  Internal friction in oxide thin films imposes a critical limitation to the
sensitivity and stability of ultra-high finesse optical cavities for
gravitational wave detectors. Strategies like doping or creating nanolaminates
are sought to introduce structural modifications that reduce internal friction.
This work describes an investigation of the morphological changes SiO2/Ta2O5
and TiO2/Ta2O5 nanolaminates undergo with annealing and their impact on room
temperature internal friction. It is demonstrated that thermal treatment
results in a reduction of internal friction in both nanolaminates, but through
different pathways. In the SiO2/Ta2O5 nanolaminate, which layers remain intact
after annealing, the total reduction in internal friction follows the reduction
in the composing SiO2 and Ta2O5 layers. Instead, interdiffusion initiated by
annealing at the interface of the TiO2/Ta2O5 nanolaminate and the formation of
a mixed phase dictate a more significant reduction in internal friction to ~
2.6 * 10-4, a value lower than any other Ta2O5 mixture coating with similar
cation concentration.
