Massively strained VO 2 thin film growth on RuO 2 Simon Fischer, † Jon-Olaf Krisponeit, *,†,‡ Michael Foerster, ¶ Lucia Aballe, ¶ Jens Falta, †,‡ and Jan Ingo Flege § †Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany ‡MAPEX Center for Materials and Processes, University of Bremen, 28359 Bremen, Germany ¶ALBA Synchrotron Light Facility, Carrer de la Llum 2-26, 08290 Cerdanyola del Vall` es, Barcelona, Spain §Applied Physics and Semiconductor Spectroscopy, Brandenburg University of Technology Cottbus-Senftenberg, Konrad-Zuse-Str. 1, 03046 Cottbus, Germany E-mail: krisponeit@ifp.uni-bremen.de Abstract Strain engineering vanadium dioxide thin films is one way to alter this material’s charac- teristic 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 VO 2 /RuO 2 system along the c axis of the rutile structure. We have grown VO 2 thin films on single domain RuO 2 islands of two distinct surface orientations by atomic oxygen-supported reactive 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 VO 2 films to grow indeed fully strained on RuO 2 (110), exhibiting a previously unreported (2×2) reconstruction. On TiO 2 (110) substrates, we reproduce this reconstruction and attribute it to an oxygen- rich termination caused by the high oxygen chemical potential. On RuO 2 (100) on the other hand, 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. Introduction Bulk vanadium dioxide exhibits a temperature- induced semiconductor-metal transition at 68 ◦ C. 1 This change in resistivity is accom- panied by a structural transition from a semi- conducting monoclinic (M) phase to a metallic phase of rutile (R) structure (see Figure 1a and b) where the vanadium atoms along the rutile c R axis are dimerized in the monoclinic phase with the dimers additionally tilting in a zigzag-like pattern perpendicular to that di- rection. Because photoemission studies showed that the lattice change alone cannot account for the band gap opening, 2 this is commonly interpreted as a Peierls-assisted Mott transi- tion. 3 Applying moderate (epitaxial) stress along the aforementioned c R axis changes the tem- perature of the lattice transition and in con- sequence also the electronic transition. This tunability enables and facilitates applications in switching devices like “smart” windows that change their IR reflectivity at a desired temper- ature 4 or in micro actuators utilizing the struc- tural transition. 5 Applying larger stress how- ever may separate the electronic and the lattice- driven transition, making an insulating phase of rutile structure possible. 6,7 On the other hand, a metallic monoclinic phase can be achieved by doping the VO 2 with electrons. 8 This charge- 1 arXiv:2003.02723v1 [cond-mat.mtrl-sci] 5 Mar 2020