Blocking of a threading dislocation

When the threading segment is still far from the misfit dislocation in its path compared to h, its behavior is that of an isolated threading dislocation in the strained film. As it approaches the misfit dislocation, however, the shear stress acting on its glide plane is altered from the field 

This is the total work that must be done to overcome the retarding force as the threading dislocation moves past the misfit dislocation. This result provides no direct information on how this work is distributed over the path taken by the threading dislocation. However, it is evident that the stress field of the misfit dislocation acts to some degree to retard the progress of the threading dislocation. 

If hif is then required to be the critical thickness corresponding to the effective mismatch strain 7, that is, if 

This criterion can be interpreted in the following way. For thin film 

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The issue of blocking of a threading dislocation is investigated here on the basis of screw dislocation models, following the general approach outlined by Freund (1990) for a buried layer and by Freund (1990) for interaction of 60 degree dislocations in an equi-biaxial strain field in a film with a free surface. The model system is illustrated in Figure 7.5. A film of thickness h is bonded to a relatively thick substrate. The film supports a mismatch shear strain e = An interface misfit dislocation with its... 

Fig. 7.7. Curve in the plane of film thickness hi h/b) versus mismatch strain 7m showing the criterion described by (7.17) for blocking of a threading dislocation due to a misfit dislocation in its path. Also shown by the dashed curve is the critical thickness criterion for the same system as given in (7.13). Blocking is predicted for states below and to the left of the solid curve, whereas the blocking mechanism should not be effective for states above and to the right of the solid curve.

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