Origin of Mechanical and Dielectric Losses from Two-Level Systems in Amorphous Silicon

TL;DR

This study investigates the origins of mechanical and dielectric losses in amorphous silicon, revealing they are caused by different types of two-level systems related to atomic density and dangling bonds.

Contribution

It provides experimental evidence that mechanical and dielectric losses in amorphous silicon originate from different two-level systems, linked to atomic density and dangling bonds respectively.

Findings

Mechanical loss decreases significantly with higher growth temperature.

Dielectric loss shows a smaller reduction and depends on film thickness.

Mechanical and dielectric losses are correlated with atomic density and dangling bonds, respectively.

Abstract

Amorphous silicon contains tunneling two-level systems, which are the dominant energy loss mechanisms for amorphous solids at low temperatures. These two-level systems affect both mechanical and electromagnetic oscillators and are believed to produce thermal and electromagnetic noise and energy loss. However, it is unclear whether the two-level systems that dominate mechanical and dielectric losses are the same; the former relies on phonon-TLS coupling, with an elastic field coupling constant, $\gamma$, while the latter depends on a TLS dipole moment, $p_0$, which couples to the electromagnetic field. Mechanical and dielectric loss measurements as well as structural characterization were performed on amorphous silicon thin films grown by electron beam deposition with a range of growth parameters. Samples grown at 425 $^{\circ}$C show a large reduction of mechanical loss (34 times) and a far smaller reduction of dielectric loss (2.3 times) compared to those grown at room temperature. Additionally, mechanical loss shows lower loss per unit volume for thicker films, while dielectric loss shows lower loss per unit volume for thinner films. Analysis of these results indicate that mechanical loss correlates with atomic density, while dielectric loss correlates with dangling bond density, suggesting a different origin for these two energy dissipation processes in amorphous silicon.