Total ionizing dose test with DEPFET sensors for Athena's WFI

The focal plane of Athena's WFI consists of spectroscopic single photon X-ray detectors that contain arrays of DEPFETs (DEpleted P-channel Field-Effect Transistor) as well as ASICs that are used for steering, readout and analog signal shaping. These components have to be examined regarding the effect of ionizing radiation. A Total Ionizing Dose (TID) test was done with prototype detector modules with 64x64 DEPFETs and one SWITCHER and VERITAS ASIC each. The current design of the WFI detector head features a proton shield equivalent to 4 cm of aluminum in order to prevent a strong increase of leakage current in the fully depleted 450 $\mu$m thick bulk of the sensor. This keeps the expected doses and dose rates during the nominal mission relatively low ($\sim$5 Gy). It is nevertheless important to study the current system in a dedicated TID test in order to exclude unforeseen effects and to study any radiation related changes that can have an effect on the very sensitive readout chain and the detector performance. The combination of low doses, low dose rates, low operating temperature (<-60{\deg}C) but high sensitivity on small changes of the threshold voltages represent somehow unusual boundary conditions in comparison to TID tests for standard radiation hard electronic components. Under these circumstances it was found beneficial to do the test in our own laboratory with an X-ray source in order to realize irradiation during nominal operation conditions. Furthermore, it facilitated to take annealing effects into account. Reasonably accurate dosimetry is achieved by measuring the X-ray spectrum and intensity with the device under test. After irradiation to a total dose of 14 Gy and subsequent annealing the threshold voltage of the DEPFETs were shifted by a mean value of 80 mV, the performance remained unchanged apart from a slight increase in readout noise by 10%.Comment: 8 pages, 6 figures, to appear in SPIE Proceeding of Astronomical Telescopes + Instrumentation, 202

Modeling the conductivity around the dimensionality-controlled metal-insulator transition in LaNiO3/LaAlO3 (100) superlattices

A dimensionality controlled metal insulator transition in epitaxial [LaNiO3 (d nm)/LaAlO3(2nm)]10 (100) superlattices (thereafter [d/2]10 SLs) is demonstrated for decreasing LaNiO3 single layer thickness from 4nm down to 1.2 nm. The [4/2]10 SL shows metallic behavior with positive resistivity temperature coefficient, while the [2/2]10 SL shows a metal-insulator transition with crossover from 3D to two-dimensional single-layer dimensionality. Strong localization appears for the [1.2/2]10 SL with the resistivity being dominated by two-dimensional variable range hopping with a localization length of about 0.035 nm

Magnetic spin structure and magnetoelectric coupling in BiFeO3-BaTiO3 multilayer

Magnetic spin structures in epitaxial BiFeO3 single layer and an epitaxial BaTiO3/BiFeO3 multilayer thin film have been studied by means of nuclear resonant scattering of synchrotron radiation. We demonstrate a spin reorientation in the 15 x[BaTiO3/BiFeO3] multilayer compared to the single BiFeO3 thin film. Where as in the BiFeO3 film, the net magnetic moment m→ lies in the (1–10) plane, identical to the bulk, m→ in the multilayer points to different polar and azimuthal directions. This spin reorientation indicates that strain and interfaces play a significant role in tuning the magnetic spin order. Furthermore, large difference in the magnetic field dependence of the magnetoelectric coefficient observed between the BiFeO3 single layer and multilayer can be associated with this magnetic spin reorientation

Magnetic Tunnel Junctions based on spinel ZnxFe3-xO4: Magnetic Tunnel Junctions based onspinel ZnxFe3-xO4

Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4). Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen. Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen

Modeling the conductivity around the dimensionality-controlled metal-insulator transition in LaNiO3/LaAlO3 (100) superlattices

A dimensionality controlled metal insulator transition in epitaxial [LaNiO3 (d nm)/LaAlO3(2nm)]10 (100) superlattices (thereafter [d/2]10 SLs) is demonstrated for decreasing LaNiO3 single layer thickness from 4nm down to 1.2 nm. The [4/2]10 SL shows metallic behavior with positive resistivity temperature coefficient, while the [2/2]10 SL shows a metal-insulator transition with crossover from 3D to two-dimensional single-layer dimensionality. Strong localization appears for the [1.2/2]10 SL with the resistivity being dominated by two-dimensional variable range hopping with a localization length of about 0.035 nm

Temperature dependence of the dielectric function in the spectral range (0.5–8.5) eV of an In2O3 thin film

We present the dielectric function of a bcc-In2O3 thin film in the wide spectral range from nearinfrared to vacuum-ultraviolet and for temperatures 10 K–300K, determined by spectroscopic ellipsometry. From the temperature dependence of electronic transition energies, we derive electron-phonon coupling properties and found hints that the direct parabolic band-band transitions involve In-d states. Further we discuss possible excitonic contributions to the dielectric function

Temperature dependence of the dielectric function in the spectral range (0.5–8.5) eV of an In2O3 thin film

We present the dielectric function of a bcc-In2O3 thin film in the wide spectral range from nearinfrared to vacuum-ultraviolet and for temperatures 10 K–300K, determined by spectroscopic ellipsometry. From the temperature dependence of electronic transition energies, we derive electron-phonon coupling properties and found hints that the direct parabolic band-band transitions involve In-d states. Further we discuss possible excitonic contributions to the dielectric function

Temperature dependence of the dielectric function in the spectral range (0.5–8.5) eV of an In2O3 thin film

We present the dielectric function of a bcc-In2O3 thin film in the wide spectral range from nearinfrared to vacuum-ultraviolet and for temperatures 10 K–300K, determined by spectroscopic ellipsometry. From the temperature dependence of electronic transition energies, we derive electron-phonon coupling properties and found hints that the direct parabolic band-band transitions involve In-d states. Further we discuss possible excitonic contributions to the dielectric function