Andreas Ruediger

The influence of copper top electrodes on the resistive switching effect in TiO2 thin films studied by conductive atomic force microscopy

Titanium dioxide thin films (30 nm) are deposited on platinized substrates by atomic layer deposition and locally studied by conductive atomic force microscopy showing repetitive bipolar resistive switching. Experiments using macroscopic copper top electrodes, which are electroformed, bipolar switched, and removed again from the TiO2–Pt stack, prove the formation of local conductive filaments with bipolar switching properties. The localized filaments can be switched repetitively with a resistance ratio of 30. Our findings underline that Cu diffusion and the formation of filaments are the major mechanism for the resistive switching in Cu/TiO2/Pt cells.

Epitaxial Patterning of Bi2FeCrO6 Double Perovskite Nanostructures: Multiferroic at Room Temperature

There is an increasing interest in developing and characterizing multifunctional materials, as they exhibit rich physical and chemical properties and offer exciting opportunities, for example, to miniaturize integrated devices and extend the potential of establishing nanoarchitecture concepts. [ 1 , 2 ] In this context, multiferroic materials [ 3 ] which combine two or more ferroic functionalities are promising for applications in fi elds such as spintronics and non-volatile data storage. [ 4 ] As recently demonstrated [ 5 ] ferromagnetic–ferroelectric multiferroics can be advantageously used to encode the information in electric polarization and magnetization giving rise to a four logic state memory. The coupling between magnetic and electrical properties leads to additional versatility for related devices, such as electric fi eld-controlled magnetic data storage. [ 6 ] The recent emergence of Bi-based double perovskite thin fi lms, such as Bi 2 FeCrO 6 (BFCO) [ 7– 9 ] and Bi 2 CoMnO 6 , [ 10 ] with strong magnetic behavior at room temperature, create opportunities to practically apply multiferroics. In competition with rival technologies, downscaling the feature size of multifunctional materials is an important step to achieve very high-density memory devices. [ 11 , 12 ]

Imaging and modeling collagen architecture from the nano to micro scale

The collagen meshwork plays a central role in the functioning of a range of tissues including cartilage, tendon, arteries, skin, bone and ligament. Because of its importance in function, it is of considerable interest for studying development, disease and regeneration processes. Here, we have used second harmonic generation (SHG) to image human tissues on the hundreds of micron scale, and developed a numerical model to quantitatively interpret the images in terms of the underlying collagen structure on the tens to hundreds of nanometer scale. Focusing on osteoarthritic changes in cartilage, we have demonstrated that this combination of polarized SHG imaging and numerical modeling can estimate fibril diameter, filling fraction, orientation and bundling. This extends SHG microscopy from a qualitative to quantitative imaging technique, providing a label-free and non-destructive platform for characterizing the extracellular matrix that can expand our understanding of the structural mechanisms in disease.

Note: Tip enhanced Raman spectroscopy with objective scanner on opaque samples

We report on 14 nm lateral resolution in tip-enhanced Raman spectroscopy mapping of carbon nanotubes with an experimental setup that has been designed for the analysis of opaque samples in confocal side-access through a novel piezo-driven objective scanner. The objective scanner allows for fast and stable laser-to-tip alignment and for the adjustment of the focus position with sub-wavelength precision to optimize the excitation of surface plasmons. It also offers the additional benefit of imaging the near-field generated Raman scattering at the gap between tip and sample as direct control of the tip enhancement.

Realization of single-termination SrTiO3 (100) surfaces by a microwave-induced hydrothermal process

A microwave-induced hydrothermal etching of SrTiO3 (100) single crystal surfaces in deionized water and subsequent annealing in oxygen atmosphere results in single-terminated and atomically flat terraces for pure and niobium-doped substrates as confirmed through one unit-cell step height and uniform phase by atomic force microscopy. This process that requires 3 min of moderate microwave radiation completely avoids the use of hydrofluoric acid (HF) and related point defects due to fluorine in the crystal surface. The advantages of a safe, inexpensive, and environmentally neutral process hold promise to replace the existing standard protocol for substrate preparation based on buffered HF.

Tunneling electroresistance effect in a Pt/Hf0.5Zr0.5O2/Pt structure

The present work reports the fabrication of a ferroelectric tunnel junction based on a CMOS compatible 2.8 nm-thick Hf0.5Zr0.5O2 tunnel barrier. It presents a comprehensive study of the electronic properties of the Pt/Hf0.5Zr0.5O2/Pt system by X-ray photoelectron and UV-Visible spectroscopies. Furthermore, two different scanning probe techniques (Piezoresponse Force Microscopy and conductive-AFM) were used to demonstrate the ferroelectric behavior of the ultrathin Hf0.5Zr0.5O2 layer as well as the typical current-voltage characteristic of a ferroelectric tunnel junction device. Finally, a direct tunneling model across symmetric barriers was used to correlate electronic and electric transport properties of the ferroelectric tunnel junction system, demonstrating a large tunnel electroresistance effect with a tunneling electroresistance effect ratio of 20.

Fully inkjet printed flexible resistive memory

Resistively switching memory cells (ReRAM) are strong contenders for next-generation non-volatile random access memories. In this paper, we present ReRAM cells on flexible substrates consisting of Ag/spin-on-glass/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate). The complete cell is fabricated using a standard inkjet printer without additional process steps. Investigations on the spin-on-glass insulating layer showed that low sintering temperatures are sufficient for good switching behavior, providing compatibility with various foils. The cells feature low switching voltages, low write currents, and a high ratio between high and low resistance state of 104. Combined with excellent switching characteristics under bending conditions, these results pave the way for low-power and low-cost memory devices for future applications in flexible electronics.

A Complementary Metal Oxide Semiconductor Process-Compatible Ferroelectric Tunnel Junction

In recent years, experimental demonstration of ferroelectric tunnel junctions (FTJ) based on perovskite tunnel barriers has been reported. However, integrating these perovskite materials into conventional silicon memory technology remains challenging due to their lack of compatibility with the complementary metal oxide semiconductor process (CMOS). This communication reports the fabrication of an FTJ based on a CMOS-compatible tunnel barrier Hf0.5Zr0.5O2 (6 unit cells thick) on an equally CMOS-compatible TiN electrode. Analysis of the FTJ by grazing angle incidence X-ray diffraction confirmed the formation of the noncentrosymmetric orthorhombic phase (Pbc21, ferroelectric phase). The FTJ characterization is followed by the reconstruction of the electrostatic potential profile in the as-grown TiN/Hf0.5Zr0.5O2/Pt heterostructure. A direct tunneling current model across a trapezoidal barrier was used to correlate the electronic and electrical properties of our FTJ devices. The good agreement between the experimental and theoretical model attests to the tunneling electroresistance effect (TER) in our FTJ device. A TER ratio of ∼15 was calculated for the present FTJ device at low read voltage (+0.2 V). This study suggests that Hf0.5Zr0.5O2 is a promising candidate for integration into conventional Si memory technology.

Manipulating multiple order parameters via oxygen vacancies: The case of Eu0.5Ba0.5TiO3−δ

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (V-O) is a key issue for the future development of transition-metal oxides. Progress in this field is currently addressed through V-O variations and their impact on mainly one order parameter. Here we reveal a mechanism for tuning bothmagnetism and ferroelectricity simultaneously by using V-O. Combining experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-delta , we demonstrate that oxygen vacancies create Ti3+ 3d(1) defect states, mediating the ferromagnetic coupling between the localized Eu 4f(7) spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the Eu0.5Ba0.5TiO3-delta.

Dependence of Apertureless Scanning Near-Field Spectroscopy on Nanoscale Refractive Index Changes

We numerically investigate the spectral dependence of the electric field enhancement on a gold tip above PbTiO3 on platinum systems by means of a finite element approach. The localized surface plasmon resonance (LSPR) is verified to change with the incident angle, the tip radius, and the tip-sample distance as well as with the refractive index of the sample underneath the tip. The refractive index sensitivity reveals detectable variations of the LSPR peak’s wavelength and maximum field enhancement, respectively, large enough to discriminate, e.g., between ferroelectric and paraelectric PbTiO3 in tip-enhanced Raman spectroscopy (TERS) configuration.