Imaging screw dislocations at atomic resolution by aberration-corrected electron optical sectioning

H Yang,P B Hirsch,J G Lozano,L Jones,P D Nellist,T J Pennycook

doi:10.1038/ncomms8266

Abstract

Screw dislocations play an important role in materials' mechanical, electrical and optical properties. However, imaging the atomic displacements in screw dislocations remains challenging. Although advanced electron microscopy techniques have allowed atomic-scale characterization of edge dislocations from the conventional end-on view, for screw dislocations, the atoms are predominantly displaced parallel to the dislocation line, and therefore the screw displacements are parallel to the electron beam and become invisible when viewed end-on. Here we show that screw displacements can be imaged directly with the dislocation lying in a plane transverse to the electron beam by optical sectioning using annular dark field imaging in a scanning transmission electron microscope. Applying this technique to a mixed [a+c] dislocation in GaN allows direct imaging of a screw dissociation with a 1.65-nm dissociation distance, thereby demonstrating a new method for characterizing dislocation core structures.

Highlights

Screw dislocations play an important role in materials’ mechanical, electrical and optical properties
We show that the helicoidal displacements around a screw can be imaged directly with the dislocation lying in a plane transverse to the electron beam by optical sectioning using annular dark field (ADF) imaging in a scanning transmission electron microscope (STEM)
It should be noted that specimens used here and in the previous work[11] involved different synthesis methods but show the same dissociation reaction for the GaN [a þ c] mixed dislocations. These results show that the characteristic helicoid structure of screw dislocations can be imaged more generally in cross-section using the STEM optical sectioning technique, which provides a direct measurement of the screw Burgers vectors

Summary

Introduction

Screw dislocations play an important role in materials’ mechanical, electrical and optical properties. A model of the c-type screw dislocation along [0001] in GaN is used, and atomic positions in single layers parallel to ð2110Þ planes normal to the electron beam are shown at various depths relative to the dislocation line.

Results

Conclusion