THE DETERMINATION OF STRAIN IN SI-GE SUPERLATTICES BY ELECTRON-DIFFRACTION IN A SCANNING-TRANSMISSION ELECTRON-MICROSCOPE

The nanometre scale of the novel strained layer electronic devices now being grown requires characterisation techniques of a corresponding resolution. This work employs the subnanometre probe of a dedicated scanning transmission electron microscope to investigate individual layers in a cross-sectioned SiGe superlattice. Using recently developed instrumentation, microdiffraction patterns have been obtained at very high resolution and the strains in each layer quantified by analysing the position of the deficit higher order Laue zone lines in the zero order beam. The experimental patterns are fitted to computer simulations incorporating possible dynamical effects. The results from a 10 nm SiGe layer are shown to be in good agreement with bulk X-ray diffraction analysis, with an accuracy limited only by the fundamental constraints of diffraction from a laterally finite sample. Hence surface relaxation, a major complication with previous applications of electron microscopy to strain measurement, can be ignored for the specimen geometry that the small probe allows. It is anticipated that the technique can in future be applied to multilayer structures which are not amenable to bulk characterisation