The growth of nitride heterostructures along semipolar directions can help to partially alleviate the problem of carrier separation due to polarization-induced electrostatic fields, as well as the problem of the quantum-confined Stark effect. Further enhancement of the internal quantum efficiency of optoelectronic device active regions can be achieved by combining semipolar orientations with quantum dot (QD) nanostructures. Their morphology and strain state affect the band structure and internal efficiency of optoelectronic devices significantly, and it is thus required to study them in detail.  In the present work, semipolar (11-22) and (10-11)-oriented GaN QDs were grown on (11-22) AlN by plasma-assisted molecular-beam epitaxy, following a modified Stranski–Krastanow growth mode [1]. Cross sectional and plan-view high-resolution transmission and scanning transmission electron microscopy (HRTEM and STEM) observations were employed in order to elucidate the nanostructures and their morphologies. The dominant QD morphology was that of truncated trigonal or tetragonal pyramid with chemically sharp interfaces. Three-dimensional structural models consistent with the experimental observations were constructed indicating that both types of QDs are bounded by (11-2n) and (10-1n) pyramidal planes. The energetically degenerate variants consistent with bicrystal symmetry were identified. Geometrical phase analysis [2] was employed for the elucidation of the internal strain state of the QDs and structural models of the pyramidal interfaces bounding the QDs were obtained using ab initio DFT.

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

References

[1] L. Lahourcade, S. Valdueza-Felip, T. Kehagias, G.P. Dimitrakopulos, P. Komninou, and E. Monroy, Appl. Phys. Lett. 94, 111901 (2009)

[2]   M.J. Hÿtch, E. Snoeck, and R. Kilaas, Ultramicroscopy 74, 131 (1998)

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