  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.
