Fabrication of exfoliable epitaxial thin films via Remote and Van der Waals epitaxy for strain engineering

Abstract

Strain engineering is a powerful approach to modify the band structure, magnetic interactions, and transport properties of thin films. Strain distorts the lattice and can lower its symmetries. Strain can make a non - superconducting material to superconducting, a boring material to ferroelectric and non - magnetic material to magnetic. Thin films can even sustain larger amount of strain than their bulk parts. Mn3Pt is one such material which exhibit strain sensitive magnetic and transport properties. It is a non-collinear Kagome antiferromagnetic material. It shows an anomalous Hall effect that can be tuned by strain. In conventional epitaxial thin films, strain is typically imposed by lattice mismatch with the substrate; however, this epitaxial strain is often limited in magnitude and tunability due to strain relaxation beyond a critical thickness. An alternative approach involves the fabrication of exfoliable thin-film membranes, which can sustain larger and tunable strains. In this work, single-crystal epitaxial thin films of Mn3Pt are fabricated using Molecular Beam Epitaxy through graphene-mediated quasi–van der Waals epitaxy. Graphene-terminated substrates, such as MgO(001) and c-Al2O3(0001), are employed with Pt and Pd buffer layers. The growth and structural properties of Mn3Pt/Pt/Pd/c-Al2O3 stacks with and without single-crystal graphene are investigated. Due to the low adatom sticking coefficient on pristine graphene, controlled defect engineering is employed to enhance nucleation and achieve continuous film growth. A thicker Pd layer (40 nm) is deposited to achieve a uniform and fully coalesced film. Subsequently, Pt and Mn3Pt layers are grown. However, due to lattice mismatch and strain energy, the films tend to delaminate and exhibit rolling behavior. To mitigate this effect, thick Al2Ox capping layer is deposited, which improves the structural stability of the films and suppresses delamination. These films are then exfoliated onto various polymer substrates for straining. Finally, to see the possibility of remote epitaxy, we have grown films of different material systems, and observed some signatures of remote epitaxy in GaSb growth.