What can we Learn from Urbach Energy of Amorphous Oxides Thin Films: Experimental Correlation Between Optical and Mechanical Properties

Abstract : We have investigated by spectroscopic ellipsometry (SE) the optical properties (190 - 1700 nm) of uniform, amorphous thin films of Ta2O5 and Nb2O5 as a function of the annealing temperature and after so-called "doping" with Ti atoms which leads to mixed Ti:Nb and Ti:Ta oxides. Ta2O5 and Ti:Ta2O5 are currently used as high index components in Bragg reflectors of interference Gravitational Wave Detectors (GWD). Parallel to the optical investigation we measured the mechanical energy dissipation of the same coatings, through the so-called "loss angle", a parameter which is widely used to quantify 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 and the energy width of the exponential Urbach tail (the Urbach energy EU). We have found that EU is neatly reduced by suitable annealing as is also perceptible from direct inspection of SE data. Doping also points to a minor decrease of EU. The reduction of EU parallels a lowering of the mechanical losses quantified by the loss angle. The observed correlation between band-tail energy width and mechanical loss is tentatively explained in terms of concepts introduced in recent theoretical studies on a-Si and other systems related to forms of structural correlation in the amorphous state which can be linked to Urbach tails. The Urbach energy reduction we observe, promoted by annealing and to a lesser extent, by doping, is likely due to an atomic organization active on a medium/long range scale which corresponds to the same scale of the molecular clusters whose dynamics is responsible of both thermal noise and mechanical losses in these amorphous materials. In this respect these observations may contribute to a better...
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Submitted on : Thursday, August 1, 2019 - 12:00:02 AM
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Alex Amato, Silvana Terreni, Massimo Granata, Christophe Michel, Benoit Sassolas, et al.. What can we Learn from Urbach Energy of Amorphous Oxides Thin Films: Experimental Correlation Between Optical and Mechanical Properties. 2019. ⟨hal-02220137⟩



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