Research projects

Tailored domain structures in ferroelectric heterostructures (DFG - FOR 5044, 2nd period)

Project TP05 within the DFG Research Unit FOR 5044 - Periodic low-dimensional defect structures in polar oxides
Project management: Prof. Dr. Lukas Eng, Prof. Dr. Christine Silberhorn (Paderborn University)
Research Teams: FERROIX
Funding: DFG
Period: 10/2024-09/2028
This subproject of the research group "Periodic low-dimensional defect structures in polar oxides", which focuses on the correlation of defect structure, electron and ion transport, as well as electromechanical properties in ferroelectric solid solutions using the model system lithium niobate-lithium tantalate, is dedicated to investigating and fabricating optical waveguide structures and novel ferroelectric heterostructures using solid-state bonding, as well as tailoring domain structures in such systems. Heterostructures and their interfaces, such as in (epitaxial) layered structures, play a central role in semiconductor technology to tailor specific properties like 2D electron gases, PN junctions, or optoelectronic properties. However, such hetero-systems have not played a role in ferroelectric materials commonly used in optics or piezotechnology. Here, the focus has been on producing remarkably homogeneous or defect-free crystals while tailoring properties through physical structuring or controlling domain structures. In contrast, this project plans to produce ferroelectric heterostructures using solid-state bonding. Heterostructures, for example, arbitrary stacks of lithium niobate and lithium tantalate (including different cuts), would enable both tailoring macroscopic properties in terms of an effective medium, which is particularly interesting for integrated and quantum optics, as well as the tailoring interfaces and their electronic properties of single crystalline ferroelectrics. Lithium tantalate, for instance, differs in spontaneous polarization. An interface would thus lead to the formation of a space charge layer based on the difference in screening charge carriers, even with the same ferroelectric domain alignment. In combination with domain structures and considering other material properties like electronic bandgaps, defect levels, or polarons, this could enable previously unattainable components, such as PN junctions, through the connection of conductive domain walls in different host systems. Preparing such interfaces and hetero systems in ferroelectrics holds excellent potential for electronics, optics, or piezotronics.

Domain Walls in LNT Mixed Crystals (DFG - FOR 5044, 2nd period)

Project TP06 within the DFG Research Unit FOR 5044 - Periodic low-dimensional defect structures in polar oxides
Project management: Prof. Dr. Lukas M. Eng
Research Teams: FERROIX
Funding: DFG
Period: 10/2024-09/2028
The nanoscopic picture of Lithium Niobate-Tantalate (LNT) solid solutions, i.e. crystalline mixtures of Lithium Niobate and Lithium Tantalate to form LNT, comes with a couple of surprises, hence demanding for a much deeper analysis of how the overall and macroscopically-determined LNT structure correlates to the local dielectric polarization, the local conductivity, and the apparent domain distribution. Defects and local chemical heterogeneity hence dramatically influence the local electric field distribution inside LNT single crystals, resulting in both a finite electronic and ionic conductivity. As a consequence, electrically poling LNT and thus inducing domains and domain walls (DWs) becomes very challenging. – The first goal (a) of this subproject TP06 within the FOR5044 hence is to shed light onto this correlation at the 10-nanometer length scale, by applying a set of dedicated scanning-force-microscopy (SFM)-based tools to locally quantify the possible origins and behaviors of both the local conductivity and the dielectric function. Optical, electronic, phononic properties and their anisotropies will be recorded to provide correlated local information from one and the same spot. These local fingerprints then should converge and be able to equally interpret the macroscopic, integral findings. – Our second goal (b) focuses on applying both the above SFM-tools as well as macroscopic methods (Hall-transport, Second-Harmonic-Generation microscopy, Raman spectroscopy) to DW structures prepared in a three-fold set of LNT samples: (iii) Naturally-grown DWs in LNT; (ii) as-poled DWs into LNT-single-crystals, including the use of finger electrodes; and (iii) DWs induced into wafer-bonded LNT-systems; the latter technique, direct wafer-bonding, provides a completely novel approach of how to engineer charged DWs and interfaces into ferroelectrics, which becomes especially interesting when using purposely doped LNT-systems (Mg, Hf, Zn) or LNT-crystals with an altered Ta- or Li-concentration. Besides the local-scale SFM measurements, we will especially investigate the domain wall conductivity (DWC) by applying external stimuli while measuring Hall-transport in single DWs; these are: (i) photo-induced generation of electron-hole-pairs at the DW; (ii) applying mechanical (tensile, compressive) strain using a piezoelectric strain-cell; and (iii) recording the Hall-signatures at variable temperatures between ambient and 4 K. – Lastly, (c), by making use of the broad DWC tunability from above, we will propose three novel device concepts especially applicable with LNT: a DW-pn-diode, a DW-based transistor device, and a photovoltaic cell. All three applications will be thoroughly investigated and characterized with both our nanoscale and macroscopic tools mentioned in (a) and (b), and evaluated for their best performance with respect to the three-fold set of how DWs can be engineered into LNT samples.

Local-Scale Optoelectronics of Synthetic 2D Materials (DFG - CRC 1415, 2nd period)

Scientific project B06 of the Collaborative Research Center CRC 1415 - Chemistry of Synthetic Two-Dimensional Materials
Project management: Prof. Lukas M. Eng
Research Team:  SNOM
Funding: DFG
Period: 07/2024-06/2028
The project aims at quantifying the two relevant parameters relevant for optoelectronic applications of low-dimensional 2DMs at the nanometer length scale, that is the anisotropic tensors of both the electronic conductivity δ(ω) and the dielectric permittivity ε(ω) (also known as the “optical” conductivity).

TiNaII - Time-resolved Nanoscopy in the deep THz regime - part II: Development of ultrafast pump-probe near-field methods for THz-driven processes (BMBF)

Project management: Prof. Lukas M. Eng
Research team:  SNOM
Funded by: BMBF
Period: 07/2019-06/2022
Goal of this project is the implementation of optical near-field microscopy for the THz-frequency regime (abbreviated: T-SNOM) from 10 THz to 100 GHz. This T-SNOM will enable the examination of transient dynamic processes with a time-resolution within the sub-30-femto-second regime and with a spatial resolution of view 10 nanometers only.

NanOMapII -  Nanoscopic Optical Material Probing at FELBE - phase II: Implementation of a New Use-Friendly Near-Field Microscope (BMBF)

Project management: Dr. Susanne C. Kehr
Research team: SNOM
Funding: BMBF
Period: 07/2010-06/2022
Near-field microscopy (SNOM) allows for the spectroscopic surface mapping of solids with a wavelength-independent resolution of about 30 nm. Particularly in the IR- to THz-regime this enables us to study nanoscopic structures on a scale way below the examination wavelength, a dimension that is usually inaccessible due to the wave-nature of light. Here, we utilize two unique near-field microscopes at the free-electron-laser source FELBE for optical examinations of semiconductors and complex oxides on a spatial scale of about 30 nm, within a wavelength regime of 4 to 250 μm, and at temperatures from 4 to 300 K.

Scanning probe microscopy: Magnetic structures and topological states (DFG  - CRC 1143)

Subproject C05 of CRC 1143: Correlated Magnetism: From Frustration To Topology 
Project management: Prof. Lukas M. Eng
Research team: SKY
Funded by: DFG
Period: 01/2015-12/2022
This project investigates the electronic and magnetic real-space structure of frustrated materials down to the atomic length scale using scanning probe techniques. Scanning-tunneling microscopy and spectroscopy (STM/STS) will be used for extracting local electronic densities of states and quasiparticle interference patterns. Magnetic force microscopy (MFM) and Kelvin-probe force microscopy (KPFM) will enable us to study magnetic textures and magnetoelectric effects. Novel quantitative methods at low temperature will be employed, in particular for MFM. Materials at focus are quantum spin liquid candidates, skyrmion systems, and Weyl semimetals.

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Finished projects