Orientation Control of Anatase Nb-doped TiO₂ Epitaxial Thin Films by Mist Chemical Vapor Deposition

[Introduction] Anatase titanium dioxide (TiO₂) thin films attract considerable attention in the field of catalysis^[1], sensors^[2], and electronic devices^[3]. The physical properties of anatase TiO₂ thin film generally depend on their crystallographic orientation. The underlying substrate plays a crucial role in controlling the orientation. To date, orientation control has been achieved using LaAlO₃ (LAO), SrTiO₃ (STO), [LaAlO₃]_0.3[SrAl_0.5Ta_0.5O₃]_0.7 (LSAT), and Y-stabilized zirconia (YSZ) substrates with pulse laser deposition and sputtering methods^[4]. However, few reports exist on controlling crystallographic orientation besides [001] and [012] orientation.<br/>The mist chemical vapor deposition (CVD) method is excellent at controlling the crystal growth orientation. In the film formation process, the chemical reactions of precursors in the mist and the epitaxial effects from the substrate work concertedly. As a result, metastable crystal phases often appear^[5]. On this basis, we employed mist CVD to fabricate epitaxial anatase Nb-doped TiO₂ (TNO) thin films and investigate the influence of the substrate, aiming to control the crystallographic orientation.<br/><br/>[Experimental method] TNO epitaxial thin films were fabricated using a hot-wall type mist-CVD. LAO (100) and α-Al₂O₃ (0001) single-crystal substrates were used. To prepare the precursor solution, titanium tetra-isopropoxide, Ti(OC₃H₇)₄, and niobium ethoxide, Nb(OC₂H₅)₅ were dissolved in hexane. The substrate was put in the furnace, and the precursor solution was atomized in the glass bubbler and introduced to the electric furnace by the flow of nitrogen gas (99.99%). The 20 nm thick film was deposited on the substrate by keeping the temperature at 500 °C for 40 minutes. Subsequently, the as-deposited films were subjected to vacuum annealing treatment. The crystal structure, crystallinity, and crystallographic orientation of the thin films were characterized by X-ray diffraction (XRD) measurement. The electrical transport properties were evaluated using the van der Pauw method.<br/><br/>[Results and Discussion] When TNO was deposited on LAO (001) single-crystal substrate, (001)-oriented epitaxial thin films with (112)-orientation secondary domains were formed. This is consistent with cases in other deposition methods. In contrast, (112)-orientated anatase-TNO were formed when the films were deposited on α-Al₂O₃ (0001) single-crystal substrate. Interestingly, although α-Al₂O₃ (0001) single-crystal substrate has been used for rutile TiO₂ epitaxial thin films in physical vapor deposition, (112)-oriented anatase TiO₂ is formed in mist-CVD. Furthermore, we measured resistivity along the in-plane direction. The obtained films on α-Al₂O₃ (0001) and LAO (100) show 2.7 × 10⁻² and 7.6 × 10⁻⁴ Ω cm at room temperature, respectively. This is reasonable based on a larger effective mass of TiO₂ for [001] direction than for [100] directions^[6]. These results provide fundamental insights for accessing unexplored physical properties by achieving epitaxial thin film growth with unprecedented orientations by selecting single-crystal substrates in the mist-CVD process.<br/><br/>1. A. Fujishima, and K. Honda, <i>Nature</i> <b>37</b>, 238 (1972).<br/>2. O. Carp, C. L. Huisman, and A. Reller, <i>Prog. Solid State Chem.</i> <b>32</b>, 33 (2004).<br/>3. K. Hashimoto, H. Irie, and A. Fujishima, <i>Jpn. J. Appl. Phys.</i> <b>44</b>, 8269 (2005).<br/>4. S. Yamamoto, T. Sumita, T. Yamaki, A. Miyashita, and H. Naramoto, <i>J. Cryst. Growth.</i> <b>237</b>, 569 (2002).<br/>5. K. Kaneko, S. Fujita, and T. Hitora, <i>Jpn. J. Appl. Phys.</i> <b>57</b>, 02CB18 (2018).<br/>6. Y. Hirose, N. Yamada, S. Nakao, T. Hitosugi, T. Shimada, and T. Hasegawa, <i>Phys. Rev. B. </i><b>79</b>, 165108 (2009).<br/><br/>Acknowledgments<br/>This research is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).