Insulator-to-metal transition of SrTiO3:Nb single crystal surfaces induced by Ar+ bombardment

In this paper, the effect of Ar+ bombardment of SrTiO3:Nb surface layers is investigated on the macro- and nanoscale using surface-sensitive methods. After bombardment, the stoichiometry and electronic structure are changed distinctly leading to an insulator-to-metal transition related to the change of the Ti "d" electron from d0 to d1 and d2. During bombardment, conducting islands are formed on the surface. The induced metallic state is not stable and can be reversed due to a redox process by external oxidation and even by self-reoxidation upon heating the sample to temperatures of 300{\deg}C.Comment: 4 pages, 4 figure

Hafnium carbide formation in oxygen deficient hafnium oxide thin films

On highly oxygen deficient thin films of hafnium oxide (hafnia, HfO$_{2-x}$) contaminated with adsorbates of carbon oxides, the formation of hafnium carbide (HfC$_x$) at the surface during vacuum annealing at temperatures as low as 600 {\deg}C is reported. Using X-ray photoelectron spectroscopy the evolution of the HfC$_x$ surface layer related to a transformation from insulating into metallic state is monitored in situ. In contrast, for fully stoichiometric HfO$_2$ thin films prepared and measured under identical conditions, the formation of HfC$_x$ was not detectable suggesting that the enhanced adsorption of carbon oxides on oxygen deficient films provides a carbon source for the carbide formation. This shows that a high concentration of oxygen vacancies in carbon contaminated hafnia lowers considerably the formation energy of hafnium carbide. Thus, the presence of a sufficient amount of residual carbon in resistive random access memory devices might lead to a similar carbide formation within the conducting filaments due to Joule heating

Inhomogeneity of donor doping in SrTiO3 substrates studied by fluorescence-lifetime imaging microscopy

Fluorescence-lifetime imaging microscopy (FLIM) was applied to investigate the donor distribution in SrTiO3 single crystals. On the surfaces of Nb- and La-doped SrTiO3, structures with different fluorescence intensities and lifetimes were found that could be related to different concentrations of Ti3+. Furthermore, the inhomogeneous distribution of donors caused a non-uniform conductivity of the surface, which complicates the production of potential electronic devices by the deposition of oxide thin films on top of doped single crystals. Hence, we propose FLIM as a convenient technique (length scale: 1 $\mu$m) for characterizing the quality of doped oxide surfaces, which could help to identify appropriate substrate materials

Self-reduction of the native TiO2(110) surface during cooling after thermal annealing - in-operando investigations

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells

Umwandlung cyclischer Hydrazine

Im ersten Teil dieser Arbeit wird die „stickstoffanaloge Hofmann-Eliminierung“ an Hydraziniumsalzen, die durch Umsetzung von cyclischen Hydrazinen mit verschiedenen Alkylierungsmitteln erhalten werden, beschrieben. Als Basen werden hierbei Natriumalkoholate oder Natriumamid eingesetzt. Die bei der Eliminierung gebildete Iminfunktion wird durch Zugabe von Natriumborhydrid, Natriumbordeuterid oder Kaliumcyanid in einer nucleophilen Addition abgefangen. Aus den erhaltenen Diaminen wird auf den Mechanismus der Eliminierung zurückgeschlossen. Der zweite Teil der Arbeit beschäftigt sich mit der „von-Braun-Reaktion“ an cyclischen Hydrazinen. Die Reaktion des jeweiligen Hydrazins mit Bromcyan liefert neben einem Cyanamid als weiteres Produkt das Hydrobromid des Hydrazins

Cluster-like resistive switching of SrTiO 3 :Nb surface layers

The understanding of the resistive switching mechanisms in perovskites is of particular importance for the development of novel non-volatile memories. Nanoscale investigations recently revealed that in the model material SrTiO 3 a filamentary type of switching is present. In this paper, we show that upon donor doping with Nb the switching type changes fundamentally. We report on the observation of conducting clusters that can be switched independently between a high resistance and a low resistance state when applying a voltage. Furthermore, we show that the resistive switching takes place in a semiconducting surface layer on top of the metallic bulk of SrTiO3:Nb single crystals, which can change its properties easily under external gradients. Based on various measurements, we postulate that ionic movements leading to the creation of secondary phases as nano-filaments between the clusters have to be taken into account in modelling the resistive switching

In situ study of redox processes on the surface of SrTiO_{3} single crystals

In this paper, we report on surface transformations under high-temperature (up to 1000 °C) annealing of SrTiO3(100) single crystals under reducing conditions and in situ oxidation. We compare macroscale electrical measurements with nanoscale investigations of as-reduced and oxidized surfaces. On the nanoscale, annealing in ultra-high-vacuum (UHV) conditions causes a restoration of the long-range atomic order of the (1 × 1) pattern. However, above annealing temperatures of 900 °C, a complex reconstruction of (×)R 33.7° and subsequently (×)R 26.6° appears. The surface becomes Ti-rich and residual carbon desorbs. Electrical surface conductivity increases with the annealing temperature, revealing an inhomogeneous spot-like structure on the nanoscale. Mapping of the surface potential also reveals comparable spatial variations, marking exits of dislocations. The estimated surface work function is increased upon reoxidation by 0.55 eV in the case of annealing at 900 °C, when (×)R 33.7° dominates. Our results show that in contrast to the macroscopic resistance of the crystal, the nanoscale surface conductivity and surface potential are significantly influenced by redox processes at room temperature