Epitaxial growth and stabilization of perovskite phase EuNiO3 thin films through RF sputtering

TL;DR

This paper presents a scalable method for growing high-quality epitaxial EuNiO3 thin films via RF sputtering, demonstrating control over phase stabilization and transition temperatures relevant for electronic applications.

Contribution

It introduces a novel RF sputtering and post-annealing process to stabilize EuNiO3 in the perovskite phase, enabling tunable metal-insulator transition temperatures.

Findings

Higher oxygen partial pressures improve crystallinity but induce RP phases.

Post-annealing transforms RP phases into perovskite phase.

Strain and oxygen vacancies influence transition temperatures.

Abstract

Phase change materials (PCMs) that exhibit volatile resistive switching are promising for emulating neuronal oscillators. Charge transfer insulators, such as ReNiO3 (where Re represents rare earth metals like Pr, Nd, Sm, Eu...), form a family of PCMs with tunable metal-insulator transition (MIT) temperatures across a broad range. Notably, MIT can be adjusted via chemical doping or strain engineering. EuNiO3, in particular, is an attractive choice for oscillator devices given its bulk transition temperature (TMI) of approximately 190{\deg}C, which is well above room temperature, reducing crosstalk issues while remaining low enough to support energy-efficient applications. We demonstrate a method to stabilize high-quality epitaxial EuNiO3 thin films through scalable reactive RF sputtering and post-annealing, optimizing oxygen partial pressures and annealing temperatures across two substrates. Growth at low or zero oxygen partial pressures resulted in amorphous samples, while higher pressures improved crystallinity but led to the stabilization of Ruddlesden-Popper (RP) phases [An+1BnO3n+1] and associated faults. Post-annealing enhanced crystallinity in all samples, transforming RP phases and faults toward the perovskite phase. We conducted operando XRD and transport measurements on our films, finding the transition temperatures of perovskite phase samples grown on LAO and LSAT to be approximately 300{\deg}C and 250{\deg}C, respectively. We attribute the increase in TMI for LAO samples to in-plane compressive strain (-0.76%), which reduces the Ni-O-Ni bond angles in-plane. Similarly, LSAT samples experience in-plane tensile strain (2.1%), which decreases out-of-plane Ni-O-Ni bond angles, increasing TMI compared to bulk. However, this is counteracted by oxygen vacancies due to lowered formation energies. Films that stabilized in RP phases did not exhibit any transition.