TY - GEN A1 - Weißhaupt, David A1 - Sürgers, Christoph A1 - Bloos, Dominik A1 - Funk, Hannes Simon A1 - Oehme, Michael A1 - Fischer, Gerda A1 - Schubert, Markus Andreas A1 - Wenger, Christian A1 - van Slageren, Joris A1 - Fischer, Inga Anita A1 - Schulze, Jörg T1 - Lateral Mn5Ge3 spin-valve in contact with a high-mobility Ge two-dimensional hole gas T2 - Semiconductor Science and Technology N2 - Abstract Ge two-dimensional hole gases (2DHG) in strained modulation-doped quantum-wells represent a promising material platform for future spintronic applications due to their excellent spin transport properties and the theoretical possibility of efficient spin manipulation. Due to the continuous development of epitaxial growth recipes extreme high hole mobilities and low effective masses can be achieved, promising an efficient spin transport. Furthermore, the Ge 2DHG can be integrated in the well-established industrial complementary metal-oxide-semiconductor (CMOS) devices technology. However, efficient electrical spin injection into a Ge 2DHG—an essential prerequisite for the realization of spintronic devices—has not yet been demonstrated. In this work, we report the fabrication and low-temperature magnetoresistance (MR) measurements of a laterally structured Mn5Ge3/Ge 2DHG/ Mn5Ge3 device. The ferromagnetic Mn5Ge3 contacts are grown directly into the Ge quantum well by means of an interdiffusion process with a spacing of approximately 130 nm, forming a direct electrical contact between the ferromagnetic metal and the Ge 2DHG. Here, we report for the first time a clear MR signal for temperatures below 13 K possibly arising from successful spin injection into the high mobility Ge 2DHG. The results represent a step forward toward the realization of CMOS compatible spintronic devices based on a 2DHG. KW - two-dimensional hole gas Y1 - 2024 U6 - https://doi.org/10.1088/1361-6641/ad8d06 SN - 0268-1242 VL - 39 IS - 12 SP - 1 EP - 10 PB - IOP Publishing ER - TY - GEN A1 - Fischer, Simon A1 - Foerster, Michael A1 - Aballe, Lucia A1 - Vonk, Vedran A1 - Falta, Jens A1 - Krisponeit, Jon-Olaf A1 - Flege, Jan Ingo T1 - Vanadium dioxide thin films on (100)- and (110)-oriented ruthenium dioxide islands T2 - Verhandlungen der DPG N2 - Bulk vanadium dioxide exhibits a metal-insulator transition at 68 ∘C. Because this change in resistivity is linked to a structural transition, the transition temperature can be tuned by epitaxial strain. Due to the high lattice mismatch of RuO2 with VO2, this substrate promises a highly shifted transition temperature, which enables new types of switching devices, smart coatings, and sensors. It is prepared by oxidizing a Ru(0001) crystal, providing micron-sized, (110)- as well as (100)-oriented RuO2 islands simultaneously. One of the challenges in preparing VO2 is to reliably meet the desired stoichiometry. We investigated the vanadium oxidation state using local XAS and XPS measurements taken at a synchrotron-based XPEEM instrument, revealing that VO2 stoichiometry is maintained throughout the entire film. This was found not only on RuO2(100) and RuO2(110), but also on the bare ruthenium substrate. The VO2 film thickness was determined to 3.6 nm by ex situ XRR measurements. Financial support from the DFG is acknowledged. KW - transition temperature KW - strain KW - synchrotron-based XPEEM KW - local X-Ray absorption spectroscopy (XAS) KW - local X-Ray photoelectron spectroscopy (XPS) Y1 - 2019 UR - https://www.dpg-verhandlungen.de/year/2019/conference/regensburg/part/o/session/37/contribution/5 SN - 0420-0195 SP - 330 EP - 330 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Krisponeit, Jon-Olaf A1 - Fischer, Simon A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - Surface reconstructions on VO2(110) T2 - Verhandlungen der DPG N2 - Vanadium dioxide features a pronounced metal-insulator transition at 340 K. For thin films, this transition is adjustable via substrate-induced strain, making VO2 highly attractive for oxide electronic applications like switching devices and sensors. In addition, VO2 is considered for smart coatings and as catalytic material. For such applicational efforts, a thorough understanding of the relevant surfaces is an important prerequisite. Among the low-indexed surfaces orientations of VO2, the (110) surface is energetically favorable. We have prepared VO2(110) thin films by means of reactive molecular beam epitaxy on two different substrate types: TiO2(110) single crystals as well as on RuO2(110) islands grown on Ru(0001) crystals. The film stoichiometry was confirmed via XPS, while the surface structure was investigated by µLEED. For both substrate types diffraction patterns of three distinct symmetries have been observed in dependence on temperature. The corresponding surface reconstructions will be discussed in terms of surface oxygen content. Financial support from the DFG and the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative, is acknowledged. KW - metal-insulator transition KW - substrate-induced strain KW - X-Ray photoelectron spectroscopy KW - µLEED KW - surface reconstruction KW - surface oxygen content Y1 - 2019 UR - https://www.dpg-verhandlungen.de/year/2019/conference/regensburg/part/o/session/37/contribution/7 SN - 0420-0195 SP - 330 EP - 330 PB - Deutsche Physikalische Gesellschaft CY - Bad Honnef ER - TY - GEN A1 - Fischer, Simon A1 - Krisponeit, Jon-Olaf A1 - Foerster, Michael A1 - Aballe, Lucia A1 - Falta, Jens A1 - Flege, Jan Ingo T1 - Massively Strained VO2 Thin Film Growth on RuO2 T2 - Crystal Growth & Design N2 - Strain engineering vanadium dioxide thin films is one way to alter this material’s characteristic first order transition from semiconductor to metal. In this study, we extend the exploitable strain regime by utilizing the very large lattice mismatch of 8.78% occurring in the VO2/RuO2 system along the c axis of the rutile structure. We have grown VO2 thin films on single-domain RuO2 islands of two distinct surface orientations by atomic oxygen-supported reactive molecular beam epitaxy (MBE). These films were examined by spatially resolved photoelectron and X-ray absorption spectroscopy, confirming the correct stoichiometry. Low energy electron diffraction then reveals the VO2 films grow indeed fully strained on RuO2(110), exhibiting a previously unreported (2 × 2) reconstruction. On TiO2(110) substrates, we reproduce this reconstruction and attribute it to an oxygen-rich termination caused by the high oxygen chemical potential. On RuO2(100), on the contrary, the films grow fully relaxed. Hence, the presented growth method allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to massively strained regions. KW - strain engineering KW - vanadium dioxide (VO2) KW - ruthenium dioxide (RuO2) KW - reactive molecular beam epitaxy (MBE) KW - low energy electron microscopy (LEEM) KW - photoemission electron microscopy (PEEM) KW - low energy electron diffraction (LEED) KW - XPEEM KW - µLEED Y1 - 2020 U6 - https://doi.org/10.1021/acs.cgd.0c00120 SN - 1528-7483 SN - 1528-7505 VL - 20 IS - 4 SP - 2734 EP - 2741 ER - TY - GEN A1 - Krisponeit, Jon-Olaf A1 - Fischer, Simon A1 - Esser, Sven A1 - Moshnyaga, Vasily A1 - Schmidt, Thomas A1 - Piper, Louis F. J. A1 - Flege, Jan Ingo A1 - Falta, Jens T1 - The morphology of VO2/TiO2(001): terraces, facets, and cracks T2 - Scientific Reports N2 - Vanadium dioxide (VO2) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile TiO2(001) substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of VO2/TiO2(001) films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches. KW - Vanadium dioxide (VO2) KW - rutile TiO2(001) KW - metal-to-insulator transition KW - morphology KW - crystallography KW - low-energy electron microscopy (LEEM) KW - low-energy electron diffraction (LEED) KW - scanning probe microscopy Y1 - 2020 U6 - https://doi.org/10.1038/s41598-020-78584-9 SN - 2045-2322 VL - 10 ER - TY - GEN A1 - Fischer, Simon A1 - Flege, Jan Ingo A1 - Foerster, Michael A1 - Aballe, Lucia A1 - Falta, Jens A1 - Krisponeit, Jon-Olaf T1 - Strongly Strained VO2 Thin Film Growth T2 - Verhandlungen der DPG - SurfaceScience21 N2 - The semiconductor-metal transition temperature of VO2 strongly shifts as a result of strain applied along the rutile c axis, making it interesting for various switching applications, as smart coatings and as sensors. In the past, this has been demonstrated, for instance, through the application of epitaxial strain on TiO2 substrates. We extend this tailoring approach by utilizing the much larger lattice mismatch of 8.78 % occurring in the VO2/RuO2 system for orientations where the c axis lies in-plane. Depositing vanadium oxide by atomic oxygen-supported reactive MBE on an oxidized Ru(0001) template, we have grown VO2 thin films on single domain RuO2 islands with distinct orientations. Locally resolved electron spectroscopy was used to ascertain the correct stoichiometry of the grown VO2 films on all template island types. Low energy electron diffraction reveals the VO2 films to grow indeed fully strained on RuO2(110) but fully relaxed on RuO2(100). Hence, the presented template allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to regions of massive tensile strain. KW - vanadium oxide (VO2) KW - strain KW - ruthenium oxide KW - Locally resolved electron spectroscopy KW - Low energy electron diffraction (LEED) KW - MBE Y1 - 2021 UR - https://www.dpg-verhandlungen.de/year/2021/conference/surfacescience/part/o/session/74/contribution/3 VL - 2021 PB - Deutsche Physikalische Gesellschaft e.V. CY - Bad Honnef ER -