Evidence of Ba-rich surface segregation in Ba_1-xSr_xTiO₃ and Ba-rich surfactant in SrTiO₃/ Ba_1-xSr_xTiO₃ stacks grown by combinatorial pulsed laser deposition

¹ SPEC, CEA, UMR 3680 CNRS, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France
² GREMAN, UMR 7347 CNRS, Univ. de Tours, Parc de Grandmont, F-37200 Tours, France
³ ICMN, UMR 7374 CNRS, Univ. d’Orléans, 1b rue de la Férollerie, F-45071 Orléans, France
⁴ CRISMAT, UMR6508 CNRS, ENSICAEN, 6 Bd du Maréchal Juin, F-14050 Caen, France

^* Corresponding author: santiago.agudeloestrada@cea.fr

Published online: 2 December 2022

Abstract

The interface of a La_0.7Sr_0.3MnO₃/SrTiO₃ bilayer was modulated by introducing 3 unit cells of Ba_1-xSr_xTiO₃ using Combinatorial Pulsed Laser Deposition. A wide range of chemical compositions was studied within the same sample, with BSTx stoichiometry variable from 0.5 to 1 along Y-axis, while the SrTiO3 overlayer thickness was modified along the X direction [Fig. 1(a)]. We performed high-resolution, laboratory-based angle-resolved XPS studies of the BSTx film surface providing information on the thickness and composition of the surface and sub-surface layers. Based on the attenuation of the La 3d corelevel photoemission signal from the La_0.7Sr_0.3MnO₃ bottom layer, the BST layer is 1.2 nm thick. XPS Ba 3d_5/2 core-level spectra were acquired at positions corresponding to different nominal Ba/Sr stoichiometry. In all measurements, the Ba 3d_5/2 core-level spectra can be represented by two main components, i.e. one component at higher binding energy (BE = 780.54 eV) corresponding to surface contribution and the other one at lower binding energy (BE = 778.92 eV) corresponding to sub-surface contribution (Figs. 2 and 3). Going from normal to 60° emission angle and using a 3-unit cell thick film model, the surface to sub-surface intensity ratio clearly evolves providing evidence of a Ba-rich surfactant. The surfactant effect is more significant for lower nominal Ba stoichiometry.