Thermally-Activated Epitaxy of NbO

TL;DR

This work establishes a thermally activated epitaxy window for NbO growth at $T_G > 1000^{\circ}C$, achieving high-quality NbO films with robust transport properties across a wide $P_{O_2}$ range and reduced impurity effects. By combining MBE with CO$_2$ laser heating and detailed structural/electrical characterization, the authors map a $T_G$-$P_{O_2}$ phase diagram showing sharp Nb/NbO/NbO$_2$ boundaries and non-monotonic optimal oxygen pressures. They argue that high-temperature, diffusion-assisted growth drives reproducible NbO formation, and they propose prototypical NbO properties, including a Hall coefficient $R_H$ that changes sign with temperature and a superconducting transition temperature $T_c$ around 1.32–1.37 K, illuminating the electronic structure and potential applications of NbO in oxide electronics. The findings highlight the importance of thermal activation for the synthesis of refractory metal oxides and establish NbO as a model system for thermally activated epitaxy and defect-controlled transport.

Abstract

We demonstrate a thermally-activated epitaxy window for the growth of NbO at temperatures exceeding 1000 $^o$C. NbO films grown in this mode display superior structural and transport properties, which are reproducible across a window of oxygen partial pressure. Through comprehensive analysis, we propose the prototypical electrical properties of NbO, for which a consensus has not yet been made. This study unequivocally demonstrates the utility of high temperatures in the thin film synthesis of refractory metal compounds.

Figures (12)

• Figure 1: Growth dynamics of the Nb-O system. (a) Kinetic and thermodynamic considerations for the formation of Nb oxides. The shaded contour plot represents the reaction rate ($K$) for the oxidation of Nb for various values of temperature and oxygen partial pressure. The black curves represent the Ellingham diagram for the formation of NbO, NbO$_2$, and Nb$_2$O$_5$ from the constituent elements. Thermodynamic data have been taken from Refs. chaseNISTJANAFThemochemicalTables1998jacobThermodynamicPropertiesNiobium2010 using standard conditions. (b-d) XRD $2\theta$-$\omega$ scans of Nb-O films grown at $T_\mathrm{G}$ = 600, 800 and 1100 $^\circ$C with varying $P_\mathrm{O_2}$. (e) Growth phase diagram of Nb-O on Al$_2$O$_3$ (0001) in the $T_\mathrm{G}$-$P_\mathrm{O_2}$ parameter space. Symbols correspond to body-centered-cubic Nb (green circle), asymmetric NbO (cyan rounded square), polycrystalline NbO (open blue square), single-phase, single-crystal NbO (filled blue square), rutile NbO$_2$ (purple triangle), and Nb$_2$O$_5$ (pink hexagon). (f) Schematic illustrating the effect of $T_\mathrm{G}$ on the amount of phase separation between Nb and NbO. (g) Optimal $P_\mathrm{O_2}$ for forming the highest crystal quality NbO as a function of $T_\mathrm{G}$.
• Figure 2: Deterministic role of $T_\mathrm{G}$ on the structural and transport properties of NbO. (a) Narrow-range XRD $2\theta$-$\omega$ scans of NbO films grown at varying $T_\mathrm{G}$ and the corresponding optimal $P_\mathrm{O_2}$. (b) (Top) in-plane [11$\overline{2}$] lattice constant ($a_\sslash$) and (bottom) out-of-plane [111] lattice constant ($a_\perp$) of the same samples as a function of $T_\mathrm{G}$. Black dashed lines correspond to the respective bulk NbO values. (c) Rocking curve measurements of the NbO (111) peak and (d) ratio of the full width at half maximum (FWHM) of the narrow component (N) to the FWHM of the broad component (B) of the rocking curves. The curves were fitting using a Gaussian function for the narrow component and a squared Lorentzian function for the broad component. (e) Temperature-dependent longitudinal resistivity and (f) residual resistivity ($\rho_\mathrm{0}$) at 1.7 K of the same samples. (g) $\rho_\mathrm{0}$ and (h) residual resistivity ratio (RRR) of NbO samples in the $T_\mathrm{G}$-$P_\mathrm{O_2}$ parameter space. Gray circles represent insulating samples.
• Figure 3: Comparison of normal state transport properties of NbO films grown at $T_\mathrm{G}$ = 600 $^\circ$C (top row), 800 $^\circ$C (middle row), and 1100 $^\circ$C (bottom row). (a-c) Hall resistivity at different temperatures of NbO films grown at the optimal $P_\mathrm{O_2}$ for each $T_\mathrm{G}$. (d-f) Hall coefficient ($R_\mathrm{H}$) versus temperature of samples grown at varying $P_\mathrm{O_2}$. (g-i) $R_\mathrm{H}$ at 2 K (squares) and 300 K (triangles) of the same samples in (d-f). Dashed gray lines at $R_\mathrm{H}$ = 0 serve as a guide to the eye.
• Figure 4: Superconducting properties of NbO. (a) Temperature-dependent longitudinal resistivity of select samples grown at $T_\mathrm{G}$ = 900, 1000, and 1100 $^\circ$C and the corresponding optimal $P_\mathrm{O_2}$. (b) Superconducting transition temperature ($T_\mathrm{c}$) of NbO films as a function of $P_\mathrm{O_2}$ for samples grown at varying $T_\mathrm{G}$.
• Figure S1: Identification of impurity phases in the Nb-O system. Wide-range XRD $2\theta$-$\omega$ scans of Nb-O films grown at $P_\mathrm{O_2}$ = 0.4 mbar and varying $T_\mathrm{G}$. Dashed lines correspond to the bulk peak positions for NbO (blue), rutile NbO$_2$ (purple), and H-Nb$_2$O$_5$ (pink). As there are numerous polymorphs and crystal structures of Nb$_2$O$_5$, it is difficult to unambiguously assign a phase.