Growth mechanism of Nb-doped TiO₂ sol–gel multilayer films characterized by SEM and focus/defocus TEM

TiO₂ and Nb-doped TiO₂ films were prepared by sol–gel processing, their microstructure was adjusted by varying the number of subsequent coating-firing cycles that resulted in final total film thickness of ~100 nm. When only few subsequent coatings are stacked (large single layer thickness) granular polycrystalline microstructures are observed. Doping with Nb reduces the crystallite size compared to the respective pure anatase films. When the single layer thickness is reduced, the film growth is successively dominated by the nucleation of subsequent films on the underlying crystalline material resulting in a columnar dense film structure. The multilayer architecture of such films can be demonstrated by defocus TEM imaging even if crystalline columns exceed single film boundaries. Results indicate that Nb is homogeneously incorporated into the anatase lattice by substitution of Ti, nevertheless the electric conductivity after H₂ post annealing is significantly lower than reported for analogous films prepared by magneton sputtering or pulsed laser deposition.