Tunneling states in amorphous silica films grown at different temperatures*

In this work we report low temperature mechanical loss, specific heat and structural characterization of amorphous silica (a-SiO₂) films grown by UHV electron-beam evaporation at substrate temperatures from 50 to 900 °C. The films’ density increases with increasing growth temperature. Through positron annihilation studies and shear modulus measurements, we also observe an increase in the average nanovoid size with growth temperature while the network becomes more compact and rigid. Mechanical loss and specific heat below 10 K decrease with increasing growth temperature; both properties are associated with tunneling states whose density decreases with increasing growth temperature, correlated with the structural changes discussed above. However, the density of tunneling states lies within the universal glassy range for all growth temperatures, unlike what was seen in a-Si, and is larger than that reported in bulk a-SiO₂. We also study the effect of hydrogen due to water absorption, in capped and uncapped films, and conclude that water does not affect the tunneling states in these a-SiO₂ films.