arXiv subjects

Michael Seibt

Publications and source records attributed to Michael Seibt.

16 recordsLinked to original sources

arXiv2026

Quantitative models for excess carrier diffusion and recombination in STEM-EBIC experiments on semiconductor nanostructures

The increased complexity and reduced size of (opto-)electronic devices demands for quantitative descriptions of excess carrier transport and recombination via various mechanisms. In addition, experimental methods capable of resolving carrier dynamics on the nanometer scale are required. In this paper, we present a quantitative model of a confined geometry including recombination at two surfaces, which is very generic for electron beam induced current measurements in a scanning transmission electron microscope - a method which offers atomic scale spatial resolution. The model is based on analytical considerations as well as finite element simulations and underlying assumptions are subjected to an in-depth discussion. Finally, the successfull application to experimental data obtained on the complex oxide SrTi0.995Nb0.005O3 demonstrates the practicality and robustness of the approach, which enables the precise determination of its bulk diffusion length of L = 10.2 +- 0.1 nm.

cond-mat.mes-hall↗

arXiv2025

Sequential tilting 4D-STEM for improved momentum-resolved STEM field mapping

Momentum-resolved scanning transmission electron microscopy (MRSTEM) is a powerful phase-contrast technique that can map lateral magnetic and electric fields ranging from the micrometer to the subatomic scale. Resolving fields ranging from a few nanometers to a few hundred nanometers, as well as across material junctions, is particularly important since these fields often determine the functional properties of devices. However, it is also challenging since they are orders of magnitude smaller than atomic electric fields. Thus, subtle changes in diffraction conditions lead to significant changes in the measured MRSTEM signal. One established approach to partially overcome this problem is precession electron diffraction, in which the incident electron beam is continuously precessed while precession-averaged diffraction patterns are acquired. Here, we present an alternative approach in which we sequentially tilt the incident electron beam and record a full diffraction pattern for each tilt and spatial position. This approach requires no hardware modification of the instrument and enables the use of arbitrary beam tilt patterns that can be optimized for specific applications. Furthermore, recording diffraction patterns for every beam tilt allows access to additional information. In this work, we use this information to create virtual large-angle convergent beam electron diffraction (vLACBED) patterns to assess MRSTEM data quality and improve field measurements by applying different data analysis methods beyond simple averaging. The presented data acquisition concept can readily be applied to other 4D-STEM applications.

cond-mat.mtrl-sci↗

arXiv2022

Microstructural analysis of GaN films grown on (1 0 0) MgF$_2$ substrate by 4D nanobeam diffraction and energy-dispersive X-ray spectrometry

The use of highly efficient and solarblind GaN photocathodes as part of multichannel plate UV detectors for applications in astronomy would strongly benefit from the direct growth of GaN on typical window materials with high transmission down to the deep UV range. GaN growth on MgF$_2$ substrates by plasma-assisted molecular beam epitaxy has recently been demonstrated. Here, we report an extensive scanning transmission electron microscopy study of the thin film microstructure for growth at 525 $^{\circ}$C and 650 $^{\circ}$C on (100) MgF$_2$. These results are systematically supported by X-ray diffraction reciprocal space maps. For both growth temperatures predominant cubic (111), (115) and (110) GaN is found with no preferred nucleation on the substrate and typical grain sizes of 100-200 nm. All observed orientations can be understood as the result of first and second order twins on different $\{111\}$ planes related to the underlying substrate. The higher growth temperature shows a strongly increased twin density along with a higher surface roughness. Furthermore, grains with cubic (110) GaN growth show a reduced density and a reduced size of about 20 nm. Furthermore, in-diffusion of Mg and F into the GaN is observed, which is accompanied by the formation of cavities in the MgF$_2$ directly at the interface.

physics.app-ph↗

arXiv2022

Interface-Assisted Room-Temperature Magnetoresistance in Cu-Phenalenyl-based Magnetic Tunnel Junctions

Delocalized carbon-based radical species with unpaired spin, such as phenalenyl (PLY) radical, opened avenues for developing multifunctional organic spintronic devices. Here we develop a novel technique based on a three-dimensional shadow mask and the in-situ deposition to fabricate PLY-, Cu-PLY-, and Zn-PLY-based organic magnetic tunnel junctions (OMTJs) with area 3x8 μm2 and improved morphology. The nonlinear and weakly temperature-dependent current-voltage (I-V) characteristics in combination with the low organic barrier height suggest tunneling as the dominant transport mechanism in the structurally and dimensionally optimized OMTJs. Cu-PLY-based OMTJs, show a significant magnetoresistance up to 14 percent at room temperature due to the formation of hybrid states at the metal-molecule interfaces called spinterface, which reveals the importance of spin-dependent interfacial modification in OMTJs design. In particular, Cu-PLY OMTJs shows a stable voltage-driven resistive switching response that suggests their use as a new viable and scalable platform for building molecular scale quantum memristors and processors.

cond-mat.mtrl-sci↗

arXiv2022

Selectivity Trends and Role of Adsorbate-Adsorbate Interactions in CO Hydrogenation on Rhodium Catalysts

Predictive-quality computational modeling of heterogeneously catalyzed reactions has emerged as an important tool for the analysis and assessment of activity and activity trends. In contrast, more subtle selectivities and selectivity trends still pose a significant challenge to prevalent microkinetic modeling approaches that typically employ a mean-field approximation (MFA). Here, we focus on CO hydrogenation on Rh catalysts with the possible products methane, acetaldehyde, ethanol and water. This reaction has already been subject to a number of experimental and theoretical studies with conflicting views on the factors controlling activity and selectivity towards the more valuable higher oxygenates. Using accelerated first-principles kinetic Monte Carlo (KMC) simulations and explicitly and systematically accounting for adsorbate-adsorbate interactions through a cluster expansion approach, we model the reaction on the low-index Rh(111) and stepped Rh(211) surfaces. We find that the Rh(111) facet is selective towards methane, while the Rh(211) facet exhibits a similar selectivity towards methane and acetaldehyde. This is consistent with the experimental selectivity observed for larger, predominantly (111)-exposing Rh nanoparticles and resolves the discrepancy to earlier first-principles MFA microkinetic work that found the Rh(111) facet to be selective towards acetaldehyde. While the latter work tried to approximately account for lateral interactions through coverage-dependent rate expressions, our analysis demonstrates that this fails to sufficiently capture concomitant correlations among the adsorbed reaction intermediates that crucially determine the overall selectivity.

physics.chem-ph↗

arXiv2021

Tracing boron diffusion into a textured silicon solar cell using electron beam induced current in a scanning transmission electron microscope

The light absorption of [001] grown single-crystalline silicon wafers can be enhanced by chemical etching, e.g., with potassium hydroxide, resulting in a pyramid-like surface texture. Alongside advantageous photon harvesting in solar cells, the surface roughness leads to drawbacks when measuring diffusion behaviour of dopants in the heterogeneous structure. In this paper, we employ experimental and simulated scanning transmission electron beam induced current in combination with simulation of boron diffusion in a self-consistent framework to trace the dopant distribution underneath the pyramid-like surface texture. In order to account for surface recombination, an effective model projecting the system along the electron beam propagation direction is used in the EBIC simulation enabling a comparison to entire two-dimensional experimental maps. We find a good agreement between simulated and experimental data and thoroughly discuss how EBIC can be used in future experiments to quantify weak electric fields.

physics.app-ph↗

arXiv2020

Site-specific plan-view TEM lamella preparation of pristine surfaces with a large field of view

Transmission electron microscopy has become a major characterisation tool with an ever increasing variety of methods being applied in wide range of scientific fields. However, the probably most famous pitfall in related workflows is the preparation of high-quality electron-transparent lamellae enabling for extraction of valuable and reliable information. Particularly in the field of solid state physics and materials science, it is often required to study the surface of a macroscopic specimen with plan-view orientation. Nevertheless, despite tremendous advances in instrumentation, i.e. focused ion beam, the yield of existing plan-view lamellae preparation techniques is relatively low compared to cross-sectional extraction methods. Furthermore, techniques relying on mechanical treatments, i.e. conventional preparation, compromise site-specifity. In this paper, we demonstrate that by combining a mechanical grinding step prior to backside lift-out in the focused ion beam plan-view lamellae preparation becomes increasingly easy. The suggested strategy combines site-specifity with micrometer precision as well as possible investigation of pristine surfaces with a field of view of several hundred square micrometers.

physics.app-ph↗

arXiv2020

Phase transitions in a perovskite thin film studied by environmental in-situ heating nano-beam electron diffraction

The rich phase diagram of bulk Pr$_{1-x}$Ca$_{x}$MnO$_3$ resulting in a high tunability of physical properties gave rise to various studies related to fundamental research as well as prospective applications of the material. Importantly, as a consequence of strong correlation effects, electronic and lattice degrees of freedom are vigorously coupled. Hence, it is debatable whether such bulk phase diagrams can be transferred to inherently strained epitaxial thin films. In this paper, the structural orthorhombic to pseudo-cubic transition for $x=0.1$ is studied in ion-beam sputtered thin films and point out differences to the respective bulk system by employing in-situ heating nano-beam electron diffraction to follow the temperature dependence of lattice constants. In addition, it is demonstrated that controlling the environment during heating, i.e. preventing oxygen loss, is crucial in order to avoid irreversible structural changes, which is expected to be a general problem of compounds containing volatile elements under non-equilibrium conditions.

cond-mat.mtrl-sci↗

arXiv2019

Plasma Profiling Time-of-Flight Mass Spectrometry for Fast Elemental Analysis of Semiconductor Structures with Depth Resolution in the Nanometer Range

Plasma profiling time of flight mass spectrometry (PP-TOFMS) has recently gained interest, as it enables the elemental profiling of semiconductor structures with high depth resolution in short acquisition times. As recently shown by Tempez et al., PP-TOFMS can be used to obtain the composition in the structures for modern field effect transistors [1]. There, the results were compared to conventional SIMS measurements. In the present study, we compare PP-TOFMS measurements of an Al-/In-/GaN quantum well multi stack to established micro- and nano-analysis techniques like cathodoluminescence (CL), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). We show that PP-TOFMS is able to resolve the layer structure of the sample even more than 500 nm deep into the sample and allows the determination of a relative elemental composition with an accuracy of about 10 rel. %. Therefore, it is an extremely rapid alternative method to obtain semiconductor elemental depth profiles without expensive and time consuming sample preparation as it is needed for TEM. Besides, PP-TOFMS offers better depth resolution and more elemental information than for example electrochemical capacitance-voltage (ECV), as the acquisition of all elements occurs in parallel and not only electrically (ECV) or optically (CL) active elements are observed.

physics.app-ph↗

arXiv2015

Spin-transfer torque switching at ultra low current densities

The influence of the tantalum buffer layer on the magnetic anisotropy of perpendicular Co-Fe-B/MgO based magnetic tunnel junctions is studied using magneto-optical Kerr-spectroscopy. Samples without a tantalum buffer are found to exhibit no perpendicular magnetization. The transport of Boron into the tantalum buffer is considered to play an important role on the switching currents of those devices. With the optimized layer stack of a perpendicular tunnel junction, a minimal critical switching current density of only 9.2 kA/cm$^2$ is observed. As of today, this value is the lowest reported value for current-induced magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2014

Controlling boron redistribution in CoFeB/MgO magnetic tunnel junctions during annealing by variation of cap layer materials and MgO deposition methods

Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphous CoFeB electrodes received much attention due to their high tunnel magneto resistance ratio at room temperature. One important parameter for achieving high tunnel magneto resistance ratios is to control the boron diffusion from the electrodes especially during post growth annealing. By high resolution transmission electron microscopy and electron energy loss spectroscopy techniques we show that the cap layer material adjacent to the electrodes and the MgO deposition method are crucial to control boron redistribution. It is pointed out, that Ta cap layers acts as sinks for boron during annealing in contrast to Ru layers. Furthermore, radio frequency sputtered MgO tunneling barriers contain a rather high concentraion of boron in trigonal [BO$_3$]$^{3-}$ - environment after annealing in contrast to electron beam evaporated MgO which is virtually free from any boron. Our data further indicate that neither boron nor oxygen-vacancy-related gap states in the bulk of MgO barriers affect spin polarized transport for tunnel magneto resistance ratios at the level of 200%.

cond-mat.mtrl-sci↗

arXiv2013

Spin-transfer torque switching below 20 kA/cm$^2$ in perpendicular magnetic tunnel junctions

We demonstrate the reduction of critical spin-transfer torque (STT) switching currents in Co-Fe-B/MgO based magnetic tunnel junctions (MTJ) with perpendicular magnetization anisotropy (PMA). The junctions yield tunnel magnetoresistance (TMR) ratios of up to 64% at 4 monolayer (ML) tunnel barrier thickness. In this paper, the reduction of the critical switching current density is studied. By optimizing the applied bias field during DC-STT measurements, ultra low critical switching current densities of less than 20 kA/cm$^2$, even down to 9 kA/cm$^2$, are found. With the reduced switching currents, our samples are ideal candidates for further experimental studies such as the theoretical predicted thermally driven spin-transfer torque effect.

cond-mat.mes-hall↗

arXiv2013

Parameter space for thermal spin-transfer torque

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with 3 monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magneto resistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm$^2$. The thermally generated torque is calculated ab initio using the Korringa-Kohn-Rostoker and non-equilibrium Green's function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

cond-mat.mes-hall↗

arXiv2011

Seebeck Effect in Magnetic Tunnel Junctions

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, i.e., the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In that respect, it is the analog to the tunneling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configuration are in the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. Experimentally, we realized 8.8 % magneto-Seebeck effect, which results from a voltage change of about -8.7 μV/K from the antiparallel to the parallel direction close to the predicted value of -12.1 μV/K.

cond-mat.mtrl-sci↗

arXiv2010

Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions

Using magnetron sputtering, we have prepared Co-Fe-B/tunnel barrier/Co-Fe-B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia-alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The similar tunnel magnetoresistance ratios of the tunnel junctions containing alumina provide evidence that coherent tunneling is suppressed by the alumina layer in the composite tunnel barrier.

cond-mat.mtrl-sci↗

arXiv2006

Nanofabrication of spin-transfer torque devices by a PMMA mask one step process: GMR versus single layer devices

We present a method to prepare magnetic spin torque devices of low specific resistance in a one step lithography process. The quality of the pillar devices is demonstrated for a standard magnetic double layer device. For single layer devices, we found hysteretic switching and a more complex dynamical excitation pattern in higher fields. A simple model to explain the resistance spikes is presented.

cond-mat.mtrl-sci↗