Origin of mechanical and dielectric losses from two-level systems in amorphous silicon

Tunneling two-level systems (TLSs), which are the main contributors to energy loss of amorphous solids at low temperatures, are found to vary widely in amorphous silicon (a-Si), depending strongly and reproducably on controllable growth parameters. TLSs affect both mechanical and electromagnetic resonators and produce thermal and electromagnetic noise and energy loss. It is unclear whether the TLSs that dominate mechanical and dielectric losses are the same; while the former relies on the coupling between TLSs and elastic fields, determined by coupling constant gamma, the latter relies on the coupling between TLSs and electromagnetic fields, determined by dipole moment ??0. We prepared a-Si films by ultra-high vacuum electron-beam deposition with a range of growth parameters and characterized them structurally (including atomic density, short and medium range order, and dangling bond density) and by low temperature mechanical and dielectric loss measurements using double paddle high Q oscillators and superconducting microwave resonators respectively. Films show a large reduction of mechanical loss (>30 times) and a far smaller reduction of dielectric loss (~2 times) with increased growth temperature. Additionally, mechanical loss shows lower loss for thicker films, while dielectric loss shows lower loss for thinner films. Analysis of these results indicates that mechanical loss correlates with atomic density, while dielectric loss correlates with dangling-bond density, which suggests a different structural origin for these two energy dissipation processes in a-Si. Alternatively, since mechanical and dielectrics loss results are obtained for two quite different frequency ranges (kHz and GHz), these data could support the idea that the standard tunneling model for TLS requires modification.