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Projects

Quantification of internal electric fields in semiconductor nanostructures by transmission electron microscopy

Funding: Deutsche Forschungsgemeinschaft DFG

Duration: 02/01/2021 - 01/31/2024

Description: The goal of the present project is the direct quantitative determination of internal electric fields (IEFs) in semiconductor nanostructures using up-to-date techniques based on transmission electron microscopy (TEM) in combination with controlled superposition of external electric fields. As an ideal experimental model system we focus on group III-nitride nanowire (III-N NW) pn-junctions and axial group III-nitride nanowire heterostructures (III-N NWHs) synthesized by plasma-assisted molecular beam epitaxy. Such NWHs can be grown with high precision and reproducibility and exhibit high polarization-induced IEFs of the order of several MV/cm that can be controlled in magnitude by adjusting the geometrical dimensions, the chemical composition and the dopant concentration. Furthermore, the IEF strength and spatial distribution in individually contacted NWs and NWHs can be modified in a controlled manner by superposition of an externally applied bias. As the optical characteristics of III-N NWHs are influenced by IEFs due to the quantum-confined Stark effect we will investigate the IEFs of specific NWHs by the novel TEM-based techniques and we will characterize the same individual nanostructures by bias-dependent photocurrent and micro-photoluminescence analysis. Comparison of the TEM and optical characterization results for different electron beam intensities and excitation powers, respectively, in combination with simulation of the electron/hole states will allow for quantitative determination of the IEFs as well as an improved understanding of the electronic band profiles, shielding of internal fields by free charge carriers and influence of preparation-induced surface states. The availability and optimization of a TEM-based technique to reliably quantify IEFs is not only important for analyzing polarization-induced IEFs in the AlN/GaN and InGaN/GaN nanostructures in this study. It is a promising and useful method that is applicable for other complex nanostructures and nanoscale devices of other semiconductor materials (such as GaAs(AlGaAs) but also for the analysis different systems such as ferroelectric tunnel junctions.

Contact: Prof. Dr. Martin Eickhoff

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Fluorescence spectroscopy of tree rings: new climate proxys for dendroclimatology

Funding: Central Research Development Fund (CRDF) of the University of Bremen, funding line 01 Impulse Grants for Research Proposals

Duration: 05/01/2021 - 04/30/2022

Description: Fluorescence spectroscopy of tree rings is applied to obtain new climate proxys to extend the possibilities of climate reconstruction by dendroclimatology. Goal of the project is the non-destructive and reproducible analysis of tree-rings by UV-VIS fluorescence spectroscopy. Emission bands in the fluorescence spectrum of tree rings can be assigned to different wood components (cellulose, lignin). Qualitative and possibly quantitative changes in the concentration of the wood components can be analyzed from the intensity of the emission bands. It is expected that these informations and further analysis (tree ring width etc.) will refine and improve the reconstruction of climate parameters such as temperature, precipitation and solar irradiation.

Contact: Dr. Christian Tessarek

 

Chemical vapor deposition and laser-induced modification of nano- and heterostructurs from two-dimensional, atomically thin layers for photonic applications

Funding: Central Research Development Fund (CRDF) of the University of Bremen, funding line 04 Impulse Grants for Postdocs

Duration: 10/01/2019 - 09/30/2022

Description: Goal of the project is the precise manipulation of the optical properties of heterostructures from two-dimensionalen (2D) atomically thin materials such as graphene, boron nitride (BN) and transition metal dichalcogenides, e.g. molybdenum disulfide (MoS2) und tungsten diselenide (WSe2). Heterolayers consisting of different 2D materials will be grown by van-der-Waals epitaxy using chemical vapor deposition. The dielectric environment will be modified by the deposition of graphene and/or BN below and/or above a transistion metal dichalcogenide layer. The challenge is the identification of specific growth windows which enable deposition and stacking of different 2D materials of high quality and free of impurities. Furthermore, growth parameters for ternary transition metal dichalcogenides (MoWS2, Mo(SSe)2) will be developed. Beside the 2D layer growth, also the self-organized growth of quantum dot-like nanostructures will be investigated, e.g. relaxed MoS2 nanoislands embedded in a WS2 matrix or strained MoSe2 nanoislands in an MoS2 matrix. This will enable the analysis of strain and carrier trapping on the optical properties. An important focus of this project proposal is the laser-induced manipulation of the layers. Laser irradiation will be used to control the optical properties by defect engineering, e.g. laser-induced S vacancies in the transition metal dichalcogenide layers. An already developed method for the local and precise laser-thinning of transition metal dichalcogenides will be applied on any 2D heterostructures to obtain periodic nanostructures towards photonic crystals.

Contact: Dr. Christian Tessarek

 

Einzelpunkt-Sensorsystem für die nicht-invasive, dynamische Messung der Herzfunktion (SINDynamik)

Förderung: Bundesministerium für Bildung und Forschung (BMBF)

Laufzeit: 01.07.2018 bis 30.06.2020

Beschreibung: Kardio-vaskuläre Erkrankungen sind in Deutschland die häufigste Todesursache. Zentrale Voraussetzung für eine Verbesserung der Therapie, ist eine effiziente Langzeitdiagnostik. Die Echtzeitmessung des zentralen Venendrucks (ZVD) im rechten Vorhof des Herzens in Bezug zum EKG-Signal stellt eines der wichtigsten Verfahren in der Herz-Lungen Diagnostik dar. Um die dynamischen ZVD-Werte im zeitlichen Verlauf exakt zu erfassen, kommen invasive und sehr kostenintensive Ansätze mit drucksensorintegriertem Herz-Katheter zum Einsatz, die nicht für eine zugängliche präventive Diagnostik eingesetzt werden können. So ist bislang insbesondere nach einem Herzinfarkt oder einer Herzoperation eine Langzeitüberwachung der Herzkammerdynamik und des Herzventilzyklus, nicht möglich.

Ziel des Verbundprojekts SINDynamik ist deshalb die Erforschung und die vorklinische Validierung einer neuartigen Diagnose-Methode zur simultanen und nicht-invasiven Bestimmung des ZVD-Echtzeitsignals und des EKG-Signals. Dabei soll eine bio-elektrodynamische Erfassung der Herzaktivität an einem einzelnen Körperpunkt (Single Point Cardio-Dynamics, SPC) erfolgen. Der Sensor detektiert die bio-elektrische Herzgewebe-Depolarisation, überlagert mit der mechanischen Bewegung des elektrisch geladenen Herzgewebes und kann damit ein vollständiges Ladungsabbild der Herz-Lungen-Wechselwirkung mit der Atemdynamik liefern. Das SPC-Messverfahren ermöglicht prinzipiell eine kontinuierliche, nicht-invasive, personifizierte Herzdiagnostik, generiert damit einen erheblichen Patientennutzen und soll zur Verringerung der Mortalitätsrate nach einem Herzinfarkt beitragen.

Die Leistungsfähigkeit des SPC-Verfahrens wird am Ende des Projekts durch den Vergleich mit Standardverfahren zu EKG- und ZVD-Messungen an einer kleinen Testgruppe von Herz-Lungen Patienten demonstriert.

Kontakt: Prof. Dr. Martin Eickhoff

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