Scanning-gate microscopy of semiconductor nanostructures: an overview

This paper presents an overview of scanning-gate microscopy applied to the imaging of electron transport through buried semiconductor nanostructures. After a brief description of the technique and of its possible artifacts, we give a summary of some of its most instructive achievements found in the literature and we present an updated review of our own research. It focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. In all of the given examples, we emphasize how a local-probe approach is able to shed new, or complementary, light on transport phenomena which are usually studied by means of macroscopic conductance measurements.Comment: Invited talk by SH at 39th "Jaszowiec" International School and Conference on the Physics of Semiconductors, Krynica-Zdroj, Poland, June 201

Electromechanical Lifting Actuation of a MEMS Cantilever and Nano-Scale Analysis of Diffusion in Semiconductor Device Dielectrics

This dissertation presents experimental and theoretical studies of physical phenomena in micro- and nano-electronic devices. Firstly, a novel and unproven means of electromechanical actuation in a micro-electro-mechanical system (MEMS) cantilever was investigated. In nearly all MEMS devices, electric forces cause suspended components to move toward the substrate. I demonstrated a design with the unusual and potentially very useful property of having a suspended MEMS cantilever lift away from the substrate. The effect was observed by optical micro-videography, by electrical sensing, and it was quantified by optical interferometry. The results agree with predictions of analytic and numerical calculations. One potential application is infrared sensing in which absorbed radiation changes the temperature of the cantilever, changing the duty cycle of an electrically-driven, repetitively closing micro-relay. Secondly, ultra-thin high-k gate dielectric layers in two 22 nm technology node semiconductor devices were studied. The purpose of the investigation was to characterize the morphology and composition of these layers as a means to verify whether the transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS) could sufficiently resolve the atomic diffusion at such small length scales. Results of analytic and Monte-Carlo numerical calculations were compared to empirical data to validate the ongoing viability of TEM EDS as a tool for nanoscale characterization of semiconductor devices in an era where transistor dimensions will soon be less than 10 nm

Untersuchung von Magnetostriktiven und Piezotronischen Mikrostrukturen und Materialien für biomagnetische Sensoren mittels Röntgenstrahlen

Detecting electric potential differences from the human physiology is an established technique in medical diagnosis, e.g., as electrocardiogram. It arises from a changing electrical polarization of living cells. Simultaneously, biomagnetism is induced and can be utilized for medical examinations, as well. Benefits in using magnetic signals are, no need for direct skin contact and an increased spatial resolution, e.g., for mapping brain activity, especially in combination with electrical examinations. But biomagnetic signals are very weak and, thus, highly sensitive devices are necessary. The development of small and easy to use biomagnetic sensors, with a sufficient sensitivity, is the goal of the Collaborative Research Centre 1261 - Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics. This thesis was written as part of this collaboration, with the main focus on the investigation of crystalline structures and structure related properties of piezotronic and magnetostrictive materials by utilizing a selection of X-ray techniques, i.e., X-ray diffraction (XRD), X-ray reflectivity (XRR) and coherent X-ray diffraction imaging (CXDI). Piezotronics, realized by combining piezoelectricity and Schottky contacts in one structure, provides a promising path to enhance sensor sensitivity. A first study investigated the crystalline structure of three piezotronic ZnO rods, spatially resolved by scanning nano XRD and combined with electrical examinations of their Schottky contact properties. It is found that the crystalline quality has a clear impact on the electrical properties of the related Schottky contact, probably due to crystalline defects. A complementary transmission electron microscopy (TEM) and XRD study performed on hybride vapor phase epitaxy (HVPE) grown GaN showed a slight, photoelectrochemical etching related relaxion of strain originating from crystal growth. In a separate study, CXDI was utilized for three-dimensional visualization of strain in a gold coated ZnO rod, with spatial resolution below 30 nm. A distinct strain distribution was found inside the rod, denoted to depletion and screening effects occurring in bent piezotronic structures, and a high strain at the interface may be related to Schottky contact formation. This interface strain agrees with results obtained from TEM. A succeeding CXDI study was conducted on a ZnO rod coated with magnetostrictive FeCoSiB and the possibility for the investigation of the Schottky contacts electrical properties. It was found that FeCoSiB sputtered on ZnO results in an ohmic contact and that an external magnetic field causes a change of the electrical properties, probably due to a strain change, visualized by CXDI. In a fifth study, magnetostrictive FeCo/TiN multilayer structures were investigated by a combined TEM and XRD/XRR approach, showing a relaxation of the structure due to an annealing process and a cube-on-cube structure of the FeCo and TiN layers

Characterization of Oxide Thin Films and Interfaces Using Transmission Electron Microscopy

abstract: Multifunctional oxide thin-films grown on silicon and several oxide substrates have been characterized using High Resolution (Scanning) Transmission Electron Microscopy (HRTEM), Energy-Dispersive X-ray Spectroscopy (EDX), and Electron Energy-Loss Spectroscopy (EELS). Oxide thin films grown on SrTiO3/Si pseudo-substrate showed the presence of amorphised SrTiO3 (STO) at the STO/Si interface. Oxide/oxide interfaces were observed to be atomically clean with very few defects. Al-doped SrTiO3 thin films grown on Si were of high crystalline quality. The Ti/O ratio estimated from EELS line scans revealed that substitution of Ti by Al created associated O vacancies. The strength of the crystal field in STO was measured using EELS, and decreased by ~1.0 eV as Ti4+ was substituted by Al3+. The damping of O-K EELS peaks confirmed the rise in oxygen vacancies. For Co-substituted STO films grown on Si, the EDS and EELS spectra across samples showed Co doping was quite random. The substitution of Ti4+ with Co3+ or Co2+ created associated oxygen vacancies for charge balance. Presence of oxygen vacancies was also confirmed by shift of Ti-L EELS peaks towards lower energy by ~0.4 eV. The crystal-field strength decreased by ~0.6 eV as Ti4+ was partially substituted by Co3+ or Co2+. Spinel Co3O4 thin films grown on MgAl2O4 (110) were observed to have excellent crystalline quality. The structure of the Co3O4/MgAl2O4 interface was determined using HRTEM and image simulations. It was found that MgAl2O4 substrate is terminated with Al and oxygen. Stacking faults and associated strain fields in spinel Co3O4 were found along [111], [001], and [113] using Geometrical Phase Analysis. NbO2 films on STO (111) were observed to be tetragonal with lattice parameter of 13.8 Å and NbO films on LSAT (111) were observed to be cubic with lattice parameter of 4.26 Å. HRTEM showed formation of high quality NbOx films and excellent coherent interface. HRTEM of SrAl4 on LAO (001) confirmed an island growth mode. The SrAl4 islands were highly crystalline with excellent epitaxial registry with LAO. By comparing HRTEM images with image simulations, the interface structure was determined to consist of Sr-terminated SrAl4 (001) on AlO2-terminated LAO (001).Dissertation/ThesisDoctoral Dissertation Physics 201

Composition Determination of Semiconductors at Different Scattering Angles with the Help of Energy-Filtered STEM and Four-Dimensional STEM

In this study, the structural characterization of nanomaterials is performed by an extension of the established method which is used for quantitative STEM based on comparing the ADF-STEM image simulations and experiments. The limitations and capabilities of the method towards single atom accuracy are investigated. The method is further elaborated by determining more complex material systems as well as optimizing the imaging conditions to increase the accuracy. Accessing the composition determination method with high resolution in a single atom accuracy and potential of estimating the accuracy of the method emerges the idea to optimize the experimental condition by conducting a simulation study. A performed simulation study on several critical imaging parameters leads to optimization of the imaging condition and enhances the accuracy of the method. The result reveals the critical role of two ADF-STEM imaging parameters: the semi-convergence angle of the impinging beam as well as the detection angle of ADF detectors. The former can be simply tuned experimentally by the choice of STEM condenser aperture, while the latter demands a fast, pixelated detector allowing the flexible choice of detection angle. The study however indicated that the optimum imaging condition differs by sample thickness and material systems. This becomes more apparent in the case of material systems containing light elements, e.g. GaNxAs1-x. Due to their low amount of protons, the light elements do not efficiently scatter to the commonly used detection angles in Z-contrast HAADF STEM micrographs. Accordingly, lower detection angles should be chosen. In contrast to the HAADF in which the image intensity is dominated by only elastically scattered electrons leading to a perfect match between ADF-STEM image simulations and experimental results so many other parameters play a role in the intensity of the micrograph at a low angular regime. In this study, as the main source of discrepancy between STEM experiments and simulations at low scattering angles, the effect of plasmon excitations on the angular distribution of the STEM intensities is investigated. The comparison of energy-filtered and unfiltered diffraction patterns indicates the significant effect of inelastic scattering at angles in the range of 0-40 mrad. Considering the effect of plasmon excitation at low scattering angles, a further method is developed for the composition determination of material systems containing light elements at a low scattering angle based on EFSTEM. It is confirmed that the strain contrast induced by SADs causes higher scattering intensity at low scattering angles. Consequently, a material system containing SADs, i.e. GaNxAs1-x, is intentionally chosen to make the composition determination more reliable. There are also other sources of discrepancy between simulated and experimental STEM images such as neglecting the phonon correlations in image simulations, the effect of mistilt from the targeted crystalline zone-axis, and the existence of surface amorphous layers on ADF images. These errors are resolved either experimentally or by optimizing the detection angle with the help of a fast, pixelated detector. Here, Si as a model material is used to obtain the angular range with the perfect match between experiment and simulation. The method was applied on a sample containing GaNxAs1-x QWs embedded in GaAs barriers. The composition and the width of the QWs obtained by the new method are in very good agreement with XRD results. The new advanced four-dimensional detector in hand enables recording a full diffraction pattern for every electron probe position. So far the camera is utilized as an annular detector with the flexibility of choosing the optimum detection angle. However, it can be further expanded in optimizing the composition determination by the flexible choice of regions on diffraction pattern for which the intensity suits the best for quantitative STEM. As Pennycook suggests, the incoherent nature of HAADF-STEM results in simply interpretable micrographs with independent information at every atomic column of the crystalline materials. However, this interpretability is lost in the conventional high resolution TEM micrographs due to the dynamical scattering and the coherent nature of the image formation. Hence, avoiding any coherent information in the diffraction pattern such as Laue zones may lead to more localized information in real space and consequently a higher accuracy in composition determination. The combination of an in-column energy filter a fast pixelated detector is utilized to quantify the composition of different material systems at high accuracy. The fourdimensional detectors however, are capable of detecting the shift of the diffraction pattern's center-of-mass (COM) which is correlated to the local electric field within the crystal such as atomic potentials. The effect of quasi-elastic TDS on electric field determination is investigated in a simulation study showing a significant effect on electric field measurements. Future researches can be focused on utilizing the combination of EFSTEM and 4D-STEM to investigate the effect of another source of inelastic scattering, i.e. plasmon excitation, on COM shift.In dieser Studie wird eine Erweiterung der quantitativen Kompositionsbestimmung mittels STEM, die auf dem Vergleich von ADF-STEM-Bildsimulationen und Experimenten beruht, für die strukturelle Charakterisierung von Nanomaterialien untersucht. Die Grenzen und Möglichkeiten der Methode werden in Bezug auf die Bestimmung der Materialzusammensetzung mit atomarer Genauigkeit untersucht. Die Methode wird weiter optimiert, indem komplexere Materialsysteme bestimmt und die Abbildungsbedingungen angepasst werden, um die Genauigkeit zu optimieren. Aus dem Vorgang zur Methode der Kompositionsbestimmung mit hoher Auflösung bei Einzelatomgenauigkeit und dem Potenzial zur Abschätzung der Genauigkeit der Methode ergibt sich die Idee, die experimentellen Bedingungen durch eine Simulationsstudie zu optimieren. Eine Simulationsstudie zu mehreren kritischen Abbildungsparametern führt zur Optimierung der Abbildungsbedingungen und verbessert die Genauigkeit der Methode. Das Ergebnis zeigt die kritische Rolle von zwei ADF-STEM-Abbildungsparametern: der Semikonvergenzwinkel des auftreffenden Strahls und der Detektionswinkel der ADFDetektoren. Ersterer kann einfach experimentell durch die Wahl der STEM-Kondensor Aperturblende eingestellt werden, während letzterer einen schnellen, gepixelten Detektor erfordert, der eine flexible Wahl des Detektionswinkels ermöglicht. Die Studie hat jedoch gezeigt, dass die optimalen Abbildungsbedingungen je nach Probendicke und Materialsystem unterschiedlich sind. Dies wird besonders bei Materialsystemen, wie. GaNxAs1-x deutlich, die leichte Elemente enthalten. Aufgrund ihrer geringeren Anzahl an Protonen (Kernladungszahl: Z) streuen die leichten Elemente nicht e zient zu den üblicherweise verwendeten Detektionswinkeln in den HAADF-STEM-Bildern, welche auf dem Z-Kontrast basieren. Dementsprechend sollten niedrigere Detektionswinkel gewählt werden. Im Gegensatz zur HAADF Bildgebung, bei der die Bildintensität nur von elastisch gestreuten Elektronen dominiert wird und somit zu einer nahezu perfekten Übereinstimmung zwischen ADF-STEM-Bildsimulationen und experimentellen Ergebnissen führt, spielen viele andere Parameter eine Rolle für die Intensität der Beugungsmuster in einem niedrigen Winkelbereich. In dieser Studie wird als Hauptursache für die Diskrepanz zwischen STEM-Experimenten und -Simulationen bei niedrigen Streuwinkeln die Wirkungvon Plasmonenanregungen auf die Winkelverteilung der STEM-Intensitäten untersucht. Der Vergleich von energiegefilterten und ungefilterten Beugungsmustern zeigt den signifikanten Effekt von inelastischer Streuung bei Winkeln im Bereich von 0-40 mrad. Unter Berücksichtigung des Effekts der Plasmonenanregung bei niedrigen Streuwinkeln wird eine weitere Methode zur Bestimmung der Zusammensetzung auf der Grundlage von EFSTEM für Materialsysteme, welche leichte Elemente enthalten, entwickelt. Es wird bestätigt, dass der durch SADs induzierte Dehnungskontrast eine höhere Streuintensität bei kleinen Streuwinkeln verursacht. Daher wird absichtlich ein Materialsystem wie beispielsweise GaNxAs1-x gewählt, das SADs enthält, um die Bestimmung der Zusammensetzung zuverlässiger zu machen. Es gibt auch andere Quellen für die Diskrepanzen zwischen simulierten und experimentellen STEM-Bildern, wie z. B. die Vernachlässigung der Phononen-Korrelationen in den Bildsimulationen, die Auswirkung der Fehlorientierung der angestrebten kristallinen Zonenachse und das Vorhandensein von amorphen Oberflächenschichten auf ADF-Bildern. Diese Fehler werden entweder experimentell oder durch Optimierung des Erfassungswinkels mit Hilfe eines schnellen, gepixelten Detektors behoben. Hier wird Si als Modellmaterial verwendet, um den Winkelbereich mit der perfekten Übereinstimmung zwischen Experiment und Simulation zu erhalten. Die Methode wurde auf eine Probe angewendet, welche aus GaNxAs1-x QWs, eingebettet in GaAsBarrieren besteht. Die Zusammensetzung und die Breite der QWs, die mit der neuen Methode ermittelt wurden, stimmen sehr gut mit den XRD-Ergebnissen überein. Der neue fortschrittliche vierdimensionale Detektor ermöglicht die Aufnahme eines vollständigen Beugungsmusters für jede Scanposition des Elektronenstrahls. Bislang wird die Kamera als ringförmiger Detektor eingesetzt, bei dem der optimale Detektionswinkel flexibel gewählt werden kann. Sie kann jedoch weiter ausgebaut werden, um die Bestimmung der Zusammensetzung durch die flexible Wahl der Regionen im Beugungsmuster, für die die Intensität am besten für die quantitative STEM geeignet ist, zu optimieren, Wie Pennycook andeutet, führt die inkohärente Natur der HAADF STEM Bilden zu einfach interpretierbaren Beugungsmustern mit unabhängigen Informationen an jeder Atomsäule der kristallinen Materialien. Diese Interpretierbarkeit geht jedoch bei konventionellen hochauflösenden TEM-Aufnahmen aufgrund der dynamischen Streuung und der kohärenten Natur der Bilderzeugung verloren. Der Verzicht auf kohärente Informationen im Beugungsmuster, wie z. B. Laue-Zonen, kann daher zu einer besseren Lokalisierung der Informationen im realen Raum und folglich zu einer höheren Genauigkeit bei der Bestimmung der Materialzusammensetzung führen. Die Kombination eines Energiefilters mit einem schnellen Pixeldetektor wird genutzt, um die Zusammensetzung verschiedener Materialsysteme mit hoher Genauigkeit zu quantifizieren. Die vierdimensionalen Detektoren sind in der Lage, die Verschiebung des Massenschwerpunkts des Beugungsmusters (COM) zu erfassen, die mit dem lokalen elektrischen Feld innerhalb des Kristalls, z. B. den atomaren Potenzialen, korreliert ist. Die Auswirkung der quasi-elastischen thermisch diffusen Streuung auf die Bestimmung des elektrischen Feldes wurde in einer Simulationsstudie untersucht, die eine signifikante Auswirkung auf die Messungen des elektrischen Feldes zeigt. Zukünftige Forschungen können sich darauf konzentrieren, die Kombination von EFSTEM und 4DSTEM zu nutzen, um die Auswirkung anderer Quellen von inelastischer Streuung, wie z.B. Plasmonenanregung, auf die COM-Verschiebung zu untersuchen

Field effect enhancement in buffered quantum nanowire networks

III-V semiconductor nanowires have shown great potential in various quantum transport experiments. However, realizing a scalable high-quality nanowire-based platform that could lead to quantum information applications has been challenging. Here, we study the potential of selective area growth by molecular beam epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The buffered geometry allows for substantial elastic strain relaxation and a strong enhancement of field effect mobility. We show that the networks possess strong spin-orbit interaction and long phase coherence lengths with a temperature dependence indicating ballistic transport. With these findings, and the compatibility of the growth method with hybrid epitaxy, we conclude that the material platform fulfills the requirements for a wide range of quantum experiments and applications