Growth, structure, and thermal stability of epitaxial BaTiO₃ films on Pt(111)

Abstract

The growth of epitaxial ultrathin BaTiO₃ films upon rf magnetron sputter deposition on a Pt(111) substrate has been studied by scanning tunnelling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. The BaTiO₃ films have been characterized from the initial stages of growth up to a film thickness of 4 unit cells. The deposited films develop a long-range order upon annealing at 1050 K in UHV. In the submonolayer regime a wetting layer is formed on Pt(111). Thicker films reveal a Stranski–Krastanov-like structure as observed with STM. By XPS a good agreement of the thin film stoichiometry with BaTiO₃ single crystal data is determined. Due to annealing at 1150 K BaTiO₃ forms large two-dimensional islands on the Pt(111) substrate. Different surface structures develop on the islands depending on the O₂ partial pressure during annealing.

Introduction

Besides its outstanding properties with respect to piezoelectricity, pyroelectricity, and high dielectric constant, BaTiO₃ shows ferroelectricity at room temperature. This makes BaTiO₃ to the prototypical ferroelectric perovskite. Between 275 K and the paraelectric-to-ferroelectric transition temperature (T_C) of 403 K, BaTiO₃ has a tetragonal symmetry where the Ba and the Ti sublattices are slightly shifted relative to the negatively charged O ions. This shift produces a remanent electrical polarization. A number of experimental and theoretical studies have shown that ferroelectricity persists down to nanometer scales. As part of the oxide superlattices even single unit cell high BaTiO₃ layers were found to be ferroelectric under appropriate boundary conditions [1], [2], [3], [4]. These properties turn BaTiO₃ to an interesting material for applications in non-volatile ferroelectric memories (FERAMs), ferroelectric tunnel junctions (FTJs), optical wave guide devices, layered multiferroics, and oxide superlattices [5], [6]. As a consequence research on BaTiO₃ focusses on the dielectric properties of thin films and the influence of structural changes and preparation conditions [7], [8], [9], [10].

Surface science investigations of BaTiO₃ surfaces under ultra-high vacuum (UHV) conditions are still rather scarce. The single crystal surfaces have been studied by LEED, XPS, and STM for the BaTiO₃(100) surface [11], [12], [13], [14], [15], and the BaTiO₃(111) surface [16], [17]. However, surface science characterization of epitaxial BaTiO₃ thin films is still at the beginning. There have been studies on (100)-oriented BaTiO₃ films on various substrates using He⁺ ion scattering [18], photoelectron spectroscopy [19], and LEED I-V [20]. However, no studies of (111)-oriented thin films prepared and studied under UHV conditions have been reported so far.

For the (111) surface of BaTiO₃ one would expect – in the absence of any surface reconstruction – a hexagonal structure as schematically indicated in Fig. 1 for the case of a Ti termination. The Ba and O ions are located within the same plane with a 1:3 ratio, whereas the terminating Ti ions are positioned in the three-fold hollow sites above three O ions. As indicated in Fig. 1, this unit cell has a formal BaTiO₃ stoichiometry. In the following, the term monolayer equivalent (MLE) for this smallest structural unit with BaTiO₃ stoichiometry is used. Since the lattice mismatch between the BaTiO₃(111) and the Pt(111)-(2 × 2) unit cells is only 2% one might expect epitaxial growth as indicated in Fig. 1. Based on the BaTiO₃ bulk structure the next layer is expected to be 2.3 Å above this interface layer.

In this work, we present a scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) investigation of rf magnetron sputter deposited epitaxial BaTiO₃ films prepared on a Pt(111) substrate and report on the morphology, the atomic structure, and the stoichiometry as well as the annealing behaviour of ultrathin BaTiO₃ films starting from the submonolayer regime. In the following the experimental data for BaTiO₃ coverages of 0.4, 0.8, 1, and 4 MLE will be presented. In the second part of the paper the changes in structure and stoichiometry are discussed which occur upon annealing in UHV and O₂ atmosphere, respectively. The deposited BaTiO₃ films develop a long-range order upon annealing at 1050 K in UHV. At 1150 K the BaTiO₃ thin films form large two-dimensional islands on the Pt substrate which reveal different surface structures depending on the O₂ partial pressure during annealing. This includes a BaTiO${}_3\left(111\right)-(\sqrt{3}\times \sqrt{3})\text{R3}0°$ which is well known for the BaTiO₃(111) single crystal surface under mildly reducing conditions [16], [17].