**4. Conclusions**

Pure BiFeO3 and Bi0.9La0.1FeO3 thin films were prepared on Pt/TiO2/SiO2/Si substrates by a modified sol–gel technique using a separate hydrolysis procedure. The effects of final crystallization temperature and La doping on phase structure, film surface quality, and nanoscale piezoelectric properties were investigated. Major conclusions of the work can be formulated as follows: (i) La doping and higher crystallization temperature lead to the increase of the grain size and apparent (102) texture of the films. (ii) Simultaneously, a decrease in the average effective piezoelectric coefficient (about 2 times in La-doped films) and an increase in the concentration of surface non-polar phase (up to 60%) are observed. (iii) Phase separation on the surface is related to either the appearance of second phases due to Bi loss at the surface or to the formation of the pseudocubic perovskite phase. (iv) As compared with the undoped BiFeO3, La-doping also increases the average grain size and self-polarization that is important for future piezoelectric applications. Piezoelectric property is shown to be directly connected to the films' growth conditions and doping, emphasizing that both should be thoroughly controlled in order to use BFO-based films in micromechanical applications.

**Author Contributions:** D.L.K., S.A.K., A.L.K. conceived and designed the experiment; A.V.S., V.V.S. prepared the samples; I.B., V.E.G., S.K. performed the experiments; I.B., S.K., O.P., A.V.S. analyzed the data and prepared the original draft; A.L.K., O.P., D.L.K., S.A.K. reviewed and edited the draft. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, refs. UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the FCT/MCTES. The work was performed within the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie gran<sup>t</sup> (agreement No 778070). The project was partly supported by RFBR and BRFBR, project numbers 20-58-0061 and T20R-359, respectively. Part of this work was funded by national funds (OE), through FCT—Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of 19 July.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
