Thickness-dependent structural transformation in PbSe-SnSe artificial superlattice

Abstract

Artificial superlattices (ASLs), composed of PbSe and SnSe, (PbSe)_m(SnSe)_n, have been characterized by X-ray diffraction and cross-sectional high-resolution transmission electron microscopy, where m and n denote the atomic plane number of each PbSe and SnSe layer. These two materials in their bulk form possess different crystal structures, and the lattice mismatch between them is fairly large. The large misfit-strain arising at the interface induces structural transformations of the constituent layers, and the resulting structures are classified into four types depending on m and n; (I) Single-crystalline ASL with the orthorhombic GeS-type structure, (II) Single-crystalline ASL with the cubic NaCl-type structure, (III) ASL with transitional structures between I and IV, and (IV) ASL made of NaCl-type PbSe layers and GeS-type SnSe layers. Single-crystalline ASLs (I and II) are stabilized when the layer thickness of either constituent is less than a critical value, irrespective of the counter layer thickness. The critical thicknesses are 1.8 nm (m = 6) for PbSe in I and 1.2 nm (n = 4) for SnSe in II. Pseudomorphic PbSe and SnSe layers are retained in the ASLs of I and II respectively.