Abstract
The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.
Highlights
Intentional modification of a crystal structure by application of strain has been used for several decades to tune functional properties of materials[1,2,3]
The two broader features at Qz = 1.45, 3.26 Å−1 are interpreted as the first order satellite peaks (reflections (00.12) and (00.27)) of the vacancy layers (VLs); the large width of these two reflections is due to the coexistence of GST blocks with different number of layers[10]
The block distribution is even sharper than the case of highly ordered GST realized on singular Si substrates with optimized molecular beam epitaxy (MBE) growth fluxes[10,12]
Summary
Intentional modification of a crystal structure by application of strain has been used for several decades to tune functional properties of materials[1,2,3]. In van der Waals (vdW) epitaxy[6], due to the weak interaction associated with the vdW bonded atomic species, an epilayer grows from the beginning fully relaxed and the lattice matching condition is lifted This allows for obtaining sharper interfaces and exploiting the new physical phenomena in two-dimensional (2D) heterostructures[7]. The absence of both defects and strain renders the control of some functional properties not accessible In such materials it would be desirable to modulate between classical and vdW epitaxy by taking advantage of the interaction of the film with the substrate and at the same time maintaining high crystalline quality. This allows the in-plane twin domains to be nearly suppressed preserving the epilayer quality
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