Strongly strained growth of vanadium dioxide thin films on microscale ruthenium dioxide islands

Abstract

Vanadium dioxide exhibits a metal-insulator transition (MIT) which comprises an electronic and a structural component. Accordingly, it is often understood as a cooperative effect of a structure-induced Peierls transition and a electron correlations-induced Mott transition. The structural transition can be exploited by subjecting VO2 thin films to epitaxial stress, which stabilizes either the low temperature insulating or the high temperature metallic phase. Through this strain engineering approach, the transition temperature can be tuned from its bulk value of 68 °C, tailoring the material towards technological applications.

In the present thesis, massively strained thin films of VO2 on micron-sized RuO2 islands are grown and analyzed. This is done, in large parts, in a low energy electron microscope (LEEM) instrument. The instrument allows for following surface processes in situ during oxidation and deposition experiments, giving microscopic and structural information on the material.

First, the RuO2 islands are fabricated by oxidizing a Ru(0001) surface using atomic oxygen from a thermal cracker. The resulting complex island morphology, which encompasses four different phases of RuO2, is studied during and after growth, assessing the kinetic and thermodynamic aspects that lead to their formation. It is found that a microcrystalline oxide phase serves as a nucleation hub for adjacent (110)- and (101)-oriented RuO2 structures, which then outgrow the incubator phase. The structural registry of a separate RuO2(100) phase to the substrate has been resolved and is found to lead to the distinct growth behavior that this phase exhibits compared to the others.

On samples prepared in this way, VO2 was grown, again with the aid of atomic oxygen. This, as confirmed by x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS), ensures that the stoichiometry of the films is correct. In situ low energy electron diffraction (LEED) measurements showed that during the growth of VO2 on RuO2(110), the lattice parameters stay constant. This indicates a very high strain near the pseudomorphic case (8.78 %). The VO2(110) surface was also found to exhibit a (2 × 2) reconstruction due to an oxygen-rich surface termination. Conversely, VO2 was found to grow relaxed on the (100)-oriented islands. Its VO2(100) surface is heavily faceted, indicating a high surface energy.

Complementary measurement of the x-ray linear dichroism in these films finds that the VO2(110)/RuO2(110) islands exhibit spectra that are characteristic for the metallic phase. This may indicate that the MIT is suppressed in high-strain conditions. On VO2(100)/RuO2(100) islands, indications of a MIT are found. However, the VO2 films experience reduction due to the synchrotron beam, which can also induce the transition into the metallic state.

Alongside a deeper understanding of Ru oxidation kinetics using atomic oxygen, this work opens up a remarkably high window of accessible strain for VO2 thin film growth and gives important insights into the surface of VO2, which until recently was often neglected.