Investigation of the anatase-to-rutile transition for TiO$_2$ sol-gel coatings with refractive index up to 2.7

TL;DR

This study investigates the phase transition from anatase to rutile in TiO2 sol-gel coatings, achieving high refractive indices up to 2.7 with low optical loss, relevant for integrated photonics applications.

Contribution

It demonstrates controlled crystallization of TiO2 coatings with high refractive indices and low optical losses, elucidating the phase transition mechanisms during thermal annealing.

Findings

Rutile coatings with refractive index 2.7 and 1% optical loss were achieved.

The anatase-to-rutile transition involves nanocrystal growth and phase transformation.

Formation of rutile platelets affects optical properties and loss.

Abstract

This work describes the elaboration of rutile titanium dioxide films with high refractive indices and low scattering by sol-gel process and controlled crystallization. The evolutions of the optical properties and crystalline structure of sol-gel processed titania coatings on fused silica were investigated for different thermal budgets of the annealing post-treatment using ellipsometry, spectrophotometry, X-ray diffraction and electronic microscopy. It reveals that anatase and rutile coatings with refractive indices of 2.5 and 2.7 can be prepared with associated optical loss of 0.5% and 1%, respectively, which are excellent compromise for applications in integrated photonics. These evolutions are associated to the thermally induced mass transfer and phase transitions occurring during thermal annealing that involves first the nucleation growth and sintering of anatase polyoriented nanocrystals, followed by the transformation into rutile polyoriented nanocrystals. Concomitantly, rutile crystals with (110) faces parallel to the surface consume surrounding anatase and rutile nanocrystals by diffusive sintering to yield micron-size rutile monocrystalline and monooriented platelets patchwork, exhibiting refractive index of 2.73 and 1.2% optical loss. The formation of these platelets is governed by surface energies and is responsible for the increase in optical loss.