Surface Morphology of Heterogeneous Nanocrystalline Rutile/Amorphous Anatase TiO₂ Films Grown by Reactive Pulsed Magnetron Sputtering

Abstract

Titanium dioxide films were grown on unheated substrates by mid‐frequency (100 kHz) reactive pulsed magnetron sputtering at different O2 partial pressures in an Ar/O2 atmosphere. X‐ray diffraction and absorption measurements reveal a transition from amorphous anatase to nanocrystalline rutile with [O2], with a mixture of both phases at intermediate values. Atomic force microscopy shows that the promotion of rutile is accompanied by surface roughening due to the apparition of hollow structures (holes) on the surface, in contrast with the extremely smooth morphology of amorphous anatase films. The number and size of holes increase with [O2] and growth time in such a way that they eventually coalesce and percolate to lead to a rough surface for those growth conditions for which a rutile rich film is obtained. Transmission electron microscopy analysis shows the growth evolution of a heterogeneous phase mixture and provides unambiguous correlation between the hole (smooth) morphology and the underlying rutile (anatase) phase. Therefore, the surface morphology results from the slower local growth rate of nanocrystalline rutile compared to that of the surrounding amorphous anatase flat regions as well as from the fast (slow) lateral (vertical) growth rate of the rutile domains.

Controlling surface morphology by nanocrystalline/amorphous competitive self-phase separation in thin films: Thickness-modulated reflectance and interference phenomena

“…Hence, competitive self-separation is not restricted to the Zr-Mo system and is likely general to other binary alloys exhibiting a composition-driven transition from an amorphous to a nanocrystalline phases. A similar separation has also been reported in the case of AlN [45] and TiO 2 [14] films.…”

“…Alternatively, thin films showing lateral self-separation of ceramic phases could be obtained in case of specific epitaxial relationships between phases or between at least one phase and the substrate [11,12,13]. Apart for some specific cases (see for instance [14,15]), the topography of the film surface has scarcely been reported to be markedly affected by the phase separation. To the best of our knowledge, surface morphology modification induced by phase separation has not been exploited so far to manipulate the functional properties.…”

Phase Selectivity in Cr and N Co-Doped TiO2 Films by Modulated Sputter Growth and Post-Deposition Flash-Lamp-Annealing

“…Figure 2 shows the GI-XRD patterns from monolithic and modulated TiO 2 :Cr,N coatings as well as from TiO 2 and TiO 2 :N monolithic films grown under equivalent conditions. The TiO 2 coating shows a mixture of rutile and anatase phases for this particular growth condition, although the relative phase contribution can be tuned, among other parameters, by the oxygen partial pressure [26]. Note that the broad bump around 55 • comes from the underlying Si(100) substrate.…”

“…Regarding N sites, the XANES N 1s shows a main contribution of interstitial sites in the form of -NO x radicals (x~2), as evidenced by the intense peak at~401 eV. However, as shown for the case of N-doped TiO 2 [26], such environments may have a relatively large cross-section and some N can also be in substitutional sites (N-Ti). This is further discussed below when presenting the XPS results.…”

“…Basically, the individual XANES spectra can be interpreted as a picture of the density of Ti-3d, O-2p, N-2p, and Cr-3d states according to dipole selection rules. A detailed description of the origin of the different spectral features and regions can be found in previous publications for binary [26,35] and ternary [17,20] oxides. The most direct information about the oxide matrix can be found in the Ti 2p edge, where anatase and rutile TiO 2 display clear distinct features, especially in the double-peak structure within the 459-462 eV region.…”