Structural and Optical Properties of GaN Quantum Dots
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Structural and Optical Properties of GaN Quantum Dots B. Daudin, N. Gogneau, C. Adelmann, , E. Sarigiannidou, T. Andreev, F. Enjalbert, E. Monroy, F. Fossard, J. L. Rouvière, Y. Hori1, X. Biquard, D. Jalabert,Le Si Dang, M. Tanaka1, O. Oda1 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. 1 NGK Insulators, LTD. 2-54 Sudacho, Mizuhoku, Nagoya, Japan ABSTRACT Growth conditions, structural and optical properties of GaN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value and on growth temperature, the growth mode of GaN deposited on AlN can be either of the Stranski-Krastanow or of the Frank-Van der Merwe type. It will be shown that vertical correlation results in a red shift and in a narrowing of the photoluminescence spectra. Growth of Eu-doped GaN quantum dots embedded in AlN will be described. Intense photoluminescence associated with Eu has been measured, with no GaN band-edge emission, as an evidence that carrier recombination mostly occurs through rare earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of temperature has been put in evidence, as a further confirmation of the recombination of confined carriers through Eu ion excitation.
INTRODUCTION The sustained interest in quantum dots (QDs) of semiconductors is largely due to the peculiar physical properties expected from carrier confinement. For instance, it has been predicted that lasers with QDs in the active layer should present a low threshold current and no temperature dependence of the threshold current [1]. Such properties combined with the wide emission/absorption spectral range of nitride semiconductors, extending from UV to visible, make the realization of GaN or GaInN QDs a highly challenging issue . Whatever the semiconductor family, the synthesis of quantum dots is often achieved by taking advantage of the Stranski-Krastanow (SK) growth mode. In this mode, the deposition of a strained two-dimensional (2D) wetting layer is followed by elastic strain relaxation through threedimensional islanding above a given critical thickness. This growth mode has been used to grow quantum dots of (In, Ga)As on GaAs [2-4], of InP on GaInP [5], of Ge on Si [6, 7] or of GaN on AlN [8-10]. In the specific case of III-N semiconductors, the SK mode is observed when growing GaN by plasma-assisted molecular beam epitaxy (PAMBE), at high temperature (710–750 °C) under Nrich conditions, the critical thickness being about 2 monolayers (ML) [8, 9]. The formation of three-dimensional (3D) GaN islands has also been observed in ammonia–MBE [10]. However, in this case, island formation does not obey the SK growth mode and a growth interruption under vacuum is required to initiate the 2D/3D transition. The formation of 3D GaN islands has also been demonstrated in metal-organic chemical va
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