(Pb,Sr)TiO3 Films on SrTiO3 (100) Substrates Grown by Liquid Phase Epitaxy
Résumé
PbTiO3 is a ferroelectric perovskite with a tetragonal structure at room temperature. It is extensively studied with different substitutions to obtain different polar phases and tunable piezoelectric and ferroelectric properties. One strategy is substitution on the Pb-site with Sr, within the (Pb,Sr)TiO3 solid solution, leading to a reduction in unit cell tetragonality, reaching cubic structure at room temperature for x=0.5 [Xing2003]. Controlling the Sr-content and thereby the structure gives the possibility to tune the electrical properties including reducing the Curie temperature, but also to stabilize a-domains depicting interesting nanoscale switching phenomena when grown as thin films [Matzen2014].
Here, we focus on presenting our work on epitaxial (Pb,Sr)TiO3 films synthesized by liquid phase epitaxy. This technique is applied for growing complex oxides on single crystalline substrates using a supersaturated solution comprising the desired oxide components of the film and a fitting solvent. Since it is a near thermodynamic equilibrium technique and the growth temperature is considerably lower than for bulk crystal growth, the defect concentration is often very low. By applying liquid phase epitaxy we have successfully grown films of Pb1-xSrxTiO3, with nominal compositions of x=0, 0.33, 0.39, and 0.43, on (100) SrTiO3 substrates. Energy dispersive X-ray measurements performed on selected films reveal an increase in the Sr-content for each nominally increase of Sr in the growth solution. Structural and spectroscopic investigations from X-ray diffraction and Raman spectroscopy at room temperature reveal all the investigated films are free of detectable impurities and possess tetragonal structure, with stable domain microscale patterning. In addition, for films with increasing Sr-content, the tetragonality decreases accordingly. These results confirm that we are able to control the composition and structure of the films by adapting the nominal composition of our growth solution. We will present and discuss the synthesis
parameters applied for growth of the (Pb,Sr)TiO3 films by liquid phase epitaxy and the challenges encountered in relation to the growth. Structural and phononic fingerprints and domain patterning from X-ray diffraction and Raman spectroscopy will be presented as well as high temperature dependent studies, where we investigate the phase transitions.
Our work underlines the successful growth of high crystalline quality films from the (Pb,Sr)TiO3 family by liquid phase epitaxy. We therefore hope these encouraging results can inspire others to
explore this synthesis route for ferroelectric materials [Wollesen].
[Xing2003] X. Xing et al., “Solid solution Pb1− xSrxTiO3 and its thermal expansion”, Journal of alloys and compounds, vol. 360, issues 1-2, pp. 286-289, 2003.
[Matzen2014] S. Matzen, et al. "Super switching and control of in-plane ferroelectric nanodomains in strained thin films.", Nature communications, vol. 5, pp. 1-8, 2014.
[Wollesen] L. Wollesen et al. in preparation “Growth of (Pb,Sr)TiO3 Films by Liquid Phase Epitaxy”.