Origin of Two-Level Systems in Amorphous Silicon*

Amorphous silicon shows anomalous properties at low temperatures due to two-level systems, which affect both mechanical and electromagnetic oscillators. However, it is unclear whether the two-level systems responsible for these effects are the same. We performed mechanical and dielectric loss measurements of amorphous silicon films grown by electron beam deposition. Samples grown at 425 °C show a large reduction of mechanical loss and a mild reduction of dielectric loss compared to those grown at room temperature. Results indicate mechanical loss is related to mass density, while dielectric loss is to dangling bond density. Moreover, mechanical loss results show lower loss per unit volume for thicker films, while dielectric loss results show lower loss per unit volume for thinner films, suggesting an underlying structural and differential origin for both types of energy dissipation processes. Additionally, specific heat measurements show that the density of two-level systems extends far above the well-known glassy range, indicating that some interactions play a crucial role in creating the universal values of mechanical losses at low temperatures.