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

High-quality growth manifests the exotic properties of correlated oxides (e.g., VO) 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 VO epilayers on thick TiO substrates above the critical thickness (). A new approach must be developed to overcome the fundamental degradation by strain relaxation during the pseudomorphic growth of VO films. Herein, we utilize thin TiO nanomembranes (NM) as a strain-sharing layer to allow the formation of crack-free VO epitaxial films exceeding . While the inhomogeneous strain relaxation induced by cracks occurs in VO films on thick TiO substrates (∼0.5 mm), the homogeneous and relaxed 50-nm-thick VO films are grown by simply converting thick TiO substrates to thin TiO NM (∼8 nm) as a growth template. Atomic-scale characterization reveals that a strong strain gradient was observed in underlying TiO NM as well as VO epilayers at the interface; unlike VO on a thick TiO substrate, this strain sharing by compliant TiO NM suppresses the formation of catastrophic cracks in VO epitaxial layers. Due to the absence of the cracks, excellent MIT steepness (Δ = 4.5 K) and cycle endurance without resistance degradation were achieved in VO films above on TiO 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.