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Interfacial structure of semipolar AlN grown on m-plane sapphire by PAMBE

Thomas Kehagias 1Lise Lahourcade 2Antiopi Lotsari 1Eva Monroy 2George P. Dimitrakopulos 1Philomela Komninou 1

1. Dept. of Physics, Aristotle University of Thessaloniki, Thessaloniki 54 124, Greece
2. CEA-CNRS, 17 avenue des Martyrs, Grenoble F-38054, France


Interfacial properties of AlN grown on m-plane sapphire by plasma-assisted MBE have been investigated in the context of achieving high-quality GaN/AlN heterostructures with reduced internal polarization. We investigate ~200 nm-thick epilayers deposited either directly on m-sapphire or after 30 min of sapphire nitridation at the growth temperature (~720oC). The active nitrogen flux was tuned to obtain a growth rate of 0.3 ML/s (~280 nm/h) under Al-rich conditions. AlN was synthesized under a III/V ratio of ~0.95 to achieve two-dimensional layers. High-resolution transmission electron microscopy (HRTEM) identified the dominant film orientation as (11-22) semipolar, with the AlN basal planes aligned parallel to the (1-104) plane of sapphire. This result is consistent with in situ RHEED observations and x-ray diffraction studies. An interfacial zone comprising additional AlN nanocrystals with the m-plane orientation was observed. In the case of growth without nitridation, the average m-plane nanocrystal size was 30 nm, but it was increased significantly in the case of nitridated sapphire, with some nonpolar crystallites exceeding 100 nm. A good epitaxial relationship with the substrate was observed both for the case of semipolar as well as for nonpolar AlN, comprising patches of regularly spaced misfit dislocations which were characterized using circuit mapping. However, the periodicity of the interfacial structure was perturbed by interfacial roughness comprising nanofacets on the (1-104), (1-10-2) and {1-210} sapphire planes. Often these nanofacets were found arranged in v-shaped formations, and were correlated to the introduction of misoriented nanocrystals. The nonpolar and semipolar orientations are interrelated by a 90o<11-20> relative rotation leading to the formation of low-energy grain boundaries between them.

Acknowledgement: Work supported under the 7th European Framework Project DOTSENSE (Grant No. STREP 224212).


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Related papers

Presentation: Poster at E-MRS Fall Meeting 2009, Symposium C, by George P. Dimitrakopulos
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-11 15:24
Revised:   2009-06-07 00:48