Amorphous-to-crystalline transition of thin-film TiO2 precursor films to brookite, anatase, and rutile polymorphs

We observe the isothermal amorphous-to-crystalline transition of TiO2 thin films deposited by reactive RF magnetron sputtering with in-situ optical microscopy at several temperatures. The crystalline fraction of single-phase anatase, brookite, and rutile films as a function of time is analyzed within the Johnson-Mehl-Avrami-Kolmogorov framework to extract model parameters. Avrami exponents near 3 for anatase are consistent with 2-dimensional growth and a constant nucleation rate but are significantly higher for brookite and rutile, which we attribute to an increasing nucleation rate. The number of crystallization centers per unit area for rutile is higher than for brookite which in turn is higher than anatase. Total crystallization times for brookite and anatase decrease with increasing film thickness, consistent with a volume-induced nucleation model. Brookite grows anisotropically in contrast to the circular morphology observed for anatase and rutile.