Tunneling Two-Level Systems in Amorphous Silica Thin Films

Amorphous silica (silicon dioxide) is one of the strongest glass formers known, and silicate glasses are by far the most widely produced for applications. Liquid-quenched silica shows tunneling two-level system density, as probed by low-temperature specific heat and internal friction, in the so-called universal glassy range observed for nearly all amorphous materials. We present low-temperature (down to 300 mK) specific heat and mechanical loss measurements on silica thin films grown at temperatures from 60 to 800 °C by e-beam evaporation. Results indicate that silica grown near room temperature shows TLS density, extracted from cryogenic specific heat, above the universal range. The mechanical loss, though high, is still within the universal range, and falls to the level of bulk silica as the growth temperature is increased to 800 °C. This is consistent with the observed increase in atomic density of approximately 10% as the growth temperature is raised 60 to 800 °C. These results are compared to previous work on amorphous silicon and tantala, with attention drawn to the somewhat unusual relative change of the sound velocity with temperature and how this changes systematically with growth temperature for the silica films.