Electron-Microscopy Studies of the Structure of Thin Epitaxial Ge₂Sb₂Te₅ Layers Grown on Si(111) Substrates

Abstract

For the creation of memory cells of new generation, structurally perfect epitaxial Ge₂Sb₂Te₅ (GST) layers and multilayered crystalline structures based on GeTe/Sb₂Te₃ superlattices grown on Si substrates are of interest. This initiates the studies of specific features of formation of such materials by various methods, including molecular-beam epitaxy. In this study, the structure of a thin (13-nm-thick) epitaxial GST layer to be used for the production of phase-change memory cells is investigated. The layers are grown by molecular-beam epitaxy on Sb-passivated Si(111) substrates. The studies are conducted by transmission electron microscopy and electron diffraction analysis of cross- and planar-section samples. The high resolution images of cross-section samples and the diffraction patterns for planar-section thin foils and their bright-field micrographs are obtained. It is found that the layer is composed of structurally perfect crystalline grains consisting mainly of the hexagonal phase and, in some local regions, of the ordered GST cubic phase, whose basal planes are parallel to the substrate surface. From the quantitative analysis of the moiré pattern appearing in bright-field electron-microscopy images, it is established that the grains, for which the GST($11\bar {2}0$) and Si(220) planes are rotated with respect to each other about the growth direction through up to 2°, occupy about 60% of the surface area of the epitaxial layer and 26% of the area is accounted for practically nonrotated grains. The fraction of the area occupied by grains misoriented with respect to the substrate by angles from 2° to 8° is close to 33%, and the grains occupying about 7% of the layer area are rotated through angles larger than 8°. The average angle of rotation angle is about 2.6°. The estimated average dimension of nonrotated grains is about 150 nm and decreases with increasing angle of rotation relative to the substrate. The experimentally established systematic features of the grain structure of the epitaxial GST layer suggest that the Si(111) substrate has an orienting effect upon the formation of the layer.