Abstract
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” ϕ = 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 (Eg) 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. Ti-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 ϕ. The correlation highlights that both the electronic states responsible of Urbach tail and the internal friction are sensitive to a self-correlation of defects on a medium-range scale, which is promoted by annealing and in our case, to a lesser extent, by doping. These observations may contribute to a better understanding of the relationship between structural and mechanical properties in amorphous oxides.
Highlights
Www.nature.com/scientificreports of GWD detectors, necessary to investigate deeper portions of the universe, calls for a lowering of coating thermal noise and a better knowledge of the structure of AO coatings at the molecular level[12]
It has been shown through Radial density function (RDF) measurements on Ti:Ta2O5 coating that a decrease of the measured loss angle is accompanied by a small improvement at the short-range scale inferred by a width reduction of the main RDF peak, related to metal-oxygen distances
A very recent paper[20] on the Zr:Ta2O5 system confirms that annealing produces systematic changes at the intermediate range scale; atomic modelling shows that such changes are to be related to well definite changes of the connections between Primary Structural Units (PSU)
Summary
Www.nature.com/scientificreports of GWD detectors, necessary to investigate deeper portions of the universe, calls for a lowering of coating thermal noise and a better knowledge of the structure of AO coatings at the molecular level[12]. In the context of optical measurements, so called Urbach tails[13], routinely observed by optical absorption measurements in crystalline and amorphous semiconductors[14], describe a sub-gap exponential broadening of the absorption edge that is related to structural and thermal disorder. The concept of band-tails states finds interesting application in the study of amorphous solids[15,16] where the structural disorder is dominant with respect to thermal one. In this view, the structural origin of the exponential tails[17,18] can give precious insight in atomic organization of the system. The correlation is observed analysing different oxide coating materials, suggesting a general rather than specific validity of such properties as suggested by the conclusions of a recent paper on Zr-doped tantala[20]
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