Defects In Solids

01. An L shaped edge dislocation ABC in a crystal is shown in Figure below. The stresses acting on the

crystal are also shown. Determine which dislocation segment moves and which dislocation segment

does not move. What will be the shape of the crystal when the dislocation segment moves continuously.

Can you observe these movements in the TEM in-situ if the dislocation is present in a foil. If you can

only apply tensile components in the TEM stage, show how the tensile stresses should be acting.

2. A screw dislocation in an fcc crystal with Burgers vector of the type a12 [1101 has split into Shockley

partial dislocations of the a/2 <112> type. Thus AB=A8i-6b. Does the split configuration of the

dislocation have any hydrostatic component of stress. If it has, determine the hydrostatic stress

assuming a separation, d, between the partial dislocations. Does it vary as hr or as 1r 2 at large

distances compared to d. Here, r, is the radial distance from the center of the position of the partial

dislocations.

3. In a GaAs film grown on Si crystal, two 60 degree dislocations combine to form a 90 degree

dislocation with Burgers vector [(a/2)[1-10]. The sense vectors of all the dislocations are parallel to each

other and remain along [110] as shown in figure below. Assign the Burgers vectors of all the three

dislocations using the Thompson tetrahedron. Is it energetically favorable or a stress is needed to assist

the combination. Construct an image dislocation model with respect to the free surface shown. Assume

the two dislocations are situated at a height h 1 from the free surface. Write the total energy in terms of

self and interaction energy terms without actually calculating the total energy. Note the right handed

coordinate system is chosen with [110] perpendicular to the plane and [1-10] lying in the plane. [001] is

perpendicular to the free surface of the GaAs film.

001

Free surface

GaAs

110 Si

110