Growth of nanocrystalline magnetron sputtered TiO₂ thin films studied by X-ray scattering

R. Kužel¹, L. Nichtová¹, Z. Matěj, D. Heřman², J. Musil²

¹Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic;
²Faculty of Applied Sciences, University of West Bohemia in Pilsen, Czech Republic

Titanium dioxide films are widely used because of their excellent properties as chemical stability, mechanical hardness and optical transmittance with high refractive index. The photocatalytic activity of TiO₂ can result in the decomposition of organic compounds on the TiO₂ surface or the reduction of the contact angle between water and the TiO₂ surface under ultraviolet irradiation. The films can be prepared by several techniques but the magnetron deposition is favourable from the point of view of mechanical durability required for practical applications. Depending on the deposition conditions, the films can be prepared as amorphous or nanocrystalline.

In the present work, a complex XRD study was performed on the films sputtered by dual magnetron on the glass and single-crystal silicon substrates. A set of nanocrystalline films with different thickness in the interval of 0.1 mm to 2 mm was investigated after the deposition. The measurement was performed on Philips X'Pert MRD in parallel beam setup, 2q scans with very low angles of incidence with collimators and Goebel mirror in the primary beam. X-ray reflectivity curves were measured too. The curves dropped rapidly for all the samples except the thinnest one (0.1 mm). In addition, contact angles of water drops on the film surface were measured after different periods of films irradiation by UV light.

The films deposited on glass subrate shown similar behaviour to the ones on Si substrate in many cases but there are also significant differences. The films on silicon show usually more pronounced texture. The most important differences were found in the phase composition for the thinnest samples, though. The thinnest film (0.1 mm) on glass substrate was amorphous while for the same film on silicon clear broad peaks of rutile were detected. Similarly, for the film with the thickness of 0.2 mm (Fig. 1a) for which small rutile peaks can also be seen on glass substrate. For none of these thin films any significant decrease of the drop contact angle after UV irradiation was found even though there are visible differences in their surface roughness. The angles remained on high values (65 - 75°). For the films thicker than 0.5 mm, anatase is clearly dominating phase (Fig. 1b, 1c) and for the 2 mm thick film only pure anatase was detected.

A set of measurements with different angles of incidence 0.5 – 5° and consequently also different penetration depths (the penetration depth for the smallest angle of incidence is below 200 nm) clearly show that rutile grows on the interface with both Si and glass substrates in all cases but its growth is supressed for thicker films by anatase which is mainly on the top (Fig. 1d). Since for all the thicker films the angles of water drops were drastically reduced after UV irradiation (5 hrs irradiation, down to 10 – 20°), it seems that only anatase is favourable in this case for hydrophilicity and by contrast rutile makes films hydrophobic. Differences between both substrates were also found. The films on silicon have better hydrophilicity than corresponding films on glass. From the structural point of view, the former have slightly better crystallinity and more pronounced texture.

Because of very broad peaks, the detailed measurement of residual stresses could be performed on the Eulerian cradle by the sin²ψ method only for two different peaks of anatase phase (101, 200). The dependences were strongly curved but with the absence of splitting for negative and positive inclination angle which could indicate triple-axis stresses. The curvature can be ascribed to the texture. The pltos are also very different for both peaks. For the peaks 101 and 200 the pole figures have also been measured and shown the presence of (100) texture. The texture was not completely fiber and for samples on Si it was significantly inclined with respect to the surface (Fig. 2). The asymmetry of the texture can be well understood, if we take into account that the deposition was made by dual magnetron.

This work is supported by the Grant Agency of the Czech Republic, no. 106/06/0327.

a)	b)
c)	d)
Fig. 1. Diffraction patterns of TiO2 films with different thickness – a) 0.1 mm, b) 0.5 mm, c) 2 mm. The patterns were obtained in parallel beam optics and 2q scan and fixed angle of incidence – 1.5º. In Fig. 1d) – patterns from the sample with the thickness 0.9 mm, obtained at different angles of incidence – lower thick line - 0.5º, middle thin line - 1.0º, upper thick line - 1.5º. Thick bars indicate theoretical peak positions for anatase, thin bars correspond to the peaks of rutile.

Fig. 2. Pole figure 200 for the sample 2 mm thick. The asymmetry of the texture can be seen in 2.5D plot (left) as well as the inclination of the texture on Si substrate.