www.advmatinterfaces.de www.MaterialsViews.com FULL PAPER STO/BTO Modulated Superlattice Multilayer Structures with Atomically Sharp Interfaces Peter K. Petrov,* Bin Zou, Yiqian Wang, James M. Perkins, David W. McComb, and Neil McN. Alford Dr. P. K. Petrov, Dr. B. Zou, Dr. J. M. Perkins, Prof. N. M. Alford Department of Materials, Imperial College London Prince Consort Road, London, SW7 2AZ, UK E-mail: p.petrov@imperial.ac.uk Prof. Y. Q. Wang Qingdao University Cultivat Base State Key Lab Qingdao 266071, China Prof. D. W. McComb Department of Materials Science and Engineering The Ohio State University Columbus OH 43212, USA DOI: 10.1002/admi.201300116 1. Introduction In recent years ultrathin SrTiO 3 (STO)/BaTiO 3 (BTO) structures have attracted interest [1,2] because of the fundamental scientific importance of their properties [3] (e.g. quasi–two-dimensional electron gases generated at their interfaces) [4] and as a way to fabricate electrically controllable microwave devices [5,6] with enhanced properties. [7,8] The mutual consensus between all authors investigating such paraelectric, ferroelectric, antiferroelectric, ferromagnetic, or multiferroic, based superlattice (SL) struc- tures both theoretically [9–11] and experi- mentally [12,13] is that the main reason for their better physical and even unique properties with respect to the parent mate- rials, origins from the microstructure structure of the layers (e.g. residual strain, strain gradient, atoms arrangement etc.) and the quality of their interfaces (defects, dead layers etc). [14] Therefore most of the recent research was devoted to evaluate the relationship between individual SL layers stoichiometry and quality, and the whole structure properties. The interface quality of the BSTO films and STO/BTO SL structures was studied using high-resolution transmission elec- tron microscopy. [15,16] It was reported that misfit dislocations, threading disloca- tions, and stacking faults were observed in the single-layered film, while only few threading dislocations were found in the multilayered film. Also, the density of defects in the single-layered film was much higher than in the multilayered film, which was related with the improved thermal stability of the multilayered film. Most recently, Ortega et al., [12] investigated the lattice distor- tions to clarify the effect of strain on dielectric and ferroelectric properties of compositionally tailored layers in symmetric BT/ (Ba,Sr)TiO 3 SLs. In [17] it was demonstrated that the strain gradient in STO thin films could be gradually reduced and their electrical properties significantly improved when they were grown as homoepitaxial multilayers using an intermediate oxygen relaxation technique. A pioneering work on development of temperature stable ferro- electric devices was carried out by Gevorgian et al. [18] The prin- ciple of the proposed method was to use Ba x Sr 1-x TiO 3 (BSTO) layers with different barium (Ba) content that have Curie points at different temperatures. By having a bilayer structure where each layer was made of BSTO with different Ba content, the effective operation temperature of the device would be between the Curie temperatures of the two layers. Recently Ortega et al reported [19] a novel application of the BTO/BSTO SLs as high breakdown field and high energy density capacitors in harsh environments; while McMillen et al. [14] by introducing an inter- facial dead layer beneath (BiFeO 3 ) 0.6 -(SrTiO 3 ) 0.4 SLs obtained significant improvement in the energy density. The aim of the present study was to investigate interfaces of modulated STO/BTO SL structures (method 2), and focusing on A comparative study is carried out investigating the microstructure and the electrical properties of Ba x Sr 1-x TiO 3 films with x = (0.25, 0.5, 0.75) deposited as modulated superlattice (SL) multilayer structures by laser ablation on both LaAlO 3 and MgO substrates. The SL structures are examined using high-res- olution transmission electron microscopy and scanning transmission electron microscopy. Their interfaces and chemical composition are investigated using energy dispersive X-ray spectroscopy, complemented with electron energy loss spectra analysis performed to give insight to the local chemistry, structure and bonding. It is found that all modulated SL samples consisted of continuous well defined 1 nm SrTiO 3 and 4 nm BaTiO 3 layers. When modulated SL multi- layered structures are compared with their single target deposited equivalents, they exhibit similar electrical properties (e.g. dielectric constant and dielectric loss) but undergo phase transition in a broader temperature region. A very important observation is that the oxygen K-edges in SrTiO 3 and BaTiO 3 layers are distinctive. Therefore it can be used as finger-print signature for analysis of ultra-thin SrTiO 3 /BaTiO 3 layers and their interfaces. Finally it is demonstrated that by varying the modulation period it is possible to develop structures with engineered ferroelectric properties and improved thermal stability. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 7) 1300116 wileyonlinelibrary.com Adv. Mater. Interfaces 2014, 1, 1300116