In III-Nitride compound semiconductor nanotechnology, a lot of emphasis is placed on achieving good quality epilayers grown along nonpolar and semipolar orientations, in order to reduce the polarization-induced internal electrostatic fields and the quantum-confined Stark effect. During growth of nonpolar or semipolar nitrides on r-plane or m-plane sapphire respectively, misoriented nanocrystals are introduced at the epilayer/substrate interface causing formation of threading dislocations (TDs) [1]. This phenomenon is facilitated by the high order of common symmetry conserved by the 90^o<11-20> rotation which characterizes the misorientation between the nanocrystals and the matrix epilayer. In the present work we investigate by atomistic simulations, combined with high resolution transmission electron microscopy (HRTEM), the (10-10)/(2-1-12) grain boundary (GB) which was observed in a-plane GaN grown on r-plane sapphire. For the atomistic simulations a modified Stillinger –Weber empirical interatomic potential was employed. Two principal structural configurations were compared, in particular one comprising interfacial dislocations and a GB with interfacial disconnections i.e. dislocations with step character. It was found that the bicrystal comprising dislocations exhibits strain due to curvature at the points where the defects are located. On the other hand the GB comprising disconnections exhibits no curvature and is consistent the HRTEM observations. The Burgers vector of the disconnections is 1/6<2-203> as verified using circuit mapping. Due to nodal balance, two such defects can combine to give a TD emanating into the epilayer, in agreement with the experimental observations.

Acknowledgement: Support under the 7th European Framework STREP Project DOTSENSE (Grant No 224212) and MRTN-CT-2004-005583 (PARSEM) is greatly acknowledged

References

[1] J. Smalc-Koziorowska, G.P. Dimitrakopulos, S.-L. Sahonta, G. Tsiakatouras, A. Georgakilas, and Ph. Komninou, Appl. Phys. Lett. 89, 021910 (2008)

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