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Failure mechanism of AlN nanocaps used to protect RE-implanted GaN during high temperature annealing

Nogales, E. and Martin, R.W. and O'Donnell, K.P. and Lorenz, K. and Alves, E. and Ruffenach, S. and Briot, O. (2006) Failure mechanism of AlN nanocaps used to protect RE-implanted GaN during high temperature annealing. Applied Physics Letters, 88. p. 31902. ISSN 0003-6951

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The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 °C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 m which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.