Image contrast of dislocations and atomic steps on (111) silicon surface in reflection electron microscopy

Abstract

Observations on dislocations emergent at the (111) surfaces of silicon crystals and on atomic surface steps by means of reflection electron microscopy (REM) are described and analysed in detail in terms of Bragg contrast in in-focus images and Fresnel contrast in out of focus images. Screw dislocations were clearly identified as places at which surface steps terminate accompanying characteristic line contrasts. Their image is a dark line for the exact Bragg setting with a width of about 0.3—0.7 μm. At settings of off-Bragg condition, their images consist of bright and dark wings. In out of focus conditions, the images consist of pairs of bright and dark Eresnel fringes. The sense of the Burgers vectors can be determined from the image contrast. Edge dislocations were also seen as similar line images but without the termination of the surface steps. The contrast of surface steps was shown to be due to a particular distortion of the lattices around the steps.