Ferroelectric Superlattices: untangling Electrostatic and Strain Effects
Khestanova,1 N. Dix,1 I. Fina,1 M. Scigaj,1,2 J.M. Rebled,1,3 C. Magén,4 S. Estradé,3 F. Peiró,3 G. Herranz,1 J. Fontcuberta,1 F. Sánchez1
Untangling electrostatic and strain effects on the polarization of ferroelectric superlattices Advanced Functional Materials26, 6446-6453, 2016
Ferroelectric superlattice: Cross-sectional transmission electron microscopy (left panel); RHEED intensity oscillations during layer-by-layer growth (middle panel) and RHEED pattern at the end (inset); and AFM topographic image of the superlattice showing terraces and steps morphology (right panel).
Superlattices combining ferroelectric and paraelectric nanometric layers are artificial materials in which electrostatic coupling can induce polarization in the paraelectric material. The ferroelectric polarization is determined by both electrical boundary conditions at the ferroelectric/paraelectric interfaces and lattice strain. The combined influence of both factors offers new opportunities to tune ferroelectricity. However, the experimental investigation of their individual impact has been elusive because of their complex interplay. We have developed a simple growth strategy that has permitted to disentangle both contributions by an independent control of strain in symmetric superlattices. Using reflection high energy electron (RHEED) assisted pulsed laser deposition (PLD) we have deposited by layer-by layer growth epitaxial symmetric superlattices, formed by paraelectric SrTiO3 and ferroelectric BaTiO3. The superlattices are fully coherent for a wide range of superlattice period, from 1 unit cell to around 10 unit cells. We have also achieved controlling the lattice strain in fully coherent superlattices of a fixed period by changing the deposition rate. Ferroelectric polarization loops were measured for all the superlattices, and it was found that their polarization displays a strong dependence on the superlattice period and, in comparison, a tiny dependence on the lattice strain. It is thus concluded that electrostatic boundary conditions in multilayers rule the ferroelectric response, whereas the lattice strain plays a relatively minor role. The high polarization in short period superlattices associated to uniform ferroelectricity decreases quickly as the period increases and the cost to polarize the paraelectric SrTiO3 becomes excessive. Thus, a uniform ferroelectric entity, artificial-like ferroelectric material, is limited exclusively to ultra-short period superlattices.
1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain 2 Dep. de Física Universitat Autònoma de Barcelona, Spain 3 Laboratory of Electron Nanoscopy (LENS-UB), Institute of Nanoscience and Nanotechnology (In2UB), University of Barcelona, Spain 4 Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragón (INA) – ARAID and Departamento de Física de la Materia Condensada Universidad de Zaragoza, Spain
Institut de Ciència de Materials de Barcelona Campus de la UAB 08193 Bellaterra, SPAIN