A. R. Smith, R. M. Feenstra, D. W. Greve, J. Neugebauer, and J. E. Northrup "Reconstructions of the GaN(0001-bar) Surface" Phys. Rev. Lett. 79, 3934–3937 (1997).
 

Description:

Most optoelectronic devices malfunction when dislocations are present because these defects cause rapid recombination of holes (positive charge) with electrons (negative charge), the charged particles that carry electric current, without conversion of their available energy into photons.  Gallium nitride, with properties closer to a ceramic material than a proper semiconductor, has become the optimal choice for blue laser-light and blue light-emitting diodes. The dislocation density in this material has been huge--more than 10⁹ to 10¹¹ cm^-2, which is unprecedented for a traditional semiconductor device--but amazingly, efficient light-emitting diodes and lasers are feasible in these heavily dislocated crystals.  One spectacular example of crystal growth at a single screw dislocation on the nitrogen face of a GaN surface is shown in this scanning tunneling micrograph.  A single screw dislocation on a GaN crystal surface of wurtzite structure can be seen. Two Ga-N bilayer growth fronts surround the dislocation, and several reconstructed surface domains are clearly visible.

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