Electron Microscopy of Gex, Si1-x/Si Strained Layer Superlattices

Abstract

Problems associated with the interpretation of conventional electron diffraction images from Gex, Si1-x/Si strained layer superlattice structures (SLS) and relaxation of the strain field of a SLS by dislocation generation and dislocation filtering mechanisms are reviewed. Surface relaxation in cross-sectional TEM samples is modelled using a Fourier series solution and good agreement is found between experimental and computed diffraction contrast images for a single layer structure. Methods to handle the extension to a multiperiod SLS structure are suggested. In a 40-period SLS of relatively low Ge fraction (x= 0.17), loss of coherency is typically observed on annealing at the first Gex, Si1-x/Si interface. For higher Ge concentrations (x= 0.25) loss of coherency occurs at several Gex, Si1-x/Si interfaces in the superlattice. The use of SLS as dislocation “filters” is briefly reviewed and the limitations of this approach to obtaining low dislocation density epitaxial films are considered.