Thursday, November 5, 2026
4:00 pm-5:00 pm
Exploring new substrates for the thin film growth of rutiles
Oxides with the rutile structure exhibit the entire spectrum of electronic behavior, from insulators and ultrawide-bandgap semiconductors to spin-polarized half-metals, transparent conductors, and superconductors, efficient catalysts, ferromagnets, and ultrafast metal-insulator transitions.
The epitaxial synthesis of rutile oxides is severely limited by the lack of suitable substrates. In fact, there are currently only two commercially available substrates, TiO2 and MgF2, with the rutile structure. Beyond the lattice mismatch, a major challenge for the stabilization of TaO2 and other exotic binary oxides with rutile-like polymorphs, e.g., ReO2 and MoO2, is preventing the unwanted oxidation of such desired rutile structures into a more oxidized version of the binary oxide (e.g., ReO3 and MoO3) due to oxygen transfer from the substrate. To achieve the targeted rutile structure of the thin film, growth must take place at very low oxygen partial pressures, e.g. < 10-8 Torr in the case of TaO2. Viable substrates to stabilize such rutile thin films must hence be able to withstand aggressively reducing conditions at temperatures frequently exceeding 1000 °C, dramatically limiting the number of candidate materials. [1]
To overcome this fundamental limitation, I demonstrate a paradigm shift, focusing on structural motif matching rather than simple crystal structure matching. The key structural feature of rutile is its edge-sharing oxygen octahedral network along the c-axis, see Fig. 1. In my talk, I will
showcase three chemically stable, non-rutile substrates that share this motif: Mg2SiO4 (forsterite) and isostructural BeAl2O4 (chrysoberyl/alexandrite), as well as Al2SiO4(OH,F)2 (topaz). I will present thermal annealing and chemical etching protocols to prepare uniformly flat step-and-terrace surfaces (see example in Fig. 2) on these materials. [2] As a proof-of-concept, I will highlight the epitaxial stabilization of NbO2 thin films grown by suboxide molecular-beam epitaxy on Mg2SiO4, and RuO2 on BeAl2O4 and Al2SiO4(OH,F)2. Finally, I will show our first steps towards the development of an entirely new substrate, MgNb2O6, a rutile derivative that crystallizes in the columbite structure.
[1] Y. A. Birkhölzer, A.S. Park. et al., “Synthesis of epitaxial TaO2 thin films on Al2O3 by suboxide molecular-beam epitaxy and thermal laser epitaxy”, APL Materials 14, 071106 (2026)
[2] M. Kubovsky, Y.A. Birkhölzer, L.B. Mitrovic, H. Paik, G.R. Rossman, D.G. Schlom, “Preparation and evaluation of alexandrite, forsterite, and topaz substrates for the epitaxial growth of rutile oxides”, APL Materials 14, 031103 (2026)
