Interpretation of permittivity values in vapor deposited thin films of organic glasses

Abstract

Permittivity values of thin glassy films obtained by physical vapor deposition onto interdigitated electrode cells have been reported to increase considerably after annealing the sample above its glass transition temperature, which cannot be explained by effects of density. While such a change might be interpreted as a transformation to a different structure or altered dynamics, we employ a nonpolar molecular liquid to demonstrate that the apparent permittivity derived from impedance experiments can increase without changes in properties inherent in the sample material. More specifically, it is shown that annealing induces surface flattening that leads to material relocating from on top of the metallic electrode digits to between these digits. For films of thicknesses below 10% of the digit spacing, we demonstrate that the impedance measurement senses predominantly the material located between electrode digits, while being blind to the part of the sample deposited onto the digits. This implies that the apparent permittivity can increase without a change in material properties. A relation that determines film height and deposition rate from the observed capacitance increment is provided.