Formation of quantum dots by self-rearrangement of metastable 2D GaN
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Formation of quantum dots by self-rearrangement of metastable 2D GaN N. Gogneau, D. Jalabert, E. Monroy, C. Adelmann and B. Daudin. CEA / CNRS / UJF Research Group “Nanophysique et Semiconducteurs” Dept. de Recherche Fondamentale sur la Matière Condensée, SP2M/PSC CEA - Grenoble, 17 rue des Martyrs, 38054 - Grenoble cedex 9, France. ABSTRACT We propose a new procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum from a GaN 2D layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the GaN coverage. We demonstrate that the QD density can be controlled in the 1010 cm-2 to 2 x 1011 cm-2 range. It is shown that beyond a given GaN thickness there is a coexistence between elastic and plastic relaxation. INTRODUCTION The Stranski-Krastanow (SK) growth mode is a widely used method to growth semiconductor self-organized quantum dots. The deposition of a strained two-dimensional (2D) wetting layer is followed by elastic relaxation through three-dimensional (3D) islanding above a given critical thickness (∼2.0 monolayers (ML) for GaN on AlN) [1-3]. In the specific case of nitride semiconductors grown by plasma-assisted molecular beam epitaxy (PAMBE), SK mode is observed when growing GaN at high temperature (710 – 750 °C) under N-rich conditions. In contrast, Ga-rich growth conditions inhibit the SK growth mode due to the formation of a selfregulated Ga film, about 2 ML thick, on the growing surface [4]. Taking advantage of Ga surfactant effect, we report in this article a new method to growth GaN QDs. We describe the structural properties of these “modified SK GaN islands” and demonstrate the possibility to control their density and size as a function of the GaN coverage. In addition, it will be shown that beyond a given GaN thickness there is a coexistence between elastic and plastic relaxation. EXPERIMENTAL Samples were grown on AlN templates deposited by metal-organic chemical vapor deposition (MOCVD) on c-sapphire. After a standard cleaning procedure, they were introduced in a MECA2000 molecular beam epitaxy chamber equipped with standard effusion cells for Al and Ga evaporation. Active nitrogen was produced by dissociation of N2 in a radio-frequency plasma cell. The experimental procedure is schematized in figure 1. Prior to GaN deposition, a thin (∼100 nm) AlN buffer layer is deposited on the pseudo-substrate. The GaN layer is grown under Ga-rich conditions, at a fixed substrate temperature of 750 °C and a growth rate of 0.23 ML/s, leading to the formation of a continuous Ga bilayer on the growing surface (figure 1-a). This Ga-
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film inhibits the SK growth mode and favors a layer-by layer growth (Frank-Van der Merwe mode). When we shutter the N-flux, while maintaining the Ga flux, the Ga bilayer is regenerated and the surface remains 2D. The RHEED pattern is unchanged (figure 1-b). Under vacuum, the Ga-film desorbs rap
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