Piezo-Strain-Controlled Phase Transition in Single-Crystalline Mott Switches for Threshold-Manipulated Leaky-Integrate-And-Fire Neurons

Correlated oxides with strong correlations between the charge, spin, lattice, and orbital degrees of freedom present a wide array of emergent properties, such as metal-insulator transition, ferroelectricity, and superconductivity. These exotic functionalities give an opportunity to overcome the limitations of electronic devices; thus heteroepitaxial growth have been employed to synthesize these functional ionic crystals. However, unrestricted integration of emergent functionality from single-crystal oxide films is inhibited owing to the limited platforms to grow epitaxial films. Therefore, unrestricted integration of single-crystal oxide films has been of great interest to exploit emerging phenomena in artificial heterostructure.
Here, we demonstrate a new strategy to integrate single-crystal (002) VO₂ thin films on highly lattice-mismatched (002) PMN-PT converse piezoelectric substrate (f_a > 10 %) by utilizing freestanding oxide nanomembrane (NM) as epitaxial template [1]. By selective dissolution of VO₂ sacrificial layers from TiO₂/VO₂ heterostructure grown on TiO₂ substrate by H₂O₂ solution, millimeter-size TiO₂ single-crystalline layers are integrated on PMN-PT substrate. After subsequent epitaxial growth of VO₂ films on the transferred TiO₂ NM, we create single-crystalline VO₂/PMN-PT heterostructures with excellent sharpness of metal-insulator transition (Δρ/ρ > 10³) even in ultrathin (<10 nm) VO₂ films. Moreover, piezoelectric-mediated reversible strain modulation of single-crystalline VO₂ films showed unprecedented modulation of T_MI (5.2 K) and isothermal resistance of VO₂ (ΔR/R (E_g) = 20 % at 300 K) via effective stran transfer, which is not possible using directly grown VO₂/PMN-PT heterostructure. Owing to the steep phase transition near the phase boundary and effective strain transfer, small changes in the lattice parameters can cause the strain induced abrupt phase transition in our VO₂ films on TiO₂ NM/PMN-PT substrate (ΔR/R (E_g) ≈ 18000% at 315K). The massive and reversible strain modulation of VO₂ epitaxial films by the VO₂/PMN-PT heterostructure offers a new type of artificial neurons with a real-time-tuned threshold values. Our strategy to utilize freestanding oxide NMs as epitaxial template for realizing single-crystalline artificial heterojunction will provide an unique opportunity to investigate unprecedented exotic functionlity for novel interfacial physics and next-generation devices.

References
[1] D. K. Lee et al., Nat. Commun. 12 (2019) 5019

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