Strain Relaxation in "2D/2D and 2D/3D Systems": Highly Textured Mica/Bi₂Te₃, Sb₂Te₃/Bi₂Te₃, and Bi₂Te₃/GeTe...

Strain engineering as a method to control functional properties has seen in the last decades a surge of interest. Heterostructures comprising 2D-materials and containing van der Waals(-like) gaps were considered unsuitable for strain engineering. However, recent work on heterostructures based on Bi₂Te₃, Sb₂Te₃, and GeTe showed the potential of a different type of strain engineering due to long-range mutual straining. Still, a comprehensive understanding of the strain relaxation mechanism in these telluride heterostructures is lacking due to limitations of the earlier analyses performed. Here, we present a detailed study of strain in two-dimensional (2D/2D) and mixed dimensional (2D/3D) systems derived from mica/Bi₂Te₃, Sb₂Te₃/Bi₂Te₃, and Bi₂Te₃/GeTe heterostructures, respectively. We first clearly show the fast relaxation process in the mica/Bi₂Te₃ system where the strain was generally transferred and confined up to the second or third van der Waals block and then abruptly relaxed. Then we show, using three independent techniques, that the long-range exponentially decaying strain in GeTe and Sb₂Te₃ grown on the relaxed Bi₂Te₃ and Bi₂Te₃ on relaxed Sb₂Te₃ as directly observed at the growth surface is still present within these three different top layers a long time after growth. The observed behavior points at immediate strain relaxation by plastic deformation without any later relaxation and rules out an elastic (energy minimization) model as was proposed recently. Our work advances the understanding of strain tuning in textured heterostructures or superlattices governed by anisotropic bonding.