Titanium dioxide (TiO₂) is a widely used material for photocatalytic, optical, electrical and medical applications. Atomic layer deposition (ALD) represents an excellent technique for the synthesis of thin films due to its precise thickness control, simplicity, high conformity of obtained films and reproducible growth of defect-free films.

It was shown recently that photo-catalytic TiO₂ films grown on cellulose-based and porous substrates can be used in water purification systems [1]. Its photo-catalytic activity strongly depends on the crystal structure and the grain size of the film, i.e. the TiO₂ films must have a well-defined anatase phase with large polycrystalline grains [2]. It was recognized that plasma enhanced ALD (PEALD) growth of the TiO₂ film can produce the anatase phase even at low deposition temperatures [3]. This result is important for the growth of thin polycrystalline films on temperature-sensitive materials, such as organic substrates. On the other hand, the grain size is shown to depend critically on the type and the morphology of substrates [4].

We have investigated the effects of thin intermediate layers, grown by ALD on silicon substrates, on the grain size of the TiO₂ films, grown by thermal ALD and PEALD technique in a wide temperature range, from room temperature up to 300^oC. Amorphous TiO₂ films were obtained with thermal ALD for temperatures below 150^oC, while the anatase phase with crystalline aggregates has been identified on films synthesised with PEALD at low temperatures. We show that the size of crystallites can be greatly enlarged if grown on an intermediate layer of amorphous Al₂O₃. The films were characterised by a range of analytical techniques, including scanning electron microscopy with energy dispersive x-ray spectroscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and x-ray diffraction.

References

[1] M. Knez, K. Nielsch, L. Niinistö, Adv. Mater. 19, 3425 (2007).

[2] J. G. Chen, Surf. Sci. Rep. 30, 1 (1997).

[3] S. Ratzsch, E.-B. Kley, A. Tünnermann, A. Szeghalmi, Nanotechnology 26, 024003, (2015).

[4] R. L. Pruurunen, et al., J. Nanosci. Nanotechnol. 11, 8101 (2011).

Figures:

Fig. 1: (a) SEM image of surface morpholoy for a thin TiO2 film grown at 250oC with visible large grains; (b) SIMS profile from the same sample, showing changes in Ti, O and Si signals, and (c) XPS showing formation of stoichiometric TiO2.

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EMC Abstracts - https://emc-proceedings.com/abstract/controlling-the-grain-size-of-polycrystalline-tio2-films-grown-by-atomic-layer-deposition/