Florian Pfaff (PhD thesis)

Oxide heterostructures can exhibit a variety of unexpected electronic and magnetic phenomena at their interfaces. A prominent example is the interface in LaAlO₃/SrTiO₃ heterostructures where a two-dimensional electron system (2DES) forms if the LaAlO₃ thickness equals or exceeds a critical thickness of four unit cells. Similar to LaAlO₃/SrTiO₃ an interface 2DES above a critical overlayer thickness has been observed in γ-Al₂O₃/SrTiO₃. However, the electron mobility as well as the sheet carrier density exceed those of LaAlO₃/SrTiO₃ heterostructures by more than one order of magnitude. This thesis is concerned with the growth and the characterization of these two types of interface systems with the main focus on the analysis of the physical properties at the interface and the understanding of their leading mechanisms.
In regard to the sample fabrication it is demonstrated in the present thesis that the hitherto established growth routine of LaAlO₃/SrTiO₃ by pulsed laser deposition has to be altered and optimized for the growth of γ-Al₂O₃. It is shown that growth monitoring by analyzing reflection high energy electron diffraction (RHEED) intensity oscillations is hindered by the formation of surface wave resonances. In order to avoid this effect, a modified growth geometry has to be used whereby also in this heterostructucture systems monitoring of the layer-by-layer growth of γ-Al₂O₃/SrTiO₃ heterostructures by electron diffraction can be achieved.
A so-called electronic reconstruction is discussed as the possible driving mechanism for the 2DES formation in LaAlO₃/SrTiO₃ . In this scenario, the built-up potential within the polar LaAlO₃ overlayer is compensated by a charge transfer from the sample surface to the top most layers of the non-polar SrTiO₃ substrate. Furthermore, the properties of these heterostructures strongly depend on the used growth conditions. In the present work, for instance, a significant increase in the charge carrier concentration as well as the 2DES spatial extension can be observed for samples grown at very low oxygen pressures, which is related to the creation of oxygen vacancies in SrTiO₃ substrate. It is microscopically shown for the first time that sharp interfaces with a very low density of defects can also be grown at very low oxygen partial pressures. In addition, no significant effect of oxygen vacancies on specific structural properties is seen. Furthermore, a detailed analysis of the atomic spacing reveales a lattice distortion within the LaAlO₃ film which shows a significant dependence on the used growth parameters and, supported by density functional theory, points towards a complex interplay of electronic reconstruction, surface oxygen vacancies and lattice distortions as the driving mechanism for the 2DES formation.
Beside the study of the structural properties of the interface in LaAlO₃/SrTiO₃ heterostructures by means of transmission electron microscopy, the electronic structure of the 2DES is analyzed by resonant inelastic x-ray scattering (RIXS) measurements which show clear indications for localized charge carriers below the critical thickness for conductivity of four unit cells. Moreover, a Raman- and a fluorescence-like signal can be identified by excitation energy dependent RIXS and attributed to the electronic character of the intermediate state. Similar results are obtained on γ-Al₂O₃/SrTiO₃ heterostructures which fortifies this interpretation and could be a hint for a similar ground state in both heterostructures and interface magnetism also to be present in this system.
By using resonant photoelectron spectroscopy the Ti 3d valence electrons can directly be observed and analyzed. Comparative measurements on LaAlO₃/SrTiO₃ and γ-Al₂O₃/SrTiO₃ indicate the existence of different types of electronic states with Ti 3d character in both systems which can be attributed to mobile carriers forming the 2DES and carriers localized in states adjacent to oxygen vacancies. By analyzing the resonance behavior of the electronic states and their relative intensities and spectral shape substantial differences are revealed which point to a different mechanism at play for forming the 2DES in  LaAlO₃/SrTiO₃ and γ-Al₂O₃/SrTiO₃ . These observations are discussed in terms of the influence of oxygen vacancies on the two interface systems. Additionally, momentum-resolved measurements are performed to resolve the metallic states at the chemical potential and to map out the Fermi surface of LaAlO₃/SrTiO₃ and γ-Al₂O₃/SrTiO₃ . Here, significantly different intensity distributions in k-space are observed and discussed with respect to matrix element effects while the results can most likely be ascribed to photoelectron diffraction due to the different crystal structure of the overlayer material.

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