Synthesis of epitaxial \ce{TaO2} thin films on \ce{Al2O3} by suboxide molecular-beam epitaxy and thermal laser epitaxy

TL;DR

This work demonstrates buffer-free epitaxial stabilization of rutile TaO$_2$ on $r$-plane Al$_2$O$_3$ using two growth routes, $S$-MBE and TLE, achieving single-domain, anisotropically strained thin films. Through an integrated set of structural, spectroscopic, and optical analyses, TaO$_2$ is shown to host a tetravalent interior with a surface Ta$_2$O$_5$ overlayer, and exhibits a small indirect optical gap of about $0.3$ eV consistent with a NbO$_2$-like distortion predicted by first-principles calculations. X-ray and electron spectroscopy confirm an intricate interplay between structure, valence states, and dielectric response, while ellipsometry reveals a highly anisotropic dielectric function with sub-eV transitions, suggesting potential metal–insulator phenomena under suitable conditions. Density-functional theory supports a distorted, insulating $I4_1/a$ phase close in energy to the rutile $P4_2/mnm$ phase, hinting at a proximity to a metal–insulator transition and highlighting TaO$_2$ as a platform for oxide electronics and photonics. Further advancement will require higher-quality substrates and improved control over Ta$^{4+}$ stabilization to realize the intrinsic MIT behavior in TaO$_2$.

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.

Figures (37)

• Figure 1: Cross-sectional schematics of TaO2 (101) (blue) on Al2O3 ($1\bar{1}2$) (yellow) perpendicular to the rutile $b$ and $c$-axes are shown in (a) and (b), respectively. The view direction into the plane for panel (a) is TaO2 [$\bar{1}01$] $\parallel$Al2O3 [$\bar{1}101$] and for panel (b) is TaO2 [$010$] $\parallel$Al2O3 [$11\bar{2}0$].
• Figure 2: Growth diagram summarizing the oxygen partial pressure ($p\text{O}_2$) and substrate temperature ($T_\text{sub}$) dependence of the TLE growth of tantalum oxide. Phase-pure TaO2 was obtained in the blue-shaded region of the diagram between 800 - 1300. Scatter markers denote experimental data, colored backgrounds serve as guide to the eye.
• Figure 3: X-ray diffraction of MBE-grown TaO2 (101) thin film on Al2O3 ($1\bar{1}02$) scans showing pronounced Laue fringes for $T_\text{sub}$ between 900 and 1100. Peaks highlighted with an asterisk are due to the substrate.
• Figure 4: XRD reciprocal space map revealing the anisotropic strain state of MBE-grown TaO2 on Al2O3 ($1\bar{1}02$). Panel (a) shows that the film is commensurately strained to the substrate lattice parameter along the rutile $b$ direction (see \ref{['Fig_Cartoon']} for illustration); the film is fully relaxed in the orthogonal high-misfit direction (b).
• Figure 5: Cross-sectional HAADF-STEM images of MBE-grown TaO2 taken (a) perpendicular and (b) parallel to the rutile $b$-axis. In panel (a), the film appears commensurately strained, as evidenced by the perfect alignment between tantalum ions in the film and aluminum ions in the substrate. In contrast, panel (b) reveals diagonal striations and an incommensurate interface marked by numerous dislocations. Panel (c) presents a magnified view of a defect-free region in the same orientation as (b), reconstructed using multislice electron ptychography to simultaneously resolve tantalum (depicted in yellow) and oxygen (purple) atomic columns.