@phdthesis{Gabel2019,
  author    = {Gabel, Judith},
  title     = {Interface Engineering of Functional Oxides: A Photoemission Study},
  doi       = {10.25972/OPUS-19227},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-192275},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Due to their complex chemical structure transition metal oxides display many fascinating properties which conventional semiconductors lack. For this reason transition metal oxides hold a lot of promise for novel electronic functionalities. Just as in conventional semiconductor heterostructures, the interfaces between different materials play a key role in oxide electronics. The textbook example is the (001) interface between the band insulators LaAlO\(_3\) and SrTiO\(_3\) at which a two-dimensional electron system (2DES) forms. In order to utilize such a 2DES in prospective electronic devices, it is vital that the electronic properties of the interface can be controlled and manipulated at will. Employing photoelectron spectroscopy as well as electronic transport measurements, this thesis examines how such interface engineering can be realized in the case of the LaAlO\(_3\)/SrTiO\(_3\) heterostructure: By photoemission we manage to unambiguously distinguish the different mechanisms by which SrTiO\(_3\) can be doped with electrons. An electronic reconstruction is identified as the driving mechanism to render stoichiometric LaAlO\(_3\)/SrTiO\(_3\) interfaces metallic. The doping of the LaAlO\(_3\)/SrTiO\(_3\) heterointerface can furthermore be finely adjusted by changing the oxygen vacancy \(V_{\mathrm{O}}\) concentration in the heterostructure. Combining intense x-ray irradiation with oxygen dosing, we even achieve control over the \(V_{\mathrm{O}}\) concentration and, consequently, the doping in the photoemission experiment itself. Exploiting this method, we investigate how the band diagram of SrTiO\(_3\)-based heterostructures changes as a function of the \(V_{\mathrm{O}}\) concentration and temperature by hard x-ray photoemission spectroscopy. With the band bending in the SrTiO\(_3\) substrate changing as a function of the \(V_{\mathrm{O}}\) concentration, the interfacial band alignment is found to vary as well. The relative permittivity of the SrTiO\(_3\) substrate and, in particular, its dependence on temperature and electric field is identified as one of the essential parameters determining the electronic interface properties. That is also why the sample temperature affects the charge carrier distribution. The mobile charge carriers are shown to shift toward the SrTiO\(_3\) bulk when the sample temperature is lowered. This effect is, however, only pronounced if the total charge carrier concentration is small. At high charge carrier concentrations the charge carriers are always confined to the interface, independent of the sample temperature. The dependence of the electronic interface properties on the \(V_{\mathrm{O}}\) concentration is also investigated by a complementary method, viz. by electronic transport measurements. These experiments confirm that the mobile charge carrier concentration increases concomitantly to the \(V_{\mathrm{O}}\) concentration. The mobility of the charge carriers changes as well depending on the \(V_{\mathrm{O}}\) concentration. Comparing spectroscopy and transport results, we are able to draw conclusions about the processes limiting the mobility in electronic transport. We furthermore build a memristor device from our LaAlO\(_3\)/SrTiO\(_3\) heterostructures and demonstrate how interface engineering is used in practice in such novel electronic applications. This thesis furthermore investigates how the electronic structure of the 2DES is affected by the interface topology: We show that, akin to the (001) LaAlO\(_3\)/SrTiO\(_3\) heterointerface, an electronic reconstruction also renders the (111) interface between LaAlO\(_3\) and SrTiO\(_3\) metallic. The change in interface topology becomes evident in the Fermi surface of the buried 2DES which is probed by soft x-ray photoemission. Based on the asymmetry in the Fermi surface, we estimate the extension of the conductive layer in the (111)-oriented LaAlO\(_3\)/SrTiO\(_3\) heterostructure. The spectral function measured furthermore identifies the charge carriers at the interface as large polarons.},
  subject      = {{\"U}bergangsmetalloxide},
  language  = {en}
}
@phdthesis{Glawion2010,
  author    = {Glawion, Sebastian},
  title     = {Spectroscopic Investigations of Doped and Undoped Transition Metal Oxyhalides},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-53169},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {In this thesis the electronic and magnetic structure of the transition metal oxyhalides TiOCl, TiOBr and VOCl is investigated. The main experimental methods are photoemission (PES) and x-ray absorption (XAS) spectroscopy as well as resonant inelastic x-ray scattering (RIXS). The results are compared to density-functional theory, and spectral functions from dynamical mean-field theory and different kinds of model calculations. Questions addressed here are those of the dimensionality of the magnetic and electronic interactions, the suitability of the oxyhalides as prototypical strongly correlated model systems, and the possibility to induce a filling-controlled insulator-metal transition. It turns out that TiOCl is a quasi-one-dimensional system with non-negligible two-dimensional coupling, while the one-dimensional character is already quite suppressed in TiOBr. In VOCl no signatures of such one-dimensional behavior remain, and it is two-dimensional. In all cases, frustrations induced by the crystal lattice govern the magnetic and electronic properties. As it turns out, although the applied theoretical approaches display improvements compared to previous studies, the differences to the experimental data still are at least partially of qualitative instead of quantitative nature. Notably, using RIXS, it is possible for the first time in TiOCl to unambiguously identify a two-spinon excitation, and the previously assumed energy scale of magnetic excitations can be confirmed. By intercalation of alkali metal atoms (Na, K) the oxyhalides can be doped with electrons, which can be evidenced and even quantified using x-ray PES. In these experiments, also a particular vertical arrangement of dopants is observed, which can be explained, at least within experimental accuracy, using the model of a so-called "polar catastrophe". However, no transition into a metallic phase can be observed upon doping, but this can be understood qualitatively and quantitatively within an alloy Hubbard model due to the impurity potential of the dopants. Furthermore, in a canonical way a transfer of spectral weight can be observed, which is a characteristic feature of strongly correlated electron systems. Overall, it can be stated that the transition metal oxyhalides actually can be regarded as prototypical Mott insulators, yet with a rich phase diagram which is far from being fully understood.},
  subject      = {{\"U}bergangsmetall},
  language  = {en}
}
@phdthesis{Haeming2010,
  author    = {H{\"a}ming, Marc},
  title     = {Electronic Many-Body Effects in organic Thin-Films and Interfaces},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-55494},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {The results of this thesis contribute to the understanding of the electronic properties of organic thin-films and interfaces. It is demonstrated that photoemission spectroscopy is very useful for studying surfaces and interfaces. Additionally it is shown, that many-body effects can be relevant for organic thin films, in particular at interfaces with strong interaction. These effects can have general implications for the material properties. In the first part of this thesis a systematic series of polyacene molecules is investigated with NEXAFS spectroscopy. The comparison of the data with core level and IPES data indicates that core excitations and core excitons need to be understood as many-body excitations. This finding implies for example that a high exciton binding energy is not necessarily associated with strong localization of the excited electron at the hole. As these effects apply also for valence excitons they can be relevant for the separation of charges and for the electron-hole recombination at interfaces. In the next chapter some fundamental effects in organic multilayer films and at organic-metal interfaces are studied with core level and NEXAFS spectroscopy. In this context a series of selected molecules is investigated, namely BTCDA, BTCDI, PTCDA and PTCDI. It is shown that in case of strong interface interaction a density of adsorbate-substrate states is formed which can lead to significant charge transfer satellites in the PES and NEXAFS spectra, similar to what is known for transition metal compounds. Moreover, it is demonstrated that the data can be modeled qualitatively by a basic approach which fuses the single impurity Anderson model with the description of charge transfer satellites by Sawatzky et al. This approach, which is equivalent to that of Gunnarsson and Sch{\"o}nhammer, allows even a relatively simple semi-quantitative analysis of the experimental data. The comparison of different adsorbate layers indicates that these many-body effects are particularly strong in case of partial occupation of the LUMO derived DOS. In the third part an organic multilayer film (SnPc), an organic-metal interface with strong coupling (SnPc/Ag) and an organic-organic interface (SnPc/PTCDA/Ag) are studied exemplarily with resonant Auger spectroscopy. The comparison of the data gives evidence for the contribution of many-body effects to the autoionization spectra. Furthermore, it is found that the electron-vibration coupling and the substrate-adsorbate charge transfer occurs on the time scale of the core hole life time. Moreover, the interaction at the organic-organic interface is weak, comparable to the intermolecular interaction in the multilayer films, despite a considerable rigid level shift for the SnPc layer. Furthermore, weak but significant electron-electron correlation is found for the molecular frontier orbitals, which are important for the substrate-adsorbate charge transfer. Therefore, these strongly coupled adsorbate films are briefly discussed within the context of the Hubbard model in the last part of this thesis. From the data derived in this work it can be estimated that such monolayer films are in the regime of medium correlations. Consequently one can expect for these adsorbate films properties which are related to the extraordinary behavior of strongly correlated materials, for which Mott metal-insulator transitions, sophisticated magnetic properties and superconductivity can be observed. Additionally some results from the investigation of alkyl/Si self-assembled monolayers are briefly discussed in the appendix. It is demonstrated exemplarily for the alkyl chains that the electronic band structure of short, finitely repeating units can be well modeled by a comparatively simple quantum well approach. In principle this approach can also be applied to higher dimensional systems, which makes it very useful for the description of E(k) relations in the regime of repeating units of intermediate length. Furthermore, the photoelectron and NEXAFS spectra indicate strong interaction at the alkyl/Si interface. It was found that the interface states can be modified by moderate x-ray irradiation, which changes the properties for charge transport through the SAM.},
  subject      = {Organischer Stoff},
  language  = {en}
}