Compliant Nanomembranes Enable a Crack-Free Heteroepitaxial Film with a Steep Metal-Insulator Transition beyond the...

High-quality growth manifests the exotic properties of correlated oxides (e.g., VO₂) in thin film forms, but the defects and cracks that relax the misfit strain energy deteriorate the quality of the metal-insulator transition (MIT) in VO₂ epilayers on thick TiO₂ substrates above the critical thickness (t_c). A new approach must be developed to overcome the fundamental degradation by strain relaxation during the pseudomorphic growth of VO₂ films. Herein, we utilize thin TiO₂ nanomembranes (NM) as a strain-sharing layer to allow the formation of crack-free VO₂ epitaxial films exceeding t_c. While the inhomogeneous strain relaxation induced by cracks occurs in VO₂ films on thick TiO₂ substrates (∼0.5 mm), the homogeneous and relaxed 50-nm-thick VO₂ films are grown by simply converting thick TiO₂ substrates to thin TiO₂ NM (∼8 nm) as a growth template. Atomic-scale characterization reveals that a strong strain gradient was observed in underlying TiO₂ NM as well as VO₂ epilayers at the interface; unlike VO₂ on a thick TiO₂ substrate, this strain sharing by compliant TiO₂ NM suppresses the formation of catastrophic cracks in VO₂ epitaxial layers. Due to the absence of the cracks, excellent MIT steepness (ΔT_H = 4.5 K) and cycle endurance without resistance degradation were achieved in VO₂ films above t_c on TiO₂ NM. Our design of thin film growth will provide a new strategy to utilize a compliance effect to release misfit strain energy and offer a novel platform for advanced epitaxial growth techniques to unrestrictedly design multifunctional heterostructures for advanced electronics.