Heterogeneous Nucleation of YBCO Film Deposited by MOD: Experimental Verification of a Thermodynamic and Kinetic Model

The deposition of superconducting YBa₂Cu₃O_7-<i>δ</i> (YBCO) thin film can be performed through physical and chemical methods, being the latter ones really advantageous in terms of cost-effectiveness, versatility, and simplicity. Therefore, within the last decade, the use of chemical solution deposition method, in particular the metal organic decomposition (MOD) approach, for producing epitaxial YBCO films has played an increasing role in the development of scalable processes.<br/>In recent years, the need for a quantitative model for the YBCO heterogeneous nucleation emerged in order to better control the parameters affecting the formation of epitaxial film deposited by fluorine based MOD process. In fact, despite the great number of articles on YBCO deposition, the nucleation has been scarcely studied in the literature and only at qualitative level. However, it is a crucial step because the best superconducting properties are exhibited by YBCO film with the <i>c</i>-axis orientation. Recently, a thermodynamic and kinetic study on the heterogenous nucleation of YBCO film has been proposed in [1]. The model focused on fluorine based MOD deposition on SrTiO₃ (STO) single crystal and was developed on the basis of the classical nucleation theory providing an estimate of <i>c</i>-axis grain fraction as a function of water partial pressure and temperature. This latter calculation could constitute a useful tool to <i>a priori</i> define the proper experimental conditions for depositing highly epitaxial <i>c</i>-axis film when low crystallization temperatures may be necessary.<br/>In the present work, the experimental verification of the model is proposed. The deposition of YBCO films on STO single crystal has been performed at different crystallization temperatures and water partial pressure values. The prepared samples have been fully characterized through the analysis of microstructure, morphology, and superconducting properties. The <i>c</i>-axis fraction has been derived by x-ray diffraction analysis and the experimental data have been described by the theoretical curves calculated from the model.<br/>The deposition on a different substrate, namely LaAlO₃ (LAO), has been also carried out to study the effect of the substrate on YBCO nucleation kinetics. In fact, the expression for the free energy barriers includes the work of adhesion of the YBCO/substrate interface. The preliminary results obtained for YBCO/LAO samples evidenced the critical importance of this quantity and its technological implications.<br/>In general, the present study shows that a deeper comprehension of the nucleation mechanism can lead to the improvement not only of the YBCO superconducting properties, but also of the process robustness and reproducibility.<br/>[1] V. Pinto, G. Celentano, and M. Tomellini, <i>Supercond. </i><i>Sci. Technol.</i>, vol. 33, no. 11, p. 115006, Nov. 2020, doi: 10.1088/1361-6668/abb201.