Synthesis of epitaxial TaO$_2$ thin films on Al$_2$O$_3$ by suboxide molecular-beam epitaxy and thermal laser epitaxy

TL;DR

This study demonstrates the epitaxial growth of metastable TaO2 thin films on sapphire substrates using advanced epitaxy techniques, revealing their structural, electronic, and optical properties, and suggesting potential for electronic applications.

Contribution

It reports the first epitaxial stabilization of TaO2 on sapphire using suboxide MBE and TLE, with detailed microstructural and electronic characterization, and theoretical insights into phase stability.

Findings

Successful epitaxial growth of TaO2 on sapphire

Confirmation of tetravalent Ta oxidation state

Identification of a 0.3 eV Mott gap in TaO2

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.