Probing surfaces and interfaces in complex oxide films via in situ X-ray photoelectron spectroscopy (original) (raw)
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Atomic scale characterization of complex oxide interfaces
Journal of Materials Science, 2006
Complex oxides exhibit the most disparate behaviors, from ferroelectricity to high T c superconductivity, colossal magnetoresistance to insulating properties. For these reasons, oxide thin films are of interest for electronics and the emerging field of spintronics. But epitaxial complex oxide ultrathin films and heterostructures can be significantly affected or even dominated by the presence of interfaces and may exhibit intriguing new physical properties quite different from the bulk. A study of the relations between structure and chemistry at the atomic scale is needed to understand the macroscopic properties of such ''interface-controlled'' materials. For this purpose, the combination of aberration-corrected Z-contrast scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) represents a very powerful tool. The availability of sub-Å ngströ m probes allows a level of unprecedented detail when analyzing not only the interface structure with sensitivity to single atoms, but also the interface chemistry. In this work state of the art STEM-EELS will be applied to the study of different oxide interfaces in heterostructures with titanates, manganites and cuprates based on the perovskite structure.
Determination of Electronic Structure of Oxide–Oxide Interfaces by Photoemission Spectroscopy
Advanced Materials, 2010
A method has been developed to use the finite escape depth of the photoelectrons emitted in ultraviolet photoemission spectroscopy (UPS) to determine the electronic density‐of‐states at the interface between two dissimilar metal oxides. Ultrathin films of one oxide are grown heteroepitaxially, one monolayer at a time, on a single‐crystal substrate of the other oxide, and UPS spectra are taken after each complete monolayer. By comparing experimental UPS spectra with calculated spectra based on specific models of the interfacial structure, the interfacial density‐of‐states can be extracted. The two oxide systems studied here are NiO–Fe3O4 and CoO–Fe3O4. The former system is found to have an atomically abrupt interface, with no significant density of interface states. For CoO, however, an interfacial electronic spectrum, different from that of either the substrate or the overlayer, is found. The spatial extent and possible origin of those interfacial states is discussed.
Electrons and Polarons at Oxide Interfaces Explored by Soft-X-Ray ARPES
Spectroscopy of Complex Oxide Interfaces
Soft-X-ray ARPES (SX-ARPES) with its enhanced probing depth and chemical specificity allows access to fundamental electronic structure characteristics-momentum-resolved spectral function, band structure, Fermi surface-of systems difficult and even impossible for the conventional ARPES such as three-dimensional materials, buried interfaces and impurities. After a recap of the spectroscopic abilities of SX-ARPES, we review its applications to oxide interfaces, focusing on the paradigm LaAlO 3 /SrTiO 3 interface. Resonant SX-ARPES at the Ti Ledge accentuates photoemission response of the mobile interface electrons and exposes their d xy-, d yz-and d xz-derived subbands forming the Fermi surface in the interface quantum well. After a recap of the electron-phonon interaction physics, we demonstrate that peak-dip-hump structure of the experimental spectral function manifests the Holstein-type large polaron nature of the interface charge carriers, explaining their fundamentally reduced mobility. Coupling of the charge carriers to polar soft phonon modes defines dramatic drop of mobility with temperature. Oxygen deficiency adds another dimension to the rich physics of LaAlO 3 /SrTiO 3 resulting from coexistence of mobile and localized electrons introduced by oxygen vacancies. Oxygen deficiency allows tuning of the polaronic coupling and thus mobility of the charge carriers, as well as of interfacial ferromagnetism connected with various atomic configurations of the vacancies. Finally, we discuss spectroscopic evidence of phase separation at the LaAlO 3 /SrTiO 3 interface. Concluding, we put prospects of SX-ARPES for complex heterostructures, spin-resolving experiments opening the totally unexplored field of interfacial spin structure, and inoperando field-effect experiments paving the way towards device applications of the reach physics of oxide interfaces.
Theory and Application of Photoelectron Diffraction for Complex Oxide Systems
Journal of the Physical Society of Japan, 2018
X-ray photoelectron diffraction (XPD) has been used to investigate film structures and local sites of surface and dopant atoms in complex oxide materials. We have performed angular-resolved measurements of intensity distribution curves (ADCs) and patterns (ADPs) of elemental core level intensities from binary to quaternary mixed oxide samples and compared them to multiple-scattering cluster (MSC) calculations in order to derive information on structural models and related parameters. MSC calculations permitted to describe both bulk diffraction features of binary oxide MnO(001) and the thickness-dependence of the tetragonal distortion of epitaxial MnO films on Ag(001). XPD was further used to investigate the surface termination of perovskite SrTiO 3 and BaTiO 3 substrates in order to evaluate influence of different ex situ and in situ preparation procedures on the surface layers, which are crucial for quality of following film growth. Despite the similarity of local environments of Sr (Ba) and Ti atoms in the perovskite film structure an angular region in the ADCs was identified as a fingerprint with the help of MSC simulations which provided clear conclusions on the perovskite oxide surfaces. Dopant sites in quaternary perovskite manganites La 1−x Ca x MnO 3 , La 1−x Sr x MnO 3 , and La 1−x Ce x MnO 3 were studied with polar angle scans of the photoemission intensities of host and dopant atoms. Both direct comparison of experimental ADCs and to the simulations within MSC models confirm the occupation of A sites by the dopants and the structural quality of the complex oxide films.
Direct k-Space Mapping of the Electronic Structure in an Oxide-Oxide Interface
Physical Review Letters, 2013
The interface between LaAlO3 and SrTiO3 hosts a two-dimensional electron system of itinerant carriers, although both oxides are band insulators. Interface ferromagnetism coexisting with superconductivity has been found and attributed to local moments. Experimentally, it has been established that Ti 3d electrons are confined to the interface. Using soft x-ray angle-resolved resonant photoelectron spectroscopy we have directly mapped the interface states in k-space. Our data demonstrate a charge dichotomy. A mobile fraction contributes to Fermi surface sheets, whereas a localized portion at higher binding energies is tentatively attributed to electrons trapped by Ovacancies in the SrTiO3. While photovoltage effects in the polar LaAlO3 layers cannot be excluded, the apparent absence of surface-related Fermi surface sheets could also be fully reconciled in a recently proposed electronic reconstruction picture where the built-in potential in the LaAlO3 is compensated by surface O-vacancies serving also as charge reservoir.
Valence-state reflectometry of complex oxide heterointerfaces
npj Quantum Materials, 2016
Emergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO 3 film on NdGaO 3 reveals a pronounced valence-state reconstruction from Co 3+ in the bulk to Co 2+ at the surface, with an areal density close to 0.5 Co 2+ ions per unit cell. An identical film capped with polar (001) LaAlO 3 maintains the Co 3+ valence over its entire thickness. We interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e − per unit cell to the subsurface CoO 2 layer at its LaO-terminated polar surface.
Physical Review B, 2009
We present an ab initio study of the (001) interfaces between two insulating perovskites, the polar LaAlO 3 and the non-polar SrTiO 3 . We observe an insulating-to-metallic transition above a critical LaAlO 3 thickness. We explain that the high conductivity observed at the TiO 2 /LaO interface, and the lack of similar conductivity at the SrO/AlO 2 interface, are inherent in the atomic geometry of the system. A large interfacial hopping matrix element between cations causes the formation of a bound electron state at the TiO 2 /LaO interface. This novel mechanism for the formation of interfacial bound states suggests a robust means for tuning conductivities at various oxide heterointerfaces.
Electronic and electrical properties of functional interfaces studied by hard X-ray photoemission
As the device downscaling in nanoelectronics has reached the 10 nm range, the functionality of materials employed in multilayered structures to be used in future logic and memory devices is largely defined by their interface properties. In particular, the electrical properties of the functional stacks are directly related to the electronic band line-up which is affected by the electric dipoles building up at the interface(s). In this work, hard X-ray photoelectron spectroscopy is applied to probe the electronic conditions at the interfaces of several relevant multilayered functional structures and to correlate the results with their electrical (transport) properties.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2011
In this report, we briefly describe the general design principles and construction of a newly developed ambient pressure X-ray photoelectron spectroscopy system. This system provides an imaging mode with o 20 mm spatial resolution in one dimension as well as an angle-resolved mode. The new imaging mode enables us to study structured surfaces under catalytically and environmentally relevant conditions. To illustrate this capability, in situ studies on a Au-SiO 2 heterojunction and Rh-TiO 2 metal-support system are presented. This new system can probe structured surfaces near ambient pressure as a function of temperature, pressure, electrical potential, local position, and time. It is a valuable in situ tool to detect material transformations at the micrometer scale.