Resumen de Creixement i estructura local de capes primes epitaxials de BaZrxTi?-xO?
Jofre Ventura Altozano
BaZrxTi1-xO3 (BZT) presents a wide range of dielectric properties interesting from a fundamental point of view and for applications. Bulk BZT studies have shown that ferroelectric BaTiO3 and paraelectric BaZrO3 precipitate at the nanometer scale in polar (PNR) and non-polar regions, respectively. The long range ferroelectricity of BaTiO3 is disrupted by the non-polar regions, causing a short range interaction between the PNR's in a behavior called relaxor. As opposed to conventional relaxors of heterovalent solid solutions, where the PNR's generate random electric fields, in the homovalent substitution of Ti4+ by Zr4+ of BZT, the polar order extent is controlled exclusively by the volume fraction and size of the PNR's. The study of BZT may, thus, be useful towards a better comprehension of the relaxor behavior. With the purpose of studying these behaviors in epitaxial thin films and the influence of the substrate, as well as analyzing the crystal structure and growth of the films, in this thesis, thin films of BaZrxTi1-xO3 covering the complete range of compositions have been deposited on SrTiO3 (001) by the laser ablation technique. Their chemical composition, average and local crystal structure, microstructure, polar order and bandgap energy have been systematically characterized by means of wavelength dispersive spectroscopy, atomic force microscopy, transmission electron microscopy, Raman spectroscopy and spectroscopic ellipsometry. The optimization of deposition parameters has allowed obtaining high crystal quality epitaxial thin films with a low content of oxygen vacancies. Despite the high lattice mismatch between BZT and SrTiO3 (from 2.2% to 6.9%), the films present a single crystal domain with a cube-on-cube epitaxial relationship. Regardless of their composition, the BZT films are fully relaxed, with lattice parameters similar to those of bulk material. Their growth is 3D, with a morphology which is columnar in volume and granular in surface. The strain relief is achieved through a major formation of a<100> edge dislocation half loops during the first growth stages. As a consequence of the similarity between their respective crystal lattices, the compositional ranges of the diverse ferroelectric behaviors of the BZT films coincide with those of bulk material. Regardless of the Zr molar fraction, the Ti atomic environment remains almost invariant with regard to that of BaTiO3 and, thus, the polar nanoregions have a rhombohedral symmetry. The bandgap energy versus composition of BZT is asymmetric, indicating its non-ideal solid solution nature. Therefore, the inherent tendency to phase segregation of the material may cause the formation of the detected polar nanoregions.
