Resumen de Estudi nanoscòpic de capes primes ferroelèctriques epitaxials de hf0.5zr0.5o2 y superxarxes de batio3/srtio3
Ferroelectricity is a functional property that can be exploited in microelectronics devices such as non-volatile memories. In this thesis, the ferroelectric properties of nanometric thin films and superlattices have been studied by combining macroscopic and nanoscale characterization techniques. Firstly, the configuration of ferroelectric domains of BaTiO3 has been studied in a set of BaTiO3/SrTiO3 superlattices of different periods by Scanning Transmission Electron Microscopy (STEM). A strong dependence of the superlattice period (layers’ thickness) has been observed, with a decreasing polar distortion for longer periods, in agreement with the macroscopic polarization. Polarization rotations have been observed inside the BaTiO3 layers of the superlattice with longest period (10 unit-cell thick BaTiO3 layers), contrasting with the single oriented (out-of-plane oriented) domains in superlattices of shorter period. Secondly, Hf0.5Zr0.5O2 thin films grown epitaxially on perovskite substrates have been studied. The films generally show a mixture of orthorhombic (ferroelectric) and monoclinic (non-polar) epitaxial crystallites, with more of one or the other phase depending on different conditions. The role of the bottom electrode on the stabilization of phases and the ferroelectricity of the films has also been studied. In order to stabilize the orthorhombic phase, the substrates require to be buffered with a manganite (i.e. La0.7Sr0.3MnO3); otherwise, the amount of the orthorhombic phase is very small, and so is the ferroelectricity of the films. Besides, the effect of the epitaxial stress has been explored by growing Hf0.5Zr0.5O2/La0.7Sr0.3MnO3 films on a set of substrates with different lattice parameters, achieving pure orthorhombic films with enhanced ferroelectricity on films grown on those substrates that impose a tensile strain to the La0.7Sr0.3MnO3 electrode. Furthermore, the epitaxy of Hf0.5Zr0.5O2 on La0.7Sr0.3MnO3 as well as the Hf0.5Zr0.5O2/La0.7Sr0.3MnO3 interface have been explored and described with atomic resolution by means of STEM.
