Cross Slip of Dislocation Loops in GaN Under Shear

Report No. ARL-RP-476
Authors: Chi-Chin Wu; Scott N. Weingarten; Peter W. Chung
Date/Pages: March 2014; 10 pages
Abstract: This work explores possible cross-slip mechanisms of gliding type <a> = a/3[1 -2 1 0] dislocation loops in wurtzite gallium nitride (GaN) as a function of slip plane. A modified form of the dislocation dynamics code Para-DiS was employed using isotropic linear elasticity and dislocation mobilities estimated in part from molecular dynamics (MD) simulations. Under an externally applied uniform stress, the occurrence of cross slip events is highly dependent on the initial dislocation slip plane. The basal plane is the preferred active plane, owing to the greater mobility of <a> type segments on that plane, over the other planes considered including the prismatic (-1 0 1 0) and two equivalent pyramidal planes (-1 0 1 1) and (1 0 -1 1). For an applied stress state, cross-slip processes are more readily seen from the prismatic-to-basal planes or the pyramidal-to-basal planes, and neither is found to occur in reverse. Cross slip by climb is not presently considered. In all cases, cross-slip events occur after the loop expands until a greater number of screw-oriented segments are able to access the cross-slip plane and the resolved stresses on the plane become sufficiently large. In comparison with dislocations found in GaN previously, the calculations suggest that some threading dislocations along the [0001] direction that have edge character may have been formed from loops whose screw segments slip and escape on basal planes leaving only the edge segments.
Distribution: Approved for public release
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Last Update / Reviewed: March 1, 2014