Observation of a Correlation Between Internal friction and Urbach Energy in Amorphous Oxides Thin Films

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We have investigated by spectroscopic ellipsometry (SE, 190-1700 nm) the optical properties of uniform, amorphous thin films of Ta2O5 and Nb2O5 as deposited and after annealing, and after so-called "doping" with Ti atoms which leads to mixed oxides.

• Ta2O5 and Ti:Ta2O5 are currently used as high-index components in Bragg reflectors for Gravitational Wave Detectors.
• Parallel to the optical investigation, we measured the mechanical energy dissipation of the same coatings, through the so-called "loss angle" phi = Q^-1, which quantifies the energy loss in materials.
• By applying the well-known Cody-Lorentz model in the analysis of SE data we have been able to derive accurate information on the fundamental absorption edge through important parameters related to the electronic density of states, such as the optical gap (E_g) and the energy width of the exponential Urbach tail (the Urbach energy E_U).