Grazing incidence x-ray scattering study of the structure of epitaxial Cr/Sn multilayers

The structure of epitaxial Cr/Sn multilayers has been studied experimentally using x-ray reflectivity and x-ray diffuse scattering measurements, as well as theoretically using linear muffin-tin orbital (LMTO) calculations. Measurements show a distinct variation in the structure of the multilayers as the Sn layer thickness increases from 0.4 to 0.6 nm. A decrease in the electron density of the Sn layer and an increase in the jaggedness of the interfaces accompany a partial transformation of the Sn layer from an epitaxial bcc structure to a β-Sn structure, as observed using in situ reflection high energy electron diffraction measurements [K. Mibu, S. Tanaka, and T. Shinjo, J. Phys. Soc. Jpn. 67, 2633 (1998)]. Present measurements along with the LMTO calculations support a structure for the multilayer in which Sn layers grow epitaxially with Cr in a bcc structure with a finite density of steps at the interfaces which causes the average electron density of a Sn layer to decrease. Diffuse scattering measurements give the average distance between steps to be about 50-100 nm. Taking the structure of the epitaxial Sn layer to be bcc, the calculated variation in the lattice strain and Sn layer energy with the thickness of Cr and Sn layers explains qualitatively the range of Cr layer thicknesses for which epitaxial growth is observed.