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Abstract: We combine state-of-the-art oxide epitaxial growth by hybrid molecular beam epitaxy with transport, x-ray photoemission, and surface diffraction, along with classical and first-principles quantum mechanical modeling to investigate the nuances of insulating layer formation in otherwise high-mobility homoepitaxial n-SrTiO3(001) films. Our analysis points to charge immobilization at the buried n-SrTiO3/undoped SrTiO3(001) interface as well as within the surface contamination layer resulting from air exposure as the drivers of electronic dead-layer formation. As Fermi level equilibration occurs at the surface and the buried interface, charge trapping reduces the sheet carrier density (n2D) and renders the n-STO film insulating if n2D falls below the critical value for the metal-to-insulator transition.

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Abstract: The electronic structures of semiconducting heterojunctions are critically dependent on composition including the presence and concentrations of dopants, both intended and unintended. Dopant profiles in the interfacial region can have major effects on band energies which in turn drive transport properties. Here we use core-level photoelectron line shapes excited with hard x rays to extract information about electric fields resulting from internal charge transfer in epitaxial La 0.03 Sr 0.97 Zr x Ti 1 – x O 3 / Ge ( 001 ) ( 0.1 ≤ x ≤ 0.7 ) heterostructures. Experiments were carried out for heterojunctions involving both n - and p -type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure, and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform n - and p -type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited.

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Abstract: We demonstrate that the interfacial dipole associated with bonding across the SrTi O 3 / Si heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into SrTi O 3 . The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.

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Abstract: A heterostructure consisting of the Mott insulator LaVO3 and the band insulator SrTiO3 is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO3 match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the nonpolar/polar interface when integrated with SrTiO3, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition are scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard x-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO3/SrTiO3 heterostructures. We find a critical LaVO3 thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3d states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO3/SrTiO3.

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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 LaAlO3 and SrTiO3 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 LaAlO3/SrTiO3 heterostructure: By photoemission we manage to unambiguously distinguish the different mechanisms by which SrTiO3 can be doped with electrons. An electronic reconstruction is identified as the driving mechanism to render stoichiometric LaAlO3/SrTiO3 interfaces metallic. The doping of the LaAlO3/SrTiO3 heterointerface can furthermore be finely adjusted by changing the oxygen vacancy (VO) concentration in the heterostructure. Combining intense x-ray irradiation with oxygen dosing, we even achieve control over the VO concentration and, consequently, the doping in the photoemission experiment itself. Exploiting this method, we investigate how the band diagram of SrTiO3-based heterostructures changes as a function of the VO concentration and temperature by hard x-ray photoemission spectroscopy. With the band bending in the SrTiO3 substrate changing as a function of the VO concentration, the interfacial band alignment is found to vary as well. The relative permittivity of the SrTiO3 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 SrTiO3 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 VO 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 VO concentration. The mobility of the charge carriers changes as well depending on the VO 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 LaAlO3/SrTiO3 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) LaAlO3/SrTiO3 heterointerface, an electronic reconstruction also renders the (111) interface between LaAlO3 and SrTiO3 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 LaAlO3/SrTiO3 heterostructure. The spectral function measured furthermore identifies the charge carriers at the interface as large polarons.