Synthesis of epitaxial TaO$_2$ thin films on Al$_2$O$_3$ by suboxide molecular-beam epitaxy and thermal laser epitaxy
cond-mat.mtrl-sci
/ Authors
Yorick A. Birkhölzer, Anna S. Park, Noah Schnitzer, Jeffrey Z. Kaaret, Benjamin Z. Gregory, Tomas A. Kraay, Tobias Schwaigert, Matthew R. Barone, Brendan D. Faeth, Felix V. E. Hensling
and 21 more authors
Iris C. G. van den Bosch, Ellen M. Kiens, Christoph Baeumer, Enrico Bergamasco, Markus Grüninger, Alexander Bordovalos, Suresh Chaulagain, Nikolas J. Podraza, Waldemar Tokarz, Wojciech Tabis, Matthew J. Wahila, Suchismita Sarker, Christopher J. Pollock, Shun-Li Shang, Zi-Kui Liu, Nongnuch Artrith, Frank M. F. de Groot, Nicole A. Benedek, Andrej Singer, David A. Muller, Darrell G. Schlom
/ Abstract
Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (1-102) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy (MBE) and thermal laser epitaxy (TLE), demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron X-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using X-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal-insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our work expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.