Preparation and evaluation of alexandrite, forsterite, and topaz substrates for the epitaxial growth of rutile oxides

TL;DR

This work addresses the limited substrate options for epitaxial growth of rutile-structured oxides and the ensuing strain-engineering opportunities. It introduces three single-crystal substrates—BeAl2O4, Mg2SiO4, and Al2SiO4(F,OH)2—prepared to atomically smooth, well-defined terminations to support rutile thin-film heteroepitaxy. The study analyzes orientational matching for (001), (010), and (101) rutile growth, demonstrates effective substrate surface preparation, and reports initial epitaxial growth of RuO2, TiO2, and NbO2 on these unconventional substrates, with BeAl2O4 offering superior thermal/chemical stability. The findings expand the substrate toolkit for rutile oxides, enabling strain-tuning and exploration of their anisotropic properties, potentially advancing superconductivity, metal-insulator transitions, and related phenomena in rutile thin films.

Abstract

Metal-insulator transitions and superconductivity in rutile-structured oxides hold promise for advanced electronic applications, yet their thin film synthesis is severely hindered by limited substrate options. Here, we present three single- crystalline substrates, BeAl2O4, Mg2SiO4, and Al2SiO4(F,OH)2, prepared via optimized thermal and chemical treatments to achieve atomically smooth surfaces suitable for epitaxial growth. Atomic force microscopy confirms atomic step-and-terrace surface morphologies, and oxide molecular-beam epitaxy growth on these substrates demonstrates successful heteroepitaxy of rutile TiO2, VO2, NbO2, and RuO2 films. Among these unconventional substrates, BeAl2O4 exhibits exceptional thermal and chemical stability, making it a versatile substrate candidate. These findings introduce new substrate platforms that facilitate strain engineering and exploration of rutile oxide thin films, potentially advancing the study of their strain-dependent physical properties.

Figures (26)

• Figure 1: Schematics of the (a) rutile, (b) topaz, (c) forsterite, and (d) alexandrite crystal structures. Edge-sharing oxygen octahedra forming the common motif along the $c$-axis are highlighted in yellow. Octahedral Mg- and Al-sites outside the chains are shown in gray in (c) and (d).
• Figure 2: In-plane lattice parameters of select (001)-oriented rutile-like oxides, commercially available rutile substrates, alexandrite, and topaz.
• Figure 3: Cross-sectional schematic of TiO2 (001) on BeAl2O4 (001) viewed along the substrate [100] (a) and [010] directions (b); VO2 (001) on Al2SiO4(F,OH)_2 (001) viewed along the substrate [001] (c) and [010] direction (d).
• Figure 4: In-plane lattice parameters of rutile (010) materials compared with commercially available rutile substrates, alexandrite, forsterite, and topaz.
• Figure 5: Cross-sectional schematic of RuO2 (010) on Mg2SiO4 (010) (a,b) and RuO2 (001) on Al2SiO4(F,OH)_2 (001) viewed along the substrate [001] (a) and [100] directions (c,d).