MBE-growth peculiarities of fluoride (CdF2–CaF2) thin film structures

Abstract

Calcium and cadmium fluorides have the same fluorite-type crystal structure with relatively close to each other lattice parameters (Δa/a∼1.4%). The specific features of the MBE growth of the CdF2 and CaF2 layers as well as of CdF2–CaF2 heterostructures, have been analysed in this work, using both previously published and recently obtained data. The surface morphology of the CdF2 layers, studied by AFM, was found to depend strongly on the growth temperature. The surface of the high temperature layers (400°C and higher) consisted of wide atomically flat terraces corresponding to the 2D growth mode. In contrast to that, well pronounced pyramidal mounds indicating the 3D growth mode were observed on the layers grown at below 300°C. Similar 3D growth was found for 10–30 monolayers thick CaF2 films grown at 500–650°C. The CdF2–CaF2 superlattices with the period ranging from 2 to 40 nm were also studied. Though all the SLs were single crystalline, their surface morphology varied with the period. While the short period SLs had a smooth surface, the long-period SLs revealed multilayer columns likely due to rotational twins. The RHEED investigations of the short-period SLs showed an asymmetry of the ‘CaF2 on CdF2’ and ‘CdF2 on CaF2’ interfaces. The possible mechanisms relevant to this phenomenon have been discussed.