Impact of Phase Composition on Bipolar Resistive Switching Performance in Polycrystalline ErMnO₃ Based Devices

Rare-earth hexagonal manganites, h-<i>R</i>MnO₃ (<i>R</i>=Y, Er, Ho to Lu) have been widely studied for their multiferroic properties. Polycrystalline hexagonal YMnO₃ thin films with promising resistive switching performance were reported recently [1], which gained interest for neuromorphic applications owing to the peculiar ferroelectric domain pattern and vortex lines in hexagonal <i>R</i>MnO₃ [2].<br/>In this work, we report the demonstration of bipolar resistive switching behavior in polycrystalline ErMnO₃-based capacitors. The ErMnO₃ films (~ 60 nm) were prepared by room temperature RF sputtering on Pt/Ti/SiO₂/Si substrates, followed by post-deposition annealing. The Al/Ti top electrodes were patterned by photolithography. The post-deposition annealing resulted in the formation of polycrystalline ErMnO₃ thin films with orthorhombic and hexagonal phases as shown by X-ray diffraction. We investigated the influence of these different crystalline phases on the switching performance and, for this purpose, developed a way to quantify the relative amount of both phases by scanning electron microscopy, Raman spectroscopy and conductive atomic force microscopy. Through a detailed structural evaluation combined with electrical characterization we developed an understanding of the physical origin of resistive switching in polycrystalline ErMnO₃ and the role of the different phases. The Au/Ti/ErMnO₃/Pt devices exhibit a bipolar resistive switching with a R_OFF/R_ON ratio larger than 10⁴ and an ultra-low resistance of only 10 Ω in the low resistance state, which can be of interest for CMOS circuitry with low power consumption [3] and for RF power switches [4]. We attribute the switching performance to the formation and rupture of conductive oxygen vacancy-based filaments. We find that a higher fraction of orthorhombic phase reduces the operation voltage but leads to a decrease of the memory window, which can be ascribed to higher Mn⁴⁺ concentration and more oxygen vacancies in the orthorhombic phase. These findings emphasize the potential of polycrystalline ErMnO₃ films as promising candidates for neuromorphic applications and provide valuable insights for enhancing the device performances by engineering the ErMnO₃ switching layer.<br/><br/><br/><br/><b>References</b><br/>[1] V. R. Rayapati <i>et al.</i>, J. Appl. Phys. 126 (2019), doi:10.1063/1.5094748.<br/>[2] H. Schmidt, Appl. Phys. Lett. 118, 140502 (2021), doi: 10.1063/5.0032988.<br/>[3] H. Cao, <i>et al.</i>, Appl. Phys. Lett. 120, 133502 (2022), doi: 10.1063/5.0085045.<br/>[4] N. Wainstein, <i>et al.</i>, Proc. IEEE. 109, 77–95 (2021), doi:10.1109/JPROC.2020.3011953.