Epitaxial thin film growth in the U-Ge binary system

TL;DR

This work demonstrates epitaxial growth of U-Ge thin films on MgO, CaF$_2$, and SrTiO$_3$, revealing a tendency toward mixed phases dominated by UGe$_3$ and UGe, with occasional UGe$_2$ and oxide formation (notably UO$_2$) at elevated temperatures due to oxygen gettering from substrates. By varying growth temperature and Ge content through co-deposition, the authors map substrate- and condition-dependent phase formation, observing texture changes and high crystallinity for certain UGe$_3$-rich films, particularly on MgO. A resistivity study indicates metallic behavior and relatively high residual resistivity ratios (up to ~6.1 for MgO), with signatures around 74 K pointing to spin fluctuations in U-Ge richer films. The study highlights the challenge of achieving phase-pure UGe$_2" and proposes buffer-layer strategies to mitigate oxygen uptake, setting a path toward controlled epitaxial U-Ge thin films for exploring heavy-fermion physics and potential superconductivity.

Abstract

We explore the U-Ge phase diagram using thin film growth by co-deposition of U and Ge via d.c. magnetron sputtering. Using three different single crystal substrates - MgO, CaF$_2$ and SrTiO$_3$ - we have stabilised mixed phase films of mostly UGe$_3$ and UGe, with evidence of UGe$_2$ as well. At higher temperatures UO$_2$ forms as a consequence of gettering of oxygen from several types of substrate. Several UGe$_3$ dominated samples grown on MgO substrates have also been characterised electrically, showing residual resistivity ratios up to six.

Figures (9)

• Figure 1: Possible epitaxial matches between UGe$_2$ on the $\{100\}$ surfaces of SrTiO$_3$ (STO, top) and MgO (bottom). Lattice directions $a$ and $c$ are shown for UGe$_2$.
• Figure 2: Specular XRD measurement of U-Ge thin films grown on STO for $T_{\mathrm{g}} = 300$$^\circ$C (top) and $525$$^\circ$C (bottom). Indexed planes are colour-coded according to the corresponding material.
• Figure 3: High angle XRD measurement of U:Ge thin films grown on MgO with varying temperatures. Indexed planes are colour-coded according to the corresponding material. At higher temperatures, as in the case of STO, UO$_2$ peaks start to appear.
• Figure 4: $\omega$ scan comparison of UGe$_3$$(100)$ and MgO $(200)$ peaks for sample SN-7. Open circles are the data and the solid lines are best fits using the Voigt function.
• Figure 5: Pole figure data for the {310} reflections of UGe$_3$ for sample SN-7.