimethoxysilane prior to deposition. Void-free fi lms with deposition thickness as low as 5.4 nm are realized and remain structurally stable at room temperature. Based on detailed structural analysis of the fi lms by specular and diffuse X-ray reflectivity measurements, it is shown that optical transmission properties of continuous ultra-thin films can be accounted for using the bulk dielectric function of gold. However, it is important to take into account the non-abrupt transition zone between the metal and the surrounding dielectrics, which extends through several lattice constants for the laterally continuous ultra-thin films (film thickness below 10 nm). This results in a significant reduction of optical transmission, as compared to the case of abrupt interfaces. These findings imply that the atomic-scale interface structure plays an important role when continuous ultra-thin films are considered, e.g., as semi-transparent electrical contacts, since optical transmission deviates significantly from the theoretical predictions for ideal films.