Influence of AIN Overgrowth on GaN Nanostructures Grown by Molecular Beam Epitaxy
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Influence of AlN Overgrowth on GaN Nanostructures Grown by Molecular Beam Epitaxy N. Gogneau, E. Monroy, D. Jalabert, E. Sarigiannidou, J. L. Rouvière, and B. Daudin
Equipe CEA-CNRS-UJF Nanophysique et Semiconducteurs, Département de Recherche Fondamentale sur la Matière Condensée, SP2M/PSC, CEA-Grenoble, 17 rue des Martyrs, 38054-Grenoble Cedex 9, France ABSTRACT The effects of AlN overgrowth on the structural properties of GaN nanostructures grown at 750°C by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy and transmission electron microscopy. The capping process induces a remarkable change in the dimension of the nanostructures. We demonstrate that the thickness/size reduction occurs at the first stage of AlN overgrowth and affects only the top GaN/AlN interface. This phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures and depends on the strain state of the nanostructures. INTRODUCTION The use of nanostructures in devices requires a precise control of their dimensions. In the specific case of GaN nanostructures grown on AlN by plasma-assisted molecular beam epitaxy (PAMBE) the growth mode strongly depends on the substrate temperature and on the Ga/N ratio, allowing us to grow either quantum wells (QWs) or quantum dots (QDs) [1]. The impact of the AlN cap layer on the structural properties of GaN nanostructures has not been addressed so far, although it is a crucial issue for the development of reproducible devices. In this work, we demonstrate that the embedding of GaN nanostructures into an AlN matrix induces a thickness/size reduction of these nanostructures, which occurs at the first stage of the AlN overgrowth. This process is associated with an exchange mechanism at the top GaN/AlN interface between the Al atoms from the cap layer and Ga atoms from the nanostructure. EXPERIMENTAL Samples were grown on AlN templates (deposited by MOCVD on c-sapphire) in a MECA2000 plasma-assisted MBE chamber, active nitrogen being supplied by a radio-frequency plasma cell. The substrate temperature was fixed at 750°C and the growth rate at 0.27 ML/s for both GaN nanostructures and AlN cap layer. Fluxes in monolayers per second (ML/s) have been deduced from reflection high-electron energy diffraction (RHEED) intensity oscillations at low temperature to prevent a possible overestimation due to metal desorption. The growth of GaN is performed under Ga-rich conditions, leading to the inhibition of the 2D/3D transition due to the formation of a Ga-film on the growing surface during the growth of GaN on AlN. The GaN QWs were covered with a 50–100 nm layer of AlN under strongly Alrich conditions by switching from Ga to Al flux while maintaining the N flux. During the capping process, the Ga excess segregates on the AlN surface and desorbs. By contrast, the GaN QDs were embedded after a growth interruption under vacuum, which is required to make possible the re-arrangement of the 2D GaN layer into 3
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