Engineering the Synthesis and Electronic Properties of Hybrid MBE-Grown Epitaxial IrO₂ Thin Films

Oxidation of metals like Pt, Ir and other refractory and noble metals, particularly in thin film synthesis, has been a major obstacle in incorporating their unconventional electronic and magnetic properties in modern quantum technologies. The low oxidation potential of these metals along with ultra-low vapor pressure, makes them challenging to work with, when it comes to state-of-the-art thin film synthesis techniques like Molecular Beam Epitaxy (MBE). Conventionally, in MBE, these metals have necessitated the need for electron beam evaporators and reactive oxidants like ozone for achieving the high-temperature evaporation/sublimation and complete metal oxidation. However, these approaches come with challenges relating to flux stability and process safety. Here, using Ir as a candidate platinum group metal, we present a systematic investigation of a hybrid MBE approach, that enhances the ease of synthesis of iridium-based oxides. First, we demonstrate the use of Ir(acac)₃ as a solid metal-organic source for efficient vapor flux delivery at sublimation temperatures as low as 170 ^oC, as opposed to the >2000 ^oC required in electron beam evaporation of pure Ir metal. High-quality synthesis of epitaxial IrO₂ (a = b = 4.49 Å, c = 3.15 Å) is achieved on single crystalline TiO₂ (a = b = 4.59 Å, c = 2.96 Å) substrates with varying out of plane facets using oxygen plasma as the oxidant and substrate temperatures as low as 250 ^oC. Atomic force microscope images show that the films are atomically smooth with an average root mean square roughness of ~2 Å, allowing precision synthesis of thin films down to unit cell thickness. Second, in addition to the already known tuning knobs to control or enhance metal oxidation during film growth, we discovered an additional effect of substrate induced epitaxial strain on the metal oxidation process [1]. Films grown on TiO₂ (110), (101) and (001) facets result in very different epitaxial strain states on the growing IrO₂ film due to differences in lattice mismatch. These strain states were experimentally observed to shift the stability of the metal-oxide, specifically leading to enhanced oxidation on the (001) oriented substrate, despite identical growth conditions. This behaviour is further explained using a DFT-based thermodynamic framework that captures the shift in formation enthalpy when the epitaxial strain from the substrate is applied to the growing crystals. The effect of strain on surface reactions can be extended to a variety of material systems, potentially allowing for synthesis of materials that have been traditionally challenging. Lastly, we observed novel electronic properties in the hybrid MBE grown IrO₂ thin films. The IrO₂ (110) and (101) epitaxial films were found to show a previously unobserved non-linear Hall effect at 1.8 K [2]. The Hall effect data was found to show excellent agreement with a non-interacting two-carrier model, attesting to the complex Fermi surface of IrO₂ having multiple electron and hole pockets. Further, the two-carrier model fit revealed the presence of low mobility (< 10 cm²/V.s) majority carriers (~ 10²² cm^-3) along with high mobility minority carriers (~ 10¹⁶ cm^-3) with mobility reaching 3000 cm²/V.s. We also observed that these hybrid MBE grown IrO₂ films, when engineered using heterostructure design, show persistent metallic conductivity in the sub-nm thickness regime, pushing the boundaries of 3D electronic materials and their integration into next-generation electronics.



References:

1] S. Nair, Z. Yang, D. Lee, S. Guo, J. T. Sadowski, S. Johnson, A. Saboor, Y. Li, H. Zhou, R. B. Comes, W. Jin, K. A. Mkhoyan, A. Janotti and B. Jalan, “Engineering metal oxidation using epitaxial strain”, Nat. Nanotechnol. 18, 1005-1011 (2023)

2] S. Nair, Z. Yang, K. Storr and B. Jalan, “High-mobility carriers in epitaxial IrO₂ films grown using hybrid molecular beam epitaxy”, Nano Lett. 24, 35, 10850-10857 (2024)