Christian Lenser

Spectroscopic Proof of the Correlation between Redox-State and Charge-Carrier Transport at the Interface of Resistively Switching Ti/PCMO Devices

By using hard X-ray photoelectron spectroscopy experimentally, proof is provided that resistive switching in Ti/Pr₀.₄₈ Ca₀.₅₂ MnO₃ (PCMO) devices is based on a redox-process that mainly occurs on the Ti-side. The different resistance states are determined by the amount of fully oxidized Ti-ions in the stack, implying a reversible redox-reaction at the interface, which governs the formation and shortening of an insulating tunnel barrier.

Spectroscopic study of the electric field induced valence change of Fe-defect centers in SrTiO3

The electrochemical changes induced by an electric field in Fe-doped SrTiO(3) have been investigated by X-ray absorption spectroscopy (XANES and EXAFS), electron paramagnetic resonance (EPR) and Raman spectroscopy. A detailed study of the Fe dopant in the regions around the anode and cathode reveals new insights into the local structure and valence state of Fe in SrTiO(3) single crystals. The ab initio full multiple-scattering XANES calculations give an evidence of the oxygen vacancy presence in the first coordination shell of iron. Differences in the length and disorder of the Fe-O bonds as extracted from EXAFS are correlated to the unequivocal identification of the defect type by complementary spectroscopical techniques to identify the valence state of the Fe-dopant and the presence of the Fe - V(Ö) complexes. Through this combinatorial approach, novel structural information on Fe - V(Ö) complexes is provided by X-ray absorption spectroscopy, and the relation of Fe-O bond length, doping level and oxidation state in SrTi(1-x)Fe(x)O(3) is briefly discussed.

Determination of the electrostatic potential distribution in Pt/Fe:SrTiO3/Nb:SrTiO3 thin-film structures by electron holography

We determined the electrostatic potential distribution in pristine Pt/Fe:SrTiO3/Nb:SrTiO3 structures by electron holography experiments, revealing the existence of a depletion layer extending into the Nb-doped bottom electrode. Simulations of potential profiles in metal-insulator-metal structures were conducted assuming different types and distributions of dopants. It is found that the presence of acceptor-type dopant concentrations at the Fe:SrTiO3/Nb:SrTiO3 interface with a donor-doped insulating layer provides a good match to the measured profile. Such acceptor-type interface concentrations may be associated with Sr vacancies on the Nb:SrTiO3 side of the bottom interface.

Feasibility studies for filament detection in resistively switching SrTiO3 devices by employing grazing incidence small angle X-ray scattering

We report on fundamental studies of grazing incidence small angle X-ray scattering (GISAXS) on resistively switching SrTiO3 thin film metal-insulator-metal (MIM) devices. Different influence factors on the GISAXS scattering pattern, e.g., surface morphology and top electrode material, were evaluated by simulations and compared with subsequent measurements. Pt top electrodes cause a strong background scattering which covers any information from the underlying SrTiO3 layer. In order to reduce this undesired background scattering, the lighter elements Al and Ti have been used. In case of Ti top electrodes, we observed that a laterally formed structure occurs in the SrTiO3 prior to any electrical treatment, which is consistent with the forming-free properties of the MIM structures. For Al top electrodes, we could detect a significant influence of an electroforming step on the scattered intensity.

Charge-transfer in B-site-depleted NdGaO3/SrTiO3 heterostructures

Cation stoichiometry has been identified as a major key in establishing 2-dimensional electron gases (2DEGs) in oxide heterostructures. Here, we discuss a 2DEG formation scenario in B-site deficient perovskite/perovskite heterostructures, which previously were predicted to show insulating behavior. We elaborate an ionic picture based on oxygen-vacancy-buffered B-site vacancy defects in the polar oxide layer that yields a continuous transition from 2DEG formation to less conducting interfaces to insulating interfaces with increasing B-site deficiency. Experimentally, a corresponding modulation of charge transfer across NdGaO3/SrTiO3 interfaces is inferred using hard x-ray photoelectron spectroscopy analysis and transport experiments. With increasing B-site deficiency, we observe a decrease of the interfacial Ti3+ core level contribution, indicating a reduced charge transfer at the interface. This result is corroborated by temperature-dependent transport measurements, revealing increased low temperature resistance, with a dominant influence of a reduced electron density in the Ga-depleted sample. We consider a redistribution of oxygen vacancies in the B-site deficient polar oxide layer to explain the alleviated interface reconstruction, adding a new perspective on potential built-up in polar-oxide thin films.Cation stoichiometry has been identified as a major key in establishing 2-dimensional electron gases (2DEGs) in oxide heterostructures. Here, we discuss a 2DEG formation scenario in B-site deficient perovskite/perovskite heterostructures, which previously were predicted to show insulating behavior. We elaborate an ionic picture based on oxygen-vacancy-buffered B-site vacancy defects in the polar oxide layer that yields a continuous transition from 2DEG formation to less conducting interfaces to insulating interfaces with increasing B-site deficiency. Experimentally, a corresponding modulation of charge transfer across NdGaO3/SrTiO3 interfaces is inferred using hard x-ray photoelectron spectroscopy analysis and transport experiments. With increasing B-site deficiency, we observe a decrease of the interfacial Ti3+ core level contribution, indicating a reduced charge transfer at the interface. This result is corroborated by temperature-dependent transport measurements, revealing increased low temperature res...