Michael Zapf (PhD thesis): High-Z Perovskite Oxides: Thin Film Deposition and Spectroscopic Investigations

Abstract

Perovskite oxides are a very versatile material class with a large variety of outstanding physical properties. A subgroup of these compounds particularly tempting to investigate are oxides involving high-Z elements, where spin-orbit coupling is expected to give rise to new intriguing phases and potential application-relevant functionalities. This thesis deals with the preparation and characterization of two representatives of high-Z oxide sample systems based on KTaO₃ and BaBiO₃. KTaO₃ is a band insulator with an electronic valence con guration of Ta 5d⁰. It is shown that by pulsed laser deposition of a disordered LaAlO₃ lm on the KTaO₃(001) surface, through the creation of oxygen vacancies, a Ta 5d^0+d state is obtained in the upmost crystal layers of the substrate. In consequence a quasi two dimensional electron system (q2DES) with large spin-orbit coupling emerges at the heterointerface. Measurements of the Hall e ect establish sheet carrier densities in the range of 0.1-1.2
10¹⁴ cm², which can be controlled by the applied oxygen background pressure during deposition and the LaAlO₃ lm thickness. When compared to the prototypical oxide q2DESs based on SrTiO₃ crystals, the investigated system exhibits exceptionally large carrier mobilities of up to 30 cm²/Vs (7000 cm²/Vs) at room temperature (below 10K). Through a depth pro ling by photoemission spectra of the Ta 4f core level it is shown that the majority of the Ta 5d charge carriers, consisting of mobile and localized electrons, is situated within 4 nm from the interface at low temperatures. Furthermore, the momentum-resolved electronic structure of the q2DES buried underneath the LaAlO₃ lm is probed by means of hard X-ray angle-resolved photoelectron spectroscopy. It is inferred that, due to a strong con nement potential of the electrons, the band structure of the system is altered compared to n-doped bulk KTO. Despite the constraint of the electron movement along one direction, the Fermi surface exhibits a clear three dimensional momentum dependence, which is related to a depth extension of the conduction channels of at least 1 nm.
The second material, BaBiO₃, is a charge-ordered insulator, which has recently been predicted to emerge as a large-gap topological insulator upon n-doping. This study reports on the thin lm growth of pristine BaBiO₃ on Nb:SrTiO₃(001) substrates by means of pulsed laser deposition. The mechanism is identi ed that facilitates the development of epitaxial order in the heterostructure despite the presence of an extraordinary large lattice mismatch of 12%. At the heterointerface, a structurally modi ed layer of about 1.7 nm thickness is formed that gradually relieves the in-plane strain and serves as the foundation of a relaxed BBO lm. The thereupon formed lattice orders laterally in registry with the substrate with the orientation  BaBiO₃(001)||SrTiO₃(001) by so-called domain matching, where 8 to 9 BaBiO₃ unit cells align with 9 to 10 unit cells of the substrate. Through the optimization of the deposition conditions in regard to the cation stoichiometry and the structural lattice quality, BaBiO3 thin lms with bulk-like electronic properties are obtained, as is inferred from a comparison of valence band spectra with density functional theory calculations. Finally, a spectroscopic survey of BaBiO₃ samples of various thicknesses resolves that a recently discovered lm thickness-controlled phase transition in BaBiO₃ thin lms can be traced back to the structural and concurrent stoichiometric modi cations occuring in the initially formed lattice on top of the SrTiO₃ substrate rather than being purely driven by the smaller spatial extent of the BBO lattice.

You find the complete pdf version here.