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Pavel Ondračka, David Holec, David Nečas, Eva Kedroňová, Stéphane Elisabeth, Antoine Goullet, Lenka Zajíčková

Optical properties of TixSi1−xO2 solid solutions

Physical Review B 95 (2017) 195163

In this work, we use the Density Functional Theory to predict optical properties of TixSi1−xO2 solid solutions. The Special Quasi-random Structure method and the Simulated Annealing procedure were used to produce models of crystalline and amorphous TixSi1−xO2. These were fully structurally optimised by the VASP package, while their electronic structure and optical properties were subsequently calculated using the Wien2k package employing the TB-mBJ potential. To further support the calculations, experimental samples were synthesized by means of plasma-enhanced chemical vapour deposition. The calculated band gaps for a-TixSi1−xO2 evaluated using the Tauc-like fitting approach are 8.53 eV for SiO2, quickly decreasing to 4.0 eV at x = 0.19, 3.52 eV at x = 0.34 and 3.24 eV at x = 1.0. Ellipsometry and spectrophotometry yielded a compositional trend for the experimental optical band gap comparable with our predictions: a quick decrease from value of 7.94 eV for pure SiO2, to 3.91 eV at x = 0.15 and then a much slower decrease over the rest of the composition range ending at 3.26 eV for pure TiO2 . A detailed analysis of anatase and rutile based solid solutions reveals introduction of silicon-induced oxygen states into the band gap in the TiO2-rich composition region, which results in the predicted reduction of the fundamental band gap. However we have showed that the optical absorption on those states is negligible. Moreover, we have also obtained a good agreement between calculated and measured imaginary part of the dielectric function εi, especially for the TiO2 rich compositions. Finally, we predicted an almost linear refractive index change at 632.8 nm between a-SiO2 (1.36) and a-TiO2 (2.34), which was experimentally confirmed.

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DOI: 10.1103/PhysRevB.95.195163

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