L05B: Line defects (dislocations) Two basic types – edge and screw, plus a mixture of these. The edge dislocation can be imagined as being formed by an extra half-plane of atoms inserted into the structure (or removal of half a plane) Note distortion of lattice caused by its presence. Characterized by its direction and Burgers vector, which is found by making a circuit around the core. The Burgers vector and direction are perpendicular for edge dislocations. Shear stress parallel to the Burgers vector causes the dislocation to move.  VMSE Last revised September 25, 2013 by W.R. Wilcox, Clarkson University

Screw dislocations Can imagine as being formed by cutting half way through the crystal along a plane, and then shifting one part relative to the other in the direction of the cut. Again, the lattice is distorted near the dislocation core. Now the Burgers vector is parallel to the dislocation. Again, shear stress parallel to the Burgers vector causes the dislocation to move through the crystal.  VMSE Burgers vector b Dislocation

Edge, Screw, and Mixed Dislocations  VMSE Notice that the Burgers vector does not change along the dislocation

Dislocations in non-metals Much more complex because of covalent and ionic bonding. If the unit cell is large, then the Burgers vector is also large. Often carry an electric charge, which can depend on impurity doping. Example of one model for a <110> edge dislocation in silicon: http://www.ornl.gov/info/ornlreview/v30n3-4/edge.htm

Observation of dislocations Many methods have been used to observe dislocations, either directly or indirectly. Most rely on observation of the strain field around the dislocation core. For example, transmission electron microscopy X-ray topography (using Brookhaven synchrotron “light source” Clarkson grad student observed dislocation motion in GaAs.) Transmission optical microscopy using crossed polarizers for ceramic crystals. “Decoration” of dislocations by impurities. Etch pits where dislocations emerge on surface. Growth spirals where screw dislocations emerge on surfaces. (0001) face of carborundum (SiC)