Impact of the Growth Mechanisms on Si and Glass Substrates on the Structural, Optical and Electrical Properties of Anatase TiO2 Thin Films Synthetized By ALD Technique (Oral)

Titanium dioxide is a multipurpose material leading to many applications such as Li ion batteries, photocatalysis, dielectric in capacitors or transparent conductive oxides 1–3. Indeed, TiO2 can be stabilized under different crystallographic structures and the possibility to tune the oxygen stoichiometry offers a large range of electronic properties 1. Furthermore, among all allotropic forms of TiO2, the crystalline anatase phase is particularly interesting in optoelectronics and photocatalysis because of its long exciton lifetime and its semiconductor behaviour with a wide indirect bandgap of 3.2 eV 2. Depending on the application purpose, we will show the possibility to deposit anatase TiO2 films on single crystalline silicon substrates for microelectronics applications and on transparent glass substrates for photoelectrochemical cells or for TiO2-based TCOs materials, which require high transparency in the visible range. For both substrates, the growth mechanism of TiO2 is required to be understood and its impact on the optical end dielectric properties. TiO2 films were grown by atomic layer deposition (ALD) on (100) oriented p-type silicon and Schott glass substrates in a low deposition temperature (TD) range from 100°C to 300°C with titanium tri-isopropoxide (TTIP) precursor and water as oxidizing agent. A structural (TEM, XRD, AFM, Raman and FTIR), optical (ellipsometry, UV-vis-NIR spectroscopy) and electrical study was carried out. As-deposited TiO2 films were successfully crystallized on both silicon and glass substrates at temperatures ≥ 250°C without post-annealing treatment. All characterizations show that the crystallization on silicon and glass substrates is promoted by the TD increase but also depends of the film thickness. Specifically, it has been found that the growth mechanism follows the one observed elsewhere 4: (i) TiO2 growth begins as amorphous, then after a certain thickness, (ii) crystalline nuclei form randomly at the surface. (iii) These germs grow isotropically through the crystallization of the already deposited surrounding amorphous TiO2, as well as the newly deposited matter. Optical properties such as refractive index, absorption coefficient and band gap were simulated with new amorphous and Tauc-Lorentz dispersion models from ellipsometric measurement on silicon and UV-vis-NIR spectroscopy transmission measurement on glass. Results from both methods are in adequacy, showing an increase of the refractive index and the absorption coefficient with TD. These results are consistent with the densification and crystallization of the material characterized by the structural analysis. Finally, a decrease of the band gap is also observed with the increase of TD which tends to 3.2 eV confirming the formation of the anatase phase 2. For the dielectric properties, the relative permittivity ε’ and the loss angle tan δ of TiO2 films were studied with frequency. The relative permittivity increases with TD, due to the higher crystalline quality of the films. Low losses are observed, indicating a low density of charged defects in the films and a good quality of the dielectric properties. All observed structural, optical and electrical properties of TiO2 films are consistent, showing the possibility to stabilize the wide band gap anatase TiO2 on different substrates by ALD at low temperature. This work paves the way for the integration of this extremely versatile oxide for future devices. (1) Elbahri, M. B.; Kahouli, A.; Mercey, B.; Prellier, W.; Lüders, U. Effects of Oxygen Pressure during Deposition on the Dielectric Properties of Amorphous Titanium Dioxide Thin Films. J. Phys. Appl. Phys. 2019, 52 (17), 175308. https://doi.org/10.1088/1361-6463/ab06a1. (2) Niemelä, J.-P.; Marin, G.; Karppinen, M. Titanium Dioxide Thin Films by Atomic Layer Deposition: A Review. Semicond. Sci. Technol. 2017, 32 (9), 093005. https://doi.org/10.1088/1361-6641/aa78ce. (3) Mo, S.-D.; Ching, W. Y. Electronic and Optical Properties of Three Phases of Titanium Dioxide: Rutile, Anatase, and Brookite. Phys. Rev. B 1995, 51 (19), 13023–13032. https://doi.org/10.1103/PhysRevB.51.13023. (4) Puurunen, R. L.; Sajavaara, T.; Santala, E.; Miikkulainen, V.; Saukkonen, T.; Laitinen, M.; Leskelä, M. Controlling the Crystallinity and Roughness of Atomic Layer Deposited Titanium Dioxide Films. J. Nanosci. Nanotechnol. 2011, 11 (9), 8101–8107. https://doi.org/10.1166/jnn.2011.5060.