Transition metal oxides (TMO)
The transition metal oxides, such as TiO2, Nb2O5, Ta2O5, HfO2 are wide band gap materials (EG>3 eV) and their thin films grown by magnetron sputtering cover a wide range of structures from amorphous to single crystalline that is the reason for a broad variety of properties they show. In order to extend functionality of these films, the present research on TMO at FZD is focused at:
- understanding relationship between the magnetron plasma parameters and the TMO film morphology/structure;
- achieving control over the film morphology, crystallinity, and phase ratio in mixed polymorphic materials on the nanometer scale;
- understanding the influence of the film structure on its mechanical, optical and electrical properties.
Comparison of the properties of Nb2O5, Ta2O5, HfO2 films deposited using different technologies involving bombardment of the growing film by energetic particles shows that a certain fraction of closed pores in the films is essential in order to achieve the trade-off between high refractive index, low mechanical stress and thermal shift . Increase of the pore fraction above the optimum value leads to a decrease of the film refractive index (example of Nb2O5 is shown in the Figure). Appearance of the open pores leads to absorption of the atmospheric humidity, as indicated by increased hydrogen concentration in the film (see Figure). The latter is responsible for substantial thermal variability of the refractive index.
It is shown that using high plasma ion density during reactive pulsed magnetron sputtering (RPMS) allows tailoring Nb2O5 film porosity and provides an optimum trade-off between mechanical and optical properties of the film .
Well-ordered nanorod arrays are produced by N+ ion irradiation (angle of incidence of 45°, energy of 15 keV) of polycrystalline anatase TiO2 films grown by CVD . The transition from amorphous films with anatase local-range order to nano-crystalline rutile TiO2 films is investigated with respect to the variation of the deposition conditions during RPMS . This is correlated with the film optical properties and surface morphology. An approach to produce highly crystalline anatase films of TiO2 using RPMS at high growth rates is established. It is shown, that by proper tuning of the oxygen partial pressure during the deposition the films can be made electrically conductive and transparent.
AiF Project No. 15042 BR “NANOMORPH”, 2006-2008
Fraunhofer Institut Angewandte Optik und Feinmechanik, Jena
Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Cientificas, Sevilla, Spain
Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Madrid, Spain
Partners from industry
1. O. Stenzel, S. Wilbrandt, N. Kaiser, M. Vinnichenko, F. Munnik, A. Kolitsch, A. Chuvilin, U. Kaiser, J. Ebert, S. Jakobs, A. Kaless, S. Wüthrich, O. Treichel, B. Wunderlich, M. Bitzer, and M. Grössl: The correlation between mechanical stress, thermal shift and refractive index in HfO2, Nb2O5, Ta2O5 and SiO2 layers and its relation to the layer porosity. Thin Solid Films 517, 6058 (2009).
2. M. Vinnichenko, A. Rogozin, D. Grambole, F. Munnik, A.Kolitsch, and W.Moeller, O. Stenzel, S. Wilbrandt, A.Chuvilin, and U. Kaiser: Highly dense amorphous Nb2O5 films with closed nano-sized pores. Appl. Phys. Lett. 95, 081904 (2009).
3. P. Romero-Gomez, A. Palmero, F. Yubero, M. Vinnichenko, A. Kolitsch, and A.R. Gonzalez-Elipe: Surface nanostructuring of TiO2 thin films by ion beam irradiation. Scr. Mater. 60, 574 (2009).
4. R. Gago, M. Vinnichenko, A. Redondo-Cubero, and L. Vázquez: Surface morphology of heterogeneous nanocrystalline rutile/amorphous anatase TiO2 films grown by reactive pulsed magnetron sputtering. Plasma Process. Polym. 7, 813 (2010).