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

Abstract

To create memory cells of new generation, epitaxial layers of Ge2Sb2Te5 (GST) material, with high crystalline perfection, and multilayered crystalline structures based on GeTe/Sb2Te3 superlattices grown on Si-wafers are of interest. This initiates the study of these and alike materials generation patterns, including with the involvement of molecular beam epitaxy. In this work, the structure of 13 nm thick layers of GST material, used to create phase-change memory cells was studied. These layers were grown on an Sb-passivated Si(111) substrate by molecular beam epitaxy. Research studies were carried out by transmission electron microscopy and electron diffraction analysis. Using high-resolution images of cross-sectional samples and diffraction patterns from planar thin foils, it was revealed that the layer consists of crystalline grains, mostly hexagonal, and in some local regions of the vacancy ordered cubic GST phase with the GST(0001) and GST(111) planes parallel to the Si(111). Based on an analysis of the moiré pattern appearing in bright-field electron microscopy images, it was found that the misorientation of the grains of the epitaxial layer around the Si(111) direction varies from 0 to 13.5º, and nearly 26 % of the surface area is almost non-rotated grains. Grains rotates within the angles from 0.2 to 2º occupy about 34 % of the layer surface area, from 2 to 8º occupy about 33 %, and the fraction of the area of grains rotated by more than 8º is close to 7 %. It has been found that as the rotation angle of the GST grains relative to the substrate increased, their average lateral size decreased from about 150 nm for non-rotated grains to 80 nm for grains rotated at an angle of more than 8º, and the average value of the rotation angle was approximately 2.6º. The data obtained on the grain structure of the epitaxial layer indicate that the relaxation of the misfit stresses of the crystal lattices of silicon and the GST material is provided both by the rotation of grains and, apparently, by the formation of misfit dislocations.