The search for new coating materials, particularly high-refractive-index layers, is crucial in the field of interferometric gravitational-wave (GW) detectors. These detectors require coatings with low mechanical loss to minimize thermal noise and low optical absorption to maintain measurement stability and precision. The development of novel high-index coating materials with improved mechanical and optical properties has the potential to advance GW detection and enable more precise measurements of astrophysical phenomena.
This study investigated the structural and mechanical properties of amorphous silicon (a-Si), hydrogenated amorphous silicon (a-Si:H), and amorphous silicon carbide (a-SiC) films deposited using magnetron sputtering (MS). The effects of deposition parameters, such as working gas, gas pressure, substrate temperature, and post-growth treatments including vacuum annealing and post-hydrogenation, were explored. The choice of working gas and its pressure, which influences the kinetic energy of the sputtered atoms upon arrival at the substrate, was found to significantly affect the properties of the deposited films. The impact of energetic incoming atoms is reflected in compressive film stress and film densification, a phenomenon known as the atomic peening effect. Additionally, a transition in dominant defects from nanovoids in e-beam a-Si to divacancies and self-interstitials in MS a-Si was observed, which can be attributed to the higher kinetic energy of incident atoms. By selecting appropriate working gas and deposition conditions, such as low gas pressure and high growth temperature, the structural order was improved, as indicated by lower bond angle deviation, and the mechanical loss of MS a-Si was reduced at both low temperatures and room temperature.
Introduction of hydrogen to a-Si had a significant impact on its optical absorption at infrared wavelengths and the room-temperature mechanical loss. We propose that hydrogen plays a dual role in a-Si:H, acting as a passivating agent for dangling bonds, particularly at high temperatures, and facilitating structural relaxation, and the latter is responsible for the observed changes in the physical properties of a-Si:H. The incorporation of carbon in a-SiC films resulted in higher mechanical loss and optical absorption compared to pure a-Si films. These findings provide insights into the underlying mechanisms and offer potential solutions for optimizing the performance of a-Si-based coatings for GW detector mirrors.